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Net::SSLeay
NAME SYNOPSIS DESCRIPTION COMPATIBILITY OVERVIEW EXAMPLES INSTALLATION LIMITATIONS KNOWN BUGS AND CAVEATS DIAGNOSTICS SECURITY BUGS AUTHOR COPYRIGHT LICENSE SEE ALSO
NAME
    Net::SSLeay - Perl bindings for OpenSSL and LibreSSL

SYNOPSIS
      use Net::SSLeay qw(get_https post_https sslcat make_headers make_form);

      ($page) = get_https('www.bacus.pt', 443, '/');                 # Case 1

      ($page, $response, %reply_headers)
             = get_https('www.bacus.pt', 443, '/',                   # Case 2
                    make_headers(User-Agent => 'Cryptozilla/5.0b1',
                                 Referer    => 'https://www.bacus.pt'
                    ));

      ($page, $result, %headers) =                                   # Case 2b
             = get_https('www.bacus.pt', 443, '/protected.html',
                  make_headers(Authorization =>
                               'Basic ' . MIME::Base64::encode("$user:$pass",''))
                  );

      ($page, $response, %reply_headers)
             = post_https('www.bacus.pt', 443, '/foo.cgi', '',       # Case 3
                    make_form(OK   => '1',
                              name => 'Sampo'
                    ));

      $reply = sslcat($host, $port, $request);                       # Case 4

      ($reply, $err, $server_cert) = sslcat($host, $port, $request); # Case 5

      $Net::SSLeay::trace = 2;  # 0=no debugging, 1=ciphers, 2=trace, 3=dump data

      Net::SSLeay::initialize(); # Initialize ssl library once

DESCRIPTION
    This module provides Perl bindings for libssl (an SSL/TLS API) and libcrypto (a cryptography
    API).

COMPATIBILITY
    Net::SSLeay supports the following libssl implementations:

    *   Any stable release of OpenSSL <https://www.openssl.org> in the 0.9.8 - 3.0 branches, except
        for OpenSSL 0.9.8 - 0.9.8b.

    *   Any stable release of LibreSSL <https://www.libressl.org> in the 2.0 - 3.4 series, except
        for LibreSSL 3.2.2 and 3.2.3.

    Net::SSLeay may not function as expected with releases other than the ones listed above due to
    libssl API incompatibilities, or, in the case of LibreSSL, because of deviations from the libssl
    API.

    Net::SSLeay is only as secure as the underlying libssl implementation you use. Although
    Net::SSLeay maintains compatibility with old versions of OpenSSL and LibreSSL, it is strongly
    recommended that you use a version of OpenSSL or LibreSSL that is supported by the
    OpenSSL/LibreSSL developers and/or your operating system vendor. Many unsupported versions of
    OpenSSL and LibreSSL are known to contain severe security vulnerabilities. Refer to the OpenSSL
    Release Strategy <https://www.openssl.org/policies/releasestrat.html> and LibreSSL Support
    Schedule <https://www.libressl.org/releases.html> for information on which versions are
    currently supported.

    The libssl API has changed significantly since OpenSSL 0.9.8: hundreds of functions have been
    added, deprecated or removed in the intervening versions. Although this documentation lists all
    of the functions and constants that Net::SSLeay may expose, they will not be available for use
    if they are missing from the underlying libssl implementation. Refer to the compatibility notes
    in this documentation, as well as the OpenSSL/LibreSSL manual pages, for information on which
    OpenSSL/LibreSSL versions support each function or constant. At run-time, you can check whether
    a function or constant is exposed before calling it using the following convention:

        if ( defined &Net::SSLeay::libssl_function ) {
            # libssl_function() (or SSL_libssl_function()) is available
            Net::SSLeay::libssl_function(...);
        }

OVERVIEW
    Net::SSLeay module basically comprise of:

    *   High level functions for accessing web servers (by using HTTP/HTTPS)

    *   Low level API (mostly mapped 1:1 to openssl's C functions)

    *   Convenience functions (related to low level API but with more perl friendly interface)

    There is also a related module called Net::SSLeay::Handle included in this distribution that you
    might want to use instead. It has its own pod documentation.

  High level functions for accessing web servers
    This module offers some high level convenience functions for accessing web pages on SSL servers
    (for symmetry, the same API is offered for accessing http servers, too), an "sslcat()" function
    for writing your own clients, and finally access to the SSL api of the SSLeay/OpenSSL package so
    you can write servers or clients for more complicated applications.

    For high level functions it is most convenient to import them into your main namespace as
    indicated in the synopsis.

   Basic set of functions
    *   get_https

    *   post_https

    *   put_https

    *   head_https

    *   do_https

    *   sslcat

    *   https_cat

    *   make_form

    *   make_headers

    Case 1 (in SYNOPSIS) demonstrates the typical invocation of get_https() to fetch an HTML page
    from secure server. The first argument provides the hostname or IP in dotted decimal notation of
    the remote server to contact. The second argument is the TCP port at the remote end (your own
    port is picked arbitrarily from high numbered ports as usual for TCP). The third argument is the
    URL of the page without the host name part. If in doubt consult the HTTP specifications at
    <http://www.w3c.org>.

    Case 2 (in SYNOPSIS) demonstrates full fledged use of "get_https()". As can be seen,
    "get_https()" parses the response and response headers and returns them as a list, which can be
    captured in a hash for later reference. Also a fourth argument to "get_https()" is used to
    insert some additional headers in the request. "make_headers()" is a function that will convert
    a list or hash to such headers. By default "get_https()" supplies "Host" (to make virtual
    hosting easy) and "Accept" (reportedly needed by IIS) headers.

    Case 2b (in SYNOPSIS) demonstrates how to get a password protected page. Refer to the HTTP
    protocol specifications for further details (e.g. RFC-2617).

    Case 3 (in SYNOPSIS) invokes "post_https()" to submit a HTML/CGI form to a secure server. The
    first four arguments are equal to "get_https()" (note that the empty string ('') is passed as
    header argument). The fifth argument is the contents of the form formatted according to CGI
    specification. Do not post UTF-8 data as content: use utf8::downgrade first. In this case the
    helper function "make_https()" is used to do the formatting, but you could pass any string.
    "post_https()" automatically adds "Content-Type" and "Content-Length" headers to the request.

    Case 4 (in SYNOPSIS) shows the fundamental "sslcat()" function (inspired in spirit by the
    "netcat" utility :-). It's your swiss army knife that allows you to easily contact servers, send
    some data, and then get the response. You are responsible for formatting the data and parsing
    the response - "sslcat()" is just a transport.

    Case 5 (in SYNOPSIS) is a full invocation of "sslcat()" which allows the return of errors as
    well as the server (peer) certificate.

    The $trace global variable can be used to control the verbosity of the high level functions.
    Level 0 guarantees silence, level 1 (the default) only emits error messages.

   Alternate versions of high-level API
    *   get_https3

    *   post_https3

    *   put_https3

    *   get_https4

    *   post_https4

    *   put_https4

    The above mentioned functions actually return the response headers as a list, which only gets
    converted to hash upon assignment (this assignment looses information if the same header occurs
    twice, as may be the case with cookies). There are also other variants of the functions that
    return unprocessed headers and that return a reference to a hash.

      ($page, $response, @headers) = get_https('www.bacus.pt', 443, '/');
      for ($i = 0; $i < $#headers; $i+=2) {
          print "$headers[$i] = " . $headers[$i+1] . "\n";
      }

      ($page, $response, $headers, $server_cert)
        = get_https3('www.bacus.pt', 443, '/');
      print "$headers\n";

      ($page, $response, $headers_ref)
        = get_https4('www.bacus.pt', 443, '/');
      for $k (sort keys %{$headers_ref}) {
          for $v (@{$$headers_ref{$k}}) {
              print "$k = $v\n";
          }
      }

    All of the above code fragments accomplish the same thing: display all values of all headers.
    The API functions ending in "3" return the headers simply as a scalar string and it is up to the
    application to split them up. The functions ending in "4" return a reference to a hash of arrays
    (see perlref and perllol if you are not familiar with complex perl data structures). To access a
    single value of such a header hash you would do something like

      print $$headers_ref{COOKIE}[0];

    Variants 3 and 4 also allow you to discover the server certificate in case you would like to
    store or display it, e.g.

      ($p, $resp, $hdrs, $server_cert) = get_https3('www.bacus.pt', 443, '/');
      if (!defined($server_cert) || ($server_cert == 0)) {
          warn "Subject Name: undefined, Issuer  Name: undefined";
      } else {
          warn 'Subject Name: '
              . Net::SSLeay::X509_NAME_oneline(
                     Net::SSLeay::X509_get_subject_name($server_cert))
                  . 'Issuer  Name: '
                      . Net::SSLeay::X509_NAME_oneline(
                             Net::SSLeay::X509_get_issuer_name($server_cert));
      }

    Beware that this method only allows after the fact verification of the certificate: by the time
    "get_https3()" has returned the https request has already been sent to the server, whether you
    decide to trust it or not. To do the verification correctly you must either employ the OpenSSL
    certificate verification framework or use the lower level API to first connect and verify the
    certificate and only then send the http data. See the implementation of "ds_https3()" for
    guidance on how to do this.

   Using client certificates
    Secure web communications are encrypted using symmetric crypto keys exchanged using encryption
    based on the certificate of the server. Therefore in all SSL connections the server must have a
    certificate. This serves both to authenticate the server to the clients and to perform the key
    exchange.

    Sometimes it is necessary to authenticate the client as well. Two options are available: HTTP
    basic authentication and a client side certificate. The basic authentication over HTTPS is
    actually quite safe because HTTPS guarantees that the password will not travel in the clear.
    Never-the-less, problems like easily guessable passwords remain. The client certificate method
    involves authentication of the client at the SSL level using a certificate. For this to work,
    both the client and the server have certificates (which typically are different) and private
    keys.

    The API functions outlined above accept additional arguments that allow one to supply the client
    side certificate and key files. The format of these files is the same as used for server
    certificates and the caveat about encrypting private keys applies.

      ($page, $result, %headers) =                                   # 2c
             = get_https('www.bacus.pt', 443, '/protected.html',
                  make_headers(Authorization =>
                               'Basic ' . MIME::Base64::encode("$user:$pass",'')),
                  '', $mime_type6, $path_to_crt7, $path_to_key8);

      ($page, $response, %reply_headers)
             = post_https('www.bacus.pt', 443, '/foo.cgi',           # 3b
                  make_headers('Authorization' =>
                               'Basic ' . MIME::Base64::encode("$user:$pass",'')),
                  make_form(OK   => '1', name => 'Sampo'),
                  $mime_type6, $path_to_crt7, $path_to_key8);

    Case 2c (in SYNOPSIS) demonstrates getting a password protected page that also requires a client
    certificate, i.e. it is possible to use both authentication methods simultaneously.

    Case 3b (in SYNOPSIS) is a full blown POST to a secure server that requires both password
    authentication and a client certificate, just like in case 2c.

    Note: The client will not send a certificate unless the server requests one. This is typically
    achieved by setting the verify mode to "VERIFY_PEER" on the server:

      Net::SSLeay::set_verify(ssl, Net::SSLeay::VERIFY_PEER, 0);

    See "perldoc ~openssl/doc/ssl/SSL_CTX_set_verify.pod" for a full description.

   Working through a web proxy
    *   set_proxy

    "Net::SSLeay" can use a web proxy to make its connections. You need to first set the proxy host
    and port using "set_proxy()" and then just use the normal API functions, e.g:

      Net::SSLeay::set_proxy('gateway.myorg.com', 8080);
      ($page) = get_https('www.bacus.pt', 443, '/');

    If your proxy requires authentication, you can supply a username and password as well

      Net::SSLeay::set_proxy('gateway.myorg.com', 8080, 'joe', 'salainen');
      ($page, $result, %headers) =
             = get_https('www.bacus.pt', 443, '/protected.html',
                  make_headers(Authorization =>
                               'Basic ' . MIME::Base64::encode("susie:pass",''))
                  );

    This example demonstrates the case where we authenticate to the proxy as "joe" and to the final
    web server as "susie". Proxy authentication requires the "MIME::Base64" module to work.

   HTTP (without S) API
    *   get_http

    *   post_http

    *   tcpcat

    *   get_httpx

    *   post_httpx

    *   tcpxcat

    Over the years it has become clear that it would be convenient to use the light-weight flavour
    API of "Net::SSLeay" for normal HTTP as well (see "LWP" for the heavy-weight object-oriented
    approach). In fact it would be nice to be able to flip https on and off on the fly. Thus regular
    HTTP support was evolved.

      use Net::SSLeay qw(get_http post_http tcpcat
                          get_httpx post_httpx tcpxcat
                          make_headers make_form);

      ($page, $result, %headers)
             = get_http('www.bacus.pt', 443, '/protected.html',
                  make_headers(Authorization =>
                               'Basic ' . MIME::Base64::encode("$user:$pass",''))
                  );

      ($page, $response, %reply_headers)
             = post_http('www.bacus.pt', 443, '/foo.cgi', '',
                    make_form(OK   => '1',
                              name => 'Sampo'
                    ));

      ($reply, $err) = tcpcat($host, $port, $request);

      ($page, $result, %headers)
             = get_httpx($usessl, 'www.bacus.pt', 443, '/protected.html',
                  make_headers(Authorization =>
                               'Basic ' . MIME::Base64::encode("$user:$pass",''))
                  );

      ($page, $response, %reply_headers)
             = post_httpx($usessl, 'www.bacus.pt', 443, '/foo.cgi', '',
                    make_form(OK   => '1',  name => 'Sampo' ));

      ($reply, $err, $server_cert) = tcpxcat($usessl, $host, $port, $request);

    As can be seen, the "x" family of APIs takes as the first argument a flag which indicates
    whether SSL is used or not.

  Certificate verification and Certificate Revocation Lists (CRLs)
    OpenSSL supports the ability to verify peer certificates. It can also optionally check the peer
    certificate against a Certificate Revocation List (CRL) from the certificates issuer. A CRL is a
    file, created by the certificate issuer that lists all the certificates that it previously
    signed, but which it now revokes. CRLs are in PEM format.

    You can enable "Net::SSLeay CRL" checking like this:

                &Net::SSLeay::X509_STORE_set_flags
                    (&Net::SSLeay::CTX_get_cert_store($ssl),
                     &Net::SSLeay::X509_V_FLAG_CRL_CHECK);

    After setting this flag, if OpenSSL checks a peer's certificate, then it will attempt to find a
    CRL for the issuer. It does this by looking for a specially named file in the search directory
    specified by CTX_load_verify_locations. CRL files are named with the hash of the issuer's
    subject name, followed by ".r0", ".r1" etc. For example "ab1331b2.r0", "ab1331b2.r1". It will
    read all the .r files for the issuer, and then check for a revocation of the peer certificate in
    all of them. (You can also force it to look in a specific named CRL file., see below). You can
    find out the hash of the issuer subject name in a CRL with

            openssl crl -in crl.pem -hash -noout

    If the peer certificate does not pass the revocation list, or if no CRL is found, then the
    handshaking fails with an error.

    You can also force OpenSSL to look for CRLs in one or more arbitrarily named files.

        my $bio = Net::SSLeay::BIO_new_file($crlfilename, 'r');
        my $crl = Net::SSLeay::PEM_read_bio_X509_CRL($bio);
        if ($crl) {
            Net::SSLeay::X509_STORE_add_crl(
                 Net::SSLeay::CTX_get_cert_store($ssl, $crl)
            );
        } else {
            error reading CRL....
        }

    Usually the URLs where you can download the CRLs is contained in the certificate itself and you
    can extract them with

        my @url = Net::SSLeay::P_X509_get_crl_distribution_points($cert)

    But there is no automatic downloading of the CRLs and often these CRLs are too huge to just
    download them to verify a single certificate. Also, these CRLs are often in DER format which you
    need to convert to PEM before you can use it:

        openssl crl -in crl.der -inform der -out crl.pem

    So as an alternative for faster and timely revocation checks you better use the Online Status
    Revocation Protocol (OCSP).

  Certificate verification and Online Status Revocation Protocol (OCSP)
    While checking for revoked certificates is possible and fast with Certificate Revocation Lists,
    you need to download the complete and often huge list before you can verify a single
    certificate.

    A faster way is to ask the CA to check the revocation of just a single or a few certificates
    using OCSP. Basically you generate for each certificate an OCSP_CERTID based on the certificate
    itself and its issuer, put the ids togetether into an OCSP_REQUEST and send the request to the
    URL given in the certificate.

    As a result you get back an OCSP_RESPONSE and need to check the status of the response, check
    that it is valid (e.g. signed by the CA) and finally extract the information about each
    OCSP_CERTID to find out if the certificate is still valid or got revoked.

    With Net::SSLeay this can be done like this:

        # get id(s) for given certs, like from get_peer_certificate
        # or get_peer_cert_chain. This will croak if
        # - one tries to make an OCSP_CERTID for a self-signed certificate
        # - the issuer of the certificate cannot be found in the SSL objects
        #   store, nor in the current certificate chain
        my $cert = Net::SSLeay::get_peer_certificate($ssl);
        my $id = eval { Net::SSLeay::OCSP_cert2ids($ssl,$cert) };
        die "failed to make OCSP_CERTID: $@" if $@;

        # create OCSP_REQUEST from id(s)
        # Multiple can be put into the same request, if the same OCSP responder
        # is responsible for them.
        my $req = Net::SSLeay::OCSP_ids2req($id);

        # determine URI of OCSP responder
        my $uri = Net::SSLeay::P_X509_get_ocsp_uri($cert);

        # Send stringified OCSP_REQUEST with POST to $uri.
        # We can ignore certificate verification for https, because the OCSP
        # response itself is signed.
        my $ua = HTTP::Tiny->new(verify_SSL => 0);
        my $res = $ua->request( 'POST',$uri, {
            headers => { 'Content-type' => 'application/ocsp-request' },
            content => Net::SSLeay::i2d_OCSP_REQUEST($req)
        });
        my $content = $res && $res->{success} && $res->{content}
            or die "query failed";

        # Extract OCSP_RESPONSE.
        # this will croak if the string is not an OCSP_RESPONSE
        my $resp = eval { Net::SSLeay::d2i_OCSP_RESPONSE($content) };

        # Check status of response.
        my $status = Net::SSLeay::OCSP_response_status($resp);
        if ($status != Net::SSLeay::OCSP_RESPONSE_STATUS_SUCCESSFUL())
            die "OCSP response failed: ".
                Net::SSLeay::OCSP_response_status_str($status);
        }

        # Verify signature of response and if nonce matches request.
        # This will croak if there is a nonce in the response, but it does not match
        # the request. It will return false if the signature could not be verified,
        # in which case details can be retrieved with Net::SSLeay::ERR_get_error.
        # It will not complain if the response does not contain a nonce, which is
        # usually the case with pre-signed responses.
        if ( ! eval { Net::SSLeay::OCSP_response_verify($ssl,$resp,$req) }) {
            die "OCSP response verification failed";
        }

        # Extract information from OCSP_RESPONSE for each of the ids.

        # If called in scalar context it will return the time (as time_t), when the
        # next update is due (minimum of all successful responses inside $resp). It
        # will croak on the following problems:
        # - response is expired or not yet valid
        # - no response for given OCSP_CERTID
        # - certificate status is not good (e.g. revoked or unknown)
        if ( my $nextupd = eval { Net::SSLeay::OCSP_response_results($resp,$id) }) {
            warn "certificate is valid, next update in ".
                ($nextupd-time())." seconds\n";
        } else {
            die "certificate is not valid: $@";
        }

        # But in array context it will return detailed information about each given
        # OCSP_CERTID instead croaking on errors:
        # if no @ids are given it will return information about all single responses
        # in the OCSP_RESPONSE
        my @results = Net::SSLeay::OCSP_response_results($resp,@ids);
        for my $r (@results) {
            print Dumper($r);
            # @results are in the same order as the @ids and contain:
            # $r->[0] - OCSP_CERTID
            # $r->[1] - undef if no error (certificate good) OR error message as string
            # $r->[2] - hash with details:
            #   thisUpdate - time_t of this single response
            #   nextUpdate - time_t when update is expected
            #   statusType - integer:
            #      V_OCSP_CERTSTATUS_GOOD(0)
            #      V_OCSP_CERTSTATUS_REVOKED(1)
            #      V_OCSP_CERTSTATUS_UNKNOWN(2)
            #   revocationTime - time_t (only if revoked)
            #   revocationReason - integer (only if revoked)
            #   revocationReason_str - reason as string (only if revoked)
        }

    To further speed up certificate revocation checking one can use a TLS extension to instruct the
    server to staple the OCSP response:

        # set TLS extension before doing SSL_connect
        Net::SSLeay::set_tlsext_status_type($ssl,
            Net::SSLeay::TLSEXT_STATUSTYPE_ocsp());

        # setup callback to verify OCSP response
        my $cert_valid = undef;
        Net::SSLeay::CTX_set_tlsext_status_cb($context,sub {
            my ($ssl,$resp) = @_;
            if (!$resp) {
                # Lots of servers don't return an OCSP response.
                # In this case we must check the OCSP status outside the SSL
                # handshake.
                warn "server did not return stapled OCSP response\n";
                return 1;
            }
            # verify status
            my $status = Net::SSLeay::OCSP_response_status($resp);
            if ($status != Net::SSLeay::OCSP_RESPONSE_STATUS_SUCCESSFUL()) {
                warn "OCSP response failure: $status\n";
                return 1;
            }
            # verify signature - we have no OCSP_REQUEST here to check nonce
            if (!eval { Net::SSLeay::OCSP_response_verify($ssl,$resp) }) {
                warn "OCSP response verify failed\n";
                return 1;
            }
            # check if the certificate is valid
            # we should check here against the peer_certificate
            my $cert = Net::SSLeay::get_peer_certificate();
            my $certid = eval { Net::SSLeay::OCSP_cert2ids($ssl,$cert) } or do {
                warn "cannot get certid from cert: $@";
                $cert_valid = -1;
                return 1;
            };

            if ( $nextupd = eval {
                Net::SSLeay::OCSP_response_results($resp,$certid) }) {
                warn "certificate not revoked\n";
                $cert_valid = 1;
            } else {
                warn "certificate not valid: $@";
                $cert_valid = 0;
            }
        });

        # do SSL handshake here
        ....
        # check if certificate revocation was checked already
        if ( ! defined $cert_valid) {
            # check revocation outside of SSL handshake by asking OCSP responder
            ...
        } elsif ( ! $cert_valid ) {
            die "certificate not valid - closing SSL connection";
        } elsif ( $cert_valid<0 ) {
            die "cannot verify certificate revocation - self-signed ?";
        } else {
            # everything fine
            ...
        }

  Using Net::SSLeay in multi-threaded applications
    IMPORTANT: versions 1.42 or earlier are not thread-safe!

    Net::SSLeay module implements all necessary stuff to be ready for multi-threaded environment -
    it requires openssl-0.9.7 or newer. The implementation fully follows thread safety related
    requirements of openssl library(see <http://www.openssl.org/docs/crypto/threads.html>).

    If you are about to use Net::SSLeay (or any other module based on Net::SSLeay) in multi-threaded
    perl application it is recommended to follow this best-practice:

   Initialization
    Load and initialize Net::SSLeay module in the main thread:

        use threads;
        use Net::SSLeay;

        Net::SSLeay::load_error_strings();
        Net::SSLeay::SSLeay_add_ssl_algorithms();
        Net::SSLeay::randomize();

        sub do_master_job {
          #... call whatever from Net::SSLeay
        }

        sub do_worker_job {
          #... call whatever from Net::SSLeay
        }

        #start threads
        my $master  = threads->new(\&do_master_job, 'param1', 'param2');
        my @workers = threads->new(\&do_worker_job, 'arg1', 'arg2') for (1..10);

        #waiting for all threads to finish
        $_->join() for (threads->list);

    NOTE: Openssl's "int SSL_library_init(void)" function (which is also aliased as
    "SSLeay_add_ssl_algorithms", "OpenSSL_add_ssl_algorithms" and "add_ssl_algorithms") is not
    re-entrant and multiple calls can cause a crash in threaded application. Net::SSLeay implements
    flags preventing repeated calls to this function, therefore even multiple initialization via
    Net::SSLeay::SSLeay_add_ssl_algorithms() should work without trouble.

   Using callbacks
    Do not use callbacks across threads (the module blocks cross-thread callback operations and
    throws a warning). Always do the callback setup, callback use and callback destruction within
    the same thread.

   Using openssl elements
    All openssl elements (X509, SSL_CTX, ...) can be directly passed between threads.

        use threads;
        use Net::SSLeay;

        Net::SSLeay::load_error_strings();
        Net::SSLeay::SSLeay_add_ssl_algorithms();
        Net::SSLeay::randomize();

        sub do_job {
          my $context = shift;
          Net::SSLeay::CTX_set_default_passwd_cb($context, sub { "secret" });
          #...
        }

        my $c = Net::SSLeay::CTX_new();
        threads->create(\&do_job, $c);

    Or:

        use threads;
        use Net::SSLeay;

        my $context; #does not need to be 'shared'

        Net::SSLeay::load_error_strings();
        Net::SSLeay::SSLeay_add_ssl_algorithms();
        Net::SSLeay::randomize();

        sub do_job {
          Net::SSLeay::CTX_set_default_passwd_cb($context, sub { "secret" });
          #...
        }

        $context = Net::SSLeay::CTX_new();
        threads->create(\&do_job);

   Using other perl modules based on Net::SSLeay
    It should be fine to use any other module based on Net::SSLeay (like IO::Socket::SSL) in
    multi-threaded applications. It is generally recommended to do any global initialization of such
    a module in the main thread before calling "threads->new(..)" or "threads->create(..)" but it
    might differ module by module.

    To be safe you can load and init Net::SSLeay explicitly in the main thread:

        use Net::SSLeay;
        use Other::SSLeay::Based::Module;

        Net::SSLeay::load_error_strings();
        Net::SSLeay::SSLeay_add_ssl_algorithms();
        Net::SSLeay::randomize();

    Or even safer:

        use Net::SSLeay;
        use Other::SSLeay::Based::Module;

        BEGIN {
          Net::SSLeay::load_error_strings();
          Net::SSLeay::SSLeay_add_ssl_algorithms();
          Net::SSLeay::randomize();
        }

   Combining Net::SSLeay with other modules linked with openssl
    BEWARE: This might be a big trouble! This is not guaranteed be thread-safe!

    There are many other (XS) modules linked directly to openssl library (like Crypt::SSLeay).

    As it is expected that also "another" module will call "SSLeay_add_ssl_algorithms" at some point
    we have again a trouble with multiple openssl initialization by Net::SSLeay and "another"
    module.

    As you can expect Net::SSLeay is not able to avoid multiple initialization of openssl library
    called by "another" module, thus you have to handle this on your own (in some cases it might not
    be possible at all to avoid this).

   Threading with get_https and friends
    The convenience functions get_https, post_https etc all initialize the SSL library by calling
    Net::SSLeay::initialize which does the conventional library initialization:

        Net::SSLeay::load_error_strings();
        Net::SSLeay::SSLeay_add_ssl_algorithms();
        Net::SSLeay::randomize();

    Net::SSLeay::initialize initializes the SSL library at most once. You can override the
    Net::SSLeay::initialize function if you desire some other type of initialization behaviour by
    get_https and friends. You can call Net::SSLeay::initialize from your own code if you desire
    this conventional library initialization.

  Convenience routines
    To be used with Low level API

        Net::SSLeay::randomize($rn_seed_file,$additional_seed);
        Net::SSLeay::set_cert_and_key($ctx, $cert_path, $key_path);
        $cert = Net::SSLeay::dump_peer_certificate($ssl);
        Net::SSLeay::ssl_write_all($ssl, $message) or die "ssl write failure";
        $got = Net::SSLeay::ssl_read_all($ssl) or die "ssl read failure";

        $got = Net::SSLeay::ssl_read_CRLF($ssl [, $max_length]);
        $got = Net::SSLeay::ssl_read_until($ssl [, $delimit [, $max_length]]);
        Net::SSLeay::ssl_write_CRLF($ssl, $message);

    *   randomize

        seeds the openssl PRNG with "/dev/urandom" (see the top of "SSLeay.pm" for how to change or
        configure this) and optionally with user provided data. It is very important to properly
        seed your random numbers, so do not forget to call this. The high level API functions
        automatically call "randomize()" so it is not needed with them. See also caveats.

    *   set_cert_and_key

        takes two file names as arguments and sets the certificate and private key to those. This
        can be used to set either server certificates or client certificates.

    *   dump_peer_certificate

        allows you to get a plaintext description of the certificate the peer (usually the server)
        presented to us.

    *   ssl_read_all

        see ssl_write_all (below)

    *   ssl_write_all

        "ssl_read_all()" and "ssl_write_all()" provide true blocking semantics for these operations
        (see limitation, below, for explanation). These are much preferred to the low level API
        equivalents (which implement BSD blocking semantics). The message argument to
        "ssl_write_all()" can be a reference. This is helpful to avoid unnecessary copying when
        writing something big, e.g:

            $data = 'A' x 1000000000;
            Net::SSLeay::ssl_write_all($ssl, \$data) or die "ssl write failed";

    *   ssl_read_CRLF

        uses "ssl_read_all()" to read in a line terminated with a carriage return followed by a
        linefeed (CRLF). The CRLF is included in the returned scalar.

    *   ssl_read_until

        uses "ssl_read_all()" to read from the SSL input stream until it encounters a programmer
        specified delimiter. If the delimiter is undefined, $/ is used. If $/ is undefined, "\n" is
        used. One can optionally set a maximum length of bytes to read from the SSL input stream.

    *   ssl_write_CRLF

        writes $message and appends CRLF to the SSL output stream.

  Initialization
    In order to use the low level API you should start your programs with the following incantation:

            use Net::SSLeay qw(die_now die_if_ssl_error);
            Net::SSLeay::load_error_strings();
            Net::SSLeay::SSLeay_add_ssl_algorithms();    # Important!
            Net::SSLeay::ENGINE_load_builtin_engines();  # If you want built-in engines
            Net::SSLeay::ENGINE_register_all_complete(); # If you want built-in engines
            Net::SSLeay::randomize();

  Error handling functions
    I can not emphasize the need to check for error enough. Use these functions even in the most
    simple programs, they will reduce debugging time greatly. Do not ask questions on the mailing
    list without having first sprinkled these in your code.

    *   die_now

    *   die_if_ssl_error

        "die_now()" and "die_if_ssl_error()" are used to conveniently print the SSLeay error stack
        when something goes wrong:

                Net::SSLeay::connect($ssl) or die_now("Failed SSL connect ($!)");


                Net::SSLeay::write($ssl, "foo") or die_if_ssl_error("SSL write ($!)");

    *   print_errs

        You can also use "Net::SSLeay::print_errs()" to dump the error stack without exiting the
        program. As can be seen, your code becomes much more readable if you import the error
        reporting functions into your main name space.

  Sockets
    Perl uses file handles for all I/O. While SSLeay has a quite flexible BIO mechanism and perl has
    an evolved PerlIO mechanism, this module still sticks to using file descriptors. Thus to attach
    SSLeay to a socket you should use "fileno()" to extract the underlying file descriptor:

        Net::SSLeay::set_fd($ssl, fileno(S));   # Must use fileno

    You should also set $| to 1 to eliminate STDIO buffering so you do not get confused if you use
    perl I/O functions to manipulate your socket handle.

    If you need to select(2) on the socket, go right ahead, but be warned that OpenSSL does some
    internal buffering so SSL_read does not always return data even if the socket selected for
    reading (just keep on selecting and trying to read). "Net::SSLeay" is no different from the C
    language OpenSSL in this respect.

  Callbacks
    You can establish a per-context verify callback function something like this:

            sub verify {
                my ($ok, $x509_store_ctx) = @_;
                print "Verifying certificate...\n";
                    ...
                return $ok;
            }

    It is used like this:

            Net::SSLeay::set_verify ($ssl, Net::SSLeay::VERIFY_PEER, \&verify);

    Per-context callbacks for decrypting private keys are implemented.

            Net::SSLeay::CTX_set_default_passwd_cb($ctx, sub { "top-secret" });
            Net::SSLeay::CTX_use_PrivateKey_file($ctx, "key.pem",
                                                 Net::SSLeay::FILETYPE_PEM)
                or die "Error reading private key";
            Net::SSLeay::CTX_set_default_passwd_cb($ctx, undef);

    If Hello Extensions are supported by your OpenSSL, a session secret callback can be set up to be
    called when a session secret is set by openssl.

    Establish it like this:

        Net::SSLeay::set_session_secret_cb($ssl, \&session_secret_cb, $somedata);

    It will be called like this:

        sub session_secret_cb
        {
            my ($secret, \@cipherlist, \$preferredcipher, $somedata) = @_;
        }

    No other callbacks are implemented. You do not need to use any callback for simple (i.e. normal)
    cases where the SSLeay built-in verify mechanism satisfies your needs.

    It is required to reset these callbacks to undef immediately after use to prevent memory leaks,
    thread safety problems and crashes on exit that can occur if different threads set different
    callbacks.

    If you want to use callback stuff, see examples/callback.pl! It's the only one I am able to make
    work reliably.

  Low level API
    In addition to the high level functions outlined above, this module contains straight-forward
    access to CRYPTO and SSL parts of OpenSSL C API.

    See the "*.h" headers from OpenSSL C distribution for a list of low level SSLeay functions to
    call (check SSLeay.xs to see if some function has been implemented). The module strips the
    initial "SSL_" off of the SSLeay names. Generally you should use "Net::SSLeay::" in its place.

    Note that some functions are prefixed with "P_" - these are very close to the original API
    however contain some kind of a wrapper making its interface more perl friendly.

    For example:

    In C:

            #include <ssl.h>

            err = SSL_set_verify (ssl, SSL_VERIFY_CLIENT_ONCE,
                                       &your_call_back_here);

    In Perl:

            use Net::SSLeay;

            $err = Net::SSLeay::set_verify ($ssl,
                                            Net::SSLeay::VERIFY_CLIENT_ONCE,
                                            \&your_call_back_here);

    If the function does not start with "SSL_" you should use the full function name, e.g.:

            $err = Net::SSLeay::ERR_get_error;

    The following new functions behave in perlish way:

            $got = Net::SSLeay::read($ssl);
                                        # Performs SSL_read, but returns $got
                                        # resized according to data received.
                                        # Returns undef on failure.

            Net::SSLeay::write($ssl, $foo) || die;
                                        # Performs SSL_write, but automatically
                                        # figures out the size of $foo

   Low level API: Version and library information related functions
    *   OpenSSL_version_num and SSLeay

        COMPATIBILITY: SSLeay() is not available in Net-SSLeay-1.42 and before. SSLeay() was made an
        alias of OpenSSL_version_num() in OpenSSL 1.1.0 and LibreSSL 2.7.0.

        COMPATIBILITY: OpenSSL_version_num() requires at least Net-SSLeay-1.82 with OpenSSL 1.1.0,
        or Net-SSLeay-1.88 with LibreSSL 2.7.0.

        Both functions return OPENSSL_VERSION_NUMBER constant (numeric) as defined by the underlying
        OpenSSL or LibreSSL library.

         my $ver_number = Net::SSLeay::SSLeay();
        or
         my $ver_number = Net::SSLeay::OpenSSL_version_num();
         # returns: OPENSSL_VERSION_NUMBER constant

         OpenSSL version numbering is:

         # 0x00903100 => openssl-0.9.3
         # 0x00904100 => openssl-0.9.4
         # 0x00905100 => openssl-0.9.5
         # 0x0090600f => openssl-0.9.6
         # 0x0090601f => openssl-0.9.6a
         # ...
         # 0x009060df => openssl-0.9.6m
         # 0x0090700f => openssl-0.9.7
         # 0x0090701f => openssl-0.9.7a
         # ...
         # 0x009070df => openssl-0.9.7m
         # 0x0090800f => openssl-0.9.8
         # 0x0090801f => openssl-0.9.8a
         # ...
         # 0x0090821f => openssl-0.9.8zh
         # 0x1000000f => openssl-1.0.0
         # ...
         # 0x1000014f => openssl-1.0.0t
         # 0x1000100f => openssl-1.0.1
         # ...
         # 0x1000115f => openssl-1.0.1u
         # 0x1000200f => openssl-1.0.2
         # ...
         # 0x1000215f => openssl-1.0.2u
         # 0x1010000f => openssl-1.1.0
         # ...
         # 0x101000cf => openssl-1.1.0l
         # 0x1010100f => openssl-1.1.1
         # ...
         # 0x101010df => openssl-1.1.1m
         # 0x30000000 => openssl-3.0.0
         # 0x30000010 => openssl-3.0.1

         Note that OpenSSL 3.0.0 and later do not set the status nibble in the
         least significant octet to f.

         LibreSSL returns 0x20000000 always:

         # 0x20000000 => libressl-2.2.1
         # ...
         # 0x20000000 => libressl-3.4.2

        You can use the version number like this when you know that the underlying library is
        OpenSSL:

          if (Net::SSLeay::SSLeay() < 0x0090800f) {
            die "You need OpenSSL 0.9.8 or higher";
          }

        LibresSSL 2.2.2 and later define constant LIBRESSL_VERSION_NUMBER that gives the LibreSSL
        version number. The format is the same that OpenSSL uses with OPENSSL_VERSION_NUMBER. You
        can do this if you need to check that the underlying library is LibreSSL and it's recent
        enough:

          if (Net::SSLeay::SSLeay() != 0x20000000 ||
              Net::SSLeay::LIBRESSL_VERSION_NUMBER() < 0x3040200f) {
            die "You need LibreSSL. Version 3.4.2 or higher";
          }

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OpenSSL_version_num.html>

        See OpenSSL 1.1.1 and earlier documentation for the details of status nibble and the format
        interpretation.

    *   SSLeay_version

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

        Returns different strings depending on $type.

         my $ver_string = Net::SSLeay::SSLeay_version($type);
         # $type
         #   SSLEAY_VERSION  - e.g. 'OpenSSL 1.0.0d 8 Feb 2011'
         #   SSLEAY_CFLAGS   - e.g. 'compiler: gcc -D_WINDLL -DOPENSSL_USE_APPLINK .....'
         #   SSLEAY_BUILT_ON - e.g. 'built on: Fri May  6 00:00:46 GMT 2011'
         #   SSLEAY_PLATFORM - e.g. 'platform: mingw'
         #   SSLEAY_DIR      - e.g. 'OPENSSLDIR: "z:/...."'
         #
         # returns: string

         Net::SSLeay::SSLeay_version();
         #is equivalent to
         Net::SSLeay::SSLeay_version(SSLEAY_VERSION);

        OpenSSL 1.1.0 changed SSLeay_version() to an alias of OpenSSL_version(). To ensure correct
        functionality with LibreSSL, use SSLEAY_* constants with SSLeay_version() and OPENSSL_*
        constants with OpenSSL_version().

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OpenSSL_version.html>

        OpenSSL website no longer has a manual page for SSLeay_version().

    *   OpenSSL_version

        COMPATIBILITY: requires at least Net-SSLeay-1.82 with OpenSSL 1.1.0, or Net-SSLeay-1.88 with
        LibreSSL 2.7.0.

        Returns different strings depending on $t. Available $t constants depend on the library
        version.

         my $ver_string = Net::SSLeay::OpenSSL_version($t);
         # $t
         #   OPENSSL_VERSION     - e.g. 'OpenSSL 1.1.0g  2 Nov 2017'
         #   OPENSSL_CFLAGS      - e.g. 'compiler: cc -DDSO_DLFCN -DHAVE_DLFCN_H .....'
         #   OPENSSL_BUILT_ON    - e.g. 'built on: reproducible build, date unspecified'
         #   OPENSSL_PLATFORM    - e.g. 'platform: darwin64-x86_64-cc'
         #   OPENSSL_DIR         - e.g. 'OPENSSLDIR: "/opt/openssl-1.1.0g"'
         #   OPENSSL_ENGINES_DIR - e.g. 'ENGINESDIR: "/opt/openssl-1.1.0g/lib/engines-1.1"'
         #
         # returns: string

         Net::SSLeay::OpenSSL_version();
         #is equivalent to
         Net::SSLeay::OpenSSL_version(OPENSSL_VERSION);

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OpenSSL_version.html>

    *   OPENSSL_info

        COMPATIBILITY: not available in Net-SSLeay-1.90 and before; requires at least OpenSSL
        3.0.0-alpha1

        Returns different strings depending on $t. Available $t constants depend on the library
        version.

         my $info_string = Net::SSLeay::OPENSSL_info($t);
         # $t
         #   OPENSSL_INFO_CONFIG_DIR - e.g. '/opt/openssl-3.0.1'
         #   OPENSSL_INFO_...
         #
         # returns: string

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OPENSSL_info.html>

    *   OPENSSL_version_major, OPENSSL_version_minor and OPENSSL_version_patch

        COMPATIBILITY: not available in Net-SSLeay-1.90 and before; requires at least OpenSSL
        3.0.0-alpha1, not in LibreSSL

        Return constants OPENSSL_VERSION_MAJOR, OPENSSL_VERSION_MINOR and OPENSSL_VERSION_PATCH,
        respectively.

         my $major = Net::SSLeay::OPENSSL_version_major();
         my $minor = Net::SSLeay::OPENSSL_version_minor();
         my $patch = Net::SSLeay::OPENSSL_version_patch();
         #
         # return: integer

        For example with OpenSSL 3.0.1, $major is 3, $minor is 0 and $patch is 1.

        Note: the constants record Net::SSLeay compile time values whereas the three functions
        return values from the library. Typically these are the same, but they can be different if
        the library version is updated but Net::SSLeay is not re-compiled. See the OpenSSL and
        LibreSSL API/ABI compatibility statements for more information.

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OPENSSL_version_major.html>

    *   OPENSSL_version_pre_release

        COMPATIBILITY: not available in Net-SSLeay-1.90 and before; requires at least OpenSSL
        3.0.0-alpha1, not in LibreSSL

        Return constant string defined by C macro OPENSSL_VERSION_PRE_RELEASE.

         my $pre_release = Net::SSLeay::OPENSSL_version_pre_release();
         #
         # returns: string

         For example: "-alpha3" or "" for a release version.

        When the macro is not defined, an empty string is returned instead.

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/OPENSSL_version_pre_release.html>

    *   OPENSSL_version_build_metadata()

        COMPATIBILITY: not available in Net-SSLeay-1.90 and before; requires at least OpenSSL
        3.0.0-alpha1, not in LibreSSL

        Return constant string defined by C macro OPENSSL_VERSION_BUILD_METADATA.

         my $metadata = Net::SSLeay::OPENSSL_version_build_metadata();
         #
         # returns: string

         For example: "+fips" or "".

        When the macro is not defined, an empty string is returned instead.

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/OPENSSL_version_build_metadata.html>

   Low level API: Initialization related functions
    *   library_init

        Initialize SSL library by registering algorithms.

         my $rv = Net::SSLeay::library_init();

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_library_init.html>

        While the original function from OpenSSL always returns 1, Net::SSLeay adds a wrapper around
        it to make sure that the OpenSSL function is only called once. Thus the function will return
        1 if initialization was done and 0 if not, i.e. if initialization was done already before.

    *   add_ssl_algorithms

        The alias for "library_init"

         Net::SSLeay::add_ssl_algorithms();

    *   OpenSSL_add_ssl_algorithms

        The alias for "library_init"

         Net::SSLeay::OpenSSL_add_ssl_algorithms();

    *   SSLeay_add_ssl_algorithms

        The alias for "library_init"

         Net::SSLeay::SSLeay_add_ssl_algorithms();

    *   load_error_strings

        Registers the error strings for all libcrypto + libssl related functions.

         Net::SSLeay::load_error_strings();
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/ERR_load_crypto_strings.html>

    *   ERR_load_crypto_strings

        Registers the error strings for all libcrypto functions. No need to call this function if
        you have already called "load_error_strings".

         Net::SSLeay::ERR_load_crypto_strings();
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/ERR_load_crypto_strings.html>

    *   ERR_load_RAND_strings

        Registers the error strings for RAND related functions. No need to call this function if you
        have already called "load_error_strings".

         Net::SSLeay::ERR_load_RAND_strings();
         #
         # returns: no return value

    *   ERR_load_SSL_strings

        Registers the error strings for SSL related functions. No need to call this function if you
        have already called "load_error_strings".

         Net::SSLeay::ERR_load_SSL_strings();
         #
         # returns: no return value

    *   OpenSSL_add_all_algorithms

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Add algorithms to internal table.

         Net::SSLeay::OpenSSL_add_all_algorithms();
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/OpenSSL_add_all_algorithms.html>

    *   OPENSSL_add_all_algorithms_conf

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Similar to "OpenSSL_add_all_algorithms" - will ALWAYS load the config file

         Net::SSLeay::OPENSSL_add_all_algorithms_conf();
         #
         # returns: no return value

    *   OPENSSL_add_all_algorithms_noconf

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Similar to "OpenSSL_add_all_algorithms" - will NEVER load the config file

         Net::SSLeay::OPENSSL_add_all_algorithms_noconf();
         #
         # returns: no return value

   Low level API: ERR_* and SSL_alert_* related functions
    NOTE: Please note that SSL_alert_* function have "SSL_" part stripped from their names.

    *   ERR_clear_error

        Clear the error queue.

         Net::SSLeay::ERR_clear_error();
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/ERR_clear_error.html>

    *   ERR_error_string

        Generates a human-readable string representing the error code $error.

         my $rv = Net::SSLeay::ERR_error_string($error);
         # $error - (unsigned integer) error code
         #
         # returns: string

        Check openssl doc <http://www.openssl.org/docs/crypto/ERR_error_string.html>

    *   ERR_get_error

        Returns the earliest error code from the thread's error queue and removes the entry. This
        function can be called repeatedly until there are no more error codes to return.

         my $rv = Net::SSLeay::ERR_get_error();
         #
         # returns: (unsigned integer) error code

        Check openssl doc <http://www.openssl.org/docs/crypto/ERR_get_error.html>

    *   ERR_peek_error

        Returns the earliest error code from the thread's error queue without modifying it.

         my $rv = Net::SSLeay::ERR_peek_error();
         #
         # returns: (unsigned integer) error code

        Check openssl doc <http://www.openssl.org/docs/crypto/ERR_get_error.html>

    *   ERR_put_error

        Adds an error code to the thread's error queue. It signals that the error of $reason code
        reason occurred in function $func of library $lib, in line number $line of $file.

         Net::SSLeay::ERR_put_error($lib, $func, $reason, $file, $line);
         # $lib - (integer) library id (check openssl/err.h for constants e.g. ERR_LIB_SSL)
         # $func - (integer) function id (check openssl/ssl.h for constants e.g. SSL_F_SSL23_READ)
         # $reason - (integer) reason id (check openssl/ssl.h for constants e.g. SSL_R_SSL_HANDSHAKE_FAILURE)
         # $file - (string) file name
         # $line - (integer) line number in $file
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/ERR_put_error.html> and
        <http://www.openssl.org/docs/crypto/err.html>

    *   alert_desc_string

        Returns a two letter string as a short form describing the reason of the alert specified by
        value.

         my $rv = Net::SSLeay::alert_desc_string($value);
         # $value - (integer) allert id (check openssl/ssl.h for SSL3_AD_* and TLS1_AD_* constants)
         #
         # returns: description string (2 letters)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_alert_type_string.html>

    *   alert_desc_string_long

        Returns a string describing the reason of the alert specified by value.

         my $rv = Net::SSLeay::alert_desc_string_long($value);
         # $value - (integer) allert id (check openssl/ssl.h for SSL3_AD_* and TLS1_AD_* constants)
         #
         # returns: description string

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_alert_type_string.html>

    *   alert_type_string

        Returns a one letter string indicating the type of the alert specified by value.

         my $rv = Net::SSLeay::alert_type_string($value);
         # $value - (integer) allert id (check openssl/ssl.h for SSL3_AD_* and TLS1_AD_* constants)
         #
         # returns: string (1 letter)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_alert_type_string.html>

    *   alert_type_string_long

        Returns a string indicating the type of the alert specified by value.

         my $rv = Net::SSLeay::alert_type_string_long($value);
         # $value - (integer) allert id (check openssl/ssl.h for SSL3_AD_* and TLS1_AD_* constants)
         #
         # returns: string

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_alert_type_string.html>

   Low level API: SSL_METHOD_* related functions
    *   SSLv23_method, SSLv23_server_method and SSLv23_client_method

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before.

        Returns SSL_METHOD structure corresponding to general-purpose version-flexible TLS method,
        the return value can be later used as a param of "CTX_new_with_method".

        NOTE: Consider using TLS_method, TLS_server_method or TLS_client_method with new code.

         my $rv = Net::SSLeay::SSLv2_method();
         #
         # returns: value corresponding to openssl's SSL_METHOD structure (0 on failure)

    *   SSLv2_method

        Returns SSL_METHOD structure corresponding to SSLv2 method, the return value can be later
        used as a param of "CTX_new_with_method". Only available where supported by the underlying
        openssl.

         my $rv = Net::SSLeay::SSLv2_method();
         #
         # returns: value corresponding to openssl's SSL_METHOD structure (0 on failure)

    *   SSLv3_method

        Returns SSL_METHOD structure corresponding to SSLv3 method, the return value can be later
        used as a param of "CTX_new_with_method".

         my $rv = Net::SSLeay::SSLv3_method();
         #
         # returns: value corresponding to openssl's SSL_METHOD structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_new.html>

    *   TLSv1_method, TLSv1_server_method and TLSv1_client_method

        COMPATIBILITY: Server and client methods not available in Net-SSLeay-1.82 and before.

        Returns SSL_METHOD structure corresponding to TLSv1 method, the return value can be later
        used as a param of "CTX_new_with_method".

         my $rv = Net::SSLeay::TLSv1_method();
         #
         # returns: value corresponding to openssl's SSL_METHOD structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_new.html>

    *   TLSv1_1_method, TLSv1_1_server_method and TLSv1_1_client_method

        COMPATIBILITY: Server and client methods not available in Net-SSLeay-1.82 and before.

        Returns SSL_METHOD structure corresponding to TLSv1_1 method, the return value can be later
        used as a param of "CTX_new_with_method". Only available where supported by the underlying
        openssl.

         my $rv = Net::SSLeay::TLSv1_1_method();
         #
         # returns: value corresponding to openssl's SSL_METHOD structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_new.html>

    *   TLSv1_2_method, TLSv1_2_server_method and TLSv1_2_client_method

        COMPATIBILITY: Server and client methods not available in Net-SSLeay-1.82 and before.

        Returns SSL_METHOD structure corresponding to TLSv1_2 method, the return value can be later
        used as a param of "CTX_new_with_method". Only available where supported by the underlying
        openssl.

         my $rv = Net::SSLeay::TLSv1_2_method();
         #
         # returns: value corresponding to openssl's SSL_METHOD structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_new.html>

    *   TLS_method, TLS_server_method and TLS_client_method

        COMPATIBILITY: Not available in Net-SSLeay-1.82 and before.

        Returns SSL_METHOD structure corresponding to general-purpose version-flexible TLS method,
        the return value can be later used as a param of "CTX_new_with_method". Only available where
        supported by the underlying openssl.

         my $rv = Net::SSLeay::TLS_method();
         #
         # returns: value corresponding to openssl's SSL_METHOD structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_new.html>

   Low level API: ENGINE_* related functions
    *   ENGINE_load_builtin_engines

        COMPATIBILITY: Requires an OpenSSL build with dynamic engine loading support.

        Load all bundled ENGINEs into memory and make them visible.

         Net::SSLeay::ENGINE_load_builtin_engines();
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/engine.html>

    *   ENGINE_register_all_complete

        COMPATIBILITY: Requires an OpenSSL build with dynamic engine loading support.

        Register all loaded ENGINEs for every algorithm they collectively implement.

         Net::SSLeay::ENGINE_register_all_complete();
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/engine.html>

    *   ENGINE_set_default

        COMPATIBILITY: Requires an OpenSSL build with dynamic engine loading support.

        Set default engine to $e + set its flags to $flags.

         my $rv = Net::SSLeay::ENGINE_set_default($e, $flags);
         # $e - value corresponding to openssl's ENGINE structure
         # $flags - (integer) engine flags
         #          flags value can be made by bitwise "OR"ing:
         #          0x0001 - ENGINE_METHOD_RSA
         #          0x0002 - ENGINE_METHOD_DSA
         #          0x0004 - ENGINE_METHOD_DH
         #          0x0008 - ENGINE_METHOD_RAND
         #          0x0010 - ENGINE_METHOD_ECDH
         #          0x0020 - ENGINE_METHOD_ECDSA
         #          0x0040 - ENGINE_METHOD_CIPHERS
         #          0x0080 - ENGINE_METHOD_DIGESTS
         #          0x0100 - ENGINE_METHOD_STORE
         #          0x0200 - ENGINE_METHOD_PKEY_METHS
         #          0x0400 - ENGINE_METHOD_PKEY_ASN1_METHS
         #          Obvious all-or-nothing cases:
         #          0xFFFF - ENGINE_METHOD_ALL
         #          0x0000 - ENGINE_METHOD_NONE
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/engine.html>

    *   ENGINE_by_id

        Get ENGINE by its identification $id.

        COMPATIBILITY: Requires an OpenSSL build with dynamic engine loading support.

         my $rv = Net::SSLeay::ENGINE_by_id($id);
         # $id - (string) engine identification e.g. "dynamic"
         #
         # returns: value corresponding to openssl's ENGINE structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/engine.html>

   Low level API: EVP_PKEY_* related functions
    *   EVP_PKEY_copy_parameters

        Copies the parameters from key $from to key $to.

         my $rv = Net::SSLeay::EVP_PKEY_copy_parameters($to, $from);
         # $to - value corresponding to openssl's EVP_PKEY structure
         # $from - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/EVP_PKEY_cmp.html>

    *   EVP_PKEY_new

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Creates a new EVP_PKEY structure.

         my $rv = Net::SSLeay::EVP_PKEY_new();
         #
         # returns: value corresponding to openssl's EVP_PKEY structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/EVP_PKEY_new.html>

    *   EVP_PKEY_free

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Free an allocated EVP_PKEY structure.

         Net::SSLeay::EVP_PKEY_free($pkey);
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/EVP_PKEY_new.html>

    *   EVP_PKEY_assign_RSA

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Set the key referenced by $pkey to $key

        NOTE: No reference counter will be increased, i.e. $key will be freed if $pkey is freed.

         my $rv = Net::SSLeay::EVP_PKEY_assign_RSA($pkey, $key);
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         # $key - value corresponding to openssl's RSA structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/EVP_PKEY_assign_RSA.html>

    *   EVP_PKEY_assign_EC_KEY

        COMPATIBILITY: not available in Net-SSLeay-1.74 and before

        Set the key referenced by $pkey to $key

        NOTE: No reference counter will be increased, i.e. $key will be freed if $pkey is freed.

         my $rv = Net::SSLeay::EVP_PKEY_assign_EC_KEY($pkey, $key);
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         # $key - value corresponding to openssl's EC_KEY structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/EVP_PKEY_assign_EC_KEY.html>

    *   EVP_PKEY_bits

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns the size of the key $pkey in bits.

         my $rv = Net::SSLeay::EVP_PKEY_bits($pkey);
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: size in bits

    *   EVP_PKEY_size

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns the maximum size of a signature in bytes. The actual signature may be smaller.

         my $rv = Net::SSLeay::EVP_PKEY_size($pkey);
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: the maximum size in bytes

        Check openssl doc <http://www.openssl.org/docs/crypto/EVP_SignInit.html>

    *   EVP_PKEY_id

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-1.0.0

        Returns $pkey type (integer value of corresponding NID).

         my $rv = Net::SSLeay::EVP_PKEY_id($pkey);
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: (integer) key type

        Example:

         my $pubkey = Net::SSLeay::X509_get_pubkey($x509);
         my $type = Net::SSLeay::EVP_PKEY_id($pubkey);
         print Net::SSLeay::OBJ_nid2sn($type);             #prints e.g. 'rsaEncryption'

   Low level API: PEM_* related functions
    Check openssl doc <http://www.openssl.org/docs/crypto/pem.html>

    *   PEM_read_bio_X509

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Loads PEM formatted X509 certificate via given BIO structure.

         my $rv = Net::SSLeay::PEM_read_bio_X509($bio);
         # $bio - value corresponding to openssl's BIO structure
         #
         # returns: value corresponding to openssl's X509 structure (0 on failure)

        Example:

         my $bio = Net::SSLeay::BIO_new_file($filename, 'r');
         my $x509 = Net::SSLeay::PEM_read_bio_X509($bio);
         Net::SSLeay::BIO_free($bio);

    *   PEM_read_bio_X509_REQ

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Loads PEM formatted X509_REQ object via given BIO structure.

         my $rv = Net::SSLeay::PEM_read_bio_X509_REQ($bio, $x=NULL, $cb=NULL, $u=NULL);
         # $bio - value corresponding to openssl's BIO structure
         #
         # returns: value corresponding to openssl's X509_REQ structure (0 on failure)

        Example:

         my $bio = Net::SSLeay::BIO_new_file($filename, 'r');
         my $x509_req = Net::SSLeay::PEM_read_bio_X509_REQ($bio);
         Net::SSLeay::BIO_free($bio);

    *   PEM_read_bio_DHparams

        Reads DH structure from BIO.

         my $rv = Net::SSLeay::PEM_read_bio_DHparams($bio);
         # $bio - value corresponding to openssl's BIO structure
         #
         # returns: value corresponding to openssl's DH structure (0 on failure)

    *   PEM_read_bio_X509_CRL

        Reads X509_CRL structure from BIO.

         my $rv = Net::SSLeay::PEM_read_bio_X509_CRL($bio);
         # $bio - value corresponding to openssl's BIO structure
         #
         # returns: value corresponding to openssl's X509_CRL structure (0 on failure)

    *   PEM_read_bio_PrivateKey

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Loads PEM formatted private key via given BIO structure.

         my $rv = Net::SSLeay::PEM_read_bio_PrivateKey($bio, $cb, $data);
         # $bio - value corresponding to openssl's BIO structure
         # $cb - reference to perl callback function
         # $data - data that will be passed to callback function (see examples below)
         #
         # returns: value corresponding to openssl's EVP_PKEY structure (0 on failure)

        Example:

         my $bio = Net::SSLeay::BIO_new_file($filename, 'r');
         my $privkey = Net::SSLeay::PEM_read_bio_PrivateKey($bio); #ask for password if needed
         Net::SSLeay::BIO_free($bio);

        To use password you have the following options:

         $privkey = Net::SSLeay::PEM_read_bio_PrivateKey($bio, \&callback_func); # use callback func for getting password
         $privkey = Net::SSLeay::PEM_read_bio_PrivateKey($bio, \&callback_func, $data); # use callback_func + pass $data to callback_func
         $privkey = Net::SSLeay::PEM_read_bio_PrivateKey($bio, undef, "secret"); # use password "secret"
         $privkey = Net::SSLeay::PEM_read_bio_PrivateKey($bio, undef, "");       # use empty password

        Callback function signature:

         sub callback_func {
           my ($max_passwd_size, $rwflag, $data) = @_;
           # $max_passwd_size - maximum size of returned password (longer values will be discarded)
           # $rwflag - indicates whether we are loading (0) or storing (1) - for PEM_read_bio_PrivateKey always 0
           # $data - the data passed to PEM_read_bio_PrivateKey as 3rd parameter

           return "secret";
         }

    *   PEM_X509_INFO_read_bio

        Reads a BIO containing a PEM formatted file into a STACK_OF(X509_INFO) structure.

         my $rv = Net::SSLeay::PEM_X509_INFO_read_bio($bio);
         # $bio - value corresponding to openssl's BIO structure
         #
         # returns: value corresponding to openssl's STACK_OF(X509_INFO) structure.

        Example:

         my $bio = Net::SSLeay::BIO_new_file($filename, 'r');
         my $sk_x509_info = Net::SSLeay::PEM_X509_INFO_read_bio($bio);
         Net::SSLeay::BIO_free($bio);

    *   PEM_get_string_X509

        NOTE: Does not exactly correspond to any low level API function

        Converts/exports X509 certificate to string (PEM format).

         Net::SSLeay::PEM_get_string_X509($x509);
         # $x509 - value corresponding to openssl's X509 structure
         #
         # returns: string with $x509 in PEM format

    *   PEM_get_string_PrivateKey

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Converts public key $pk into PEM formatted string (optionally protected with password).

         my $rv = Net::SSLeay::PEM_get_string_PrivateKey($pk, $passwd, $enc_alg);
         # $pk - value corresponding to openssl's EVP_PKEY structure
         # $passwd - [optional] (string) password to use for key encryption
         # $enc_alg - [optional] algorithm to use for key encryption (default: DES_CBC) - value corresponding to openssl's EVP_CIPHER structure
         #
         # returns: PEM formatted string

        Examples:

         $pem_privkey = Net::SSLeay::PEM_get_string_PrivateKey($pk);
         $pem_privkey = Net::SSLeay::PEM_get_string_PrivateKey($pk, "secret");
         $pem_privkey = Net::SSLeay::PEM_get_string_PrivateKey($pk, "secret", Net::SSLeay::EVP_get_cipherbyname("DES-EDE3-CBC"));

    *   PEM_get_string_X509_CRL

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Converts X509_CRL object $x509_crl into PEM formatted string.

         Net::SSLeay::PEM_get_string_X509_CRL($x509_crl);
         # $x509_crl - value corresponding to openssl's X509_CRL structure
         #
         # returns: no return value

    *   PEM_get_string_X509_REQ

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Converts X509_REQ object $x509_crl into PEM formatted string.

         Net::SSLeay::PEM_get_string_X509_REQ($x509_req);
         # $x509_req - value corresponding to openssl's X509_REQ structure
         #
         # returns: no return value

   Low level API: d2i_* (DER format) related functions
    *   d2i_X509_bio

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Loads DER formatted X509 certificate via given BIO structure.

         my $rv = Net::SSLeay::d2i_X509_bio($bp);
         # $bp - value corresponding to openssl's BIO structure
         #
         # returns: value corresponding to openssl's X509 structure (0 on failure)

        Example:

         my $bio = Net::SSLeay::BIO_new_file($filename, 'rb');
         my $x509 = Net::SSLeay::d2i_X509_bio($bio);
         Net::SSLeay::BIO_free($bio);

        Check openssl doc <http://www.openssl.org/docs/crypto/d2i_X509.html>

    *   d2i_X509_CRL_bio

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Loads DER formatted X509_CRL object via given BIO structure.

         my $rv = Net::SSLeay::d2i_X509_CRL_bio($bp);
         # $bp - value corresponding to openssl's BIO structure
         #
         # returns: value corresponding to openssl's X509_CRL structure (0 on failure)

        Example:

         my $bio = Net::SSLeay::BIO_new_file($filename, 'rb');
         my $x509_crl = Net::SSLeay::d2i_X509_CRL_bio($bio);
         Net::SSLeay::BIO_free($bio);

    *   d2i_X509_REQ_bio

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Loads DER formatted X509_REQ object via given BIO structure.

         my $rv = Net::SSLeay::d2i_X509_REQ_bio($bp);
         # $bp - value corresponding to openssl's BIO structure
         #
         # returns: value corresponding to openssl's X509_REQ structure (0 on failure)

        Example:

         my $bio = Net::SSLeay::BIO_new_file($filename, 'rb');
         my $x509_req = Net::SSLeay::d2i_X509_REQ_bio($bio);
         Net::SSLeay::BIO_free($bio);

   Low level API: PKCS12 related functions
    *   P_PKCS12_load_file

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Loads X509 certificate + private key + certificates of CA chain (if present in PKCS12 file).

         my ($privkey, $cert, @cachain) = Net::SSLeay::P_PKCS12_load_file($filename, $load_chain, $password);
         # $filename - name of PKCS12 file
         # $load_chain - [optional] whether load (1) or not(0) CA chain (default: 0)
         # $password - [optional] password for private key
         #
         # returns: triplet ($privkey, $cert, @cachain)
         #          $privkey - value corresponding to openssl's EVP_PKEY structure
         #          $cert - value corresponding to openssl's X509 structure
         #          @cachain - array of values corresponding to openssl's X509 structure (empty if no CA chain in PKCS12)

        IMPORTANT NOTE: after you do the job you need to call X509_free() on $privkey + all members
        of @cachain and EVP_PKEY_free() on $privkey.

        Examples:

         my ($privkey, $cert) = Net::SSLeay::P_PKCS12_load_file($filename);
         #or
         my ($privkey, $cert) = Net::SSLeay::P_PKCS12_load_file($filename, 0, $password);
         #or
         my ($privkey, $cert, @cachain) = Net::SSLeay::P_PKCS12_load_file($filename, 1);
         #or
         my ($privkey, $cert, @cachain) = Net::SSLeay::P_PKCS12_load_file($filename, 1, $password);

         #BEWARE: THIS IS WRONG - MEMORY LEAKS! (you cannot free @cachain items)
         my ($privkey, $cert) = Net::SSLeay::P_PKCS12_load_file($filename, 1, $password);

        NOTE With some combinations of Windows, perl, compiler and compiler options, you may see a
        runtime error "no OPENSSL_Applink", when calling Net::SSLeay::P_PKCS12_load_file. See
        README.Win32 for more details.

   Low level API: SESSION_* related functions
    *   d2i_SSL_SESSION

        COMPATIBILITY: does not work in Net-SSLeay-1.85 and before

        Transforms the binary ASN1 representation string of an SSL/TLS session into an SSL_SESSION
        object.

         my $ses = Net::SSLeay::d2i_SSL_SESSION($data);
         # $data - the session as ASN1 representation string
         #
         # returns: $ses - the new SSL_SESSION

        Check openssl doc <https://www.openssl.org/docs/ssl/i2d_SSL_SESSION.html>

    *   i2d_SSL_SESSION

        COMPATIBILITY: does not work in Net-SSLeay-1.85 and before

        Transforms the SSL_SESSION object in into the ASN1 representation and returns it as string.

         my $data = Net::SSLeay::i2d_SSL_SESSION($ses);
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: $data - session as string

        Check openssl doc <https://www.openssl.org/docs/ssl/d2i_SSL_SESSION.html>

    *   SESSION_new

        Creates a new SSL_SESSION structure.

         my $rv = Net::SSLeay::SESSION_new();
         #
         # returns: value corresponding to openssl's SSL_SESSION structure (0 on failure)

    *   SESSION_free

        Free an allocated SSL_SESSION structure.

         Net::SSLeay::SESSION_free($ses);
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_SESSION_free.html>

    *   SESSION_up_ref

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL
        1.1.0-pre4 or LibreSSL 2.7.0

        Increases the reference counter on a SSL_SESSION structure.

         Net::SSLeay::SESSION_up_ref($ses);
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: 1 on success else 0

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_SESSION_up_ref.html>

    *   SESSION_dup

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Duplicates a SSL_SESSION structure.

         Net::SSLeay::SESSION_dup($ses);
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: the duplicated session

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_SESSION_dup.html>

    *   SESSION_is_resumable

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Determine whether an SSL_SESSION object can be used for resumption.

         Net::SSLeay::SESSION_is_resumable($ses);
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: (integer) 1 if it can or 0 if not

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_SESSION_is_resumable.html>

    *   SESSION_cmp

        Compare two SSL_SESSION structures.

         my $rv = Net::SSLeay::SESSION_cmp($sesa, $sesb);
         # $sesa - value corresponding to openssl's SSL_SESSION structure
         # $sesb - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: 0 if the two structures are the same

        NOTE: Not available in openssl 1.0 or later

    *   SESSION_get_app_data

        Can be used to get application defined value/data.

         my $rv = Net::SSLeay::SESSION_get_app_data($ses);
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: string/buffer/pointer ???

    *   SESSION_set_app_data

        Can be used to set some application defined value/data.

         my $rv = Net::SSLeay::SESSION_set_app_data($s, $a);
         # $s - value corresponding to openssl's SSL_SESSION structure
         # $a - (string/buffer/pointer ???) data
         #
         # returns: ???

    *   SESSION_get_ex_data

        Is used to retrieve the information for $idx from session $ses.

         my $rv = Net::SSLeay::SESSION_get_ex_data($ses, $idx);
         # $ses - value corresponding to openssl's SSL_SESSION structure
         # $idx - (integer) index for application specific data
         #
         # returns: pointer to ???

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_SESSION_get_ex_new_index.html>

    *   SESSION_set_ex_data

        Is used to store application data at arg for idx into the session object.

         my $rv = Net::SSLeay::SESSION_set_ex_data($ss, $idx, $data);
         # $ss - value corresponding to openssl's SSL_SESSION structure
         # $idx - (integer) ???
         # $data - (pointer) ???
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_SESSION_get_ex_new_index.html>

    *   SESSION_get_ex_new_index

        Is used to register a new index for application specific data.

         my $rv = Net::SSLeay::SESSION_get_ex_new_index($argl, $argp, $new_func, $dup_func, $free_func);
         # $argl - (long) ???
         # $argp - (pointer) ???
         # $new_func - function pointer ??? (CRYPTO_EX_new *)
         # $dup_func - function pointer ??? (CRYPTO_EX_dup *)
         # $free_func - function pointer ??? (CRYPTO_EX_free *)
         #
         # returns: (integer) ???

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_SESSION_get_ex_new_index.html>

    *   SESSION_get_master_key

        NOTE: Does not exactly correspond to any low level API function

        Returns 'master_key' value from SSL_SESSION structure $s

         Net::SSLeay::SESSION_get_master_key($s);
         # $s - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: master key (binary data)

    *   SESSION_set_master_key

        Sets 'master_key' value for SSL_SESSION structure $s

         Net::SSLeay::SESSION_set_master_key($s, $key);
         # $s - value corresponding to openssl's SSL_SESSION structure
         # $key - master key (binary data)
         #
         # returns: no return value

        Not available with OpenSSL 1.1 and later. Code that previously used SESSION_set_master_key
        must now set $secret in the session_secret callback set with SSL_set_session_secret_cb.

    *   SESSION_get_time

        Returns the time at which the session s was established. The time is given in seconds since
        1.1.1970.

         my $rv = Net::SSLeay::SESSION_get_time($s);
         # $s - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: timestamp (seconds since 1.1.1970)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_SESSION_get_time.html>

    *   get_time

        Technically the same functionality as "SESSION_get_time".

         my $rv = Net::SSLeay::get_time($s);

    *   SESSION_get_timeout

        Returns the timeout value set for session $s in seconds.

         my $rv = Net::SSLeay::SESSION_get_timeout($s);
         # $s - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: timeout (in seconds)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_SESSION_get_time.html>

    *   get_timeout

        Technically the same functionality as "SESSION_get_timeout".

         my $rv = Net::SSLeay::get_timeout($s);

    *   SESSION_print

        NOTE: Does not exactly correspond to any low level API function

        Prints session details (e.g. protocol version, cipher, session-id ...) to BIO.

         my $rv = Net::SSLeay::SESSION_print($fp, $ses);
         # $fp - value corresponding to openssl's BIO structure
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: 1 on success, 0 on failure

        You have to use necessary BIO functions like this:

         # let us have $ssl corresponding to openssl's SSL structure
         my $ses = Net::SSLeay::get_session($ssl);
         my $bio = Net::SSLeay::BIO_new(&Net::SSLeay::BIO_s_mem);
         Net::SSLeay::SESSION_print($bio, $ses);
         print Net::SSLeay::BIO_read($bio);

    *   SESSION_print_fp

        Prints session details (e.g. protocol version, cipher, session-id ...) to file handle.

         my $rv = Net::SSLeay::SESSION_print_fp($fp, $ses);
         # $fp - perl file handle
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: 1 on success, 0 on failure

        Example:

         # let us have $ssl corresponding to openssl's SSL structure
         my $ses = Net::SSLeay::get_session($ssl);
         open my $fh, ">", "output.txt";
         Net::SSLeay::SESSION_print_fp($fh,$ses);

    *   SESSION_set_time

        Replaces the creation time of the session s with the chosen value $t (seconds since
        1.1.1970).

         my $rv = Net::SSLeay::SESSION_set_time($ses, $t);
         # $ses - value corresponding to openssl's SSL_SESSION structure
         # $t - time value
         #
         # returns: 1 on success

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_SESSION_get_time.html>

    *   set_time

        Technically the same functionality as "SESSION_set_time".

         my $rv = Net::SSLeay::set_time($ses, $t);

    *   SESSION_set_timeout

        Sets the timeout value for session s in seconds to $t.

         my $rv = Net::SSLeay::SESSION_set_timeout($s, $t);
         # $s - value corresponding to openssl's SSL_SESSION structure
         # $t - timeout (in seconds)
         #
         # returns: 1 on success

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_SESSION_get_time.html>

    *   set_timeout

        Technically the same functionality as "SESSION_set_timeout".

         my $rv = Net::SSLeay::set_timeout($ses, $t);

   Low level API: SSL_CTX_* related functions
    NOTE: Please note that the function described in this chapter have "SSL_" part stripped from
    their original openssl names.

    *   CTX_add_client_CA

        Adds the CA name extracted from $cacert to the list of CAs sent to the client when
        requesting a client certificate for $ctx.

         my $rv = Net::SSLeay::CTX_add_client_CA($ctx, $cacert);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $cacert - value corresponding to openssl's X509 structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_client_CA_list.html>

    *   CTX_add_extra_chain_cert

        Adds the certificate $x509 to the certificate chain presented together with the certificate.
        Several certificates can be added one after the other.

         my $rv = Net::SSLeay::CTX_add_extra_chain_cert($ctx, $x509);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $x509 - value corresponding to openssl's X509 structure
         #
         # returns: 1 on success, check out the error stack to find out the reason for failure otherwise

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_add_extra_chain_cert.html>

    *   CTX_add_session

        Adds the session $ses to the context $ctx.

         my $rv = Net::SSLeay::CTX_add_session($ctx, $ses);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_add_session.html>

    *   CTX_callback_ctrl

        ??? (more info needed)

         my $rv = Net::SSLeay::CTX_callback_ctrl($ctx, $cmd, $fp);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $cmd - (integer) command id
         # $fp - (function pointer) ???
         #
         # returns: ???

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_ctrl.html>

    *   CTX_check_private_key

        Checks the consistency of a private key with the corresponding certificate loaded into $ctx.

         my $rv = Net::SSLeay::CTX_check_private_key($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   CTX_ctrl

        Internal handling function for SSL_CTX objects.

        BEWARE: openssl doc says: This function should never be called directly!

         my $rv = Net::SSLeay::CTX_ctrl($ctx, $cmd, $larg, $parg);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $cmd - (integer) command id
         # $larg - (integer) long ???
         # $parg - (string/pointer) ???
         #
         # returns: (long) result of given command ???

         #valid $cmd values
          1 - SSL_CTRL_NEED_TMP_RSA
          2 - SSL_CTRL_SET_TMP_RSA
          3 - SSL_CTRL_SET_TMP_DH
          4 - SSL_CTRL_SET_TMP_ECDH
          5 - SSL_CTRL_SET_TMP_RSA_CB
          6 - SSL_CTRL_SET_TMP_DH_CB
          7 - SSL_CTRL_SET_TMP_ECDH_CB
          8 - SSL_CTRL_GET_SESSION_REUSED
          9 - SSL_CTRL_GET_CLIENT_CERT_REQUEST
         10 - SSL_CTRL_GET_NUM_RENEGOTIATIONS
         11 - SSL_CTRL_CLEAR_NUM_RENEGOTIATIONS
         12 - SSL_CTRL_GET_TOTAL_RENEGOTIATIONS
         13 - SSL_CTRL_GET_FLAGS
         14 - SSL_CTRL_EXTRA_CHAIN_CERT
         15 - SSL_CTRL_SET_MSG_CALLBACK
         16 - SSL_CTRL_SET_MSG_CALLBACK_ARG
         17 - SSL_CTRL_SET_MTU
         20 - SSL_CTRL_SESS_NUMBER
         21 - SSL_CTRL_SESS_CONNECT
         22 - SSL_CTRL_SESS_CONNECT_GOOD
         23 - SSL_CTRL_SESS_CONNECT_RENEGOTIATE
         24 - SSL_CTRL_SESS_ACCEPT
         25 - SSL_CTRL_SESS_ACCEPT_GOOD
         26 - SSL_CTRL_SESS_ACCEPT_RENEGOTIATE
         27 - SSL_CTRL_SESS_HIT
         28 - SSL_CTRL_SESS_CB_HIT
         29 - SSL_CTRL_SESS_MISSES
         30 - SSL_CTRL_SESS_TIMEOUTS
         31 - SSL_CTRL_SESS_CACHE_FULL
         32 - SSL_CTRL_OPTIONS
         33 - SSL_CTRL_MODE
         40 - SSL_CTRL_GET_READ_AHEAD
         41 - SSL_CTRL_SET_READ_AHEAD
         42 - SSL_CTRL_SET_SESS_CACHE_SIZE
         43 - SSL_CTRL_GET_SESS_CACHE_SIZE
         44 - SSL_CTRL_SET_SESS_CACHE_MODE
         45 - SSL_CTRL_GET_SESS_CACHE_MODE
         50 - SSL_CTRL_GET_MAX_CERT_LIST
         51 - SSL_CTRL_SET_MAX_CERT_LIST
         52 - SSL_CTRL_SET_MAX_SEND_FRAGMENT
         53 - SSL_CTRL_SET_TLSEXT_SERVERNAME_CB
         54 - SSL_CTRL_SET_TLSEXT_SERVERNAME_ARG
         55 - SSL_CTRL_SET_TLSEXT_HOSTNAME
         56 - SSL_CTRL_SET_TLSEXT_DEBUG_CB
         57 - SSL_CTRL_SET_TLSEXT_DEBUG_ARG
         58 - SSL_CTRL_GET_TLSEXT_TICKET_KEYS
         59 - SSL_CTRL_SET_TLSEXT_TICKET_KEYS
         60 - SSL_CTRL_SET_TLSEXT_OPAQUE_PRF_INPUT
         61 - SSL_CTRL_SET_TLSEXT_OPAQUE_PRF_INPUT_CB
         62 - SSL_CTRL_SET_TLSEXT_OPAQUE_PRF_INPUT_CB_ARG
         63 - SSL_CTRL_SET_TLSEXT_STATUS_REQ_CB
         64 - SSL_CTRL_SET_TLSEXT_STATUS_REQ_CB_ARG
         65 - SSL_CTRL_SET_TLSEXT_STATUS_REQ_TYPE
         66 - SSL_CTRL_GET_TLSEXT_STATUS_REQ_EXTS
         67 - SSL_CTRL_SET_TLSEXT_STATUS_REQ_EXTS
         68 - SSL_CTRL_GET_TLSEXT_STATUS_REQ_IDS
         69 - SSL_CTRL_SET_TLSEXT_STATUS_REQ_IDS
         70 - SSL_CTRL_GET_TLSEXT_STATUS_REQ_OCSP_RESP
         71 - SSL_CTRL_SET_TLSEXT_STATUS_REQ_OCSP_RESP
         72 - SSL_CTRL_SET_TLSEXT_TICKET_KEY_CB
         73 - DTLS_CTRL_GET_TIMEOUT
         74 - DTLS_CTRL_HANDLE_TIMEOUT
         75 - DTLS_CTRL_LISTEN
         76 - SSL_CTRL_GET_RI_SUPPORT
         77 - SSL_CTRL_CLEAR_OPTIONS
         78 - SSL_CTRL_CLEAR_MODE

         82 - SSL_CTRL_GET_EXTRA_CHAIN_CERTS
         83 - SSL_CTRL_CLEAR_EXTRA_CHAIN_CERTS

         88 - SSL_CTRL_CHAIN
         89 - SSL_CTRL_CHAIN_CERT

         90 - SSL_CTRL_GET_CURVES
         91 - SSL_CTRL_SET_CURVES
         92 - SSL_CTRL_SET_CURVES_LIST
         93 - SSL_CTRL_GET_SHARED_CURVE
         94 - SSL_CTRL_SET_ECDH_AUTO
         97 - SSL_CTRL_SET_SIGALGS
         98 - SSL_CTRL_SET_SIGALGS_LIST
         99 - SSL_CTRL_CERT_FLAGS
         100 - SSL_CTRL_CLEAR_CERT_FLAGS
         101 - SSL_CTRL_SET_CLIENT_SIGALGS
         102 - SSL_CTRL_SET_CLIENT_SIGALGS_LIST
         103 - SSL_CTRL_GET_CLIENT_CERT_TYPES
         104 - SSL_CTRL_SET_CLIENT_CERT_TYPES
         105 - SSL_CTRL_BUILD_CERT_CHAIN
         106 - SSL_CTRL_SET_VERIFY_CERT_STORE
         107 - SSL_CTRL_SET_CHAIN_CERT_STORE
         108 - SSL_CTRL_GET_PEER_SIGNATURE_NID
         109 - SSL_CTRL_GET_SERVER_TMP_KEY
         110 - SSL_CTRL_GET_RAW_CIPHERLIST
         111 - SSL_CTRL_GET_EC_POINT_FORMATS
         112 - SSL_CTRL_GET_TLSA_RECORD
         113 - SSL_CTRL_SET_TLSA_RECORD
         114 - SSL_CTRL_PULL_TLSA_RECORD

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_ctrl.html>

    *   CTX_flush_sessions

        Causes a run through the session cache of $ctx to remove sessions expired at time $tm.

         Net::SSLeay::CTX_flush_sessions($ctx, $tm);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $tm - specifies the time which should be used for the expiration test (seconds since 1.1.1970)
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_flush_sessions.html>

    *   CTX_free

        Free an allocated SSL_CTX object.

         Net::SSLeay::CTX_free($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_free.html>

    *   CTX_get_app_data

        Can be used to get application defined value/data.

         my $rv = Net::SSLeay::CTX_get_app_data($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: string/buffer/pointer ???

    *   CTX_set_app_data

        Can be used to set some application defined value/data.

         my $rv = Net::SSLeay::CTX_set_app_data($ctx, $arg);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $arg - (string/buffer/pointer ???) data
         #
         # returns: ???

    *   CTX_get0_param

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.0.2-beta1 or LibreSSL 2.7.0

        Returns the current verification parameters.

         my $vpm = Net::SSLeay::CTX_get0_param($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: value corresponding to openssl's X509_VERIFY_PARAM structure

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_CTX_get0_param.html>

    *   CTX_get_cert_store

        Returns the current certificate verification storage.

         my $rv = Net::SSLeay::CTX_get_cert_store($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: value corresponding to openssl's X509_STORE structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_cert_store.html>

    *   CTX_get_client_CA_list

        Returns the list of client CAs explicitly set for $ctx using "CTX_set_client_CA_list".

         my $rv = Net::SSLeay::CTX_get_client_CA_list($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: value corresponding to openssl's X509_NAME_STACK structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_client_CA_list.html>

    *   CTX_get_ex_data

        Is used to retrieve the information for index $idx from $ctx.

         my $rv = Net::SSLeay::CTX_get_ex_data($ssl, $idx);
         # $ssl - value corresponding to openssl's SSL_CTX structure
         # $idx - (integer) index for application specific data
         #
         # returns: pointer to ???

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_get_ex_new_index.html>

    *   CTX_get_ex_new_index

        Is used to register a new index for application specific data.

         my $rv = Net::SSLeay::CTX_get_ex_new_index($argl, $argp, $new_func, $dup_func, $free_func);
         # $argl - (long) ???
         # $argp - (pointer) ???
         # $new_func - function pointer ??? (CRYPTO_EX_new *)
         # $dup_func - function pointer ??? (CRYPTO_EX_dup *)
         # $free_func - function pointer ??? (CRYPTO_EX_free *)
         #
         # returns: (integer) ???

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_get_ex_new_index.html>

    *   CTX_get_mode

        Returns the mode set for ctx.

         my $rv = Net::SSLeay::CTX_get_mode($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: mode (bitmask)

         #to decode the return value (bitmask) use:
         0x00000001 corresponds to SSL_MODE_ENABLE_PARTIAL_WRITE
         0x00000002 corresponds to SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER
         0x00000004 corresponds to SSL_MODE_AUTO_RETRY
         0x00000008 corresponds to SSL_MODE_NO_AUTO_CHAIN
         0x00000010 corresponds to SSL_MODE_RELEASE_BUFFERS
         (note: some of the bits might not be supported by older openssl versions)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_mode.html>

    *   CTX_set_mode

        Adds the mode set via bitmask in $mode to $ctx. Options already set before are not cleared.

         my $rv = Net::SSLeay::CTX_set_mode($ctx, $mode);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $mode - mode bitmask
         #
         # returns: the new mode bitmask after adding $mode

        For bitmask details see "CTX_get_mode" (above).

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_mode.html>

    *   CTX_get_options

        Returns the options (bitmask) set for $ctx.

         my $rv = Net::SSLeay::CTX_get_options($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: options (bitmask)

        BEWARE: The available constants and their values in bitmask depend on the TLS library. For
        example, SSL_OP_NO_TLSv1_3 became available much later than SSL_OP_NO_COMPRESS which is
        already deprecated by some libraries. Also, some previously used option values have been
        recycled and are now used for newer options. See the list of constants in this document for
        options Net::SSLeay currently supports.

        You are strongly encouraged to check your TLS library if you need to use numeric values
        directly. The following is a sample of historic values. It may not be correct anymore.

         #to decode the return value (bitmask) use:
         0x00000004 corresponds to SSL_OP_LEGACY_SERVER_CONNECT
         0x00000800 corresponds to SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
         0x00004000 corresponds to SSL_OP_NO_TICKET
         0x00010000 corresponds to SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
         0x00400000 corresponds to SSL_OP_CIPHER_SERVER_PREFERENCE
         0x04000000 corresponds to SSL_OP_NO_TLSv1

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_get_options.html>

    *   CTX_set_options

        Adds the options set via bitmask in $options to ctx. Options already set before are not
        cleared.

         Net::SSLeay::CTX_set_options($ctx, $options);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $options - options bitmask
         #
         # returns: the new options bitmask after adding $options

        For bitmask details see "CTX_get_options" (above).

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_options.html>

    *   CTX_get_quiet_shutdown

        Returns the 'quiet shutdown' setting of $ctx.

         my $rv = Net::SSLeay::CTX_get_quiet_shutdown($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: (integer) the current setting

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_quiet_shutdown.html>

    *   CTX_get_read_ahead

         my $rv = Net::SSLeay::CTX_get_read_ahead($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: (integer) read_ahead value

    *   CTX_get_session_cache_mode

        Returns the currently used cache mode (bitmask).

         my $rv = Net::SSLeay::CTX_get_session_cache_mode($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: mode (bitmask)

        BEWARE: SESS_CACHE_OFF and other constants are not available in Net-SSLeay-1.82 and before.
        If the constants are not available, the following values have historically been correct. You
        are strongly encouraged to check your TLS library for the current values.

         #to decode the return value (bitmask) use:
         0x0000 corresponds to SSL_SESS_CACHE_OFF
         0x0001 corresponds to SSL_SESS_CACHE_CLIENT
         0x0002 corresponds to SSL_SESS_CACHE_SERVER
         0x0080 corresponds to SSL_SESS_CACHE_NO_AUTO_CLEAR
         0x0100 corresponds to SSL_SESS_CACHE_NO_INTERNAL_LOOKUP
         0x0200 corresponds to SSL_SESS_CACHE_NO_INTERNAL_STORE
         (note: some of the bits might not be supported by older openssl versions)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_session_cache_mode.html>

    *   CTX_set_session_cache_mode

        Enables/disables session caching by setting the operational mode for $ctx to $mode.

         my $rv = Net::SSLeay::CTX_set_session_cache_mode($ctx, $mode);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $mode - mode (bitmask)
         #
         # returns: previously set cache mode

        For bitmask details see "CTX_get_session_cache_mode" (above).

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_session_cache_mode.html>

    *   CTX_get_timeout

        Returns the currently set timeout value for $ctx.

         my $rv = Net::SSLeay::CTX_get_timeout($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: timeout in seconds

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_timeout.html>

    *   CTX_get_verify_depth

        Returns the verification depth limit currently set in $ctx. If no limit has been explicitly
        set, -1 is returned and the default value will be used.

         my $rv = Net::SSLeay::CTX_get_verify_depth($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: depth limit currently set in $ctx, -1 if no limit has been explicitly set

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_get_verify_mode.html>

    *   CTX_get_verify_mode

        Returns the verification mode (bitmask) currently set in $ctx.

         my $rv = Net::SSLeay::CTX_get_verify_mode($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: mode (bitmask)

        For bitmask details see "CTX_set_verify".

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_get_verify_mode.html>

    *   CTX_set_verify

        Sets the verification flags for $ctx to be $mode and specifies the verify_callback function
        to be used.

         Net::SSLeay::CTX_set_verify($ctx, $mode, $callback);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $mode - mode (bitmask), see OpenSSL manual
         # $callback - [optional] reference to perl callback function
         #
         # returns: no return value

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_verify.html>

    *   CTX_set_post_handshake_auth

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Enable the Post-Handshake Authentication extension to be added to the ClientHello such that
        post-handshake authentication can be requested by the server.

         Net::SSLeay::CTX_set_posthandshake_auth($ctx, $val);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $val - 0 then the extension is not sent, otherwise it is
         #
         # returns: no return value

        Check openssl doc
        https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_post_handshake_auth
        <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_post_handshake_auth.html>

    *   CTX_load_verify_locations

        Specifies the locations for $ctx, at which CA certificates for verification purposes are
        located. The certificates available via $CAfile and $CApath are trusted.

         my $rv = Net::SSLeay::CTX_load_verify_locations($ctx, $CAfile, $CApath);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $CAfile - (string) file of CA certificates in PEM format, the file can contain several CA certificates (or '')
         # $CApath - (string) directory containing CA certificates in PEM format (or '')
         #
         # returns: 1 on success, 0 on failure (check the error stack to find out the reason)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_load_verify_locations.html>

    *   CTX_need_tmp_RSA

        Return the result of "SSL_CTX_ctrl(ctx,SSL_CTRL_NEED_TMP_RSA,0,NULL)"

         my $rv = Net::SSLeay::CTX_need_tmp_RSA($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: result of SSL_CTRL_NEED_TMP_RSA command

        Not available with OpenSSL 1.1 and later.

    *   CTX_new

        The same as "CTX_v23_new"

         my $rv = Net::SSLeay::CTX_new();
         #
         # returns: value corresponding to openssl's SSL_CTX structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_new.html>

        Not available with OpenSSL 1.1 and later.

    *   CTX_v2_new

        Creates a new SSL_CTX object - based on SSLv2_method() - as framework to establish TLS/SSL
        enabled connections.

         my $rv = Net::SSLeay::CTX_v2_new();
         #
         # returns: value corresponding to openssl's SSL_CTX structure (0 on failure)

    *   CTX_v23_new

        Creates a new SSL_CTX object - based on SSLv23_method() - as framework to establish TLS/SSL
        enabled connections.

         my $rv = Net::SSLeay::CTX_v23_new();
         #
         # returns: value corresponding to openssl's SSL_CTX structure (0 on failure)

    *   CTX_v3_new

        Creates a new SSL_CTX object - based on SSLv3_method() - as framework to establish TLS/SSL
        enabled connections.

         my $rv = Net::SSLeay::CTX_v3_new();
         #
         # returns: value corresponding to openssl's SSL_CTX structure (0 on failure)

    *   CTX_tlsv1_new

        Creates a new SSL_CTX object - based on TLSv1_method() - as framework to establish TLS/SSL
        enabled connections.

         my $rv = Net::SSLeay::CTX_tlsv1_new();
         #
         # returns: value corresponding to openssl's SSL_CTX structure (0 on failure)

    *   CTX_tlsv1_1_new

        Creates a new SSL_CTX object - based on TLSv1_1_method() - as framework to establish TLS/SSL
        enabled connections. Only available where supported by the underlying openssl.

         my $rv = Net::SSLeay::CTX_tlsv1_1_new();
         #
         # returns: value corresponding to openssl's SSL_CTX structure (0 on failure)

    *   CTX_tlsv1_2_new

        Creates a new SSL_CTX object - based on TLSv1_2_method() - as framework to establish TLS/SSL
        enabled connections. Only available where supported by the underlying openssl.

         my $rv = Net::SSLeay::CTX_tlsv1_2_new();
         #
         # returns: value corresponding to openssl's SSL_CTX structure (0 on failure)

    *   CTX_new_with_method

        Creates a new SSL_CTX object based on $meth method

         my $rv = Net::SSLeay::CTX_new_with_method($meth);
         # $meth - value corresponding to openssl's SSL_METHOD structure
         #
         # returns: value corresponding to openssl's SSL_CTX structure (0 on failure)

         #example
         my $ctx = Net::SSLeay::CTX_new_with_method(&Net::SSLeay::TLSv1_method);

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_new.html>

    *   CTX_set_min_proto_version, CTX_set_max_proto_version, set_min_proto_version and
        set_max_proto_version,

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.1.0-pre2 or LibreSSL 2.6.0

        Set the minimum and maximum supported protocol for $ctx or $ssl.

         my $rv = Net::SSLeay::CTX_set_min_proto_version($ctx, $version)
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $version - (integer) constat version value or 0 for automatic lowest or highest value
         #
         # returns: 1 on success, 0 on failure

         #example: allow only TLS 1.2 for a SSL_CTX
         my $rv_min = Net::SSLeay::CTX_set_min_proto_version($ctx, Net::SSLeay::TLS1_2_VERSION());
         my $rv_max = Net::SSLeay::CTX_set_max_proto_version($ctx, Net::SSLeay::TLS1_2_VERSION());

         #example: allow only TLS 1.1 for a SSL
         my $rv_min = Net::SSLeay::set_min_proto_version($ssl, Net::SSLeay::TLS1_1_VERSION());
         my $rv_max = Net::SSLeay::set_max_proto_version($ssl, Net::SSLeay::TLS1_1_VERSION());

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_min_proto_version.html>

    *   CTX_get_min_proto_version, CTX_get_max_proto_version, get_min_proto_version and
        get_max_proto_version,

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL 1.1.0g

        Get the minimum and maximum supported protocol for $ctx or $ssl.

         my $version = Net::SSLeay::CTX_get_min_proto_version($ctx)
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: 0 automatic lowest or highest value, configured value otherwise

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_min_proto_version.html>

    *   CTX_remove_session

        Removes the session $ses from the context $ctx.

         my $rv = Net::SSLeay::CTX_remove_session($ctx, $ses);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_add_session.html>

    *   CTX_sess_accept

         my $rv = Net::SSLeay::CTX_sess_accept($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of started SSL/TLS handshakes in server mode

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_accept_good

         my $rv = Net::SSLeay::CTX_sess_accept_good($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of successfully established SSL/TLS sessions in server mode

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_accept_renegotiate

         my $rv = Net::SSLeay::CTX_sess_accept_renegotiate($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of start renegotiations in server mode

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_cache_full

         my $rv = Net::SSLeay::CTX_sess_cache_full($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of sessions that were removed because the maximum session cache size was exceeded

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_cb_hits

         my $rv = Net::SSLeay::CTX_sess_cb_hits($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of successfully retrieved sessions from the external session cache in server mode

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_connect

         my $rv = Net::SSLeay::CTX_sess_connect($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of started SSL/TLS handshakes in client mode

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_connect_good

         my $rv = Net::SSLeay::CTX_sess_connect_good($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of successfully established SSL/TLS sessions in client mode

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_connect_renegotiate

         my $rv = Net::SSLeay::CTX_sess_connect_renegotiate($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of start renegotiations in client mode

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_get_cache_size

        Returns the currently valid session cache size.

         my $rv = Net::SSLeay::CTX_sess_get_cache_size($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: current size

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_set_cache_size.html>

    *   CTX_sess_hits

         my $rv = Net::SSLeay::CTX_sess_hits($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of successfully reused sessions

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_misses

         my $rv = Net::SSLeay::CTX_sess_misses($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of sessions proposed by clients that were not found in the internal session cache in server mode

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_number

         my $rv = Net::SSLeay::CTX_sess_number($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: current number of sessions in the internal session cache

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_set_cache_size

        Sets the size of the internal session cache of context $ctx to $size.

         Net::SSLeay::CTX_sess_set_cache_size($ctx, $size);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $size - cache size (0 = unlimited)
         #
         # returns: previously valid size

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_set_cache_size.html>

    *   CTX_sess_timeouts

        Returns the number of sessions proposed by clients and either found in the internal or
        external session cache in server mode, but that were invalid due to timeout. These sessions
        are not included in the SSL_CTX_sess_hits count.

         my $rv = Net::SSLeay::CTX_sess_timeouts($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: number of sessions

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sess_number.html>

    *   CTX_sess_set_new_cb

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before

        Sets the callback function, which is automatically called whenever a new session was
        negotiated.

         Net::SSLeay::CTX_sess_set_new_cb($ctx, $func);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $func - perl reference to callback function
         #
         # returns: no return value

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_sess_set_new_cb.html>

    *   CTX_sess_set_remove_cb

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before

        Sets the callback function, which is automatically called whenever a session is removed by
        the SSL engine.

         Net::SSLeay::CTX_sess_set_remove_cb($ctx, $func);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $func - perl reference to callback function
         #
         # returns: no return value

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_sess_set_remove_cb.html>

    *   CTX_sessions

        Returns a pointer to the lhash databases containing the internal session cache for ctx.

         my $rv = Net::SSLeay::CTX_sessions($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: value corresponding to openssl's LHASH structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_sessions.html>

    *   CTX_set1_param

        COMPATIBILITY: requires at least OpenSSL 1.0.0-beta3

        Applies X509 verification parameters $vpm on $ctx

         my $rv = Net::SSLeay::CTX_set1_param($ctx, $vpm);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $vpm - value corresponding to openssl's X509_VERIFY_PARAM structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_CTX_get0_param.html>

    *   CTX_set_cert_store

        Sets/replaces the certificate verification storage of $ctx to/with $store.

         Net::SSLeay::CTX_set_cert_store($ctx, $store);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $store - value corresponding to openssl's X509_STORE structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_cert_store.html>

    *   CTX_set_cert_verify_callback

        Sets the verification callback function for $ctx. SSL objects that are created from $ctx
        inherit the setting valid at the time when "Net::SSLeay::new($ctx)" is called.

         Net::SSLeay::CTX_set_cert_verify_callback($ctx, $func, $data);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $func - perl reference to callback function
         # $data - [optional] data that will be passed to callback function when invoked
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_cert_verify_callback.html>

    *   CTX_set_cipher_list

        Sets the list of available ciphers for $ctx using the control string $str. The list of
        ciphers is inherited by all ssl objects created from $ctx.

         my $rv = Net::SSLeay::CTX_set_cipher_list($s, $str);
         # $s - value corresponding to openssl's SSL_CTX structure
         # $str - (string) cipher list e.g. '3DES:+RSA'
         #
         # returns: 1 if any cipher could be selected and 0 on complete failure

        The format of $str is described in
        <https://www.openssl.org/docs/manmaster/man1/openssl-ciphers.html>

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_cipher_list.html>

    *   CTX_set_ciphersuites

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Configure the available TLSv1.3 ciphersuites.

         my $rv = Net::SSLeay::CTX_set_ciphersuites($ctx, $str);
         # $ctx  - value corresponding to openssl's SSL_CTX structure
         # $str  - colon (":") separated list of TLSv1.3 ciphersuite names in order of preference
         #
         # returns: (integer) 1 if the requested ciphersuite list was configured, and 0 otherwise

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_ciphersuites.html>

    *   CTX_set_client_CA_list

        Sets the list of CAs sent to the client when requesting a client certificate for $ctx.

         Net::SSLeay::CTX_set_client_CA_list($ctx, $list);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $list - value corresponding to openssl's X509_NAME_STACK structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_client_CA_list.html>

    *   CTX_set_default_passwd_cb

        Sets the default password callback called when loading/storing a PEM certificate with
        encryption.

         Net::SSLeay::CTX_set_default_passwd_cb($ctx, $func);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $func - perl reference to callback function
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_default_passwd_cb.html>

    *   CTX_set_default_passwd_cb_userdata

        Sets a pointer to userdata which will be provided to the password callback on invocation.

         Net::SSLeay::CTX_set_default_passwd_cb_userdata($ctx, $userdata);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $userdata - data that will be passed to callback function when invoked
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_default_passwd_cb.html>

    *   CTX_set_default_verify_paths

        ??? (more info needed)

         my $rv = Net::SSLeay::CTX_set_default_verify_paths($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: 1 on success, 0 on failure

    *   CTX_set_ex_data

        Is used to store application data at $data for $idx into the $ctx object.

         my $rv = Net::SSLeay::CTX_set_ex_data($ssl, $idx, $data);
         # $ssl - value corresponding to openssl's SSL_CTX structure
         # $idx - (integer) ???
         # $data - (pointer) ???
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_get_ex_new_index.html>

    *   CTX_set_purpose

         my $rv = Net::SSLeay::CTX_set_purpose($s, $purpose);
         # $s - value corresponding to openssl's SSL_CTX structure
         # $purpose - (integer) purpose identifier
         #
         # returns: 1 on success, 0 on failure

         #avainable purpose identifier
         1 - X509_PURPOSE_SSL_CLIENT
         2 - X509_PURPOSE_SSL_SERVER
         3 - X509_PURPOSE_NS_SSL_SERVER
         4 - X509_PURPOSE_SMIME_SIGN
         5 - X509_PURPOSE_SMIME_ENCRYPT
         6 - X509_PURPOSE_CRL_SIGN
         7 - X509_PURPOSE_ANY
         8 - X509_PURPOSE_OCSP_HELPER
         9 - X509_PURPOSE_TIMESTAMP_SIGN

         #or use corresponding constants
         $purpose = &Net::SSLeay::X509_PURPOSE_SSL_CLIENT;
         ...
         $purpose = &Net::SSLeay::X509_PURPOSE_TIMESTAMP_SIGN;

    *   CTX_set_quiet_shutdown

        Sets the 'quiet shutdown' flag for $ctx to be mode. SSL objects created from $ctx inherit
        the mode valid at the time "Net::SSLeay::new($ctx)" is called.

         Net::SSLeay::CTX_set_quiet_shutdown($ctx, $mode);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $mode - 0 or 1
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_quiet_shutdown.html>

    *   CTX_set_read_ahead

         my $rv = Net::SSLeay::CTX_set_read_ahead($ctx, $val);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $val - read_ahead value to be set
         #
         # returns: the original read_ahead value

    *   CTX_set_session_id_context

        Sets the context $sid_ctx of length $sid_ctx_len within which a session can be reused for
        the $ctx object.

         my $rv = Net::SSLeay::CTX_set_session_id_context($ctx, $sid_ctx, $sid_ctx_len);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $sid_ctx - data buffer
         # $sid_ctx_len - length of data in $sid_ctx
         #
         # returns: 1 on success, 0 on failure (the error is logged to the error stack)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_session_id_context.html>

    *   CTX_set_ssl_version

        Sets a new default TLS/SSL method for SSL objects newly created from this $ctx. SSL objects
        already created with "Net::SSLeay::new($ctx)" are not affected, except when
        "Net::SSLeay:clear($ssl)" is being called.

         my $rv = Net::SSLeay::CTX_set_ssl_version($ctx, $meth);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $meth - value corresponding to openssl's SSL_METHOD structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_ssl_version.html>

    *   CTX_set_timeout

        Sets the timeout for newly created sessions for $ctx to $t. The timeout value $t must be
        given in seconds.

         my $rv = Net::SSLeay::CTX_set_timeout($ctx, $t);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $t - timeout in seconds
         #
         # returns: previously set timeout value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_timeout.html>

    *   CTX_set_tmp_dh

        Sets DH parameters to be used to be $dh. The key is inherited by all ssl objects created
        from $ctx.

         my $rv = Net::SSLeay::CTX_set_tmp_dh($ctx, $dh);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $dh - value corresponding to openssl's DH structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_tmp_dh_callback.html>

    *   CTX_set_tmp_dh_callback

        Sets the callback function for $ctx to be used when a DH parameters are required to
        $tmp_dh_callback.

         Net::SSLeay::CTX_set_tmp_dh_callback($ctx, $tmp_dh_callback);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # tmp_dh_callback - (function pointer) ???
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_tmp_dh_callback.html>

    *   CTX_set_tmp_rsa

        Sets the temporary/ephemeral RSA key to be used to be $rsa.

         my $rv = Net::SSLeay::CTX_set_tmp_rsa($ctx, $rsa);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $rsa - value corresponding to openssl's RSA structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_tmp_rsa_callback.html>

        Not available with OpenSSL 1.1 and later.

    *   CTX_set_tmp_rsa_callback

        Sets the callback function for ctx to be used when a temporary/ephemeral RSA key is required
        to $tmp_rsa_callback.

        ??? (does this function really work?)

         Net::SSLeay::CTX_set_tmp_rsa_callback($ctx, $tmp_rsa_callback);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $tmp_rsa_callback - (function pointer) ???
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_tmp_rsa_callback.html>

        Not available with OpenSSL 1.1 and later.

    *   CTX_set_trust

         my $rv = Net::SSLeay::CTX_set_trust($s, $trust);
         # $s - value corresponding to openssl's SSL_CTX structure
         # $trust - (integer) trust identifier
         #
         # returns: the original value

         #available trust identifiers
         1 - X509_TRUST_COMPAT
         2 - X509_TRUST_SSL_CLIENT
         3 - X509_TRUST_SSL_SERVER
         4 - X509_TRUST_EMAIL
         5 - X509_TRUST_OBJECT_SIGN
         6 - X509_TRUST_OCSP_SIGN
         7 - X509_TRUST_OCSP_REQUEST
         8 - X509_TRUST_TSA

         #or use corresponding constants
         $trust = &Net::SSLeay::X509_TRUST_COMPAT;
         ...
         $trust = &Net::SSLeay::X509_TRUST_TSA;

    *   CTX_set_verify_depth

        Sets the maximum depth for the certificate chain verification that shall be allowed for ctx.

         Net::SSLeay::CTX_set_verify_depth($ctx, $depth);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $depth - max. depth
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_verify.html>

    *   CTX_use_PKCS12_file

        Adds the certificate and private key from PKCS12 file $p12filename to $ctx.

         my $rv = Net::SSLeay::CTX_use_PKCS12_file($ctx, $p12filename, $password);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $p12filename - (string) filename
         # $password - (string) password to decrypt private key
         #
         # returns: 1 on success, 0 on failure

    *   CTX_use_PrivateKey

        Adds the private key $pkey to $ctx.

         my $rv = Net::SSLeay::CTX_use_PrivateKey($ctx, $pkey);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   CTX_use_PrivateKey_file

        Adds the first private key found in $file to $ctx.

         my $rv = Net::SSLeay::CTX_use_PrivateKey_file($ctx, $file, $type);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $file - (string) file name
         # $type - (integer) type - use constants &Net::SSLeay::FILETYPE_PEM or &Net::SSLeay::FILETYPE_ASN1
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   CTX_use_RSAPrivateKey

        Adds the RSA private key $rsa to $ctx.

         my $rv = Net::SSLeay::CTX_use_RSAPrivateKey($ctx, $rsa);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $rsa - value corresponding to openssl's RSA structure
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   CTX_use_RSAPrivateKey_file

        Adds the first RSA private key found in $file to $ctx.

         my $rv = Net::SSLeay::CTX_use_RSAPrivateKey_file($ctx, $file, $type);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $file - (string) file name
         # $type - (integer) type - use constants &Net::SSLeay::FILETYPE_PEM or &Net::SSLeay::FILETYPE_ASN1
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

    *   CTX_use_certificate

        Loads the certificate $x into $ctx

         my $rv = Net::SSLeay::CTX_use_certificate($ctx, $x);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   CTX_use_certificate_chain_file

        Loads a certificate chain from $file into $ctx. The certificates must be in PEM format and
        must be sorted starting with the subject's certificate (actual client or server
        certificate), followed by intermediate CA certificates if applicable, and ending at the
        highest level (root) CA.

         my $rv = Net::SSLeay::CTX_use_certificate_chain_file($ctx, $file);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $file - (string) file name
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   CTX_use_certificate_file

        Loads the first certificate stored in $file into $ctx.

         my $rv = Net::SSLeay::CTX_use_certificate_file($ctx, $file, $type);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $file - (string) file name
         # $type - (integer) type - use constants &Net::SSLeay::FILETYPE_PEM or &Net::SSLeay::FILETYPE_ASN1
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   CTX_get_security_level

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.0,
        not in LibreSSL

        Returns the security level associated with $ctx.

         my $level = Net::SSLeay::CTX_get_security_level($ctx);
         # $ctx   - value corresponding to openssl's SSL_CTX structure
         #
         # returns: (integer) current security level

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_get_security_level.html>

    *   CTX_set_security_level

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.0,
        not in LibreSSL

        Sets the security level associated with $ctx to $level.

         Net::SSLeay::CTX_set_security_level($ctx, $level);
         # $ssl   - value corresponding to openssl's SSL_CTX structure
         # $level - new security level
         #
         # returns: no return value

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_security_level.html>

    *   CTX_set_num_tickets

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Set number of TLSv1.3 session tickets that will be sent to a client.

         my $rv = Net::SSLeay::CTX_set_num_tickets($ctx, $number_of_tickets);
         # $ctx  - value corresponding to openssl's SSL_CTX structure
         # $number_of_tickets - number of tickets to send
         #
         # returns: 1 on success, 0 on failure

        Set to zero if you do not no want to support a session resumption.

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_num_tickets.html>

    *   CTX_get_num_tickets

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Get number of TLSv1.3 session tickets that will be sent to a client.

         my $number_of_tickets = Net::SSLeay::CTX_get_num_tickets($ctx);
         # $ctx  - value corresponding to openssl's SSL_CTX structure
         #
         # returns: (integer) number of tickets to send

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_get_num_tickets.html>

    *   CTX_set_keylog_callback

        COMPATIBILITY: not available in Net-SSLeay-1.90 and before; requires at least OpenSSL
        1.1.1pre1, not in LibreSSL

        Set the TLS key logging callback.

         Net::SSLeay::CTX_set_keylog_callback($ctx, $cb);
         # $ctx  - value corresponding to openssl's SSL_CTX structure
         # $cb - reference to a perl callback function
         #
         # returns: no return value

        The callback function will be called like this:

         keylog_cb_func($ssl, $line);
         # $ssl - value corresponding to OpenSSL's SSL object associated with the connection
         # $line - a string containing the key material in the format used by NSS for its SSLKEYLOGFILE debugging output

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_keylog_callback.html>

    *   CTX_get_keylog_callback

        COMPATIBILITY: not available in Net-SSLeay-1.90 and before; requires at least OpenSSL
        1.1.1pre1, not in LibreSSL

        Retrieve the previously set TLS key logging callback.

         my $cb = Net::SSLeay::CTX_get_keylog_callback($ctx);
         # $ctx  - value corresponding to openssl's SSL_CTX structure
         #
         # returns: a reference to a perl callback function or undef if no callback is set

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_CTX_get_keylog_callback.html>

   Low level API: SSL_* related functions
    NOTE: Please note that the function described in this chapter have "SSL_" part stripped from
    their original openssl names.

    *   new

        Creates a new SSL structure which is needed to hold the data for a TLS/SSL connection. The
        new structure inherits the settings of the underlying context $ctx: connection method
        (SSLv2/v3/TLSv1), options, verification settings, timeout settings.

         my $rv = Net::SSLeay::new($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: value corresponding to openssl's SSL structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_new.html>

    *   accept

        Waits for a TLS/SSL client to initiate the TLS/SSL handshake. The communication channel must
        already have been set and assigned to the ssl by setting an underlying BIO.

         my $rv = Net::SSLeay::accept($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: 1 = success, 0 = handshake not successful, <0 = fatal error during handshake

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_accept.html>

    *   add_client_CA

        Adds the CA name extracted from cacert to the list of CAs sent to the client when requesting
        a client certificate for the chosen ssl, overriding the setting valid for ssl's SSL_CTX
        object.

         my $rv = Net::SSLeay::add_client_CA($ssl, $x);
         # $ssl - value corresponding to openssl's SSL structure
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_client_CA_list.html>

    *   callback_ctrl

        ??? (more info needed)

         my $rv = Net::SSLeay::callback_ctrl($ssl, $cmd, $fp);
         # $ssl - value corresponding to openssl's SSL structure
         # $cmd - (integer) command id
         # $fp - (function pointer) ???
         #
         # returns: ???

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_ctrl.html>

    *   check_private_key

        Checks the consistency of a private key with the corresponding certificate loaded into $ssl

         my $rv = Net::SSLeay::check_private_key($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   clear

        Reset SSL object to allow another connection.

         Net::SSLeay::clear($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_clear.html>

    *   connect

        Initiate the TLS/SSL handshake with an TLS/SSL server.

         my $rv = Net::SSLeay::connect($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: 1 = success, 0 = handshake not successful, <0 = fatal error during handshake

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_connect.html>

    *   copy_session_id

        Copies the session structure fro $from to $to (+ also the private key and certificate
        associated with $from).

         Net::SSLeay::copy_session_id($to, $from);
         # $to - value corresponding to openssl's SSL structure
         # $from - value corresponding to openssl's SSL structure
         #
         # returns: no return value

    *   ctrl

        Internal handling function for SSL objects.

        BEWARE: openssl doc says: This function should never be called directly!

         my $rv = Net::SSLeay::ctrl($ssl, $cmd, $larg, $parg);
         # $ssl - value corresponding to openssl's SSL structure
         # $cmd - (integer) command id
         # $larg - (integer) long ???
         # $parg - (string/pointer) ???
         #
         # returns: (long) result of given command ???

        For more details about valid $cmd values check "CTX_ctrl".

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_ctrl.html>

    *   do_handshake

        Will wait for a SSL/TLS handshake to take place. If the connection is in client mode, the
        handshake will be started. The handshake routines may have to be explicitly set in advance
        using either SSL_set_connect_state or SSL_set_accept_state(3).

         my $rv = Net::SSLeay::do_handshake($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: 1 = success, 0 = handshake not successful, <0 = fatal error during handshake

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_do_handshake.html>

    *   dup

        Returns a duplicate of $ssl.

         my $rv = Net::SSLeay::dup($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's SSL structure (0 on failure)

    *   free

        Free an allocated SSL structure.

         Net::SSLeay::free($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_free.html>

    *   get0_param

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.0.2-beta1 or LibreSSL 2.7.0

        Returns the current verification parameters.

         my $vpm = Net::SSLeay::get0_param($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's X509_VERIFY_PARAM structure

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_CTX_get0_param.html>

    *   get_SSL_CTX

        Returns a pointer to the SSL_CTX object, from which $ssl was created with Net::SSLeay::new.

         my $rv = Net::SSLeay::get_SSL_CTX($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's SSL_CTX structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_SSL_CTX.html>

    *   set_SSL_CTX

        COMPATIBILITY: requires at least OpenSSL 0.9.8f

        Sets the SSL_CTX the corresponds to an SSL session.

         my $the_ssl_ctx = Net::SSLeay::set_SSL_CTX($ssl, $ssl_ctx);
         # $ssl - value corresponding to openssl's SSL structure
         # $ssl_ctx - Change the ssl object to the given ssl_ctx
         #
         # returns - the ssl_ctx

    *   get_app_data

        Can be used to get application defined value/data.

         my $rv = Net::SSLeay::get_app_data($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: string/buffer/pointer ???

    *   set_app_data

        Can be used to set some application defined value/data.

         my $rv = Net::SSLeay::set_app_data($ssl, $arg);
         # $ssl - value corresponding to openssl's SSL structure
         # $arg - (string/buffer/pointer ???) data
         #
         # returns: ???

    *   get_certificate

        Gets X509 certificate from an established SSL connection.

         my $rv = Net::SSLeay::get_certificate($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's X509 structure (0 on failure)

    *   get_cipher

        Obtains the name of the currently used cipher.

         my $rv = Net::SSLeay::get_cipher($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (string) cipher name e.g. 'DHE-RSA-AES256-SHA' or '', when no session has been established.

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_current_cipher.html>

    *   get_cipher_bits

        Obtain the number of secret/algorithm bits used.

         my $rv = Net::SSLeay::get_cipher_bits($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: number of secret bits used by current cipher

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_current_cipher.html> and
        <http://www.openssl.org/docs/ssl/SSL_CIPHER_get_name.html>

    *   get_ciphers

        COMPATIBILITY: not available in Net-SSLeay-1.88 and before

        Returns a list of SSL_CIPHER structures available for $ssl sorted by preference

         my @ciphers = Net::SSLeay::get_ciphers($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (list) SSL_CIPHER structures or nothing when $ssl is undefined or no ciphers are available

        Example:

         my @ciphers = Net::SSLeay::get_ciphers($ssl);
         foreach my $c (@ciphers) {
           print Net::SSLeay::CIPHER_get_name($c) . "\n";
         }

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_get_ciphers.html>

    *   get_cipher_list

        Returns the name (string) of the SSL_CIPHER listed for $ssl with priority $n.

         my $rv = Net::SSLeay::get_cipher_list($ssl, $n);
         # $ssl - value corresponding to openssl's SSL structure
         # $n - (integer) priority
         #
         # returns: (string) cipher name e.g. 'EDH-DSS-DES-CBC3-SHA' or undef in case of error

        Call Net::SSLeay::get_cipher_list with priority starting from 0 to obtain the sorted list of
        available ciphers, until undef is returned:

         my $priority = 0;
         while (my $c = Net::SSLeay::get_cipher_list($ssl, $priority)) {
           print "cipher[$priority] = $c\n";
           $priority++;
         }

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_get_cipher_list.html>

    *   get_client_CA_list

        Returns the list of client CAs explicitly set for $ssl using
        "Net::SSleay::set_client_CA_list" or $ssl's SSL_CTX object with
        "Net::SSLeay::CTX_set_client_CA_list", when in server mode.

        In client mode, returns the list of client CAs sent from the server, if any.

         my $rv = Net::SSLeay::get_client_CA_list($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's STACK_OF(X509_NAME) structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_client_CA_list.html>

    *   get_current_cipher

        Returns the cipher actually used.

         my $rv = Net::SSLeay::get_current_cipher($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's SSL_CIPHER structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_current_cipher.html>

    *   get_default_timeout

        Returns the default timeout value assigned to SSL_SESSION objects negotiated for the
        protocol valid for $ssl.

         my $rv = Net::SSLeay::get_default_timeout($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (long) timeout in seconds

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_default_timeout.html>

    *   get_error

        Returns a result code for a preceding call to "connect", "accept", "do_handshake", "read",
        "peek" or "write" on $ssl.

         my $rv = Net::SSLeay::get_error($ssl, $ret);
         # $ssl - value corresponding to openssl's SSL structure
         # $ret - return value of preceding TLS/SSL I/O operation
         #
         # returns: result code, which is one of the following values:
         #  0 - SSL_ERROR_NONE
         #  1 - SSL_ERROR_SSL
         #  2 - SSL_ERROR_WANT_READ
         #  3 - SSL_ERROR_WANT_WRITE
         #  4 - SSL_ERROR_WANT_X509_LOOKUP
         #  5 - SSL_ERROR_SYSCALL
         #  6 - SSL_ERROR_ZERO_RETURN
         #  7 - SSL_ERROR_WANT_CONNECT
         #  8 - SSL_ERROR_WANT_ACCEPT

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_error.html>

    *   get_ex_data

        Is used to retrieve the information for $idx from $ssl.

         my $rv = Net::SSLeay::get_ex_data($ssl, $idx);
         # $ssl - value corresponding to openssl's SSL structure
         # $idx - (integer) index for application specific data
         #
         # returns: pointer to ???

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_ex_new_index.html>

    *   set_ex_data

        Is used to store application data at $data for $idx into the $ssl object.

         my $rv = Net::SSLeay::set_ex_data($ssl, $idx, $data);
         # $ssl - value corresponding to openssl's SSL structure
         # $idx - (integer) ???
         # $data - (pointer) ???
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_ex_new_index.html>

    *   get_ex_new_index

        Is used to register a new index for application specific data.

         my $rv = Net::SSLeay::get_ex_new_index($argl, $argp, $new_func, $dup_func, $free_func);
         # $argl - (long) ???
         # $argp - (pointer) ???
         # $new_func - function pointer ??? (CRYPTO_EX_new *)
         # $dup_func - function pointer ??? (CRYPTO_EX_dup *)
         # $free_func - function pointer ??? (CRYPTO_EX_free *)
         #
         # returns: (integer) ???

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_ex_new_index.html>

    *   get_fd

        Returns the file descriptor which is linked to $ssl.

         my $rv = Net::SSLeay::get_fd($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: file descriptor (>=0) or -1 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_fd.html>

    *   get_finished

        Obtains the latest 'Finished' message sent to the peer. Return value is zero if there's been
        no Finished message yet. Default count is 2*EVP_MAX_MD_SIZE that is long enough for all
        possible Finish messages. If you supply a non-default count, the resulting return value may
        be longer than returned buf's length.

         my $rv = Net::SSLeay::get_finished($ssl, $buf, $count);
         # $ssl - value corresponding to openssl's SSL structure
         # $buf - buffer where the returned data will be stored
         # $count - [optional] max size of return data - default is 2*EVP_MAX_MD_SIZE
         #
         # returns: length of latest Finished message

    *   get_peer_finished

        Obtains the latest 'Finished' message expected from the peer. Parameters and return value
        are similar to get_finished().

         my $rv = Net::SSLeay::get_peer_finished($ssl, $buf, $count);
         # $ssl - value corresponding to openssl's SSL structure
         # $buf - buffer where the returned data will be stored
         # $count - [optional] max size of return data - default is 2*EVP_MAX_MD_SIZE
         #
         # returns: length of latest Finished message

    *   get_keyblock_size

        Gets the length of the TLS keyblock.

        NOTE: Does not exactly correspond to any low level API function.

         my $rv = Net::SSLeay::get_keyblock_size($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: keyblock size, -1 on error

    *   get_mode

        Returns the mode (bitmask) set for $ssl.

         my $rv = Net::SSLeay::get_mode($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: mode (bitmask)

        To decode the return value (bitmask) see documentation for "CTX_get_mode".

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_mode.html>

    *   set_mode

        Adds the mode set via bitmask in $mode to $ssl. Options already set before are not cleared.

         my $rv = Net::SSLeay::set_mode($ssl, $mode);
         # $ssl - value corresponding to openssl's SSL structure
         # $mode - mode (bitmask)
         #
         # returns: the new mode bitmask after adding $mode

        For $mode bitmask details see "CTX_get_mode".

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_mode.html>

    *   get_options

        Returns the options (bitmask) set for $ssl.

         my $rv = Net::SSLeay::get_options($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: options (bitmask)

        To decode the return value (bitmask) see documentation for "CTX_get_options".

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_options.html>

    *   set_options

        Adds the options set via bitmask in $options to $ssl. Options already set before are not
        cleared!

         Net::SSLeay::set_options($ssl, $options);
         # $ssl - value corresponding to openssl's SSL structure
         # $options - options (bitmask)
         #
         # returns: the new options bitmask after adding $options

        For $options bitmask details see "CTX_get_options".

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_options.html>

    *   get_peer_certificate

        Get the X509 certificate of the peer.

         my $rv = Net::SSLeay::get_peer_certificate($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's X509 structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_peer_certificate.html>

    *   get_peer_cert_chain

        Get the certificate chain of the peer as an array of X509 structures.

         my @rv = Net::SSLeay::get_peer_cert_chain($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: list of X509 structures

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_peer_certificate.html>

    *   get_quiet_shutdown

        Returns the 'quiet shutdown' setting of ssl.

         my $rv = Net::SSLeay::get_quiet_shutdown($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (integer) current 'quiet shutdown' value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_quiet_shutdown.html>

    *   get_rbio

        Get 'read' BIO linked to an SSL object $ssl.

         my $rv = Net::SSLeay::get_rbio($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's BIO structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_rbio.html>

    *   get_read_ahead

         my $rv = Net::SSLeay::get_read_ahead($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (integer) read_ahead value

    *   set_read_ahead

         Net::SSLeay::set_read_ahead($ssl, $val);
         # $ssl - value corresponding to openssl's SSL structure
         # $val - read_ahead value to be set
         #
         # returns: the original read_ahead value

    *   get_security_level

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.0,
        not in LibreSSL

        Returns the security level associated with $ssl.

         my $level = Net::SSLeay::get_security_level($ssl);
         # $ssl   - value corresponding to openssl's SSL structure
         #
         # returns: (integer) current security level

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_get_security_level.html>

    *   set_security_level

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.0,
        not in LibreSSL

        Sets the security level associated with $ssl to $level.

         Net::SSLeay::set_security_level($ssl, $level);
         # $ssl   - value corresponding to openssl's SSL structure
         # $level - new security level
         #
         # returns: no return value

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_set_security_level.html>

    *   set_num_tickets

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Set number of TLSv1.3 session tickets that will be sent to a client.

         my $rv = Net::SSLeay::set_num_tickets($ssl, $number_of_tickets);
         # $ssl  - value corresponding to openssl's SSL structure
         # $number_of_tickets - number of tickets to send
         #
         # returns: 1 on success, 0 on failure

        Set to zero if you do not no want to support a session resumption.

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_set_num_tickets.html>

    *   get_num_tickets

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Get number of TLSv1.3 session tickets that will be sent to a client.

         my $number_of_tickets = Net::SSLeay::get_num_tickets($ctx);
         # $ctx  - value corresponding to openssl's SSL structure
         #
         # returns: number of tickets to send

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_get_num_tickets.html>

    *   get_server_random

        Returns internal SSLv3 server_random value.

         Net::SSLeay::get_server_random($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: server_random value (binary data)

    *   get_client_random

        NOTE: Does not exactly correspond to any low level API function

        Returns internal SSLv3 client_random value.

         Net::SSLeay::get_client_random($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: client_random value (binary data)

    *   export_keying_material

        Returns keying material based on the string $label and optional $context. Note that with
        TLSv1.2 and lower, empty context (empty string) and undefined context (no value or 'undef')
        will return different values.

          my $out = Net::SSLeay::export_keying_material($ssl, $olen, $label, $context);
          # $ssl - value corresponding to openssl's SSL structure
          # $olen - number of bytes to return
          # $label - application specific label
          # $context - [optional] context - default is undef for no context
          #
          # returns: keying material (binary data) or undef on error

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_export_keying_material.html>

    *   get_session

        Retrieve TLS/SSL session data used in $ssl. The reference count of the SSL_SESSION is NOT
        incremented.

         my $rv = Net::SSLeay::get_session($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's SSL_SESSION structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_session.html>

    *   SSL_get0_session

        The alias for "get_session" (note that the name is "SSL_get0_session" NOT "get0_session").

         my $rv = Net::SSLeay::SSL_get0_session();

    *   get1_session

        Returns a pointer to the SSL_SESSION actually used in $ssl. The reference count of the
        SSL_SESSION is incremented by 1.

         my $rv = Net::SSLeay::get1_session($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's SSL_SESSION structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_session.html>

    *   get_shared_ciphers

        Returns string with a list (colon ':' separated) of ciphers shared between client and server
        within SSL session $ssl.

         my $rv = Net::SSLeay::get_shared_ciphers()
         #
         # returns: string like 'ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES256-SHA:DHE-DSS-AES256-SHA:...'

    *   get_shutdown

        Returns the shutdown mode of $ssl.

         my $rv = Net::SSLeay::get_shutdown($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: shutdown mode (bitmask) of ssl

         #to decode the return value (bitmask) use:
         0 - No shutdown setting, yet
         1 - SSL_SENT_SHUTDOWN
         2 - SSL_RECEIVED_SHUTDOWN

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_shutdown.html>

    *   get_ssl_method

        Returns a function pointer to the TLS/SSL method set in $ssl.

         my $rv = Net::SSLeay::get_ssl_method($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's SSL_METHOD structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_ssl_version.html>

    *   in_init, in_before, is_init_finished, in_connect_init, in_accept_init

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before.

        Retrieve information about the handshake state machine. All functions take $ssl as the only
        argument and return 0 or 1. These functions are recommended over get_state() and state().

         my $rv = Net::SSLeay::is_init_finished($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: All functions return 1 or 0

        Check openssl doc https://www.openssl.org/docs/ssl/SSL_in_init.html
        <http://www.openssl.org/docs/ssl/SSL_in_init.html>

    *   get_state

        COMPATIBILITY: OpenSSL 1.1.0 and later use different constants which are not made available.
        Use is_init_finished() and related functions instead.

        Returns the SSL connection state.

         my $rv = Net::SSLeay::get_state($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (integer) state value
         #          to decode the returned state check:
         #          SSL_ST_* constants in openssl/ssl.h
         #          SSL2_ST_* constants in openssl/ssl2.h
         #          SSL23_ST_* constants in openssl/ssl23.h
         #          SSL3_ST_* + DTLS1_ST_* constants in openssl/ssl3.h

    *   state

        Exactly the same as "get_state".

         my $rv = Net::SSLeay::state($ssl);

    *   set_state

        Sets the SSL connection state.

         Net::SSLeay::set_state($ssl,Net::SSLeay::SSL_ST_ACCEPT());

        Not available with OpenSSL 1.1 and later.

    *   get_verify_depth

        Returns the verification depth limit currently set in $ssl.

         my $rv = Net::SSLeay::get_verify_depth($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: current depth or -1 if no limit has been explicitly set

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_get_verify_mode.html>

    *   set_verify_depth

        Sets the maximum depth for the certificate chain verification that shall be allowed for
        $ssl.

         Net::SSLeay::set_verify_depth($ssl, $depth);
         # $ssl - value corresponding to openssl's SSL structure
         # $depth - (integer) depth
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_verify.html>

    *   get_verify_mode

        Returns the verification mode (bitmask) currently set in $ssl.

         my $rv = Net::SSLeay::get_verify_mode($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: mode (bitmask)

        To decode the return value (bitmask) see documentation for "CTX_get_verify_mode".

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_get_verify_mode.html>

    *   set_verify

        Sets the verification flags for $ssl to be $mode and specifies the $verify_callback function
        to be used.

         Net::SSLeay::set_verify($ssl, $mode, $callback);
         # $ssl - value corresponding to openssl's SSL structure
         # $mode - mode (bitmask)
         # $callback - [optional] reference to perl callback function
         #
         # returns: no return value

        For $mode bitmask details see "CTX_get_verify_mode".

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_verify.html>

    *   set_post_handshake_auth

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Enable the Post-Handshake Authentication extension to be added to the ClientHello such that
        post-handshake authentication can be requested by the server.

         Net::SSLeay::set_posthandshake_auth($ssl, $val);
         # $ssl - value corresponding to openssl's SSL structure
         # $val - 0 then the extension is not sent, otherwise it is
         #
         # returns: no return value

        Check openssl doc https://www.openssl.org/docs/manmaster/man3/SSL_set_post_handshake_auth
        <https://www.openssl.org/docs/manmaster/man3/SSL_set_post_handshake_auth.html>

    *   verify_client_post_handshake

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        verify_client_post_handshake causes a CertificateRequest message to be sent by a server on
        the given ssl connection.

         my $rv = Net::SSLeay::verify_client_post_handshake($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: 1 if the request succeeded, and 0 if the request failed. The error stack can be examined to determine the failure reason.

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/SSL_verify_client_post_handshake.html>

    *   get_verify_result

        Returns the result of the verification of the X509 certificate presented by the peer, if
        any.

         my $rv = Net::SSLeay::get_verify_result($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (integer)
         #      0 - X509_V_OK: ok
         #      2 - X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT: unable to get issuer certificate
         #      3 - X509_V_ERR_UNABLE_TO_GET_CRL: unable to get certificate CRL
         #      4 - X509_V_ERR_UNABLE_TO_DECRYPT_CERT_SIGNATURE: unable to decrypt certificate's signature
         #      5 - X509_V_ERR_UNABLE_TO_DECRYPT_CRL_SIGNATURE: unable to decrypt CRL's signature
         #      6 - X509_V_ERR_UNABLE_TO_DECODE_ISSUER_PUBLIC_KEY: unable to decode issuer public key
         #      7 - X509_V_ERR_CERT_SIGNATURE_FAILURE: certificate signature failure
         #      8 - X509_V_ERR_CRL_SIGNATURE_FAILURE: CRL signature failure
         #      9 - X509_V_ERR_CERT_NOT_YET_VALID: certificate is not yet valid
         #     10 - X509_V_ERR_CERT_HAS_EXPIRED: certificate has expired
         #     11 - X509_V_ERR_CRL_NOT_YET_VALID: CRL is not yet valid
         #     12 - X509_V_ERR_CRL_HAS_EXPIRED: CRL has expired
         #     13 - X509_V_ERR_ERROR_IN_CERT_NOT_BEFORE_FIELD: format error in certificate's notBefore field
         #     14 - X509_V_ERR_ERROR_IN_CERT_NOT_AFTER_FIELD: format error in certificate's notAfter field
         #     15 - X509_V_ERR_ERROR_IN_CRL_LAST_UPDATE_FIELD: format error in CRL's lastUpdate field
         #     16 - X509_V_ERR_ERROR_IN_CRL_NEXT_UPDATE_FIELD: format error in CRL's nextUpdate field
         #     17 - X509_V_ERR_OUT_OF_MEM: out of memory
         #     18 - X509_V_ERR_DEPTH_ZERO_SELF_SIGNED_CERT: self signed certificate
         #     19 - X509_V_ERR_SELF_SIGNED_CERT_IN_CHAIN: self signed certificate in certificate chain
         #     20 - X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY: unable to get local issuer certificate
         #     21 - X509_V_ERR_UNABLE_TO_VERIFY_LEAF_SIGNATURE: unable to verify the first certificate
         #     22 - X509_V_ERR_CERT_CHAIN_TOO_LONG: certificate chain too long
         #     23 - X509_V_ERR_CERT_REVOKED: certificate revoked
         #     24 - X509_V_ERR_INVALID_CA: invalid CA certificate
         #     25 - X509_V_ERR_PATH_LENGTH_EXCEEDED: path length constraint exceeded
         #     26 - X509_V_ERR_INVALID_PURPOSE: unsupported certificate purpose
         #     27 - X509_V_ERR_CERT_UNTRUSTED: certificate not trusted
         #     28 - X509_V_ERR_CERT_REJECTED: certificate rejected
         #     29 - X509_V_ERR_SUBJECT_ISSUER_MISMATCH: subject issuer mismatch
         #     30 - X509_V_ERR_AKID_SKID_MISMATCH: authority and subject key identifier mismatch
         #     31 - X509_V_ERR_AKID_ISSUER_SERIAL_MISMATCH: authority and issuer serial number mismatch
         #     32 - X509_V_ERR_KEYUSAGE_NO_CERTSIGN:key usage does not include certificate signing
         #     50 - X509_V_ERR_APPLICATION_VERIFICATION: application verification failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_verify_result.html>

    *   set_verify_result

        Override result of peer certificate verification.

         Net::SSLeay::set_verify_result($ssl, $v);
         # $ssl - value corresponding to openssl's SSL structure
         # $v - (integer) result value
         #
         # returns: no return value

        For more info about valid return values see "get_verify_result"

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_verify_result.html>

    *   get_wbio

        Get 'write' BIO linked to an SSL object $ssl.

         my $rv = Net::SSLeay::get_wbio($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: value corresponding to openssl's BIO structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_get_rbio.html>

    *   load_client_CA_file

        Load X509 certificates from file (PEM formatted).

         my $rv = Net::SSLeay::load_client_CA_file($file);
         # $file - (string) file name
         #
         # returns: value corresponding to openssl's STACK_OF(X509_NAME) structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_load_client_CA_file.html>

    *   clear_num_renegotiations

        Executes SSL_CTRL_CLEAR_NUM_RENEGOTIATIONS command on $ssl.

         my $rv = Net::SSLeay::clear_num_renegotiations($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: command result

    *   need_tmp_RSA

        Executes SSL_CTRL_NEED_TMP_RSA command on $ssl.

         my $rv = Net::SSLeay::need_tmp_RSA($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: command result

        Not available with OpenSSL 1.1 and later.

    *   num_renegotiations

        Executes SSL_CTRL_GET_NUM_RENEGOTIATIONS command on $ssl.

         my $rv = Net::SSLeay::num_renegotiations($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: command result

    *   total_renegotiations

        Executes SSL_CTRL_GET_TOTAL_RENEGOTIATIONS command on $ssl.

         my $rv = Net::SSLeay::total_renegotiations($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: command result

    *   peek

        Copies $max bytes from the specified $ssl into the returned value. In contrast to the
        "Net::SSLeay::read()" function, the data in the SSL buffer is unmodified after the
        SSL_peek() operation.

         Net::SSLeay::peek($ssl, $max);
         # $ssl - value corresponding to openssl's SSL structure
         # $max - [optional] max bytes to peek (integer) - default is 32768
         #
         # in scalar context: data read from the TLS/SSL connection, undef on error
         # in list context:   two-item array consisting of data read (undef on error),
         #                      and return code from SSL_peek().

    *   peek_ex

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Copies $max bytes from the specified $ssl into the returned value. In contrast to the
        "Net::SSLeay::read_ex()" function, the data in the SSL buffer is unmodified after the
        SSL_peek_ex() operation.

         my($got, $rv) = Net::SSLeay::peek_ex($ssl, $max);
         # $ssl - value corresponding to openssl's SSL structure
         # $max - [optional] max bytes to peek (integer) - default is 32768
         #
         # returns a list: two-item list consisting of data read (undef on error),
         #                 and return code from SSL_peek_ex().

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_peek_ex.html>

    *   pending

        Obtain number of readable bytes buffered in $ssl object.

         my $rv = Net::SSLeay::pending($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: the number of bytes pending

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_pending.html>

    *   has_pending

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.0,
        not in LibreSSL

        Returns 1 if $ssl has buffered data (whether processed or unprocessed) and 0 otherwise.

         my $rv = Net::SSLeay::has_pending($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (integer) 1 or 0

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_has_pending.html>

    *   read

        Tries to read $max bytes from the specified $ssl.

         my $got = Net::SSLeay::read($ssl, $max);
         my($got, $rv) = Net::SSLeay::read($ssl, $max);
         # $ssl - value corresponding to openssl's SSL structure
         # $max - [optional] max bytes to read (integer) - default is 32768
         #
         # returns:
         # in scalar context: data read from the TLS/SSL connection, undef on error
         # in list context:   two-item array consisting of data read (undef on error),
         #                      and return code from SSL_read().

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_read.html>

    *   read_ex

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Tries to read $max bytes from the specified $ssl.

         my($got, $rv) = Net::SSLeay::read_ex($ssl, $max);
         # $ssl - value corresponding to openssl's SSL structure
         # $max - [optional] max bytes to read (integer) - default is 32768
         #
         # returns a list: two-item list consisting of data read (undef on error),
         #                 and return code from SSL_read_ex().

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_read_ex.html>

    *   renegotiate

        Turn on flags for renegotiation so that renegotiation will happen

         my $rv = Net::SSLeay::renegotiate($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: 1 on success, 0 on failure

    *   rstate_string

        Returns a 2 letter string indicating the current read state of the SSL object $ssl.

         my $rv = Net::SSLeay::rstate_string($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: 2-letter string

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_rstate_string.html>

    *   rstate_string_long

        Returns a string indicating the current read state of the SSL object ssl.

         my $rv = Net::SSLeay::rstate_string_long($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: string with current state

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_rstate_string.html>

    *   session_reused

        Query whether a reused session was negotiated during handshake.

         my $rv = Net::SSLeay::session_reused($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: 0 - new session was negotiated; 1 - session was reused.

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_session_reused.html>

    *   set1_param

        COMPATIBILITY: requires at least OpenSSL 1.0.0-beta3

        Applies X509 verification parameters $vpm on $ssl

         my $rv = Net::SSLeay::set1_param($ssl, $vpm);
         # $ssl - value corresponding to openssl's SSL structure
         # $vpm - value corresponding to openssl's X509_VERIFY_PARAM structure
         #
         # returns: 1 on success, 0 on failure

    *   set_accept_state

        Sets $ssl to work in server mode.

         Net::SSLeay::set_accept_state($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_connect_state.html>

    *   set_bio

        Connects the BIOs $rbio and $wbio for the read and write operations of the TLS/SSL
        (encrypted) side of $ssl.

         Net::SSLeay::set_bio($ssl, $rbio, $wbio);
         # $ssl - value corresponding to openssl's SSL structure
         # $rbio - value corresponding to openssl's BIO structure
         # $wbio - value corresponding to openssl's BIO structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_bio.html>

    *   set_cipher_list

        Sets the list of ciphers only for ssl.

         my $rv = Net::SSLeay::set_cipher_list($ssl, $str);
         # $ssl - value corresponding to openssl's SSL structure
         # $str - (string) cipher list e.g. '3DES:+RSA'
         #
         # returns: 1 if any cipher could be selected and 0 on complete failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_cipher_list.html>

    *   set_ciphersuites

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Configure the available TLSv1.3 ciphersuites.

         my $rv = Net::SSLeay::set_ciphersuites($ssl, $str);
         # $ssl  - value corresponding to openssl's SSL structure
         # $str  - colon (":") separated list of TLSv1.3 ciphersuite names in order of preference
         #
         # returns: (integer) 1 if the requested ciphersuite list was configured, and 0 otherwise

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_set_ciphersuites.html>

    *   set_client_CA_list

        Sets the list of CAs sent to the client when requesting a client certificate for the chosen
        $ssl, overriding the setting valid for $ssl's SSL_CTX object.

         my $rv = Net::SSLeay::set_client_CA_list($ssl, $list);
         # $ssl - value corresponding to openssl's SSL structure
         # $list - value corresponding to openssl's STACK_OF(X509_NAME) structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_client_CA_list.html>

    *   set_connect_state

        Sets $ssl to work in client mode.

         Net::SSLeay::set_connect_state($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_connect_state.html>

    *   set_fd

        Sets the file descriptor $fd as the input/output facility for the TLS/SSL (encrypted) side
        of $ssl, $fd will typically be the socket file descriptor of a network connection.

         my $rv = Net::SSLeay::set_fd($ssl, $fd);
         # $ssl - value corresponding to openssl's SSL structure
         # $fd - (integer) file handle (got via perl's fileno)
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_fd.html>

    *   set_psk_client_callback

        Sets the psk client callback.

         Net::SSLeay::set_psk_client_callback($ssl, sub { my $hint = shift; return ($identity, $key) } );
         # $ssl - value corresponding to openssl's SSL structure
         # $hint - PSK identity hint send by the server
         # $identity - PSK identity
         # $key - PSK key, hex string without the leading '0x', e.g. 'deadbeef'
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_psk_client_callback.html>

    *   set_rfd

        Sets the file descriptor $fd as the input (read) facility for the TLS/SSL (encrypted) side
        of $ssl.

         my $rv = Net::SSLeay::set_rfd($ssl, $fd);
         # $ssl - value corresponding to openssl's SSL structure
         # $fd - (integer) file handle (got via perl's fileno)
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_fd.html>

    *   set_wfd

         my $rv = Net::SSLeay::set_wfd($ssl, $fd);
         # $ssl - value corresponding to openssl's SSL structure
         # $fd - (integer) file handle (got via perl's fileno)
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_fd.html>

    *   set_info_callback

        Sets the callback function, that can be used to obtain state information for $ssl during
        connection setup and use. When callback is undef, the callback setting currently valid for
        ctx is used.

         Net::SSLeay::set_info_callback($ssl, $cb, [$data]);
         # $ssl - value corresponding to openssl's SSL structure
         # $cb - sub { my ($ssl,$where,$ret,$data) = @_; ... }
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_info_callback.html>

    *   CTX_set_info_callback

        Sets the callback function on ctx, that can be used to obtain state information during ssl
        connection setup and use. When callback is undef, an existing callback will be disabled.

         Net::SSLeay::CTX_set_info_callback($ssl, $cb, [$data]);
         # $ssl - value corresponding to openssl's SSL structure
         # $cb - sub { my ($ssl,$where,$ret,$data) = @_; ... }
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_info_callback.html>

    *   set_msg_callback

        Sets the callback function, that can be used to obtain protocol messages information for
        $ssl during connection setup and use. When callback is undef, the callback setting currently
        valid for ctx is used. Note that set_msg_callback_arg is not provided as there is no need to
        explicitly set $arg, this is handled by set_msg_callback.

         Net::SSLeay::set_msg_callback($ssl, $cb, [$arg]);
         # $ssl - value corresponding to openssl's SSL structure
         # $cb - sub { my ($write_p,$version,$content_type,$buf,$len,$ssl,$arg) = @_; ... }
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/manmaster/man3/SSL_set_msg_callback.html>

    *   CTX_set_msg_callback

        Sets the callback function on ctx, that can be used to obtain protocol messages information
        for ssl connection setup and use. When callback is undef, the existing callback will be
        disabled. Note that CTX_set_msg_callback_arg is not provided as there is no need to
        explicitly set $arg, this is handled by CTX_set_msg_callback.

         Net::SSLeay::CTX_set_msg_callback($ssl, $cb, [$arg]);
         # $ssl - value corresponding to openssl's SSL structure
         # $cb - sub { my ($write_p,$version,$content_type,$buf,$len,$ssl,$arg) = @_; ... }
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/manmaster/man3/SSL_CTX_set_msg_callback.html>

    *   set_pref_cipher

        Sets the list of available ciphers for $ssl using the control string $str.

         my $rv = Net::SSLeay::set_pref_cipher($ssl, $str);
         # $ssl - value corresponding to openssl's SSL structure
         # $str - (string) cipher list e.g. '3DES:+RSA'
         #
         # returns: 1 if any cipher could be selected and 0 on complete failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_cipher_list.html>

    *   CTX_set_psk_client_callback

        Sets the psk client callback.

         Net::SSLeay::CTX_set_psk_client_callback($ssl, sub { my $hint = shift; return ($identity, $key) } );
         # $ssl - value corresponding to openssl's SSL structure
         # $hint - PSK identity hint send by the server
         # $identity - PSK identity
         # $key - PSK key, hex string without the leading '0x', e.g. 'deadbeef'
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_psk_client_callback.html>

    *   set_purpose

         my $rv = Net::SSLeay::set_purpose($ssl, $purpose);
         # $ssl - value corresponding to openssl's SSL structure
         # $purpose - (integer) purpose identifier
         #
         # returns: 1 on success, 0 on failure

        For more info about available $purpose identifiers see "CTX_set_purpose".

    *   set_quiet_shutdown

        Sets the 'quiet shutdown' flag for $ssl to be $mode.

         Net::SSLeay::set_quiet_shutdown($ssl, $mode);
         # $ssl - value corresponding to openssl's SSL structure
         # $mode - 0 or 1
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_quiet_shutdown.html>

    *   set_session

        Set a TLS/SSL session to be used during TLS/SSL connect.

         my $rv = Net::SSLeay::set_session($to, $ses);
         # $to - value corresponding to openssl's SSL structure
         # $ses - value corresponding to openssl's SSL_SESSION structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_session.html>

    *   set_session_id_context

        Sets the context $sid_ctx of length $sid_ctx_len within which a session can be reused for
        the $ssl object.

         my $rv = Net::SSLeay::set_session_id_context($ssl, $sid_ctx, $sid_ctx_len);
         # $ssl - value corresponding to openssl's SSL structure
         # $sid_ctx - data buffer
         # $sid_ctx_len - length of data in $sid_ctx
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_session_id_context.html>

    *   set_session_secret_cb

        Setup pre-shared secret session resumption function.

         Net::SSLeay::set_session_secret_cb($ssl, $func, $data);
         # $ssl - value corresponding to openssl's SSL structure
         # $func - perl reference to callback function
         # $data - [optional] data that will be passed to callback function when invoked
         #
         # returns: no return value

        The callback function will be called like:

         callback_function($secret, $ciphers, $pref_cipher, $data);
         # $secret is the current master session key, usually all 0s at the beginning of a session
         # $ciphers is ref to an array of peer cipher names
         # $pref_cipher is a ref to an index into the list of cipher names of
         #  the preferred cipher. Set it if you want to specify a preferred cipher
         # $data is the data passed to set_session_secret_cb

        The callback function should return 1 if it likes the suggested cipher (or has selected an
        alternative by setting pref_cipher), else it should return 0 (in which case OpenSSL will
        select its own preferred cipher).

        With OpenSSL 1.1 and later, callback_function can change the master key for the session by
        altering $secret and returning 1.

    *   CTX_set_tlsext_ticket_getkey_cb

        Setup encryption for TLS session tickets (stateless session reuse).

         Net::SSLeay::CTX_set_tlsext_ticket_getkey_cb($ctx, $func, $data);
         # $ctx  - value corresponding to openssl's SSL_CTX structure
         # $func - perl reference to callback function
         # $data - [optional] data that will be passed to callback function when invoked
         #
         # returns: no return value

        The callback function will be called like:

         getkey($data,[$key_name]) -> ($key,$current_key_name)
         # $data is the data passed to set_session_secret_cb
         # $key_name is the name of the key OpenSSL has extracted from the session ticket
         # $key is the requested key for ticket encryption + HMAC
         # $current_key_name is the name for the currently valid key

        OpenSSL will call the function without a key name if it generates a new ticket. It then
        needs the callback to return the encryption+HMAC key and an identifier (key name) for this
        key.

        When OpenSSL gets a session ticket from the client it extracts the key name and calls the
        callback with this name as argument. It then expects the callback to return the
        encryption+HMAC key matching the requested key name and and also the key name which should
        be used at the moment. If the requested key name and the returned key name differ it means
        that this session ticket was created with an expired key and need to be renewed. In this
        case OpenSSL will call the callback again with no key name to create a new session ticket
        based on the old one.

        The key must be at least 32 byte of random data which can be created with RAND_bytes.
        Internally the first 16 byte are used as key in AES-128 encryption while the next 16 byte
        are used for the SHA-256 HMAC. The key name are binary data and must be exactly 16 byte
        long.

        Example:

            Net::SSLeay::RAND_bytes(my $oldkey,32);
            Net::SSLeay::RAND_bytes(my $newkey,32);
            my $oldkey_name = pack("a16",'oldsecret');
            my $newkey_name = pack("a16",'newsecret');

            my @keys = (
                [ $newkey_name, $newkey ], # current active key
                [ $oldkey_name, $oldkey ], # already expired
            );

            Net::SSLeay::CTX_set_tlsext_ticket_getkey_cb($server2->_ctx, sub {
                my ($mykeys,$name) = @_;

                # return (current_key, current_key_name) if no name given
                return ($mykeys->[0][1],$mykeys->[0][0]) if ! $name;

                # return (matching_key, current_key_name) if we find a key matching
                # the given name
                for(my $i = 0; $i<@$mykeys; $i++) {
                    next if $name ne $mykeys->[$i][0];
                    return ($mykeys->[$i][1],$mykeys->[0][0]);
                }

                # no matching key found
                return;
            },\@keys);

        This function is based on the OpenSSL function SSL_CTX_set_tlsext_ticket_key_cb but provides
        a simpler to use interface. For more information see
        <http://www.openssl.org/docs/ssl/SSL_CTX_set_tlsext_ticket_key_cb.html>

    *   set_session_ticket_ext_cb

        Setup callback for TLS session tickets (stateless session reuse).

         Net::SSLeay::set_session_ticket_ext_cb($ssl, $func, $data);
         # $ssl  - value corresponding to openssl's SSL structure
         # $func - perl reference to callback function
         # $data - [optional] data that will be passed to callback function when invoked
         #
         # returns: no return value

        The callback function will be called like:

         getticket($ssl,$ticket,$data) -> $return_value
         # $ssl is a value corresponding to openssl's SSL structure
         # $ticket is a value of received TLS session ticket (can also be empty)
         # $data is the data passed to set_session_ticket_ext_cb
         # $return_value is either 0 (failure) or 1 (success)

        This function is based on the OpenSSL function SSL_set_session_ticket_ext_cb.

    *   set_session_ticket_ext

        Set TLS session ticket (stateless session reuse).

         Net::SSLeay::set_session_ticket_ext($ssl, $ticket);
         # $ssl    - value corresponding to openssl's SSL structure
         # $ticket - is a value of TLS session ticket which client will send (can also be empty string)
         #
         # returns: no return value

        The callback function will be called like:

         getticket($ssl,$ticket,$data) -> $return_value
         # $ssl is a value corresponding to openssl's SSL structure
         # $ticket is a value of received TLS session ticket (can also be empty)
         # $data is the data passed to set_session_ticket_ext_cb
         # $return_value is either 0 (failure) or 1 (success)

        This function is based on the OpenSSL function SSL_set_session_ticket_ext_cb.

    *   set_shutdown

        Sets the shutdown state of $ssl to $mode.

         Net::SSLeay::set_shutdown($ssl, $mode);
         # $ssl - value corresponding to openssl's SSL structure
         # $mode - (integer) shutdown mode:
         #         0 - No shutdown
         #         1 - SSL_SENT_SHUTDOWN
         #         2 - SSL_RECEIVED_SHUTDOWN
         #         3 - SSL_RECEIVED_SHUTDOWN+SSL_SENT_SHUTDOWN
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_set_shutdown.html>

    *   set_ssl_method

        Sets a new TLS/SSL method for a particular $ssl object.

         my $rv = Net::SSLeay::set_ssl_method($ssl, $method);
         # $ssl - value corresponding to openssl's SSL structure
         # $method - value corresponding to openssl's SSL_METHOD structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_ssl_version.html>

    *   set_tmp_dh

        Sets DH parameters to be used to be $dh.

         my $rv = Net::SSLeay::set_tmp_dh($ssl, $dh);
         # $ssl - value corresponding to openssl's SSL structure
         # $dh - value corresponding to openssl's DH structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_tmp_dh_callback.html>

    *   set_tmp_dh_callback

        Sets the callback function for $ssl to be used when a DH parameters are required to $dh_cb.

        ??? (does this function really work?)

         Net::SSLeay::set_tmp_dh_callback($ssl, $dh);
         # $ssl - value corresponding to openssl's SSL structure
         # $dh_cb - pointer to function ???
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_tmp_dh_callback.html>

    *   set_tmp_rsa

        Sets the temporary/ephemeral RSA key to be used in $ssl to be $rsa.

         my $rv = Net::SSLeay::set_tmp_rsa($ssl, $rsa);
         # $ssl - value corresponding to openssl's SSL structure
         # $rsa - value corresponding to openssl's RSA structure
         #
         # returns: 1 on success, 0 on failure

        Example:

         $rsakey = Net::SSLeay::RSA_generate_key();
         Net::SSLeay::set_tmp_rsa($ssl, $rsakey);
         Net::SSLeay::RSA_free($rsakey);

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_tmp_rsa_callback.html>

    *   set_tmp_rsa_callback

        Sets the callback function for $ssl to be used when a temporary/ephemeral RSA key is
        required to $tmp_rsa_callback.

        ??? (does this function really work?)

         Net::SSLeay::set_tmp_rsa_callback($ssl, $tmp_rsa_callback);
         # $ssl - value corresponding to openssl's SSL structure
         # $tmp_rsa_callback - (function pointer) ???
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_tmp_rsa_callback.html>

    *   set_trust

         my $rv = Net::SSLeay::set_trust($ssl, $trust);
         # $ssl - value corresponding to openssl's SSL structure
         # $trust - (integer) trust identifier
         #
         # returns: the original value

        For more details about $trust values see "CTX_set_trust".

    *   shutdown

        Shuts down an active TLS/SSL connection. It sends the 'close notify' shutdown alert to the
        peer.

         my $rv = Net::SSLeay::shutdown($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: 1 - shutdown was successfully completed
         #          0 - shutdown is not yet finished,
         #         -1 - shutdown was not successful

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_shutdown.html>

    *   state_string

        Returns a 6 letter string indicating the current state of the SSL object $ssl.

         my $rv = Net::SSLeay::state_string($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: 6-letter string

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_state_string.html>

    *   state_string_long

        Returns a string indicating the current state of the SSL object $ssl.

         my $rv = Net::SSLeay::state_string_long($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: state strings

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_state_string.html>

    *   set_default_passwd_cb

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.1.0f. Not needed with LibreSSL.

        Sets the default password callback called when loading/storing a PEM certificate with
        encryption for $ssl.

         Net::SSLeay::set_default_passwd_cb($ssl, $func);
         # $ssl - value corresponding to openssl's SSL structure
         # $func - perl reference to callback function
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_default_passwd_cb.html>

    *   set_default_passwd_cb_userdata

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.1.0f. Not needed with LibreSSL.

        Sets a pointer to userdata which will be provided to the password callback of $ssl on
        invocation.

         Net::SSLeay::set_default_passwd_cb_userdata($ssl, $userdata);
         # $ssl - value corresponding to openssl's SSL structure
         # $userdata - data that will be passed to callback function when invoked
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_set_default_passwd_cb.html>

    *   use_PrivateKey

        Adds $pkey as private key to $ssl.

         my $rv = Net::SSLeay::use_PrivateKey($ssl, $pkey);
         # $ssl - value corresponding to openssl's SSL structure
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   use_PrivateKey_ASN1

        Adds the private key of type $pk stored in $data to $ssl.

         my $rv = Net::SSLeay::use_PrivateKey_ASN1($pk, $ssl, $d, $len);
         # $pk - (integer) key type, NID of corresponding algorithm
         # $ssl - value corresponding to openssl's SSL structure
         # $data - key data (binary)
         # $len - length of $data
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   use_PrivateKey_file

        Adds the first private key found in $file to $ssl.

         my $rv = Net::SSLeay::use_PrivateKey_file($ssl, $file, $type);
         # $ssl - value corresponding to openssl's SSL structure
         # $file - (string) file name
         # $type - (integer) type - use constants &Net::SSLeay::FILETYPE_PEM or &Net::SSLeay::FILETYPE_ASN1
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   use_RSAPrivateKey

        Adds $rsa as RSA private key to $ssl.

         my $rv = Net::SSLeay::use_RSAPrivateKey($ssl, $rsa);
         # $ssl - value corresponding to openssl's SSL structure
         # $rsa - value corresponding to openssl's RSA structure
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   use_RSAPrivateKey_ASN1

        Adds RSA private key stored in $data to $ssl.

         my $rv = Net::SSLeay::use_RSAPrivateKey_ASN1($ssl, $data, $len);
         # $ssl - value corresponding to openssl's SSL structure
         # $data - key data (binary)
         # $len - length of $data
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   use_RSAPrivateKey_file

        Adds the first RSA private key found in $file to $ssl.

         my $rv = Net::SSLeay::use_RSAPrivateKey_file($ssl, $file, $type);
         # $ssl - value corresponding to openssl's SSL structure
         # $file - (string) file name
         # $type - (integer) type - use constants &Net::SSLeay::FILETYPE_PEM or &Net::SSLeay::FILETYPE_ASN1
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   use_certificate

        Loads the certificate $x into $ssl.

         my $rv = Net::SSLeay::use_certificate($ssl, $x);
         # $ssl - value corresponding to openssl's SSL structure
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   use_certificate_ASN1

        Loads the ASN1 encoded certificate from $data to $ssl.

         my $rv = Net::SSLeay::use_certificate_ASN1($ssl, $data, $len);
         # $ssl - value corresponding to openssl's SSL structure
         # $data - certificate data (binary)
         # $len - length of $data
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   use_certificate_chain_file

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL 1.1.0

        Loads a certificate chain from $file into $ssl. The certificates must be in PEM format and
        must be sorted starting with the subject's certificate (actual client or server
        certificate), followed by intermediate CA certificates if applicable, and ending at the
        highest level (root) CA.

         my $rv = Net::SSLeay::use_certificate_chain_file($ssl, $file);
         # $ssl - value corresponding to openssl's SSL structure
         # $file - (string) file name
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   use_certificate_file

        Loads the first certificate stored in $file into $ssl.

         my $rv = Net::SSLeay::use_certificate_file($ssl, $file, $type);
         # $ssl - value corresponding to openssl's SSL structure
         # $file - (string) file name
         # $type - (integer) type - use constants &Net::SSLeay::FILETYPE_PEM or &Net::SSLeay::FILETYPE_ASN1
         #
         # returns: 1 on success, otherwise check out the error stack to find out the reason

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_CTX_use_certificate.html>

    *   get_version

        Returns SSL/TLS protocol name

         my $rv = Net::SSLeay::get_version($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (string) protocol name, see OpenSSL manual for the full list
         #          TLSv1
         #          TLSv1.3

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_get_version.html>

    *   version

        Returns SSL/TLS protocol version

         my $rv = Net::SSLeay::version($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (integer) protocol version, see OpenSSL manual for the full list
         #          0x0301 - TLS1_VERSION  (TLSv1)
         #          0xFEFF - DTLS1_VERSION (DTLSv1)

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_version.html>

    *   client_version

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.0,
        not in LibreSSL

        Returns TLS protocol version used by the client when initiating the connection

         my $rv = Net::SSLeay::client_version($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (integer) protocol version, see OpenSSL manual for the full list
         #          0x0301 - TLS1_VERSION  (TLSv1)
         #          0xFEFF - DTLS1_VERSION (DTLSv1)

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_client_version.html>

    *   is_dtls

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.0,
        not in LibreSSL

         my $rv = Net::SSLeay::is_dtls($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (integer) zero or one
         #          0 - connection is not using DTLS
         #          1 - connection is using DTLS

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_is_dtls.html>

    *   want

        Returns state information for the SSL object $ssl.

         my $rv = Net::SSLeay::want($ssl);
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: state
         #          1 - SSL_NOTHING
         #          2 - SSL_WRITING
         #          3 - SSL_READING
         #          4 - SSL_X509_LOOKUP

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_want.html>

    *   write

        Writes data from the buffer $data into the specified $ssl connection.

         my $rv = Net::SSLeay::write($ssl, $data);
         # $ssl - value corresponding to openssl's SSL structure
         # $data - data to be written
         #
         # returns: >0 - (success) number of bytes actually written to the TLS/SSL connection
         #           0 - write not successful, probably the underlying connection was closed
         #          <0 - error

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_write.html>

    *   write_ex

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Writes data from the buffer $data into the specified $ssl connection.

         my ($len, $rv) = Net::SSLeay::write_ex($ssl, $data);
         # $ssl - value corresponding to openssl's SSL structure
         # $data - data to be written
         #
         # returns a list: two-item list consisting of number of bytes written,
         #                 and return code from SSL_write_ex()

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/SSL_write_ex.html>

    *   write_partial

        NOTE: Does not exactly correspond to any low level API function

        Writes a fragment of data in $data from the buffer $data into the specified $ssl connection.
        This is a non-blocking function like Net::SSLeay::write().

         my $rv = Net::SSLeay::write_partial($ssl, $from, $count, $data);
         # $ssl - value corresponding to openssl's SSL structure
         # $from - (integer) offset from the beginning of $data
         # $count - (integer) length of data to be written
         # $data - data buffer
         #
         # returns: >0 - (success) number of bytes actually written to the TLS/SSL connection
         #           0 - write not successful, probably the underlying connection was closed
         #          <0 - error

    *   set_tlsext_host_name

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.8f

        Sets TLS servername extension on SLL object $ssl to value $name.

         my $rv = set_tlsext_host_name($ssl, $name);
         # $ssl - value corresponding to openssl's SSL structure
         # $name - (string) name to be set
         #
         # returns: 1 on success, 0 on failure

   Low level API: RAND_* related functions
    Check openssl doc related to RAND stuff <http://www.openssl.org/docs/crypto/rand.html>

    *   RAND_add

        Mixes the $num bytes at $buf into the PRNG state.

         Net::SSLeay::RAND_add($buf, $num, $entropy);
         # $buf - buffer with data to be mixed into the PRNG state
         # $num - number of bytes in $buf
         # $entropy - estimate of how much randomness is contained in $buf (in bytes)
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/RAND_add.html>

    *   RAND_seed

        Equivalent to "RAND_add" when $num == $entropy.

         Net::SSLeay::RAND_seed($buf);   # Perlishly figures out buf size
         # $buf - buffer with data to be mixed into the PRNG state
         # $num - number of bytes in $buf
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/RAND_add.html>

    *   RAND_status

        Gives PRNG status (seeded enough or not).

         my $rv = Net::SSLeay::RAND_status();
         #returns: 1 if the PRNG has been seeded with enough data, 0 otherwise

        Check openssl doc <http://www.openssl.org/docs/crypto/RAND_add.html>

    *   RAND_bytes

        Puts $num cryptographically strong pseudo-random bytes into $buf.

         my $rv = Net::SSLeay::RAND_bytes($buf, $num);
         # $buf - buffer where the random data will be stored
         # $num - the size (in bytes) of requested random data
         #
         # returns: 1 on success, -1 if not supported by the current RAND method, or 0 on other failure

        Check openssl doc <http://www.openssl.org/docs/manmaster/man3/RAND_bytes.html>

    *   RAND_priv_bytes

        COMPATIBILITY: not available in Net-SSLeay-1.85 and before; requires at least OpenSSL 1.1.1,
        not in LibreSSL

        Puts $num cryptographically strong pseudo-random bytes into $buf.

         my $rv = Net::SSLeay::RAND_priv_bytes($buf, $num);
         # $buf - buffer where the random data will be stored
         # $num - the size (in bytes) of requested random data
         #
         # returns: 1 on success, -1 if not supported by the current RAND method, or 0 on other failure

        RAND_priv_bytes has the same semantics as RAND_bytes, but see see the documentation for more
        information.

        Check openssl doc <http://www.openssl.org/docs/manmaster/man3/RAND_priv_bytes.html>

    *   RAND_pseudo_bytes

        Puts $num pseudo-random (not necessarily unpredictable) bytes into $buf.

         my $rv = Net::SSLeay::RAND_pseudo_bytes($buf, $num);
         # $buf - buffer where the random data will be stored
         # $num - the size (in bytes) of requested random data
         #
         # returns: 1 if the bytes generated are cryptographically strong, 0 otherwise

        Check openssl doc <http://www.openssl.org/docs/crypto/RAND_bytes.html>

    *   RAND_cleanup

        Erase the PRNG state.

         Net::SSLeay::RAND_cleanup();
         # no args, no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/RAND_cleanup.html>

    *   RAND_egd_bytes

        Queries the entropy gathering daemon EGD on socket $path for $bytes bytes.

         my $rv = Net::SSLeay::RAND_egd_bytes($path, $bytes);
         # $path - path to a socket of entropy gathering daemon EGD
         # $bytes - number of bytes we want from EGD
         #
         # returns: the number of bytes read from the daemon on success, and -1 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/RAND_egd.html>

    *   RAND_file_name

        Generates a default path for the random seed file.

         my $file = Net::SSLeay::RAND_file_name($num);
         # $num - maximum size of returned file name
         #
         # returns: string with file name on success, '' (empty string) or undef on failure

        LibreSSL and OpenSSL 1.1.0a and later return undef when, for example, $num is not large
        enough to hold the filename.

        Check openssl doc <http://www.openssl.org/docs/crypto/RAND_load_file.html>

    *   RAND_load_file

        COMPATIBILITY: Is no longer functional on LibreSSL

        Reads $max_bytes of bytes from $file_name and adds them to the PRNG.

         my $rv = Net::SSLeay::RAND_load_file($file_name, $max_bytes);
         # $file_name - the name of file
         # $max_bytes - bytes to read from $file_name; -1 => the complete file is read
         #
         # returns: the number of bytes read

        Check openssl doc <http://www.openssl.org/docs/crypto/RAND_load_file.html>

    *   RAND_write_file

        Writes 1024 random bytes to $file_name which can be used to initialize the PRNG by calling
        "RAND_load_file" in a later session.

         my $rv = Net::SSLeay::RAND_write_file($file_name);
         # $file_name - the name of file
         #
         # returns: the number of bytes written, and -1 if the bytes written were generated without appropriate seed

        Check openssl doc <http://www.openssl.org/docs/crypto/RAND_load_file.html>

    *   RAND_poll

        Collects some entropy from operating system and adds it to the PRNG.

         my $rv = Net::SSLeay::RAND_poll();
         # returns: 1 on success, 0 on failure (unable to gather reasonable entropy)

   Low level API: OBJ_* related functions
    *   OBJ_cmp

        Compares ASN1_OBJECT $a to ASN1_OBJECT $b.

         my $rv = Net::SSLeay::OBJ_cmp($a, $b);
         # $a - value corresponding to openssl's ASN1_OBJECT structure
         # $b - value corresponding to openssl's ASN1_OBJECT structure
         #
         # returns: if the two are identical 0 is returned

        Check openssl doc <http://www.openssl.org/docs/crypto/OBJ_nid2obj.html>

    *   OBJ_dup

        Returns a copy/duplicate of $o.

         my $rv = Net::SSLeay::OBJ_dup($o);
         # $o - value corresponding to openssl's ASN1_OBJECT structure
         #
         # returns: value corresponding to openssl's ASN1_OBJECT structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/OBJ_nid2obj.html>

    *   OBJ_nid2ln

        Returns long name for given NID $n.

         my $rv = Net::SSLeay::OBJ_nid2ln($n);
         # $n - (integer) NID
         #
         # returns: (string) long name e.g. 'commonName'

        Check openssl doc <http://www.openssl.org/docs/crypto/OBJ_nid2obj.html>

    *   OBJ_ln2nid

        Returns NID corresponding to given long name $n.

         my $rv = Net::SSLeay::OBJ_ln2nid($s);
         # $s - (string) long name e.g. 'commonName'
         #
         # returns: (integer) NID

    *   OBJ_nid2sn

        Returns short name for given NID $n.

         my $rv = Net::SSLeay::OBJ_nid2sn($n);
         # $n - (integer) NID
         #
         # returns: (string) short name e.g. 'CN'

        Example:

         print Net::SSLeay::OBJ_nid2sn(&Net::SSLeay::NID_commonName);

    *   OBJ_sn2nid

        Returns NID corresponding to given short name $s.

         my $rv = Net::SSLeay::OBJ_sn2nid($s);
         # $s - (string) short name e.g. 'CN'
         #
         # returns: (integer) NID

        Example:

         print "NID_commonName constant=", &Net::SSLeay::NID_commonName;
         print "OBJ_sn2nid('CN')=", Net::SSLeay::OBJ_sn2nid('CN');

    *   OBJ_nid2obj

        Returns ASN1_OBJECT for given NID $n.

         my $rv = Net::SSLeay::OBJ_nid2obj($n);
         # $n - (integer) NID
         #
         # returns: value corresponding to openssl's ASN1_OBJECT structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/OBJ_nid2obj.html>

    *   OBJ_obj2nid

        Returns NID corresponding to given ASN1_OBJECT $o.

         my $rv = Net::SSLeay::OBJ_obj2nid($o);
         # $o - value corresponding to openssl's ASN1_OBJECT structure
         #
         # returns: (integer) NID

        Check openssl doc <http://www.openssl.org/docs/crypto/OBJ_nid2obj.html>

    *   OBJ_txt2obj

        Converts the text string s into an ASN1_OBJECT structure. If $no_name is 0 then long names
        (e.g. 'commonName') and short names (e.g. 'CN') will be interpreted as well as numerical
        forms (e.g. '2.5.4.3'). If $no_name is 1 only the numerical form is acceptable.

         my $rv = Net::SSLeay::OBJ_txt2obj($s, $no_name);
         # $s - text string to be converted
         # $no_name - (integer) 0 or 1
         #
         # returns: value corresponding to openssl's ASN1_OBJECT structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/OBJ_nid2obj.html>

    *   OBJ_obj2txt

        Converts the ASN1_OBJECT a into a textual representation.

         Net::SSLeay::OBJ_obj2txt($a, $no_name);
         # $a - value corresponding to openssl's ASN1_OBJECT structure
         # $no_name - (integer) 0 or 1
         #
         # returns: textual representation e.g. 'commonName' ($no_name=0), '2.5.4.3' ($no_name=1)

        Check openssl doc <http://www.openssl.org/docs/crypto/OBJ_nid2obj.html>

    *   OBJ_txt2nid

        Returns NID corresponding to text string $s which can be a long name, a short name or the
        numerical representation of an object.

         my $rv = Net::SSLeay::OBJ_txt2nid($s);
         # $s - (string) e.g. 'commonName' or 'CN' or '2.5.4.3'
         #
         # returns: (integer) NID

        Example:

         my $nid = Net::SSLeay::OBJ_txt2nid('2.5.4.3');
         Net::SSLeay::OBJ_nid2sn($n);

        Check openssl doc <http://www.openssl.org/docs/crypto/OBJ_nid2obj.html>

   Low level API: ASN1_INTEGER_* related functions
    *   ASN1_INTEGER_new

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Creates a new ASN1_INTEGER structure.

         my $rv = Net::SSLeay::ASN1_INTEGER_new();
         #
         # returns: value corresponding to openssl's ASN1_INTEGER structure (0 on failure)

    *   ASN1_INTEGER_free

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Free an allocated ASN1_INTEGER structure.

         Net::SSLeay::ASN1_INTEGER_free($i);
         # $i - value corresponding to openssl's ASN1_INTEGER structure
         #
         # returns: no return value

    *   ASN1_INTEGER_get

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns integer value of given ASN1_INTEGER object.

        BEWARE: If the value stored in ASN1_INTEGER is greater than max. integer that can be stored
        in 'long' type (usually 32bit but may vary according to platform) then this function will
        return -1. For getting large ASN1_INTEGER values consider using "P_ASN1_INTEGER_get_dec" or
        "P_ASN1_INTEGER_get_hex".

         my $rv = Net::SSLeay::ASN1_INTEGER_get($a);
         # $a - value corresponding to openssl's ASN1_INTEGER structure
         #
         # returns: integer value of ASN1_INTEGER object in $a

    *   ASN1_INTEGER_set

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sets value of given ASN1_INTEGER object to value $val

        BEWARE: $val has max. limit (= max. integer that can be stored in 'long' type). For setting
        large ASN1_INTEGER values consider using "P_ASN1_INTEGER_set_dec" or
        "P_ASN1_INTEGER_set_hex".

         my $rv = Net::SSLeay::ASN1_INTEGER_set($i, $val);
         # $i - value corresponding to openssl's ASN1_INTEGER structure
         # $val - integer value
         #
         # returns: 1 on success, 0 on failure

    *   P_ASN1_INTEGER_get_dec

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns string with decimal representation of integer value of given ASN1_INTEGER object.

         Net::SSLeay::P_ASN1_INTEGER_get_dec($i);
         # $i - value corresponding to openssl's ASN1_INTEGER structure
         #
         # returns: string with decimal representation

    *   P_ASN1_INTEGER_get_hex

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns string with hexadecimal representation of integer value of given ASN1_INTEGER
        object.

         Net::SSLeay::P_ASN1_INTEGER_get_hex($i);
         # $i - value corresponding to openssl's ASN1_INTEGER structure
         #
         # returns: string with hexadecimal representation

    *   P_ASN1_INTEGER_set_dec

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sets value of given ASN1_INTEGER object to value $val (decimal string, suitable for large
        integers)

         Net::SSLeay::P_ASN1_INTEGER_set_dec($i, $str);
         # $i - value corresponding to openssl's ASN1_INTEGER structure
         # $str - string with decimal representation
         #
         # returns: 1 on success, 0 on failure

    *   P_ASN1_INTEGER_set_hex

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sets value of given ASN1_INTEGER object to value $val (hexadecimal string, suitable for
        large integers)

         Net::SSLeay::P_ASN1_INTEGER_set_hex($i, $str);
         # $i - value corresponding to openssl's ASN1_INTEGER structure
         # $str - string with hexadecimal representation
         #
         # returns: 1 on success, 0 on failure

   Low level API: ASN1_STRING_* related functions
    *   P_ASN1_STRING_get

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns string value of given ASN1_STRING object.

         Net::SSLeay::P_ASN1_STRING_get($s, $utf8_decode);
         # $s - value corresponding to openssl's ASN1_STRING structure
         # $utf8_decode - [optional] 0 or 1 whether the returned value should be utf8 decoded (default=0)
         #
         # returns: string

         $string = Net::SSLeay::P_ASN1_STRING_get($s);
         #is the same as:
         $string = Net::SSLeay::P_ASN1_STRING_get($s, 0);

   Low level API: ASN1_TIME_* related functions
    *   ASN1_TIME_new

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

         my $time = ASN1_TIME_new();
         # returns: value corresponding to openssl's ASN1_TIME structure

    *   ASN1_TIME_free

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

         ASN1_TIME_free($time);
         # $time - value corresponding to openssl's ASN1_TIME structure

    *   ASN1_TIME_set

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

         ASN1_TIME_set($time, $t);
         # $time - value corresponding to openssl's ASN1_TIME structure
         # $t - time value in seconds since 1.1.1970

        BEWARE: It is platform dependent how this function will handle dates after 2038. Although
        perl's integer is large enough the internal implementation of this function is dependent on
        the size of time_t structure (32bit time_t has problem with 2038).

        If you want to safely set date and time after 2038 use function "P_ASN1_TIME_set_isotime".

    *   P_ASN1_TIME_get_isotime

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7e

        NOTE: Does not exactly correspond to any low level API function

        Gives ISO-8601 string representation of ASN1_TIME structure.

         my $datetime_string = P_ASN1_TIME_get_isotime($time);
         # $time - value corresponding to openssl's ASN1_TIME structure
         #
         # returns: datetime string like '2033-05-16T20:39:37Z' or '' on failure

        The output format is compatible with module DateTime::Format::RFC3339

    *   P_ASN1_TIME_set_isotime

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7e

        NOTE: Does not exactly correspond to any low level API function

        Sets time and date value of ANS1_time structure.

         my $rv = P_ASN1_TIME_set_isotime($time, $string);
         # $time - value corresponding to openssl's ASN1_TIME structure
         # $string - ISO-8601 timedate string like '2033-05-16T20:39:37Z'
         #
         # returns: 1 on success, 0 on failure

        The $string parameter has to be in full form like "2012-03-22T23:55:33" or
        "2012-03-22T23:55:33Z" or "2012-03-22T23:55:33CET". Short forms like "2012-03-22T23:55" or
        "2012-03-22" are not supported.

    *   P_ASN1_TIME_put2string

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before, has bugs with openssl-0.9.8i

        NOTE: Does not exactly correspond to any low level API function

        Gives string representation of ASN1_TIME structure.

         my $str = P_ASN1_TIME_put2string($time);
         # $time - value corresponding to openssl's ASN1_TIME structure
         #
         # returns: datetime string like 'May 16 20:39:37 2033 GMT'

    *   P_ASN1_UTCTIME_put2string

        NOTE: deprecated function, only for backward compatibility, just an alias for
        "P_ASN1_TIME_put2string"

   Low level API: X509_* related functions
    *   X509_new

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Allocates and initializes a X509 structure.

         my $rv = Net::SSLeay::X509_new();
         #
         # returns: value corresponding to openssl's X509 structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_new.html>

    *   X509_free

        Frees up the X509 structure.

         Net::SSLeay::X509_free($a);
         # $a - value corresponding to openssl's X509 structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_new.html>

    *   X509_check_host

        COMPATIBILITY: not available in Net-SSLeay-1.68 and before; requires at least OpenSSL 1.0.2.
        X509_CHECK_FLAG_NEVER_CHECK_SUBJECT requires OpenSSL 1.1.0.

        Checks if the certificate Subject Alternative Name (SAN) or Subject CommonName (CN) matches
        the specified host name.

         my $rv = Net::SSLeay::X509_check_host($cert, $name, $flags, $peername);
         # $cert - value corresponding to openssl's X509 structure
         # $name - host name to check
         # $flags (optional, default: 0) - can be the bitwise OR of:
         #   &Net::SSLeay::X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT
         #   &Net::SSLeay::X509_CHECK_FLAG_NO_WILDCARDS
         #   &Net::SSLeay::X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS
         #   &Net::SSLeay::X509_CHECK_FLAG_MULTI_LABEL_WILDCARDS
         #   &Net::SSLeay::X509_CHECK_FLAG_SINGLE_LABEL_SUBDOMAINS
         #   &Net::SSLeay::X509_CHECK_FLAG_NEVER_CHECK_SUBJECT
         # $peername (optional) - If not omitted and $host matches $cert,
         #                        a copy of the matching SAN or CN from
         #                        the peer certificate is stored in $peername.
         #
         # returns:
         #   1 for a successful match
         #   0 for a failed match
         #  -1 for an internal error
         #  -2 if the input is malformed

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_check_host.html>.

    *   X509_check_email

        COMPATIBILITY: not available in Net-SSLeay-1.68 and before; requires at least OpenSSL 1.0.2.

        Checks if the certificate matches the specified email address.

         my $rv = Net::SSLeay::X509_check_email($cert, $address, $flags);
         # $cert - value corresponding to openssl's X509 structure
         # $address - email address to check
         # $flags (optional, default: 0) - see X509_check_host()
         #
         # returns: see X509_check_host()

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_check_email.html>.

    *   X509_check_ip

        COMPATIBILITY: not available in Net-SSLeay-1.68 and before; requires at least OpenSSL 1.0.2.

        Checks if the certificate matches the specified IPv4 or IPv6 address.

         my $rv = Net::SSLeay::X509_check_ip($cert, $address, $flags);
         # $cert - value corresponding to openssl's X509 structure
         # $address - IP address to check in binary format, in network byte order
         # $flags (optional, default: 0) - see X509_check_host()
         #
         # returns: see X509_check_host()

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_check_ip.html>.

    *   X509_check_ip_asc

        COMPATIBILITY: not available in Net-SSLeay-1.68 and before; requires at least OpenSSL 1.0.2.

        Checks if the certificate matches the specified IPv4 or IPv6 address.

         my $rv = Net::SSLeay::X509_check_ip_asc($cert, $address, $flags);
         # $cert - value corresponding to openssl's X509 structure
         # $address - IP address to check in text representation
         # $flags (optional, default: 0) - see X509_check_host()
         #
         # returns: see X509_check_host()

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_check_ip_asc.html>.

    *   X509_certificate_type

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns bitmask with type of certificate $x.

         my $rv = Net::SSLeay::X509_certificate_type($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: (integer) bitmask with certificate type

         #to decode bitmask returned by this function use these constants:
         &Net::SSLeay::EVP_PKS_DSA
         &Net::SSLeay::EVP_PKS_EC
         &Net::SSLeay::EVP_PKS_RSA
         &Net::SSLeay::EVP_PKT_ENC
         &Net::SSLeay::EVP_PKT_EXCH
         &Net::SSLeay::EVP_PKT_EXP
         &Net::SSLeay::EVP_PKT_SIGN
         &Net::SSLeay::EVP_PK_DH
         &Net::SSLeay::EVP_PK_DSA
         &Net::SSLeay::EVP_PK_EC
         &Net::SSLeay::EVP_PK_RSA

    *   X509_digest

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Computes digest/fingerprint of X509 $data using $type hash function.

         my $digest_value = Net::SSLeay::X509_digest($data, $type);
         # $data - value corresponding to openssl's X509 structure
         # $type - value corresponding to openssl's EVP_MD structure - e.g. got via EVP_get_digestbyname()
         #
         # returns: hash value (binary)

         #to get printable (hex) value of digest use:
         print unpack('H*', $digest_value);

    *   X509_issuer_and_serial_hash

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sort of a checksum of issuer name and serial number of X509 certificate $x. The result is
        not a full hash (e.g. sha-1), it is kind-of-a-hash truncated to the size of 'unsigned long'
        (32 bits). The resulting value might differ across different openssl versions for the same
        X509 certificate.

         my $rv = Net::SSLeay::X509_issuer_and_serial_hash($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: number representing checksum

    *   X509_issuer_name_hash

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sort of a checksum of issuer name of X509 certificate $x. The result is not a full hash
        (e.g. sha-1), it is kind-of-a-hash truncated to the size of 'unsigned long' (32 bits). The
        resulting value might differ across different openssl versions for the same X509
        certificate.

         my $rv = Net::SSLeay::X509_issuer_name_hash($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: number representing checksum

    *   X509_subject_name_hash

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sort of a checksum of subject name of X509 certificate $x. The result is not a full hash
        (e.g. sha-1), it is kind-of-a-hash truncated to the size of 'unsigned long' (32 bits). The
        resulting value might differ across different openssl versions for the same X509
        certificate.

         my $rv = Net::SSLeay::X509_subject_name_hash($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: number representing checksum

    *   X509_pubkey_digest

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Computes digest/fingerprint of public key from X509 certificate $data using $type hash
        function.

         my $digest_value = Net::SSLeay::X509_pubkey_digest($data, $type);
         # $data - value corresponding to openssl's X509 structure
         # $type - value corresponding to openssl's EVP_MD structure - e.g. got via EVP_get_digestbyname()
         #
         # returns: hash value (binary)

         #to get printable (hex) value of digest use:
         print unpack('H*', $digest_value);

    *   X509_set_issuer_name

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sets issuer of X509 certificate $x to $name.

         my $rv = Net::SSLeay::X509_set_issuer_name($x, $name);
         # $x - value corresponding to openssl's X509 structure
         # $name - value corresponding to openssl's X509_NAME structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_set_pubkey

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sets public key of X509 certificate $x to $pkey.

         my $rv = Net::SSLeay::X509_set_pubkey($x, $pkey);
         # $x - value corresponding to openssl's X509 structure
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_set_serialNumber

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sets serial number of X509 certificate $x to $serial.

         my $rv = Net::SSLeay::X509_set_serialNumber($x, $serial);
         # $x - value corresponding to openssl's X509 structure
         # $serial - value corresponding to openssl's ASN1_INTEGER structure
         #
         # returns: 1 on success, 0 on failure

         #to create $serial value use one of these:
         $serial = Net::SSLeay::P_ASN1_INTEGER_set_hex('45ad6f');
         $serial = Net::SSLeay::P_ASN1_INTEGER_set_dec('7896541238529631478');
         $serial = Net::SSLeay::ASN1_INTEGER_set(45896);

    *   X509_set_subject_name

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sets subject of X509 certificate $x to $name.

         my $rv = Net::SSLeay::X509_set_subject_name($x, $name);
         # $x - value corresponding to openssl's X509 structure
         # $name - value corresponding to openssl's X509_NAME structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_set_version

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Set 'version' value for X509 certificate $ to $version.

         my $rv = Net::SSLeay::X509_set_version($x, $version);
         # $x - value corresponding to openssl's X509 structure
         # $version - (integer) version number
         #
         # returns: 1 on success, 0 on failure

    *   X509_sign

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sign X509 certificate $x with private key $pkey (using digest algorithm $md).

         my $rv = Net::SSLeay::X509_sign($x, $pkey, $md);
         # $x - value corresponding to openssl's X509 structure
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         # $md - value corresponding to openssl's EVP_MD structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_verify

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Verifies X509 object $a using public key $r (pubkey of issuing CA).

         my $rv = Net::SSLeay::X509_verify($x, $r);
         # $x - value corresponding to openssl's X509 structure
         # $r - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: 0 - verify failure, 1 - verify OK, <0 - error

    *   X509_get_ext_count

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns the total number of extensions in X509 object $x.

         my $rv = Net::SSLeay::X509_get_ext_count($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: count of extensions

    *   X509_get_pubkey

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns public key corresponding to given X509 object $x.

         my $rv = Net::SSLeay::X509_get_pubkey($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: value corresponding to openssl's EVP_PKEY structure (0 on failure)

        NOTE: This method returns only the public key's key bits, without the algorithm or
        parameters. Use "X509_get_X509_PUBKEY()" to return the full public key (SPKI) instead.

    *   X509_get_X509_PUBKEY

        COMPATIBILITY: not available in Net-SSLeay-1.72 and before

        Returns the full public key (SPKI) of given X509 certificate $x.

         Net::SSLeay::X509_get_X509_PUBKEY($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: public key data in DER format (binary)

    *   X509_get_serialNumber

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns serial number of X509 certificate $x.

         my $rv = Net::SSLeay::X509_get_serialNumber($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: value corresponding to openssl's ASN1_INTEGER structure (0 on failure)

        See "P_ASN1_INTEGER_get_dec", "P_ASN1_INTEGER_get_hex" or "ASN1_INTEGER_get" to decode
        ASN1_INTEGER object.

    *   X509_get0_serialNumber

        COMPATIBILITY: available in Net-SSLeay-1.86 onwards

        X509_get0_serialNumber() is the same as X509_get_serialNumber() except it accepts a const
        parameter and returns a const result.

    *   X509_get_version

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns 'version' value of given X509 certificate $x.

         my $rv = Net::SSLeay::X509_get_version($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: (integer) version

    *   X509_get_ext

        Returns X509_EXTENSION from $x509 based on given position/index.

         my $rv = Net::SSLeay::X509_get_ext($x509, $index);
         # $x509 - value corresponding to openssl's X509 structure
         # $index - (integer) position/index of extension within $x509
         #
         # returns: value corresponding to openssl's X509_EXTENSION structure (0 on failure)

    *   X509_get_ext_by_NID

        Returns X509_EXTENSION from $x509 based on given NID.

         my $rv = Net::SSLeay::X509_get_ext_by_NID($x509, $nid, $loc);
         # $x509 - value corresponding to openssl's X509 structure
         # $nid - (integer) NID value
         # $loc - (integer) position to start lookup at
         #
         # returns: position/index of extension, negative value on error
         #          call Net::SSLeay::X509_get_ext($x509, $rv) to get the actual extension

    *   X509_get_fingerprint

        Returns fingerprint of certificate $cert.

        NOTE: Does not exactly correspond to any low level API function. The implementation is based
        on openssl's "X509_digest()".

         Net::SSLeay::X509_get_fingerprint($x509, $type);
         # $x509 - value corresponding to openssl's X509 structure
         # $type - (string) digest type, currently supported values:
         #         "md5"
         #         "sha1"
         #         "sha256"
         #         "ripemd160"
         #
         # returns: certificate digest - hexadecimal string (NOT binary data!)

    *   X509_get_issuer_name

        Return an X509_NAME object representing the issuer of the certificate $cert.

         my $rv = Net::SSLeay::X509_get_issuer_name($cert);
         # $cert - value corresponding to openssl's X509 structure
         #
         # returns: value corresponding to openssl's X509_NAME structure (0 on failure)

    *   X509_get_notAfter

        Return an object giving the time after which the certificate $cert is not valid.

         my $rv = Net::SSLeay::X509_get_notAfter($cert);
         # $cert - value corresponding to openssl's X509 structure
         #
         # returns: value corresponding to openssl's ASN1_TIME structure (0 on failure)

        To get human readable/printable form the return value you can use:

         my $time = Net::SSLeay::X509_get_notAfter($cert);
         print "notAfter=", Net::SSLeay::P_ASN1_TIME_get_isotime($time), "\n";

    *   X509_get_notBefore

        Return an object giving the time before which the certificate $cert is not valid

         my $rv = Net::SSLeay::X509_get_notBefore($cert);
         # $cert - value corresponding to openssl's X509 structure
         #
         # returns: value corresponding to openssl's ASN1_TIME structure (0 on failure)

        To get human readable/printable form the return value you can use:

         my $time = Net::SSLeay::X509_get_notBefore($cert);
         print "notBefore=", Net::SSLeay::P_ASN1_TIME_get_isotime($time), "\n";

    *   X509_get_subjectAltNames

        NOTE: Does not exactly correspond to any low level API function.

        Returns the list of alternative subject names from X509 certificate $cert.

         my @rv = Net::SSLeay::X509_get_subjectAltNames($cert);
         # $cert - value corresponding to openssl's X509 structure
         #
         # returns: list containing pairs - name_type (integer), name_value (string)
         #          where name_type can be:
         #          0 - GEN_OTHERNAME
         #          1 - GEN_EMAIL
         #          2 - GEN_DNS
         #          3 - GEN_X400
         #          4 - GEN_DIRNAME
         #          5 - GEN_EDIPARTY
         #          6 - GEN_URI
         #          7 - GEN_IPADD
         #          8 - GEN_RID

        Note: type 7 - GEN_IPADD contains the IP address as a packed binary address. GEN_RID is
        available in Net-SSLeay-1.90 and later. Maximum length for returned RID string is currently
        2500. Invalid and overly long RID values are skipped and not returned. GEN_X400 and
        GEN_EDIPARTY are not supported and will not be returned even when present in the
        certificate.

    *   X509_get_subject_name

        Returns the subject of the certificate $cert.

         my $rv = Net::SSLeay::X509_get_subject_name($cert);
         # $cert - value corresponding to openssl's X509 structure
         #
         # returns: value corresponding to openssl's X509_NAME structure (0 on failure)

    *   X509_gmtime_adj

        Adjust th ASN1_TIME object to the timestamp (in GMT).

         my $rv = Net::SSLeay::X509_gmtime_adj($s, $adj);
         # $s - value corresponding to openssl's ASN1_TIME structure
         # $adj - timestamp (seconds since 1.1.1970)
         #
         # returns: value corresponding to openssl's ASN1_TIME structure (0 on failure)

        BEWARE: this function may fail for dates after 2038 as it is dependent on time_t size on
        your system (32bit time_t does not work after 2038). Consider using
        "P_ASN1_TIME_set_isotime" instead).

    *   X509_load_cert_crl_file

        Takes PEM file and loads all X509 certificates and X509 CRLs from that file into X509_LOOKUP
        structure.

         my $rv = Net::SSLeay::X509_load_cert_crl_file($ctx, $file, $type);
         # $ctx - value corresponding to openssl's X509_LOOKUP structure
         # $file - (string) file name
         # $type - (integer) type - use constants &Net::SSLeay::FILETYPE_PEM or &Net::SSLeay::FILETYPE_ASN1
         #                          if not FILETYPE_PEM then behaves as Net::SSLeay::X509_load_cert_file()
         #
         # returns: 1 on success, 0 on failure

    *   X509_load_cert_file

        Loads/adds X509 certificate from $file to X509_LOOKUP structure

         my $rv = Net::SSLeay::X509_load_cert_file($ctx, $file, $type);
         # $ctx - value corresponding to openssl's X509_LOOKUP structure
         # $file - (string) file name
         # $type - (integer) type - use constants &Net::SSLeay::FILETYPE_PEM or &Net::SSLeay::FILETYPE_ASN1
         #
         # returns: 1 on success, 0 on failure

    *   X509_load_crl_file

        Loads/adds X509 CRL from $file to X509_LOOKUP structure

         my $rv = Net::SSLeay::X509_load_crl_file($ctx, $file, $type);
         # $ctx - value corresponding to openssl's X509_LOOKUP structure
         # $file - (string) file name
         # $type - (integer) type - use constants &Net::SSLeay::FILETYPE_PEM or &Net::SSLeay::FILETYPE_ASN1
         #
         # returns: 1 on success, 0 on failure

    *   X509_policy_level_get0_node

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_policy_level_get0_node($level, $i);
         # $level - value corresponding to openssl's X509_POLICY_LEVEL structure
         # $i - (integer) index/position
         #
         # returns: value corresponding to openssl's X509_POLICY_NODE structure (0 on failure)

    *   X509_policy_level_node_count

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_policy_level_node_count($level);
         # $level - value corresponding to openssl's X509_POLICY_LEVEL structure
         #
         # returns: (integer) node count

    *   X509_policy_node_get0_parent

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_policy_node_get0_parent($node);
         # $node - value corresponding to openssl's X509_POLICY_NODE structure
         #
         # returns: value corresponding to openssl's X509_POLICY_NODE structure (0 on failure)

    *   X509_policy_node_get0_policy

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_policy_node_get0_policy($node);
         # $node - value corresponding to openssl's X509_POLICY_NODE structure
         #
         # returns: value corresponding to openssl's ASN1_OBJECT structure (0 on failure)

    *   X509_policy_node_get0_qualifiers

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_policy_node_get0_qualifiers($node);
         # $node - value corresponding to openssl's X509_POLICY_NODE structure
         #
         # returns: value corresponding to openssl's STACK_OF(POLICYQUALINFO) structure (0 on failure)

    *   X509_policy_tree_free

        ??? (more info needed)

         Net::SSLeay::X509_policy_tree_free($tree);
         # $tree - value corresponding to openssl's X509_POLICY_TREE structure
         #
         # returns: no return value

    *   X509_policy_tree_get0_level

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_policy_tree_get0_level($tree, $i);
         # $tree - value corresponding to openssl's X509_POLICY_TREE structure
         # $i - (integer) level index
         #
         # returns: value corresponding to openssl's X509_POLICY_LEVEL structure (0 on failure)

    *   X509_policy_tree_get0_policies

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_policy_tree_get0_policies($tree);
         # $tree - value corresponding to openssl's X509_POLICY_TREE structure
         #
         # returns: value corresponding to openssl's X509_POLICY_NODE structure (0 on failure)

    *   X509_policy_tree_get0_user_policies

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_policy_tree_get0_user_policies($tree);
         # $tree - value corresponding to openssl's X509_POLICY_TREE structure
         #
         # returns: value corresponding to openssl's X509_POLICY_NODE structure (0 on failure)

    *   X509_policy_tree_level_count

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_policy_tree_level_count($tree);
         # $tree - value corresponding to openssl's X509_POLICY_TREE structure
         #
         # returns: (integer) count

    *   X509_verify_cert_error_string

        Returns a human readable error string for verification error $n.

         my $rv = Net::SSLeay::X509_verify_cert_error_string($n);
         # $n - (long) numeric error code
         #
         # returns: error string

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_STORE_CTX_get_error.html>

    *   P_X509_add_extensions

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Adds one or more X509 extensions to X509 object $x.

         my $rv = Net::SSLeay::P_X509_add_extensions($x, $ca_cert, $nid, $value);
         # $x - value corresponding to openssl's X509 structure
         # $ca_cert - value corresponding to openssl's X509 structure (issuer's cert - necessary for sertting NID_authority_key_identifier)
         # $nid - NID identifying extension to be set
         # $value - extension value
         #
         # returns: 1 on success, 0 on failure

        You can set more extensions at once:

         my $rv = Net::SSLeay::P_X509_add_extensions($x509, $ca_cert,
                        &Net::SSLeay::NID_key_usage => 'digitalSignature,keyEncipherment',
                        &Net::SSLeay::NID_subject_key_identifier => 'hash',
                        &Net::SSLeay::NID_authority_key_identifier => 'keyid',
                        &Net::SSLeay::NID_authority_key_identifier => 'issuer',
                        &Net::SSLeay::NID_basic_constraints => 'CA:FALSE',
                        &Net::SSLeay::NID_ext_key_usage => 'serverAuth,clientAuth',
                        &Net::SSLeay::NID_netscape_cert_type => 'server',
                        &Net::SSLeay::NID_subject_alt_name => 'DNS:s1.dom.com,DNS:s2.dom.com,DNS:s3.dom.com',
                  );

    *   P_X509_copy_extensions

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Copies X509 extensions from X509_REQ object to X509 object - handy when you need to turn
        X509_REQ into X509 certificate.

         Net::SSLeay::P_X509_copy_extensions($x509_req, $x509, $override);
         # $x509_req - value corresponding to openssl's X509_REQ structure
         # $x509 - value corresponding to openssl's X509 structure
         # $override - (integer) flag indication whether to override already existing items in $x509 (default 1)
         #
         # returns: 1 on success, 0 on failure

    *   P_X509_get_crl_distribution_points

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Get the list of CRL distribution points from X509 certificate.

         my @cdp = Net::SSLeay::P_X509_get_crl_distribution_points($x509);
         # $x509 - value corresponding to openssl's X509 structure
         #
         # returns: list of distribution points (usually URLs)

    *   P_X509_get_ext_key_usage

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Gets the list of extended key usage of given X509 certificate $cert.

         my @ext_usage = Net::SSLeay::P_X509_get_ext_key_usage($cert, $format);
         # $cert - value corresponding to openssl's X509 structure
         # $format - choose type of return values: 0=OIDs, 1=NIDs, 2=shortnames, 3=longnames
         #
         # returns: list of values

        Examples:

         my @extkeyusage_oid = Net::SSLeay::P_X509_get_ext_key_usage($x509,0);
         # returns for example: ("1.3.6.1.5.5.7.3.1", "1.3.6.1.5.5.7.3.2")

         my @extkeyusage_nid = Net::SSLeay::P_X509_get_ext_key_usage($x509,1);
         # returns for example: (129, 130)

         my @extkeyusage_sn  = Net::SSLeay::P_X509_get_ext_key_usage($x509,2);
         # returns for example: ("serverAuth", "clientAuth")

         my @extkeyusage_ln  = Net::SSLeay::P_X509_get_ext_key_usage($x509,3);
         # returns for example: ("TLS Web Server Authentication",  "TLS Web Client Authentication")

    *   P_X509_get_key_usage

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Gets the list of key usage of given X509 certificate $cert.

         my @keyusage = Net::SSLeay::P_X509_get_key_usage($cert);
         # $cert - value corresponding to openssl's X509 structure
         #
         # returns: list of key usage values which can be none, one or more from the following list:
         #          "digitalSignature"
         #          "nonRepudiation"
         #          "keyEncipherment"
         #          "dataEncipherment"
         #          "keyAgreement"
         #          "keyCertSign"
         #          "cRLSign"
         #          "encipherOnly"
         #          "decipherOnly"

    *   P_X509_get_netscape_cert_type

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Gets the list of Netscape cert types of given X509 certificate $cert.

         Net::SSLeay::P_X509_get_netscape_cert_type($cert);
         # $cert - value corresponding to openssl's X509 structure
         #
         # returns: list of Netscape type values which can be none, one or more from the following list:
         #          "client"
         #          "server"
         #          "email"
         #          "objsign"
         #          "reserved"
         #          "sslCA"
         #          "emailCA"
         #          "objCA"

    *   P_X509_get_pubkey_alg

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns ASN1_OBJECT corresponding to X509 certificate public key algorithm.

         my $rv = Net::SSLeay::P_X509_get_pubkey_alg($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: value corresponding to openssl's ASN1_OBJECT structure (0 on failure)

        To get textual representation use:

         my $alg = Net::SSLeay::OBJ_obj2txt(Net::SSLeay::P_X509_get_pubkey_alg($x509));
         # returns for example: "rsaEncryption"

    *   P_X509_get_signature_alg

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns ASN1_OBJECT corresponding to X509 signarite key algorithm.

         my $rv = Net::SSLeay::P_X509_get_signature_alg($x);
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: value corresponding to openssl's ASN1_OBJECT structure (0 on failure)

        To get textual representation use:

         my $alg = Net::SSLeay::OBJ_obj2txt(Net::SSLeay::P_X509_get_signature_alg($x509))
         # returns for example: "sha1WithRSAEncryption"

    *   sk_X509_new_null

        Returns a new, empty, STACK_OF(X509) structure.

         my $rv = Net::SSLeay::sk_X509_new_null();
         #
         # returns: value corresponding to openssl's STACK_OF(X509) structure

    *   sk_X509_push

        Pushes an X509 structure onto a STACK_OF(X509) structure.

         my $rv = Net::SSLeay::sk_X509_push($sk_x509, $x509);
         # $sk_x509 - value corresponding to openssl's STACK_OF(X509) structure
         # $x509 - value corresponding to openssl's X509 structure
         #
         # returns: total number of elements after the operation, 0 on failure

    *   sk_X509_pop

        Pops an single X509 structure from a STACK_OF(X509) structure.

         my $x509 = NetSSLeay::sk_X509_pop($sk_x509)
         # $sk_x509 - value corresponding to openssl's STACK_OF(X509) structure
         #
         # returns: a pointer to an X509 structure, undef on failure

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/sk_TYPE_pop.html>

    *   sk_X509_shift

        Shifts an single X509 structure onto a STACK_OF(X509) structure.

         my $x509 = NetSSLeay::sk_X509_shift($sk_x509, $x509)
         # $sk_x509 - value corresponding to openssl's STACK_OF(X509) structure
         # $x509 - value corresponding to openssl's X509 structure
         #
         # returns: a pointer to an X509 structure, undef on failure

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/sk_TYPE_shift.html>

    *   sk_X509_unshift

        Unshifts an single X509 structure from a STACK_OF(X509) structure.

         my $rv = NetSSLeay::sk_X509_unshift($sk_x509)
         # $sk_x509 - value corresponding to openssl's STACK_OF(X509) structure
         #
         # returns: total number of elements after the operation, 0 on failure

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/sk_TYPE_unshift.html>

    *   sk_X509_insert

        Inserts a single X509 structure into a STACK_OF(X509) at the specified index.

         my $rv = Net::SSLeay::sk_X509_insert($sk_x509, $x509, index);
         # $sk_x509 - value corresponding to openssl's STACK_OF(X509) structure
         # $x509 - value corresponding to openssl's X509 structure
         # index - integer - 0 based index
         #
         # returns: total number of elements after the operation, 0 on failure

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/sk_TYPE_insert.html>

    *   sk_X509_delete

        Delete a single X509 structure from a STACK_OF(X509) at the specified index.

         my $x509 = Net::SSLeay::sk_X509_delete($sk_x509, index);
         # $sk_x509 - value corresponding to openssl's STACK_OF(X509) structure
         # index - integer - 0 based index
         #
         # returns: a pointer to an X509 structure, undef on failure

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/sk_TYPE_delete.html>

    *   sk_X509_value

        Return a single X509 structure from a STACK_OF(X509) at the specified index.

         my $x509 = Net::SSLeay::sk_X509_value($sk_x509, index)
         # $sk_x509 - value corresponding to openssl's STACK_OF(X509) structure
         # index - integer - 0 based index
         #
         # returns: a pointer to an X509 structure, undef on failure

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/sk_TYPE_value.html>

    *   sk_X509_num

        Return the number of X509 elements in a STACK_OF(X509).

         my $num = Net::SSLeay::sk_X509_num($sk_x509);
         # $sk_x509 - value corresponding to openssl's STACK_OF(X509) structure
         #
         # returns: the number of elements in the stack, -1 if the passed stack is NULL

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/sk_TYPE_num.html>

   Low level API: X509_REQ_* related functions
    *   X509_REQ_new

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Creates a new X509_REQ structure.

         my $rv = Net::SSLeay::X509_REQ_new();
         #
         # returns: value corresponding to openssl's X509_REQ structure (0 on failure)

    *   X509_REQ_free

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Free an allocated X509_REQ structure.

         Net::SSLeay::X509_REQ_free($x);
         # $x - value corresponding to openssl's X509_REQ structure
         #
         # returns: no return value

    *   X509_REQ_add1_attr_by_NID

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Adds an attribute whose name is defined by a NID $nid. The field value to be added is in
        $bytes.

         my $rv = Net::SSLeay::X509_REQ_add1_attr_by_NID($req, $nid, $type, $bytes);
         # $req - value corresponding to openssl's X509_REQ structure
         # $nid - (integer) NID value
         # $type - (integer) type of data in $bytes (see below)
         # $bytes - data to be set
         #
         # returns: 1 on success, 0 on failure

         # values for $type - use constants:
         &Net::SSLeay::MBSTRING_UTF8     - $bytes contains utf8 encoded data
         &Net::SSLeay::MBSTRING_ASC      - $bytes contains ASCII data

    *   X509_REQ_digest

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Computes digest/fingerprint of X509_REQ $data using $type hash function.

         my $digest_value = Net::SSLeay::X509_REQ_digest($data, $type);
         # $data - value corresponding to openssl's X509_REQ structure
         # $type - value corresponding to openssl's EVP_MD structure - e.g. got via EVP_get_digestbyname()
         #
         # returns: hash value (binary)

         #to get printable (hex) value of digest use:
         print unpack('H*', $digest_value);

    *   X509_REQ_get_attr_by_NID

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Retrieve the next index matching $nid after $lastpos ($lastpos should initially be set to
        -1).

         my $rv = Net::SSLeay::X509_REQ_get_attr_by_NID($req, $nid, $lastpos=-1);
         # $req - value corresponding to openssl's X509_REQ structure
         # $nid - (integer) NID value
         # $lastpos - [optional] (integer) index where to start search (default -1)
         #
         # returns: index (-1 if there are no more entries)

        Note: use "P_X509_REQ_get_attr" to get the actual attribute value - e.g.

         my $index = Net::SSLeay::X509_REQ_get_attr_by_NID($req, $nid);
         my @attr_values = Net::SSLeay::P_X509_REQ_get_attr($req, $index);

    *   X509_REQ_get_attr_by_OBJ

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Retrieve the next index matching $obj after $lastpos ($lastpos should initially be set to
        -1).

         my $rv = Net::SSLeay::X509_REQ_get_attr_by_OBJ($req, $obj, $lastpos=-1);
         # $req - value corresponding to openssl's X509_REQ structure
         # $obj - value corresponding to openssl's ASN1_OBJECT structure
         # $lastpos - [optional] (integer) index where to start search (default -1)
         #
         # returns: index (-1 if there are no more entries)

        Note: use "P_X509_REQ_get_attr" to get the actual attribute value - e.g.

         my $index = Net::SSLeay::X509_REQ_get_attr_by_NID($req, $nid);
         my @attr_values = Net::SSLeay::P_X509_REQ_get_attr($req, $index);

    *   X509_REQ_get_attr_count

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns the total number of attributes in $req.

         my $rv = Net::SSLeay::X509_REQ_get_attr_count($req);
         # $req - value corresponding to openssl's X509_REQ structure
         #
         # returns: (integer) items count

    *   X509_REQ_get_pubkey

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns public key corresponding to given X509_REQ object $x.

         my $rv = Net::SSLeay::X509_REQ_get_pubkey($x);
         # $x - value corresponding to openssl's X509_REQ structure
         #
         # returns: value corresponding to openssl's EVP_PKEY structure (0 on failure)

    *   X509_REQ_get_subject_name

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns X509_NAME object corresponding to subject name of given X509_REQ object $x.

         my $rv = Net::SSLeay::X509_REQ_get_subject_name($x);
         # $x - value corresponding to openssl's X509_REQ structure
         #
         # returns: value corresponding to openssl's X509_NAME structure (0 on failure)

    *   X509_REQ_get_version

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns 'version' value for given X509_REQ object $x.

         my $rv = Net::SSLeay::X509_REQ_get_version($x);
         # $x - value corresponding to openssl's X509_REQ structure
         #
         # returns: (integer) version e.g. 0 = "version 1"

    *   X509_REQ_set_pubkey

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sets public key of given X509_REQ object $x to $pkey.

         my $rv = Net::SSLeay::X509_REQ_set_pubkey($x, $pkey);
         # $x - value corresponding to openssl's X509_REQ structure
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_REQ_set_subject_name

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sets subject name of given X509_REQ object $x to X509_NAME object $name.

         my $rv = Net::SSLeay::X509_REQ_set_subject_name($x, $name);
         # $x - value corresponding to openssl's X509_REQ structure
         # $name - value corresponding to openssl's X509_NAME structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_REQ_set_version

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sets 'version' of given X509_REQ object $x to $version.

         my $rv = Net::SSLeay::X509_REQ_set_version($x, $version);
         # $x - value corresponding to openssl's X509_REQ structure
         # $version - (integer) e.g. 0 = "version 1"
         #
         # returns: 1 on success, 0 on failure

    *   X509_REQ_sign

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sign X509_REQ object $x with private key $pk (using digest algorithm $md).

         my $rv = Net::SSLeay::X509_REQ_sign($x, $pk, $md);
         # $x - value corresponding to openssl's X509_REQ structure
         # $pk - value corresponding to openssl's EVP_PKEY structure (requestor's private key)
         # $md - value corresponding to openssl's EVP_MD structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_REQ_verify

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Verifies X509_REQ object $x using public key $r (pubkey of requesting party).

         my $rv = Net::SSLeay::X509_REQ_verify($x, $r);
         # $x - value corresponding to openssl's X509_REQ structure
         # $r - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: 0 - verify failure, 1 - verify OK, <0 - error

    *   P_X509_REQ_add_extensions

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Adds one or more X509 extensions to X509_REQ object $x.

         my $rv = Net::SSLeay::P_X509_REQ_add_extensions($x, $nid, $value);
         # $x - value corresponding to openssl's X509_REQ structure
         # $nid - NID identifying extension to be set
         # $value - extension value
         #
         # returns: 1 on success, 0 on failure

        You can set more extensions at once:

         my $rv = Net::SSLeay::P_X509_REQ_add_extensions($x509_req,
                    &Net::SSLeay::NID_key_usage => 'digitalSignature,keyEncipherment',
                    &Net::SSLeay::NID_basic_constraints => 'CA:FALSE',
                    &Net::SSLeay::NID_ext_key_usage => 'serverAuth,clientAuth',
                    &Net::SSLeay::NID_netscape_cert_type => 'server',
                    &Net::SSLeay::NID_subject_alt_name => 'DNS:s1.com,DNS:s2.com',
                    &Net::SSLeay::NID_crl_distribution_points => 'URI:http://pki.com/crl1,URI:http://pki.com/crl2',
                  );

    *   P_X509_REQ_get_attr

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Returns attribute value for X509_REQ's attribute at index $n.

         Net::SSLeay::P_X509_REQ_get_attr($req, $n);
         # $req - value corresponding to openssl's X509_REQ structure
         # $n - (integer) attribute index
         #
         # returns: value corresponding to openssl's ASN1_STRING structure

   Low level API: X509_CRL_* related functions
    *   X509_CRL_new

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Creates a new X509_CRL structure.

         my $rv = Net::SSLeay::X509_CRL_new();
         #
         # returns: value corresponding to openssl's X509_CRL structure (0 on failure)

    *   X509_CRL_free

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Free an allocated X509_CRL structure.

         Net::SSLeay::X509_CRL_free($x);
         # $x - value corresponding to openssl's X509_CRL structure
         #
         # returns: no return value

    *   X509_CRL_digest

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Computes digest/fingerprint of X509_CRL $data using $type hash function.

         my $digest_value = Net::SSLeay::X509_CRL_digest($data, $type);
         # $data - value corresponding to openssl's X509_CRL structure
         # $type - value corresponding to openssl's EVP_MD structure - e.g. got via EVP_get_digestbyname()
         #
         # returns: hash value (binary)

        Example:

         my $x509_crl
         my $md = Net::SSLeay::EVP_get_digestbyname("sha1");
         my $digest_value = Net::SSLeay::X509_CRL_digest($x509_crl, $md);
         #to get printable (hex) value of digest use:
         print "digest=", unpack('H*', $digest_value), "\n";

    *   X509_CRL_get_ext

        COMPATIBILITY: not available in Net-SSLeay-1.54 and before

        Returns X509_EXTENSION from $x509 based on given position/index.

         my $rv = Net::SSLeay::X509_CRL_get_ext($x509, $index);
         # $x509 - value corresponding to openssl's X509_CRL structure
         # $index - (integer) position/index of extension within $x509
         #
         # returns: value corresponding to openssl's X509_EXTENSION structure (0 on failure)

    *   X509_CRL_get_ext_by_NID

        COMPATIBILITY: not available in Net-SSLeay-1.54 and before

        Returns X509_EXTENSION from $x509 based on given NID.

         my $rv = Net::SSLeay::X509_CRL_get_ext_by_NID($x509, $nid, $loc);
         # $x509 - value corresponding to openssl's X509_CRL structure
         # $nid - (integer) NID value
         # $loc - (integer) position to start lookup at
         #
         # returns: position/index of extension, negative value on error
         #          call Net::SSLeay::X509_CRL_get_ext($x509, $rv) to get the actual extension

    *   X509_CRL_get_ext_count

        COMPATIBILITY: not available in Net-SSLeay-1.54 and before

        Returns the total number of extensions in X509_CRL object $x.

         my $rv = Net::SSLeay::X509_CRL_get_ext_count($x);
         # $x - value corresponding to openssl's X509_CRL structure
         #
         # returns: count of extensions

    *   X509_CRL_get_issuer

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns X509_NAME object corresponding to the issuer of X509_CRL $x.

         my $rv = Net::SSLeay::X509_CRL_get_issuer($x);
         # $x - value corresponding to openssl's X509_CRL structure
         #
         # returns: value corresponding to openssl's X509_NAME structure (0 on failure)

        See other "X509_NAME_*" functions to get more info from X509_NAME structure.

    *   X509_CRL_get_lastUpdate

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns 'lastUpdate' date-time value of X509_CRL object $x.

         my $rv = Net::SSLeay::X509_CRL_get_lastUpdate($x);
         # $x - value corresponding to openssl's X509_CRL structure
         #
         # returns: value corresponding to openssl's ASN1_TIME structure (0 on failure)

    *   X509_CRL_get_nextUpdate

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns 'nextUpdate' date-time value of X509_CRL object $x.

         my $rv = Net::SSLeay::X509_CRL_get_nextUpdate($x);
         # $x - value corresponding to openssl's X509_CRL structure
         #
         # returns: value corresponding to openssl's ASN1_TIME structure (0 on failure)

    *   X509_CRL_get_version

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns 'version' value of given X509_CRL structure $x.

         my $rv = Net::SSLeay::X509_CRL_get_version($x);
         # $x - value corresponding to openssl's X509_CRL structure
         #
         # returns: (integer) version

    *   X509_CRL_set_issuer_name

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Sets the issuer of X509_CRL object $x to X509_NAME object $name.

         my $rv = Net::SSLeay::X509_CRL_set_issuer_name($x, $name);
         # $x - value corresponding to openssl's X509_CRL structure
         # $name - value corresponding to openssl's X509_NAME structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_CRL_set_lastUpdate

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Sets 'lastUpdate' value of X509_CRL object $x to $tm.

         my $rv = Net::SSLeay::X509_CRL_set_lastUpdate($x, $tm);
         # $x - value corresponding to openssl's X509_CRL structure
         # $tm - value corresponding to openssl's ASN1_TIME structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_CRL_set_nextUpdate

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Sets 'nextUpdate' value of X509_CRL object $x to $tm.

         my $rv = Net::SSLeay::X509_CRL_set_nextUpdate($x, $tm);
         # $x - value corresponding to openssl's X509_CRL structure
         # $tm - value corresponding to openssl's ASN1_TIME structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_CRL_set_version

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Sets 'version' value of given X509_CRL structure $x to $version.

         my $rv = Net::SSLeay::X509_CRL_set_version($x, $version);
         # $x - value corresponding to openssl's X509_CRL structure
         # $version - (integer) version number (1 = version 2 CRL)
         #
         # returns: 1 on success, 0 on failure

        Note that if you want to use any X509_CRL extension you need to set "version 2 CRL" -
        "Net::SSLeay::X509_CRL_set_version($x, 1)".

    *   X509_CRL_sign

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Sign X509_CRL object $x with private key $pkey (using digest algorithm $md).

         my $rv = Net::SSLeay::X509_CRL_sign($x, $pkey, $md);
         # $x - value corresponding to openssl's X509_CRL structure
         # $pkey - value corresponding to openssl's EVP_PKEY structure
         # $md - value corresponding to openssl's EVP_MD structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_CRL_sort

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Sorts the data of X509_CRL object so it will be written in serial number order.

         my $rv = Net::SSLeay::X509_CRL_sort($x);
         # $x - value corresponding to openssl's X509_CRL structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_CRL_verify

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Verifies X509_CRL object $a using public key $r (pubkey of issuing CA).

         my $rv = Net::SSLeay::X509_CRL_verify($a, $r);
         # $a - value corresponding to openssl's X509_CRL structure
         # $r - value corresponding to openssl's EVP_PKEY structure
         #
         # returns: 0 - verify failure, 1 - verify OK, <0 - error

    *   P_X509_CRL_add_revoked_serial_hex

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Adds given serial number $serial_hex to X509_CRL object $crl.

         Net::SSLeay::P_X509_CRL_add_revoked_serial_hex($crl, $serial_hex, $rev_time, $reason_code, $comp_time);
         # $crl - value corresponding to openssl's X509_CRL structure
         # $serial_hex - string (hexadecimal) representation of serial number
         # $rev_time - (revocation time) value corresponding to openssl's ASN1_TIME structure
         # $reason_code - [optional] (integer) reason code (see below) - default 0
         # $comp_time - [optional] (compromise time) value corresponding to openssl's ASN1_TIME structure
         #
         # returns: no return value

         reason codes:
         0 - unspecified
         1 - keyCompromise
         2 - CACompromise
         3 - affiliationChanged
         4 - superseded
         5 - cessationOfOperation
         6 - certificateHold
         7 - removeFromCRL

    *   P_X509_CRL_get_serial

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Returns serial number of X509_CRL object.

         my $rv = Net::SSLeay::P_X509_CRL_get_serial($crl);
         # $crl - value corresponding to openssl's X509_CRL structure
         #
         # returns: value corresponding to openssl's ASN1_INTEGER structure (0 on failure)

    *   P_X509_CRL_set_serial

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.7

        Sets serial number of X509_CRL object to $crl_number.

         my $rv = Net::SSLeay::P_X509_CRL_set_serial($crl, $crl_number);
         # $crl - value corresponding to openssl's X509_CRL structure
         # $crl_number - value corresponding to openssl's ASN1_INTEGER structure
         #
         # returns: 1 on success, 0 on failure

    *   P_X509_CRL_add_extensions

        COMPATIBILITY: not available in Net-SSLeay-1.88 and before

        Adds one or more X509 extensions to X509 CRL object $x.

         my $rv = Net::SSLeay::P_X509_CRL_add_extensions($x, $ca_cert, $nid, $value);
         # $x - value corresponding to openssl's X509 CRL structure
         # $ca_cert - value corresponding to openssl's X509 structure (issuer's cert - necessary for sertting NID_authority_key_identifier)
         # $nid - NID identifying extension to be set
         # $value - extension value
         #
         # returns: 1 on success, 0 on failure

        For more details see "P_X509_add_extensions".

   Low level API: X509_EXTENSION_* related functions
    *   X509_EXTENSION_get_critical

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns 'critical' flag of given X509_EXTENSION object $ex.

         my $rv = Net::SSLeay::X509_EXTENSION_get_critical($ex);
         # $ex - value corresponding to openssl's X509_EXTENSION structure
         #
         # returns: (integer) 1 - critical, 0 - noncritical

    *   X509_EXTENSION_get_data

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns value (raw data) of X509_EXTENSION object $ne.

         my $rv = Net::SSLeay::X509_EXTENSION_get_data($ne);
         # $ne - value corresponding to openssl's X509_EXTENSION structure
         #
         # returns: value corresponding to openssl's ASN1_OCTET_STRING structure (0 on failure)

        Note: you can use "P_ASN1_STRING_get" to convert ASN1_OCTET_STRING into perl scalar
        variable.

    *   X509_EXTENSION_get_object

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns OID (ASN1_OBJECT) of X509_EXTENSION object $ne.

         my $rv = Net::SSLeay::X509_EXTENSION_get_object($ex);
         # $ex - value corresponding to openssl's X509_EXTENSION structure
         #
         # returns: value corresponding to openssl's ASN1_OBJECT structure (0 on failure)

    *   X509V3_EXT_print

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns string representation of given X509_EXTENSION object $ext.

         Net::SSLeay::X509V3_EXT_print($ext, $flags, $utf8_decode);
         # $ext - value corresponding to openssl's X509_EXTENSION structure
         # $flags - [optional] (integer) Currently the flag argument is unused and should be set to 0
         # $utf8_decode - [optional] 0 or 1 whether the returned value should be utf8 decoded (default=0)
         #
         # returns: no return value

    *   X509V3_EXT_d2i

        Parses an extension and returns its internal structure.

         my $rv = Net::SSLeay::X509V3_EXT_d2i($ext);
         # $ext - value corresponding to openssl's X509_EXTENSION structure
         #
         # returns: pointer ???

   Low level API: X509_NAME_* related functions
    *   X509_NAME_ENTRY_get_data

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Retrieves the field value of $ne in and ASN1_STRING structure.

         my $rv = Net::SSLeay::X509_NAME_ENTRY_get_data($ne);
         # $ne - value corresponding to openssl's X509_NAME_ENTRY structure
         #
         # returns: value corresponding to openssl's ASN1_STRING structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_NAME_ENTRY_get_object.html>

    *   X509_NAME_ENTRY_get_object

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Retrieves the field name of $ne in and ASN1_OBJECT structure.

         my $rv = Net::SSLeay::X509_NAME_ENTRY_get_object($ne);
         # $ne - value corresponding to openssl's X509_NAME_ENTRY structure
         #
         # returns: value corresponding to openssl's ASN1_OBJECT structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_NAME_ENTRY_get_object.html>

    *   X509_NAME_new

        COMPATIBILITY: not available in Net-SSLeay-1.55 and before; requires at least openssl-0.9.5

        Creates a new X509_NAME structure. Adds a field whose name is defined by a string $field.
        The field value to be added is in $bytes.

         my $rv = Net::SSLeay::X509_NAME_new();
         #
         # returns: value corresponding to openssl's X509_NAME structure (0 on failure)

    *   X509_NAME_hash

        COMPATIBILITY: not available in Net-SSLeay-1.55 and before; requires at least openssl-0.9.5

        Sort of a checksum of issuer name $name. The result is not a full hash (e.g. sha-1), it is
        kind-of-a-hash truncated to the size of 'unsigned long' (32 bits). The resulting value might
        differ across different openssl versions for the same X509 certificate.

         my $rv = Net::SSLeay::X509_NAME_hash($name);
         # $name - value corresponding to openssl's X509_NAME structure
         #
         # returns: number representing checksum

    *   X509_NAME_add_entry_by_txt

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.5

        Adds a field whose name is defined by a string $field. The field value to be added is in
        $bytes.

         my $rv = Net::SSLeay::X509_NAME_add_entry_by_txt($name, $field, $type, $bytes, $len, $loc, $set);
         # $name - value corresponding to openssl's X509_NAME structure
         # $field - (string) field definition (name) - e.g. "organizationName"
         # $type - (integer) type of data in $bytes (see below)
         # $bytes - data to be set
         # $loc - [optional] (integer) index where the new entry is inserted: if it is -1 (default) it is appended
         # $set - [optional] (integer) determines how the new type is added. If it is 0 (default) a new RDN is created
         #
         # returns: 1 on success, 0 on failure

         # values for $type - use constants:
         &Net::SSLeay::MBSTRING_UTF8     - $bytes contains utf8 encoded data
         &Net::SSLeay::MBSTRING_ASC      - $bytes contains ASCII data

        Unicode note: when passing non-ascii (unicode) string in $bytes do not forget to set "$flags
        = &Net::SSLeay::MBSTRING_UTF8" and encode the perl $string via "$bytes = encode('utf-8',
        $string)".

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_NAME_add_entry_by_txt.html>

    *   X509_NAME_add_entry_by_NID

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.5

        Adds a field whose name is defined by a NID $nid. The field value to be added is in $bytes.

         my $rv = Net::SSLeay::X509_NAME_add_entry_by_NID($name, $nid, $type, $bytes, $len, $loc, $set);
         # $name - value corresponding to openssl's X509_NAME structure
         # $nid - (integer) field definition - NID value
         # $type - (integer) type of data in $bytes (see below)
         # $bytes - data to be set
         # $loc - [optional] (integer) index where the new entry is inserted: if it is -1 (default) it is appended
         # $set - [optional] (integer) determines how the new type is added. If it is 0 (default) a new RDN is created
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_NAME_add_entry_by_txt.html>

    *   X509_NAME_add_entry_by_OBJ

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-0.9.5

        Adds a field whose name is defined by a object (OID) $obj . The field value to be added is
        in $bytes.

         my $rv = Net::SSLeay::X509_NAME_add_entry_by_OBJ($name, $obj, $type, $bytes, $len, $loc, $set);
         # $name - value corresponding to openssl's X509_NAME structure
         # $obj - field definition - value corresponding to openssl's ASN1_OBJECT structure
         # $type - (integer) type of data in $bytes (see below)
         # $bytes - data to be set
         # $loc - [optional] (integer) index where the new entry is inserted: if it is -1 (default) it is appended
         # $set - [optional] (integer) determines how the new type is added. If it is 0 (default) a new RDN is created
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_NAME_add_entry_by_txt.html>

    *   X509_NAME_cmp

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Compares two X509_NAME obejcts.

         my $rv = Net::SSLeay::X509_NAME_cmp($a, $b);
         # $a - value corresponding to openssl's X509_NAME structure
         # $b - value corresponding to openssl's X509_NAME structure
         #
         # returns: 0 if $a matches $b; non zero otherwise

    *   X509_NAME_digest

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Computes digest/fingerprint of X509_NAME $data using $type hash function.

         my $digest_value = Net::SSLeay::X509_NAME_digest($data, $type);
         # $data - value corresponding to openssl's X509_NAME structure
         # $type - value corresponding to openssl's EVP_MD structure - e.g. got via EVP_get_digestbyname()
         #
         # returns: hash value (binary)

         #to get printable (hex) value of digest use:
         print unpack('H*', $digest_value);

    *   X509_NAME_entry_count

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns the total number of entries in $name.

         my $rv = Net::SSLeay::X509_NAME_entry_count($name);
         # $name - value corresponding to openssl's X509_NAME structure
         #
         # returns: (integer) entries count

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_NAME_get_index_by_NID.html>

    *   X509_NAME_get_entry

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Retrieves the X509_NAME_ENTRY from $name corresponding to index $loc. Acceptable values for
        $loc run from 0 to "Net::SSLeay::X509_NAME_entry_count($name)- 1". The value returned is an
        internal pointer which must not be freed.

         my $rv = Net::SSLeay::X509_NAME_get_entry($name, $loc);
         # $name - value corresponding to openssl's X509_NAME structure
         # $loc - (integer) index of wanted entry
         #
         # returns: value corresponding to openssl's X509_NAME_ENTRY structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_NAME_get_index_by_NID.html>

    *   X509_NAME_print_ex

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns a string with human readable version of $name.

         Net::SSLeay::X509_NAME_print_ex($name, $flags, $utf8_decode);
         # $name - value corresponding to openssl's X509_NAME structure
         # $flags - [optional] conversion flags (default XN_FLAG_RFC2253) - see below
         # $utf8_decode - [optional] 0 or 1 whether the returned value should be utf8 decoded (default=0)
         #
         # returns: string representation of $name

         #available conversion flags - use constants:
         &Net::SSLeay::XN_FLAG_COMPAT
         &Net::SSLeay::XN_FLAG_DN_REV
         &Net::SSLeay::XN_FLAG_DUMP_UNKNOWN_FIELDS
         &Net::SSLeay::XN_FLAG_FN_ALIGN
         &Net::SSLeay::XN_FLAG_FN_LN
         &Net::SSLeay::XN_FLAG_FN_MASK
         &Net::SSLeay::XN_FLAG_FN_NONE
         &Net::SSLeay::XN_FLAG_FN_OID
         &Net::SSLeay::XN_FLAG_FN_SN
         &Net::SSLeay::XN_FLAG_MULTILINE
         &Net::SSLeay::XN_FLAG_ONELINE
         &Net::SSLeay::XN_FLAG_RFC2253
         &Net::SSLeay::XN_FLAG_SEP_COMMA_PLUS
         &Net::SSLeay::XN_FLAG_SEP_CPLUS_SPC
         &Net::SSLeay::XN_FLAG_SEP_MASK
         &Net::SSLeay::XN_FLAG_SEP_MULTILINE
         &Net::SSLeay::XN_FLAG_SEP_SPLUS_SPC
         &Net::SSLeay::XN_FLAG_SPC_EQ

        Most likely you will be fine with default:

         Net::SSLeay::X509_NAME_print_ex($name, &Net::SSLeay::XN_FLAG_RFC2253);

        Or you might want RFC2253-like output without utf8 chars escaping:

         use Net::SSLeay qw/XN_FLAG_RFC2253 ASN1_STRFLGS_ESC_MSB/;
         my $flag_rfc22536_utf8 = (XN_FLAG_RFC2253) & (~ ASN1_STRFLGS_ESC_MSB);
         my $result = Net::SSLeay::X509_NAME_print_ex($name, $flag_rfc22536_utf8, 1);

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_NAME_print_ex.html>

    *   X509_NAME_get_text_by_NID

        Retrieves the text from the first entry in name which matches $nid, if no such entry exists
        -1 is returned.

        openssl note: this is a legacy function which has various limitations which makes it of
        minimal use in practice. It can only find the first matching entry and will copy the
        contents of the field verbatim: this can be highly confusing if the target is a
        multicharacter string type like a BMPString or a UTF8String.

         Net::SSLeay::X509_NAME_get_text_by_NID($name, $nid);
         # $name - value corresponding to openssl's X509_NAME structure
         # $nid - NID value (integer)
         #
         # returns: text value

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_NAME_get_index_by_NID.html>

    *   X509_NAME_oneline

        Return an ASCII version of $name.

         Net::SSLeay::X509_NAME_oneline($name);
         # $name - value corresponding to openssl's X509_NAME structure
         #
         # returns: (string) ASCII version of $name

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_NAME_print_ex.html>

    *   sk_X509_NAME_free

        Free an allocated STACK_OF(X509_NAME) structure.

         Net::SSLeay::sk_X509_NAME_free($sk);
         # $sk - value corresponding to openssl's STACK_OF(X509_NAME) structure
         #
         # returns: no return value

    *   sk_X509_NAME_num

        Return number of items in STACK_OF(X509_NAME)

         my $rv = Net::SSLeay::sk_X509_NAME_num($sk);
         # $sk - value corresponding to openssl's STACK_OF(X509_NAME) structure
         #
         # returns: number of items

    *   sk_X509_NAME_value

        Returns X509_NAME from position $index in STACK_OF(X509_NAME)

         my $rv = Net::SSLeay::sk_X509_NAME_value($sk, $i);
         # $sk - value corresponding to openssl's STACK_OF(X509_NAME) structure
         # $i - (integer) index/position
         #
         # returns: value corresponding to openssl's X509_NAME structure (0 on failure)

    *   add_file_cert_subjects_to_stack

        Add a file of certs to a stack. All certs in $file that are not already in the $stackCAs
        will be added.

         my $rv = Net::SSLeay::add_file_cert_subjects_to_stack($stackCAs, $file);
         # $stackCAs - value corresponding to openssl's STACK_OF(X509_NAME) structure
         # $file - (string) filename
         #
         # returns: 1 on success, 0 on failure

    *   add_dir_cert_subjects_to_stack

        Add a directory of certs to a stack. All certs in $dir that are not already in the $stackCAs
        will be added.

         my $rv = Net::SSLeay::add_dir_cert_subjects_to_stack($stackCAs, $dir);
         # $stackCAs - value corresponding to openssl's STACK_OF(X509_NAME) structure
         # $dir - (string) the directory to append from. All files in this directory will be examined as potential certs. Any that are acceptable to SSL_add_dir_cert_subjects_to_stack() that are not already in the stack will be included.
         #
         # returns: 1 on success, 0 on failure

   Low level API: X509_STORE_* related functions
    *   X509_STORE_CTX_new

        returns a newly initialised X509_STORE_CTX structure.

    *   X509_STORE_CTX_init

        X509_STORE_CTX_init() sets up an X509_STORE_CTX for a subsequent verification operation. It
        must be called before each call to X509_verify_cert().

         my $rv = Net::SSLeay::X509_STORE_CTX_init($x509_store_ctx, $x509_store, $x509, $chain);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure (required)
         # $x509_store - value corresponding to openssl's X509_STORE structure (optional)
         # $x509 - value corresponding to openssl's X509 structure (optional)
         # $chain - value corresponding to openssl's STACK_OF(X509) structure (optional)
         #
         # returns: 1 on success, 0 on failure
         #
         # Note: returns nothing with Net::SSLeay 1.90 and earlier.

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/X509_STORE_CTX_init.html>

    *   X509_STORE_CTX_free

        Frees an X509_STORE_CTX structure.

         Net::SSLeay::X509_STORE_CTX_free($x509_store_ctx);

        # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure

    *   X509_verify_cert

        The X509_verify_cert() function attempts to discover and validate a certificate chain based
        on parameters in ctx. A complete description of the process is contained in the verify(1)
        manual page.

        If this function returns 0, use X509_STORE_CTX_get_error to get additional error
        information.

         my $rv = Net::SSLeay::X509_verify_cert($x509_store_ctx);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure
         #
         # returns: 1 if a complete chain can be built and validated, otherwise 0

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/X509_verify_cert.html>

    *   X509_STORE_CTX_get_current_cert

        Returns the certificate in ctx which caused the error or 0 if no certificate is relevant.

         my $rv = Net::SSLeay::X509_STORE_CTX_get_current_cert($x509_store_ctx);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure
         #
         # returns: value corresponding to openssl's X509 structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_STORE_CTX_get_error.html>

    *   X509_STORE_CTX_get0_cert

        COMPATIBILITY: not available in Net-SSLeay-1.88 and before; requires at least OpenSSL
        1.1.0pre6 or LibreSSL 2.7.0

        Returns an internal pointer to the certificate being verified by the ctx.

         my $x509 = Net::SSLeay::X509_STORE_CTX_get0_cert($x509_store_ctx);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure
         #
         # returns: value corresponding to openssl's X509 structure

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/X509_STORE_CTX_get0_cert.html>

    *   X509_STORE_CTX_get1_chain

        Returns a returns a complete validate chain if a previous call to X509_verify_cert() is
        successful.

         my $rv = Net::SSLeay::X509_STORE_CTX_get1_chain($x509_store_ctx);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure
         #
         # returns: value corresponding to openssl's STACK_OF(X509) structure

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/X509_STORE_CTX_get1_chain.html>

    *   X509_STORE_CTX_get_error

        Returns the error code of $ctx.

         my $rv = Net::SSLeay::X509_STORE_CTX_get_error($x509_store_ctx);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure
         #
         # returns: (integer) error code

        For more info about erro code values check function "get_verify_result".

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_STORE_CTX_get_error.html>

    *   X509_STORE_CTX_get_error_depth

        Returns the depth of the error. This is a non-negative integer representing where in the
        certificate chain the error occurred. If it is zero it occurred in the end entity
        certificate, one if it is the certificate which signed the end entity certificate and so on.

         my $rv = Net::SSLeay::X509_STORE_CTX_get_error_depth($x509_store_ctx);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure
         #
         # returns: (integer) depth

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_STORE_CTX_get_error.html>

    *   X509_STORE_CTX_get_ex_data

        Is used to retrieve the information for $idx from $x509_store_ctx.

         my $rv = Net::SSLeay::X509_STORE_CTX_get_ex_data($x509_store_ctx, $idx);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure
         # $idx - (integer) index for application specific data
         #
         # returns: pointer to ???

    *   X509_STORE_CTX_set_ex_data

        Is used to store application data at arg for idx into $x509_store_ctx.

         my $rv = Net::SSLeay::X509_STORE_CTX_set_ex_data($x509_store_ctx, $idx, $data);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure
         # $idx - (integer) ???
         # $data - (pointer) ???
         #
         # returns: 1 on success, 0 on failure

    *   X509_STORE_CTX_set_cert

        Sets the certificate to be verified in $x509_store_ctx to $x.

         Net::SSLeay::X509_STORE_CTX_set_cert($x509_store_ctx, $x);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_STORE_CTX_new.html>

    *   X509_STORE_new

        Returns a newly initialized X509_STORE structure.

         my $rv = Net::SSLeay::X509_STORE_new();
         #
         # returns: value corresponding to openssl's X509_STORE structure (0 on failure)

    *   X509_STORE_free

        Frees an X509_STORE structure

         Net::SSLeay::X509_STORE_free($x509_store);
         # $x509_store - value corresponding to openssl's X509_STORE structure

    *   X509_STORE_add_lookup

        Adds a lookup to an X509_STORE for a given lookup method.

         my $method = &Net::SSLeay::X509_LOOKUP_hash_dir;
         my $rv = Net::SSLeay::X509_STORE_add_lookup($x509_store, $method);
         # $method - value corresponding to openssl's X509_LOOKUP_METHOD structure
         # $x509_store - value corresponding to openssl's X509_STORE structure
         #
         # returns: value corresponding to openssl's X509_LOOKUP structure

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/X509_STORE_add_lookup.html>

    *   X509_STORE_CTX_set_error

        Sets the error code of $ctx to $s. For example it might be used in a verification callback
        to set an error based on additional checks.

         Net::SSLeay::X509_STORE_CTX_set_error($x509_store_ctx, $s);
         # $x509_store_ctx - value corresponding to openssl's X509_STORE_CTX structure
         # $s - (integer) error id
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_STORE_CTX_get_error.html>

    *   X509_STORE_add_cert

        Adds X509 certificate $x into the X509_STORE $store.

         my $rv = Net::SSLeay::X509_STORE_add_cert($store, $x);
         # $store - value corresponding to openssl's X509_STORE structure
         # $x - value corresponding to openssl's X509 structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_STORE_add_crl

        Adds X509 CRL $x into the X509_STORE $store.

         my $rv = Net::SSLeay::X509_STORE_add_crl($store, $x);
         # $store - value corresponding to openssl's X509_STORE structure
         # $x - value corresponding to openssl's X509_CRL structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_STORE_set1_param

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_STORE_set1_param($store, $pm);
         # $store - value corresponding to openssl's X509_STORE structure
         # $pm - value corresponding to openssl's X509_VERIFY_PARAM structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_LOOKUP_hash_dir

        Returns an X509_LOOKUP structure that instructs an X509_STORE to load files from a directory
        containing certificates with filenames in the format *hash.N* or crls with filenames in the
        format *hash.*r*N*

         my $rv = Net::SSLeay::X509_LOOKUP_hash_dir();
         #
         # returns: value corresponding to openssl's X509_LOOKUP_METHOD structure, with the hashed directory method

        Check openssl doc <https://www.openssl.org/docs/man1.1.1/man3/X509_load_crl_file.html>

    *   X509_LOOKUP_add_dir

        Add a directory to an X509_LOOKUP structure, usually obtained from X509_STORE_add_lookup.

         my $method = &Net::SSLeay::X509_LOOKUP_hash_dir;
         my $lookup = Net::SSLeay::X509_STORE_add_lookup($x509_store, $method);
         my $type = &Net::SSLeay::X509_FILETYPE_PEM;
         Net::SSLeay::X509_LOOKUP_add_dir($lookup, $dir, $type);
         # $lookup - value corresponding to openssl's X509_LOOKUP structure
         # $dir - string path to a directory
         # $type - constant corresponding to the type of file in the directory - can be X509_FILETYPE_PEM, X509_FILETYPE_DEFAULT, or X509_FILETYPE_ASN1

    *   X509_STORE_set_flags

         Net::SSLeay::X509_STORE_set_flags($ctx, $flags);
         # $ctx - value corresponding to openssl's X509_STORE structure
         # $flags - (unsigned long) flags to be set (bitmask)
         #
         # returns: no return value

         #to create $flags value use corresponding constants like
         $flags = Net::SSLeay::X509_V_FLAG_CRL_CHECK();

        For more details about $flags bitmask see "X509_VERIFY_PARAM_set_flags".

    *   X509_STORE_set_purpose

         Net::SSLeay::X509_STORE_set_purpose($ctx, $purpose);
         # $ctx - value corresponding to openssl's X509_STORE structure
         # $purpose - (integer) purpose identifier
         #
         # returns: no return value

        For more details about $purpose identifier check "CTX_set_purpose".

    *   X509_STORE_set_trust

         Net::SSLeay::X509_STORE_set_trust($ctx, $trust);
         # $ctx - value corresponding to openssl's X509_STORE structure
         # $trust - (integer) trust identifier
         #
         # returns: no return value

        For more details about $trust identifier check "CTX_set_trust".

   Low Level API: X509_INFO related functions
    *   sk_X509_INFO_num

        Returns the number of values in a STACK_OF(X509_INFO) structure.

         my $rv = Net::SSLeay::sk_X509_INFO_num($sk_x509_info);
         # $sk_x509_info - value corresponding to openssl's STACK_OF(X509_INFO) structure
         #
         # returns: number of values in $sk_X509_info

    *   sk_X509_INFO_value

        Returns the value of a STACK_OF(X509_INFO) structure at a given index.

         my $rv = Net::SSLeay::sk_X509_INFO_value($sk_x509_info, $index);
         # $sk_x509_info - value corresponding to openssl's STACK_OF(X509_INFO) structure
         # $index - index into the stack
         #
         # returns: value corresponding to openssl's X509_INFO structure at the given index

    *   P_X509_INFO_get_x509

        Returns the X509 structure stored in an X509_INFO structure.

         my $rv = Net::SSLeay::P_X509_INFO_get_x509($x509_info);
         # $x509_info - value corresponding to openssl's X509_INFO structure
         #
         # returns: value corresponding to openssl's X509 structure

   Low level API: X509_VERIFY_PARAM_* related functions
    *   X509_VERIFY_PARAM_add0_policy

        Enables policy checking (it is disabled by default) and adds $policy to the acceptable
        policy set.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_add0_policy($param, $policy);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $policy - value corresponding to openssl's ASN1_OBJECT structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_add0_table

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_add0_table($param);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_VERIFY_PARAM_add1_host

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.0.2-beta2 or LibreSSL 2.7.0

        Adds an additional reference identifier that can match the peer's certificate.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_add1_host($param, $name);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $name - (string) name to be set
         #
         # returns: 1 on success, 0 on failure

        See also OpenSSL docs, "X509_VERIFY_PARAM_set1_host" and "X509_VERIFY_PARAM_set_hostflags"
        for more information, including wildcard matching.

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_clear_flags

        Clears the flags $flags in param.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_clear_flags($param, $flags);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $flags - (unsigned long) flags to be set (bitmask)
         #
         # returns: 1 on success, 0 on failure

        For more details about $flags bitmask see "X509_VERIFY_PARAM_set_flags".

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_free

        Frees up the X509_VERIFY_PARAM structure.

         Net::SSLeay::X509_VERIFY_PARAM_free($param);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         #
         # returns: no return value

    *   X509_VERIFY_PARAM_get0_peername

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.0.2-beta2 or LibreSSL 2.7.0

        Returns the DNS hostname or subject CommonName from the peer certificate that matched one of
        the reference identifiers.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_get0_peername($param);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         #
         # returns: (string) name e.g. '*.example.com' or undef

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_get_depth

        Returns the current verification depth.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_get_depth($param);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         #
         # returns: (ineger) depth

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_get_flags

        Returns the current verification flags.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_get_flags($param);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         #
         # returns: (unsigned long) flags to be set (bitmask)

        For more details about returned flags bitmask see "X509_VERIFY_PARAM_set_flags".

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_set_flags

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_set_flags($param, $flags);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $flags - (unsigned long) flags to be set (bitmask)
         #
         # returns: 1 on success, 0 on failure

         #to create $flags value use corresponding constants like
         $flags = Net::SSLeay::X509_V_FLAG_CRL_CHECK();

        For more details about $flags bitmask, see the OpenSSL docs below.

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_inherit

        ??? (more info needed)

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_inherit($to, $from);
         # $to - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $from - value corresponding to openssl's X509_VERIFY_PARAM structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_VERIFY_PARAM_lookup

        Finds X509_VERIFY_PARAM by name.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_lookup($name);
         # $name - (string) name we want to find
         #
         # returns: value corresponding to openssl's X509_VERIFY_PARAM structure (0 on failure)

    *   X509_VERIFY_PARAM_new

        Creates a new X509_VERIFY_PARAM structure.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_new();
         #
         # returns: value corresponding to openssl's X509_VERIFY_PARAM structure (0 on failure)

    *   X509_VERIFY_PARAM_set1

        Sets the name of X509_VERIFY_PARAM structure $to to the same value as the name of
        X509_VERIFY_PARAM structure $from.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_set1($to, $from);
         # $to - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $from - value corresponding to openssl's X509_VERIFY_PARAM structure
         #
         # returns: 1 on success, 0 on failure

    *   X509_VERIFY_PARAM_set1_email

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.0.2-beta1 or LibreSSL 2.7.0

        Sets the expected RFC822 email address to email.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_set1_email($param, $email);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $email - (string) email to be set
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_set1_host

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.0.2-beta1 or LibreSSL 2.7.0

        Sets the expected DNS hostname to name clearing any previously specified host name or names.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_set1_host($param, $name);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $name - (string) name to be set
         #
         # returns: 1 on success, 0 on failure

        See also OpenSSL docs, "X509_VERIFY_PARAM_add1_host" and "X509_VERIFY_PARAM_set_hostflags"
        for more information, including wildcard matching.

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_set1_ip

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.0.2-beta1 or LibreSSL 2.7.0

        Sets the expected IP address to ip.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_set1_ip($param, $ip);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $ip - (binary) 4 octet IPv4 or 16 octet IPv6 address
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_set1_ip_asc

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.0.2-beta1 or LibreSSL 2.7.0

        Sets the expected IP address to ipasc.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_set1_asc($param, $ipasc);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $ip - (string) IPv4 or IPv6 address
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_set1_name

        Sets the name of X509_VERIFY_PARAM structure $param to $name.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_set1_name($param, $name);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $name - (string) name to be set
         #
         # returns: 1 on success, 0 on failure

    *   X509_VERIFY_PARAM_set1_policies

        Enables policy checking (it is disabled by default) and sets the acceptable policy set to
        policies. Any existing policy set is cleared. The policies parameter can be 0 to clear an
        existing policy set.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_set1_policies($param, $policies);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $policies - value corresponding to openssl's STACK_OF(ASN1_OBJECT) structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_set_depth

        Sets the maximum verification depth to depth. That is the maximum number of untrusted CA
        certificates that can appear in a chain.

         Net::SSLeay::X509_VERIFY_PARAM_set_depth($param, $depth);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $depth - (integer) depth to be set
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_set_hostflags

        COMPATIBILITY: not available in Net-SSLeay-1.82 and before; requires at least OpenSSL
        1.0.2-beta2 or LibreSSL 2.7.0

         Net::SSLeay::X509_VERIFY_PARAM_set_hostflags($param, $flags);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $flags - (unsigned int) flags to be set (bitmask)
         #
         # returns: no return value

        See also OpenSSL docs, "X509_VERIFY_PARAM_add1_host" and "X509_VERIFY_PARAM_set1_host" for
        more information. The flags for controlling wildcard checks and other features are defined
        in OpenSSL docs.

        Check openssl doc <https://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_set_purpose

        Sets the verification purpose in $param to $purpose. This determines the acceptable purpose
        of the certificate chain, for example SSL client or SSL server.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_set_purpose($param, $purpose);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $purpose - (integer) purpose identifier
         #
         # returns: 1 on success, 0 on failure

        For more details about $purpose identifier check "CTX_set_purpose".

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_set_time

        Sets the verification time in $param to $t. Normally the current time is used.

         Net::SSLeay::X509_VERIFY_PARAM_set_time($param, $t);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $t - (time_t) time in seconds since 1.1.1970
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_set_trust

        Sets the trust setting in $param to $trust.

         my $rv = Net::SSLeay::X509_VERIFY_PARAM_set_trust($param, $trust);
         # $param - value corresponding to openssl's X509_VERIFY_PARAM structure
         # $trust - (integer) trust identifier
         #
         # returns: 1 on success, 0 on failure

        For more details about $trust identifier check "CTX_set_trust".

        Check openssl doc <http://www.openssl.org/docs/crypto/X509_VERIFY_PARAM_set_flags.html>

    *   X509_VERIFY_PARAM_table_cleanup

        ??? (more info needed)

         Net::SSLeay::X509_VERIFY_PARAM_table_cleanup();
         #
         # returns: no return value

   Low level API: Cipher (EVP_CIPHER_*) related functions
    *   EVP_get_cipherbyname

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before

        Returns an EVP_CIPHER structure when passed a cipher name.

         my $rv = Net::SSLeay::EVP_get_cipherbyname($name);
         # $name - (string) cipher name e.g. 'aes-128-cbc', 'camellia-256-ecb', 'des-ede', ...
         #
         # returns: value corresponding to openssl's EVP_CIPHER structure

        Check openssl doc <http://www.openssl.org/docs/crypto/EVP_EncryptInit.html>

   Low level API: Digest (EVP_MD_*) related functions
    *   OpenSSL_add_all_digests

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

         Net::SSLeay::OpenSSL_add_all_digests();
         # no args, no return value

        http://www.openssl.org/docs/crypto/OpenSSL_add_all_algorithms.html

    *   P_EVP_MD_list_all

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-1.0.0

        NOTE: Does not exactly correspond to any low level API function

         my $rv = Net::SSLeay::P_EVP_MD_list_all();
         #
         # returns: arrayref - list of available digest names

        The returned digest names correspond to values expected by "EVP_get_digestbyname".

        Note that some of the digests are available by default and some only after calling
        "OpenSSL_add_all_digests".

    *   EVP_get_digestbyname

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

         my $rv = Net::SSLeay::EVP_get_digestbyname($name);
         # $name - string with digest name
         #
         # returns: value corresponding to openssl's EVP_MD structure

        The $name param can be:

         md2
         md4
         md5
         mdc2
         ripemd160
         sha
         sha1
         sha224
         sha256
         sha512
         whirlpool

        Or better check the supported digests by calling "P_EVP_MD_list_all".

    *   EVP_MD_type

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

         my $rv = Net::SSLeay::EVP_MD_type($md);
         # $md - value corresponding to openssl's EVP_MD structure
         #
         # returns: the NID (integer) of the OBJECT IDENTIFIER representing the given message digest

    *   EVP_MD_size

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

         my $rv = Net::SSLeay::EVP_MD_size($md);
         # $md - value corresponding to openssl's EVP_MD structure
         #
         # returns: the size of the message digest in bytes (e.g. 20 for SHA1)

    *   EVP_MD_CTX_md

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7

         Net::SSLeay::EVP_MD_CTX_md($ctx);
         # $ctx - value corresponding to openssl's EVP_MD_CTX structure
         #
         # returns: value corresponding to openssl's EVP_MD structure

    *   EVP_MD_CTX_create

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7

        Allocates, initializes and returns a digest context.

         my $rv = Net::SSLeay::EVP_MD_CTX_create();
         #
         # returns: value corresponding to openssl's EVP_MD_CTX structure

        The complete idea behind EVP_MD_CTX looks like this example:

          Net::SSLeay::OpenSSL_add_all_digests();

          my $md = Net::SSLeay::EVP_get_digestbyname("sha1");
          my $ctx = Net::SSLeay::EVP_MD_CTX_create();
          Net::SSLeay::EVP_DigestInit($ctx, $md);

          while(my $chunk = get_piece_of_data()) {
            Net::SSLeay::EVP_DigestUpdate($ctx,$chunk);
          }

          my $result = Net::SSLeay::EVP_DigestFinal($ctx);
          Net::SSLeay::EVP_MD_CTX_destroy($ctx);

          print "digest=", unpack('H*', $result), "\n"; #print hex value

    *   EVP_DigestInit_ex

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7

        Sets up digest context $ctx to use a digest $type from ENGINE $impl, $ctx must be
        initialized before calling this function, type will typically be supplied by a function such
        as "EVP_get_digestbyname". If $impl is 0 then the default implementation of digest $type is
        used.

         my $rv = Net::SSLeay::EVP_DigestInit_ex($ctx, $type, $impl);
         # $ctx  - value corresponding to openssl's EVP_MD_CTX structure
         # $type - value corresponding to openssl's EVP_MD structure
         # $impl - value corresponding to openssl's ENGINE structure
         #
         # returns: 1 for success and 0 for failure

    *   EVP_DigestInit

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7

        Behaves in the same way as "EVP_DigestInit_ex" except the passed context $ctx does not have
        to be initialized, and it always uses the default digest implementation.

         my $rv = Net::SSLeay::EVP_DigestInit($ctx, $type);
         # $ctx - value corresponding to openssl's EVP_MD_CTX structure
         # $type - value corresponding to openssl's EVP_MD structure
         #
         # returns: 1 for success and 0 for failure

    *   EVP_MD_CTX_destroy

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7

        Cleans up digest context $ctx and frees up the space allocated to it, it should be called
        only on a context created using "EVP_MD_CTX_create".

         Net::SSLeay::EVP_MD_CTX_destroy($ctx);
         # $ctx - value corresponding to openssl's EVP_MD_CTX structure
         #
         # returns: no return value

    *   EVP_DigestUpdate

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7

         my $rv = Net::SSLeay::EVP_DigestUpdate($ctx, $data);
         # $ctx  - value corresponding to openssl's EVP_MD_CTX structure
         # $data - data to be hashed
         #
         # returns: 1 for success and 0 for failure

    *   EVP_DigestFinal_ex

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7

        Retrieves the digest value from $ctx. After calling "EVP_DigestFinal_ex" no additional calls
        to "EVP_DigestUpdate" can be made, but "EVP_DigestInit_ex" can be called to initialize a new
        digest operation.

         my $digest_value = Net::SSLeay::EVP_DigestFinal_ex($ctx);
         # $ctx - value corresponding to openssl's EVP_MD_CTX structure
         #
         # returns: hash value (binary)

         #to get printable (hex) value of digest use:
         print unpack('H*', $digest_value);

    *   EVP_DigestFinal

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7

        Similar to "EVP_DigestFinal_ex" except the digest context ctx is automatically cleaned up.

         my $rv = Net::SSLeay::EVP_DigestFinal($ctx);
         # $ctx - value corresponding to openssl's EVP_MD_CTX structure
         #
         # returns: hash value (binary)

         #to get printable (hex) value of digest use:
         print unpack('H*', $digest_value);

    *   MD2

        COMPATIBILITY: no supported by default in openssl-1.0.0

        Computes MD2 from given $data (all data needs to be loaded into memory)

         my $digest = Net::SSLeay::MD2($data);
         print "digest(hexadecimal)=", unpack('H*', $digest);

    *   MD4

        Computes MD4 from given $data (all data needs to be loaded into memory)

         my $digest = Net::SSLeay::MD4($data);
         print "digest(hexadecimal)=", unpack('H*', $digest);

    *   MD5

        Computes MD5 from given $data (all data needs to be loaded into memory)

         my $digest = Net::SSLeay::MD5($data);
         print "digest(hexadecimal)=", unpack('H*', $digest);

    *   RIPEMD160

        Computes RIPEMD160 from given $data (all data needs to be loaded into memory)

         my $digest = Net::SSLeay::RIPEMD160($data);
         print "digest(hexadecimal)=", unpack('H*', $digest);

    *   SHA1

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

        Computes SHA1 from given $data (all data needs to be loaded into memory)

         my $digest = Net::SSLeay::SHA1($data);
         print "digest(hexadecimal)=", unpack('H*', $digest);

    *   SHA256

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.8

        Computes SHA256 from given $data (all data needs to be loaded into memory)

         my $digest = Net::SSLeay::SHA256($data);
         print "digest(hexadecimal)=", unpack('H*', $digest);

    *   SHA512

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.8

        Computes SHA512 from given $data (all data needs to be loaded into memory)

         my $digest = Net::SSLeay::SHA512($data);
         print "digest(hexadecimal)=", unpack('H*', $digest);

    *   EVP_Digest

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.7

        Computes "any" digest from given $data (all data needs to be loaded into memory)

         my $md = Net::SSLeay::EVP_get_digestbyname("sha1"); #or any other algorithm
         my $digest = Net::SSLeay::EVP_Digest($data, $md);
         print "digest(hexadecimal)=", unpack('H*', $digest);

    *   EVP_sha1

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

         my $md = Net::SSLeay::EVP_sha1();
         #
         # returns: value corresponding to openssl's EVP_MD structure

    *   EVP_sha256

        COMPATIBILITY: requires at least openssl-0.9.8

         my $md = Net::SSLeay::EVP_sha256();
         #
         # returns: value corresponding to openssl's EVP_MD structure

    *   EVP_sha512

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before; requires at least openssl-0.9.8

         my $md = Net::SSLeay::EVP_sha512();
         #
         # returns: value corresponding to openssl's EVP_MD structure

    *   EVP_add_digest

         my $rv = Net::SSLeay::EVP_add_digest($digest);
         # $digest - value corresponding to openssl's EVP_MD structure
         #
         # returns: 1 on success, 0 otherwise

   Low level API: CIPHER_* related functions
    *   CIPHER_get_name

        COMPATIBILITY: not available in Net-SSLeay-1.42 and before

        Returns name of the cipher used.

         my $rv = Net::SSLeay::CIPHER_get_name($cipher);
         # $cipher - value corresponding to openssl's SSL_CIPHER structure
         #
         # returns: (string) cipher name e.g. 'DHE-RSA-AES256-SHA', '(NONE)' if $cipher is undefined.

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_CIPHER_get_name.html>

        Example:

         my $ssl_cipher = Net::SSLeay::get_current_cipher($ssl);
         my $cipher_name = Net::SSLeay::CIPHER_get_name($ssl_cipher);

    *   CIPHER_description

        COMPATIBILITY: doesn't work correctly in Net-SSLeay-1.88 and before

        Returns a textual description of the cipher used.

         my $rv = Net::SSLeay::CIPHER_description($cipher);
         # $cipher - value corresponding to openssl's SSL_CIPHER structure
         #
         # returns: (string) cipher description e.g. 'DHE-RSA-AES256-SHA SSLv3 Kx=DH Au=RSA Enc=AES(256) Mac=SHA1'

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_CIPHER_description.html>

    *   CIPHER_get_bits

        COMPATIBILITY: $alg_bits doesn't work correctly in Net-SSLeay-1.88 and before

        Returns the number of secret bits used for cipher.

         my $rv = Net::SSLeay::CIPHER_get_bits($cipher, $alg_bits);
         # $cipher - value corresponding to openssl's SSL_CIPHER structure
         # $alg_bits - [optional] empty scalar for storing additional return value
         #
         # returns: (integer) number of secret bits, 0 on error
         #          (integer) in $alg_bits for bits processed by the chosen algorithm

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_CIPHER_get_bits.html>

        Example:

         # bits and alg_bits are not equal for e.g., TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA,
         # RFC 8422 name TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
         my $alg_bits;
         my $bits = Net::SSLeay::CIPHER_get_bits($cipher, $alg_bits);
         #my $bits = Net::SSLeay::CIPHER_get_bits($cipher);
         print "bits: $bits, alg_bits: $alg_bits\n";

    *   CIPHER_get_version

        COMPATIBILITY: not available in Net-SSLeay-1.88 and before

        Returns version of SSL/TLS protocol that first defined the cipher

         my $rv = Net::SSLeay::CIPHER_get_version($cipher);
         # $cipher - value corresponding to openssl's SSL_CIPHER structure
         #
         # returns: (string) cipher name e.g. 'TLSv1/SSLv3' with some libraries, 'TLSv1.0' or 'TLSv1.3', '(NONE)' if $cipher is undefined.

        Check openssl doc <https://www.openssl.org/docs/ssl/SSL_CIPHER_get_version.html>

   Low level API: RSA_* related functions
    *   RSA_generate_key

        Generates a key pair and returns it in a newly allocated RSA structure. The pseudo-random
        number generator must be seeded prior to calling RSA_generate_key.

         my $rv = Net::SSLeay::RSA_generate_key($bits, $e, $perl_cb, $perl_cb_arg);
         # $bits - (integer) modulus size in bits e.g. 512, 1024, 2048
         # $e - (integer) public exponent, an odd number, typically 3, 17 or 65537
         # $perl_cb - [optional] reference to perl callback function
         # $perl_cb_arg - [optional] data that will be passed to callback function when invoked
         #
         # returns: value corresponding to openssl's RSA structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/RSA_generate_key.html>

    *   RSA_free

        Frees the RSA structure and its components. The key is erased before the memory is returned
        to the system.

         Net::SSLeay::RSA_free($r);
         # $r - value corresponding to openssl's RSA structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/RSA_new.html>

    *   RSA_get_key_parameters

        Returns a list of pointers to BIGNUMs representing the parameters of the key in this order:
        (n, e, d, p, q, dmp1, dmq1, iqmp)

        Caution: returned list consists of SV pointers to BIGNUMs, which would need to be blessed as
        Crypt::OpenSSL::Bignum for further use

         my (@params) = RSA_get_key_parameters($r);

   Low level API: BIO_* related functions
    *   BIO_eof

        Returns 1 if the BIO has read EOF, the precise meaning of 'EOF' varies according to the BIO
        type.

         my $rv = Net::SSLeay::BIO_eof($s);
         # $s - value corresponding to openssl's BIO structure
         #
         # returns: 1 if EOF has been reached 0 otherwise

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_ctrl.html>

    *   BIO_f_ssl

        Returns the SSL BIO method. This is a filter BIO which is a wrapper round the OpenSSL SSL
        routines adding a BIO 'flavour' to SSL I/O.

         my $rv = Net::SSLeay::BIO_f_ssl();
         #
         # returns: value corresponding to openssl's BIO_METHOD structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_f_ssl.html>

    *   BIO_free

        Frees up a single BIO.

         my $rv = Net::SSLeay::BIO_free($bio;);
         # $bio; - value corresponding to openssl's BIO structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_new.html>

    *   BIO_new

        Returns a new BIO using method $type

         my $rv = Net::SSLeay::BIO_new($type);
         # $type - value corresponding to openssl's BIO_METHOD structure
         #
         # returns: value corresponding to openssl's BIO structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_new.html>

    *   BIO_new_buffer_ssl_connect

        Creates a new BIO chain consisting of a buffering BIO, an SSL BIO (using ctx) and a connect
        BIO.

         my $rv = Net::SSLeay::BIO_new_buffer_ssl_connect($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: value corresponding to openssl's BIO structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_f_ssl.html>

    *   BIO_new_file

        Creates a new file BIO with mode $mode the meaning of mode is the same as the stdio function
        fopen(). The BIO_CLOSE flag is set on the returned BIO.

         my $rv = Net::SSLeay::BIO_new_file($filename, $mode);
         # $filename - (string) filename
         # $mode - (string) opening mode (as mode by stdio function fopen)
         #
         # returns: value corresponding to openssl's BIO structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_s_file.html>

    *   BIO_new_ssl

        Allocates an SSL BIO using SSL_CTX ctx and using client mode if client is non zero.

         my $rv = Net::SSLeay::BIO_new_ssl($ctx, $client);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $client - (integer) 0 or 1 - indicates ssl client mode
         #
         # returns: value corresponding to openssl's BIO structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_f_ssl.html>

    *   BIO_new_ssl_connect

        Creates a new BIO chain consisting of an SSL BIO (using ctx) followed by a connect BIO.

         my $rv = Net::SSLeay::BIO_new_ssl_connect($ctx);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         #
         # returns: value corresponding to openssl's BIO structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_f_ssl.html>

    *   BIO_pending

        Return the number of pending characters in the BIOs read buffers.

         my $rv = Net::SSLeay::BIO_pending($s);
         # $s - value corresponding to openssl's BIO structure
         #
         # returns: the amount of pending data

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_ctrl.html>

    *   BIO_wpending

        Return the number of pending characters in the BIOs write buffers.

         my $rv = Net::SSLeay::BIO_wpending($s);
         # $s - value corresponding to openssl's BIO structure
         #
         # returns: the amount of pending data

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_ctrl.html>

    *   BIO_read

        Read the underlying descriptor.

         Net::SSLeay::BIO_read($s, $max);
         # $s - value corresponding to openssl's BIO structure
         # $max - [optional] max. bytes to read (if not specified, the value 32768 is used)
         #
         # returns: data

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_read.html>

    *   BIO_write

        Attempts to write data from $buffer to BIO $b.

         my $rv = Net::SSLeay::BIO_write($b, $buffer);
         # $b - value corresponding to openssl's BIO structure
         # $buffer - data
         #
         # returns: amount of data successfully written
         #          or that no data was successfully read or written if the result is 0 or -1
         #          or -2 when the operation is not implemented in the specific BIO type

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_read.html>

    *   BIO_s_mem

        Return the memory BIO method function.

         my $rv = Net::SSLeay::BIO_s_mem();
         #
         # returns: value corresponding to openssl's BIO_METHOD structure (0 on failure)

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_s_mem.html>

    *   BIO_ssl_copy_session_id

        Copies an SSL session id between BIO chains from and to. It does this by locating the SSL
        BIOs in each chain and calling SSL_copy_session_id() on the internal SSL pointer.

         my $rv = Net::SSLeay::BIO_ssl_copy_session_id($to, $from);
         # $to - value corresponding to openssl's BIO structure
         # $from - value corresponding to openssl's BIO structure
         #
         # returns: 1 on success, 0 on failure

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_f_ssl.html>

    *   BIO_ssl_shutdown

        Closes down an SSL connection on BIO chain bio. It does this by locating the SSL BIO in the
        chain and calling SSL_shutdown() on its internal SSL pointer.

         Net::SSLeay::BIO_ssl_shutdown($ssl_bio);
         # $ssl_bio - value corresponding to openssl's BIO structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/BIO_f_ssl.html>

   Low level API: Server side Server Name Indication (SNI) support
    *   set_tlsext_host_name

        TBA

    *   get_servername

        TBA

    *   get_servername_type

        TBA

    *   CTX_set_tlsext_servername_callback

        COMPATIBILITY: requires at least OpenSSL 0.9.8f

        This function is used in a server to support Server side Server Name Indication (SNI).

         Net::SSLeay::CTX_set_tlsext_servername_callback($ctx, $code)
         # $ctx - SSL context
         # $code - reference to a subroutine that will be called when a new connection is being initiated
         #
         # returns: no return value

        On the client side: use set_tlsext_host_name($ssl, $servername) before initiating the SSL
        connection.

        On the server side: Set up an additional SSL_CTX() for each different certificate;

        Add a servername callback to each SSL_CTX() using CTX_set_tlsext_servername_callback();

        The callback function is required to retrieve the client-supplied servername with
        get_servername(ssl). Figure out the right SSL_CTX to go with that host name, then switch the
        SSL object to that SSL_CTX with set_SSL_CTX().

        Example:

         # set callback
         Net::SSLeay::CTX_set_tlsext_servername_callback($ctx,
            sub {
              my $ssl = shift;
              my $h = Net::SSLeay::get_servername($ssl);
              Net::SSLeay::set_SSL_CTX($ssl, $hostnames{$h}->{ctx}) if exists $hostnames{$h};
            } );

        More complete example:

         # ... initialize Net::SSLeay

         my %hostnames = (
           'sni1' => { cert=>'sni1.pem', key=>'sni1.key' },
           'sni2' => { cert=>'sni2.pem', key=>'sni2.key' },
         );

         # create a new context for each certificate/key pair
         for my $name (keys %hostnames) {
           $hostnames{$name}->{ctx} = Net::SSLeay::CTX_new or die;
           Net::SSLeay::CTX_set_cipher_list($hostnames{$name}->{ctx}, 'ALL');
           Net::SSLeay::set_cert_and_key($hostnames{$name}->{ctx},
           $hostnames{$name}->{cert}, $hostnames{$name}->{key}) or die;
         }

         # create default context
         my $ctx = Net::SSLeay::CTX_new or die;
         Net::SSLeay::CTX_set_cipher_list($ctx, 'ALL');
         Net::SSLeay::set_cert_and_key($ctx, 'cert.pem','key.pem') or die;

         # set callback
         Net::SSLeay::CTX_set_tlsext_servername_callback($ctx, sub {
           my $ssl = shift;
           my $h = Net::SSLeay::get_servername($ssl);
           Net::SSLeay::set_SSL_CTX($ssl, $hostnames{$h}->{ctx}) if exists $hostnames{$h};
           } );

         # ... later

         $s = Net::SSLeay::new($ctx);
         Net::SSLeay::set_fd($s, fileno($accepted_socket));
         Net::SSLeay::accept($s);

   Low level API: NPN (next protocol negotiation) related functions
    NPN is being replaced with ALPN, a more recent TLS extension for application protocol
    negotiation that's in process of being adopted by IETF. Please look below for APLN API
    description.

    Simple approach for using NPN support looks like this:

     ### client side
     use Net::SSLeay;
     use IO::Socket::INET;

     Net::SSLeay::initialize();
     my $sock = IO::Socket::INET->new(PeerAddr=>'encrypted.google.com:443') or die;
     my $ctx = Net::SSLeay::CTX_tlsv1_new() or die;
     Net::SSLeay::CTX_set_options($ctx, &Net::SSLeay::OP_ALL);
     Net::SSLeay::CTX_set_next_proto_select_cb($ctx, ['http1.1','spdy/2']);
     my $ssl = Net::SSLeay::new($ctx) or die;
     Net::SSLeay::set_fd($ssl, fileno($sock)) or die;
     Net::SSLeay::connect($ssl);

     warn "client:negotiated=",Net::SSLeay::P_next_proto_negotiated($ssl), "\n";
     warn "client:last_status=", Net::SSLeay::P_next_proto_last_status($ssl), "\n";

     ### server side
     use Net::SSLeay;
     use IO::Socket::INET;

     Net::SSLeay::initialize();
     my $ctx = Net::SSLeay::CTX_tlsv1_new() or die;
     Net::SSLeay::CTX_set_options($ctx, &Net::SSLeay::OP_ALL);
     Net::SSLeay::set_cert_and_key($ctx, "cert.pem", "key.pem");
     Net::SSLeay::CTX_set_next_protos_advertised_cb($ctx, ['spdy/2','http1.1']);
     my $sock = IO::Socket::INET->new(LocalAddr=>'localhost', LocalPort=>5443, Proto=>'tcp', Listen=>20) or die;

     while (1) {
       my $ssl = Net::SSLeay::new($ctx);
       warn("server:waiting for incoming connection...\n");
       my $fd = $sock->accept();
       Net::SSLeay::set_fd($ssl, $fd->fileno);
       Net::SSLeay::accept($ssl);
       warn "server:negotiated=",Net::SSLeay::P_next_proto_negotiated($ssl),"\n";
       my $got = Net::SSLeay::read($ssl);
       Net::SSLeay::ssl_write_all($ssl, "length=".length($got));
       Net::SSLeay::free($ssl);
       $fd->close();
     }
     # check with: openssl s_client -connect localhost:5443 -nextprotoneg http/1.1,spdy/2

    Please note that the selection (negotiation) is performed by client side, the server side simply
    advertise the list of supported protocols.

    Advanced approach allows you to implement your own negotiation algorithm.

     #see below documentation for:
     Net::SSleay::CTX_set_next_proto_select_cb($ctx, $perl_callback_function, $callback_data);
     Net::SSleay::CTX_set_next_protos_advertised_cb($ctx, $perl_callback_function, $callback_data);

    Detection of NPN support (works even in older Net::SSLeay versions):

     use Net::SSLeay;

     if (exists &Net::SSLeay::P_next_proto_negotiated) {
       # do NPN stuff
     }

    *   CTX_set_next_proto_select_cb

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-1.0.1

        NOTE: You need CTX_set_next_proto_select_cb on client side of SSL connection.

        Simple usage - in this case a "common" negotiation algorithm (as implemented by openssl's
        function SSL_select_next_proto) is used.

         $rv = Net::SSleay::CTX_set_next_proto_select_cb($ctx, $arrayref);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $arrayref - list of accepted protocols - e.g. ['http1.0', 'http1.1']
         #
         # returns: 0 on success, 1 on failure

        Advanced usage (you probably do not need this):

         $rv = Net::SSleay::CTX_set_next_proto_select_cb($ctx, $perl_callback_function, $callback_data);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $perl_callback_function - reference to perl function
         # $callback_data - [optional] data to passed to callback function when invoked
         #
         # returns: 0 on success, 1 on failure

         # where callback function looks like
         sub npn_advertised_cb_invoke {
           my ($ssl, $arrayref_proto_list_advertised_by_server, $callback_data) = @_;
           my $status;
           # ...
           $status = 1;   #status can be:
                          # 0 - OPENSSL_NPN_UNSUPPORTED
                          # 1 - OPENSSL_NPN_NEGOTIATED
                          # 2 - OPENSSL_NPN_NO_OVERLAP
           return $status, ['http1.1','spdy/2']; # the callback has to return 2 values
         }

        To undefine/clear this callback use:

         Net::SSleay::CTX_set_next_proto_select_cb($ctx, undef);

    *   CTX_set_next_protos_advertised_cb

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-1.0.1

        NOTE: You need CTX_set_next_proto_select_cb on server side of SSL connection.

        Simple usage:

         $rv = Net::SSleay::CTX_set_next_protos_advertised_cb($ctx, $arrayref);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $arrayref - list of advertised protocols - e.g. ['http1.0', 'http1.1']
         #
         # returns: 0 on success, 1 on failure

        Advanced usage (you probably do not need this):

         $rv = Net::SSleay::CTX_set_next_protos_advertised_cb($ctx, $perl_callback_function, $callback_data);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $perl_callback_function - reference to perl function
         # $callback_data - [optional] data to passed to callback function when invoked
         #
         # returns: 0 on success, 1 on failure

         # where callback function looks like
         sub npn_advertised_cb_invoke {
           my ($ssl, $callback_data) = @_;
           # ...
           return ['http1.1','spdy/2']; # the callback has to return arrayref
         }

        To undefine/clear this callback use:

         Net::SSleay::CTX_set_next_protos_advertised_cb($ctx, undef);

    *   P_next_proto_negotiated

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-1.0.1

        Returns the name of negotiated protocol for given SSL connection $ssl.

         $rv = Net::SSLeay::P_next_proto_negotiated($ssl)
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (string) negotiated protocol name (or undef if no negotiation was done or failed with fatal error)

    *   P_next_proto_last_status

        COMPATIBILITY: not available in Net-SSLeay-1.45 and before; requires at least openssl-1.0.1

        Returns the result of the last negotiation for given SSL connection $ssl.

         $rv = Net::SSLeay::P_next_proto_last_status($ssl)
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (integer) negotiation status
         #          0 - OPENSSL_NPN_UNSUPPORTED
         #          1 - OPENSSL_NPN_NEGOTIATED
         #          2 - OPENSSL_NPN_NO_OVERLAP

   Low level API: ALPN (application layer protocol negotiation) related functions
    Application protocol can be negotiated via two different mechanisms employing two different TLS
    extensions: NPN (obsolete) and ALPN (recommended).

    The API is rather similar, with slight differences reflecting protocol specifics. In particular,
    with ALPN the protocol negotiation takes place on server, while with NPN the client implements
    the protocol negotiation logic.

    With ALPN, the most basic implementation looks like this:

     ### client side
     use Net::SSLeay;
     use IO::Socket::INET;

     Net::SSLeay::initialize();
     my $sock = IO::Socket::INET->new(PeerAddr=>'encrypted.google.com:443') or die;
     my $ctx = Net::SSLeay::CTX_tlsv1_new() or die;
     Net::SSLeay::CTX_set_options($ctx, &Net::SSLeay::OP_ALL);
     Net::SSLeay::CTX_set_alpn_protos($ctx, ['http/1.1', 'http/2.0', 'spdy/3]);
     my $ssl = Net::SSLeay::new($ctx) or die;
     Net::SSLeay::set_fd($ssl, fileno($sock)) or die;
     Net::SSLeay::connect($ssl);

     warn "client:selected=",Net::SSLeay::P_alpn_selected($ssl), "\n";

     ### server side
     use Net::SSLeay;
     use IO::Socket::INET;

     Net::SSLeay::initialize();
     my $ctx = Net::SSLeay::CTX_tlsv1_new() or die;
     Net::SSLeay::CTX_set_options($ctx, &Net::SSLeay::OP_ALL);
     Net::SSLeay::set_cert_and_key($ctx, "cert.pem", "key.pem");
     Net::SSLeay::CTX_set_alpn_select_cb($ctx, ['http/1.1', 'http/2.0', 'spdy/3]);
     my $sock = IO::Socket::INET->new(LocalAddr=>'localhost', LocalPort=>5443, Proto=>'tcp', Listen=>20) or die;

     while (1) {
       my $ssl = Net::SSLeay::new($ctx);
       warn("server:waiting for incoming connection...\n");
       my $fd = $sock->accept();
       Net::SSLeay::set_fd($ssl, $fd->fileno);
       Net::SSLeay::accept($ssl);
       warn "server:selected=",Net::SSLeay::P_alpn_selected($ssl),"\n";
       my $got = Net::SSLeay::read($ssl);
       Net::SSLeay::ssl_write_all($ssl, "length=".length($got));
       Net::SSLeay::free($ssl);
       $fd->close();
     }
     # check with: openssl s_client -connect localhost:5443 -alpn spdy/3,http/1.1

    Advanced approach allows you to implement your own negotiation algorithm.

     #see below documentation for:
     Net::SSleay::CTX_set_alpn_select_cb($ctx, $perl_callback_function, $callback_data);

    Detection of ALPN support (works even in older Net::SSLeay versions):

     use Net::SSLeay;

     if (exists &Net::SSLeay::P_alpn_selected) {
       # do ALPN stuff
     }

    *   CTX_set_alpn_select_cb

        COMPATIBILITY: not available in Net-SSLeay-1.55 and before; requires at least openssl-1.0.2

        NOTE: You need CTX_set_alpn_select_cb on server side of TLS connection.

        Simple usage - in this case a "common" negotiation algorithm (as implemented by openssl's
        function SSL_select_next_proto) is used.

         $rv = Net::SSleay::CTX_set_alpn_select_cb($ctx, $arrayref);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $arrayref - list of accepted protocols - e.g. ['http/2.0', 'http/1.1', 'spdy/3']
         #
         # returns: 0 on success, 1 on failure

        Advanced usage (you probably do not need this):

         $rv = Net::SSleay::CTX_set_alpn_select_cb($ctx, $perl_callback_function, $callback_data);
         # $ctx - value corresponding to openssl's SSL_CTX structure
         # $perl_callback_function - reference to perl function
         # $callback_data - [optional] data to passed to callback function when invoked
         #
         # returns: 0 on success, 1 on failure

         # where callback function looks like
         sub alpn_select_cb_invoke {
           my ($ssl, $arrayref_proto_list_advertised_by_client, $callback_data) = @_;
           # ...
           if ($negotiated) {
             return 'http/2.0';
           } else {
             return undef;
           }
         }

        To undefine/clear this callback use:

         Net::SSleay::CTX_set_alpn_select_cb($ctx, undef);

    *   set_alpn_protos

        COMPATIBILITY: not available in Net-SSLeay-1.55 and before; requires at least openssl-1.0.2

        NOTE: You need set_alpn_protos on client side of TLS connection.

        This adds list of supported application layer protocols to ClientHello message sent by a
        client. It advertises the enumeration of supported protocols:

         Net::SSLeay::set_alpn_protos($ssl, ['http/1.1', 'http/2.0', 'spdy/3]);
         # returns 0 on success

    *   CTX_set_alpn_protos

        COMPATIBILITY: not available in Net-SSLeay-1.55 and before; requires at least openssl-1.0.2

        NOTE: You need CTX_set_alpn_protos on client side of TLS connection.

        This adds list of supported application layer protocols to ClientHello message sent by a
        client. It advertises the enumeration of supported protocols:

         Net::SSLeay::CTX_set_alpn_protos($ctx, ['http/1.1', 'http/2.0', 'spdy/3]);
         # returns 0 on success

    *   P_alpn_selected

        COMPATIBILITY: not available in Net-SSLeay-1.55 and before; requires at least openssl-1.0.2

        Returns the name of negotiated protocol for given TLS connection $ssl.

         $rv = Net::SSLeay::P_alpn_selected($ssl)
         # $ssl - value corresponding to openssl's SSL structure
         #
         # returns: (string) negotiated protocol name (or undef if no negotiation was done or failed with fatal error)

   Low level API: DANE Support
    OpenSSL version 1.0.2 adds preliminary support RFC6698 Domain Authentication of Named Entities
    (DANE) Transport Layer Association within OpenSSL

    *   SSL_get_tlsa_record_byname

        COMPATIBILITY: DELETED from net-ssleay, since it is not supported by OpenSSL

        In order to facilitate DANE there is additional interface, SSL_get_tlsa_record_byname,
        accepting hostname, port and socket type that returns packed TLSA record. In order to make
        it even easier there is additional SSL_ctrl function that calls SSL_get_tlsa_record_byname
        for you. Latter is recommended for programmers that wish to maintain broader binary
        compatibility, e.g. make application work with both 1.0.2 and prior version (in which case
        call to SSL_ctrl with new code returning error would have to be ignored when running with
        prior version).

         Net::SSLeay::get_tlsa_record_byname($name, $port, $type);

   Low level API: Other functions
    *   COMP_add_compression_method

        Adds the compression method cm with the identifier id to the list of available compression
        methods. This list is globally maintained for all SSL operations within this application. It
        cannot be set for specific SSL_CTX or SSL objects.

         my $rv = Net::SSLeay::COMP_add_compression_method($id, $cm);
         # $id - (integer) compression method id
         #       0 to 63:    methods defined by the IETF
         #       64 to 192:  external party methods assigned by IANA
         #       193 to 255: reserved for private use
         #
         # $cm - value corresponding to openssl's COMP_METHOD structure
         #
         # returns: 0 on success, 1 on failure (check the error queue to find out the reason)

        Check openssl doc <http://www.openssl.org/docs/ssl/SSL_COMP_add_compression_method.html>

    *   DH_free

        Frees the DH structure and its components. The values are erased before the memory is
        returned to the system.

         Net::SSLeay::DH_free($dh);
         # $dh - value corresponding to openssl's DH structure
         #
         # returns: no return value

        Check openssl doc <http://www.openssl.org/docs/crypto/DH_new.html>

    *   FIPS_mode_set

        Enable or disable FIPS mode in a FIPS capable OpenSSL.

         Net::SSLeay:: FIPS_mode_set($enable);
         # $enable - (integer) 1 to enable, 0 to disable

   Low level API: EC related functions
    *   CTX_set_tmp_ecdh

        TBA

    *   EC_KEY_free

        TBA

    *   EC_KEY_new_by_curve_name

        TBA

    *   EC_KEY_generate_key

        Generates a EC key and returns it in a newly allocated EC_KEY structure. The EC key then can
        be used to create a PKEY which can be used in calls like X509_set_pubkey.

         my $key = Net::SSLeay::EVP_PKEY_new();
         my $ec  = Net::SSLeay::EC_KEY_generate_key($curve);
         Net::SSLeay::EVP_PKEY_assign_EC_KEY($key,$ec);

         # $curve - curve name like 'secp521r1' or the matching Id (integer) of the curve
         #
         # returns: value corresponding to openssl's EC_KEY structure (0 on failure)

        This function has no equivalent in OpenSSL but combines multiple OpenSSL functions for an
        easier interface.

    *   CTX_set_ecdh_auto, set_ecdh_auto

        These functions enable or disable the automatic curve selection on the server side by
        calling SSL_CTX_set_ecdh_auto or SSL_set_ecdh_auto respectively. If enabled the highest
        preference curve is automatically used for ECDH temporary keys used during key exchange.
        This function is no longer available for OpenSSL 1.1.0 or higher.

          Net::SSLeay::CTX_set_ecdh_auto($ctx,1);
          Net::SSLeay::set_ecdh_auto($ssl,1);

    *   CTX_set1_curves_list, set1_curves_list

        These functions set the supported curves (in order of preference) by calling
        SSL_CTX_set1_curves_list or SSL_set1_curves_list respectively. For a TLS client these curves
        are offered to the server in the supported curves extension while on the server side these
        are used to determine the shared curve. These functions are only available since OpenSSL
        1.1.0.

          Net::SSLeay::CTX_set1_curves_list($ctx,"P-521:P-384:P-256");
          Net::SSLeay::set1_curves_list($ssl,"P-521:P-384:P-256");

    *   CTX_set1_groups_list, set1_groups_list

        These functions set the supported groups (in order of preference) by calling
        SSL_CTX_set1_groups_list or SSL_set1_groups_list respectively. This is practically the same
        as CTX_set1_curves_list and set1_curves_list except that all DH groups can be given as
        supported by TLS 1.3. These functions are only available since OpenSSL 1.1.1.

          Net::SSLeay::CTX_set1_groups_list($ctx,"P-521:P-384:P-256");
          Net::SSLeay::set1_groups_list($ssl,"P-521:P-384:P-256");

   Low level API: OSSL_LIB_CTX and OSSL_PROVIDER related functions
    *   OSSL_LIB_CTX_get0_global_default

        Returns a concrete (non NULL) reference to the global default library context.

         my $libctx = Net::SSLeay::OSSL_LIB_CTX_get0_global_default();
         # returns: a value corresponding to OSSL_LIB_CTX structure or false on failure

        Typically it's simpler to use undef with functions that take an OSSL_LIB_CTX argument when
        global default library context is needed.

        Check openssl doc
        <https://www.openssl.org/docs/manmaster/man3/OSSL_LIB_CTX_get0_global_default.html>

    *   OSSL_PROVIDER_load

        Loads and initializes a provider

         my $provider = Net::SSLeay::OSSL_PROVIDER_load($libctx, $name);
         # $libctx - value corresponding to OSSL_LIB_CTX structure or undef
         # $name - (string) provider name, e.g., 'legacy'
         #
         # returns: a value corresponding to OSSL_PROVIDER or false on failure

        Using undef loads the provider within the global default library context.

         my $provider = Net::SSLeay::OSSL_PROVIDER_load(undef, 'legacy');

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OSSL_PROVIDER_load.html>

    *   OSSL_PROVIDER_try_load

        Loads and initializes a provider similar to OSSL_PROVIDER_load with additional fallback
        control.

         my $provider = Net::SSLeay::OSSL_PROVIDER_try_load($libctx, $name, $retain_fallbacks);
         # $libctx - value corresponding to OSSL_LIB_CTX structure or undef
         # $name - (string) provider name, e.g., 'legacy'
         # $retain_fallbacks - (integer) 0 or 1
         #
         # returns: a value corresponding to OSSL_PROVIDER or false on failure

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OSSL_PROVIDER_try_load.html>

    *   OSSL_PROVIDER_unload

        Unloads the given provider.

         my $rv = Net::SSLeay::OSSL_PROVIDER_unload($provider);
         # $provider - a value corresponding to OSSL_PROVIDER
         #
         # returns: (integer) 1 on success, 0 on error

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OSSL_PROVIDER_unload.html>

    *   OSSL_PROVIDER_available

        Checks if a named provider is available for use.

         my $rv = Net::SSLeay::OSSL_PROVIDER_available($libctx, $name);
         # $libctx - value corresponding to OSSL_LIB_CTX structure or undef
         # $name - (string) provider name, e.g., 'legacy'
         #
         # returns: (integer) 1 if the named provider is available, otherwise 0.

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OSSL_PROVIDER_available.html>

    *   OSSL_PROVIDER_do_all

        Iterates over all loaded providers. A callback is called for each provider.

         my $rv = Net::SSLeay::OSSL_PROVIDER_do_all($libctx, $cb, $cbdata);
         # $libctx - value corresponding to OSSL_LIB_CTX structure or undef
         # $cb - reference to a perl callback function
         $ $cbdata - data that will be passed to callback function
         #
         # returns: (integer) 1 if all callbacks returned 1, 0 the first time a callback returns 0.

        Example:

         sub do_all_cb {
             my ($provider, $cbdata) = @_;

             my $name = Net::SSLeay::OSSL_PROVIDER_get0_name($provider);
             print "Callback for provider: '$name', cbdata: '$cbdata'\n";
             return 1;
          }
          my $data_for_cb = 'Hello';

          # Triggers default provider automatic loading.
          Net::SSLeay::OSSL_PROVIDER_available(undef, 'default') || die 'default provider not available';
          Net::SSLeay::OSSL_PROVIDER_load(undef, 'legacy') || die 'load legacy';
          Net::SSLeay::OSSL_PROVIDER_load(undef, 'null')   || die 'load null';
          Net::SSLeay::OSSL_PROVIDER_do_all(undef, \&do_all_cb, $data_for_cb) || die 'a callback failed';

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OSSL_PROVIDER_do_all.html>

    *   OSSL_PROVIDER_get0_name

        Returns the name of the given provider.

         my $name = Net::SSLeay::OSSL_PROVIDER_get0_name($provider);
         # $provider - a value corresponding to OSSL_PROVIDER
         #
         # returns: (string) provider name, e.g., 'legacy'

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OSSL_PROVIDER_get0_name.html>

    *   OSSL_PROVIDER_self_test

        Runs the provider's self tests.

         my $rv = Net::SSLeay::OSSL_PROVIDER_self_test($provider);
         # $libctx - value corresponding to OSSL_LIB_CTX structure or undef
         # $provider - a value corresponding to OSSL_PROVIDER
         #
         # returns: (integer) returns 1 if the self tests pass, 0 on error

        Check openssl doc <https://www.openssl.org/docs/manmaster/man3/OSSL_PROVIDER_self_test.html>

  Constants
    There are many openssl constants available in Net::SSLeay. You can use them like this:

     use Net::SSLeay;
     print &Net::SSLeay::NID_commonName;
     #or
     print Net::SSLeay::NID_commonName();

    Or you can import them and use:

     use Net::SSLeay qw/NID_commonName/;
     print &NID_commonName;
     #or
     print NID_commonName();
     #or
     print NID_commonName;

    The constants names are derived from openssl constants, however constants starting with "SSL_"
    prefix have name with "SSL_" part stripped - e.g. openssl's constant "SSL_OP_ALL" is available
    as "Net::SSleay::OP_ALL"

    The list of all available constant names:

        ASN1_STRFLGS_ESC_CTRL                   OPENSSL_VERSION_STRING
        ASN1_STRFLGS_ESC_MSB                    OP_ALL
        ASN1_STRFLGS_ESC_QUOTE                  OP_ALLOW_NO_DHE_KEX
        ASN1_STRFLGS_RFC2253                    OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
        CB_ACCEPT_EXIT                          OP_CIPHER_SERVER_PREFERENCE
        CB_ACCEPT_LOOP                          OP_CISCO_ANYCONNECT
        CB_ALERT                                OP_COOKIE_EXCHANGE
        CB_CONNECT_EXIT                         OP_CRYPTOPRO_TLSEXT_BUG
        CB_CONNECT_LOOP                         OP_DONT_INSERT_EMPTY_FRAGMENTS
        CB_EXIT                                 OP_ENABLE_MIDDLEBOX_COMPAT
        CB_HANDSHAKE_DONE                       OP_EPHEMERAL_RSA
        CB_HANDSHAKE_START                      OP_LEGACY_SERVER_CONNECT
        CB_LOOP                                 OP_MICROSOFT_BIG_SSLV3_BUFFER
        CB_READ                                 OP_MICROSOFT_SESS_ID_BUG
        CB_READ_ALERT                           OP_MSIE_SSLV2_RSA_PADDING
        CB_WRITE                                OP_NETSCAPE_CA_DN_BUG
        CB_WRITE_ALERT                          OP_NETSCAPE_CHALLENGE_BUG
        ERROR_NONE                              OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
        ERROR_SSL                               OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
        ERROR_SYSCALL                           OP_NON_EXPORT_FIRST
        ERROR_WANT_ACCEPT                       OP_NO_ANTI_REPLAY
        ERROR_WANT_CONNECT                      OP_NO_CLIENT_RENEGOTIATION
        ERROR_WANT_READ                         OP_NO_COMPRESSION
        ERROR_WANT_WRITE                        OP_NO_ENCRYPT_THEN_MAC
        ERROR_WANT_X509_LOOKUP                  OP_NO_QUERY_MTU
        ERROR_ZERO_RETURN                       OP_NO_RENEGOTIATION
        EVP_PKS_DSA                             OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
        EVP_PKS_EC                              OP_NO_SSL_MASK
        EVP_PKS_RSA                             OP_NO_SSLv2
        EVP_PKT_ENC                             OP_NO_SSLv3
        EVP_PKT_EXCH                            OP_NO_TICKET
        EVP_PKT_EXP                             OP_NO_TLSv1
        EVP_PKT_SIGN                            OP_NO_TLSv1_1
        EVP_PK_DH                               OP_NO_TLSv1_2
        EVP_PK_DSA                              OP_NO_TLSv1_3
        EVP_PK_EC                               OP_PKCS1_CHECK_1
        EVP_PK_RSA                              OP_PKCS1_CHECK_2
        FILETYPE_ASN1                           OP_PRIORITIZE_CHACHA
        FILETYPE_PEM                            OP_SAFARI_ECDHE_ECDSA_BUG
        F_CLIENT_CERTIFICATE                    OP_SINGLE_DH_USE
        F_CLIENT_HELLO                          OP_SINGLE_ECDH_USE
        F_CLIENT_MASTER_KEY                     OP_SSLEAY_080_CLIENT_DH_BUG
        F_D2I_SSL_SESSION                       OP_SSLREF2_REUSE_CERT_TYPE_BUG
        F_GET_CLIENT_FINISHED                   OP_TLSEXT_PADDING
        F_GET_CLIENT_HELLO                      OP_TLS_BLOCK_PADDING_BUG
        F_GET_CLIENT_MASTER_KEY                 OP_TLS_D5_BUG
        F_GET_SERVER_FINISHED                   OP_TLS_ROLLBACK_BUG
        F_GET_SERVER_HELLO                      READING
        F_GET_SERVER_VERIFY                     RECEIVED_SHUTDOWN
        F_I2D_SSL_SESSION                       RSA_3
        F_READ_N                                RSA_F4
        F_REQUEST_CERTIFICATE                   R_BAD_AUTHENTICATION_TYPE
        F_SERVER_HELLO                          R_BAD_CHECKSUM
        F_SSL_CERT_NEW                          R_BAD_MAC_DECODE
        F_SSL_GET_NEW_SESSION                   R_BAD_RESPONSE_ARGUMENT
        F_SSL_NEW                               R_BAD_SSL_FILETYPE
        F_SSL_READ                              R_BAD_SSL_SESSION_ID_LENGTH
        F_SSL_RSA_PRIVATE_DECRYPT               R_BAD_STATE
        F_SSL_RSA_PUBLIC_ENCRYPT                R_BAD_WRITE_RETRY
        F_SSL_SESSION_NEW                       R_CHALLENGE_IS_DIFFERENT
        F_SSL_SESSION_PRINT_FP                  R_CIPHER_TABLE_SRC_ERROR
        F_SSL_SET_FD                            R_INVALID_CHALLENGE_LENGTH
        F_SSL_SET_RFD                           R_NO_CERTIFICATE_SET
        F_SSL_SET_WFD                           R_NO_CERTIFICATE_SPECIFIED
        F_SSL_USE_CERTIFICATE                   R_NO_CIPHER_LIST
        F_SSL_USE_CERTIFICATE_ASN1              R_NO_CIPHER_MATCH
        F_SSL_USE_CERTIFICATE_FILE              R_NO_PRIVATEKEY
        F_SSL_USE_PRIVATEKEY                    R_NO_PUBLICKEY
        F_SSL_USE_PRIVATEKEY_ASN1               R_NULL_SSL_CTX
        F_SSL_USE_PRIVATEKEY_FILE               R_PEER_DID_NOT_RETURN_A_CERTIFICATE
        F_SSL_USE_RSAPRIVATEKEY                 R_PEER_ERROR
        F_SSL_USE_RSAPRIVATEKEY_ASN1            R_PEER_ERROR_CERTIFICATE
        F_SSL_USE_RSAPRIVATEKEY_FILE            R_PEER_ERROR_NO_CIPHER
        F_WRITE_PENDING                         R_PEER_ERROR_UNSUPPORTED_CERTIFICATE_TYPE
        GEN_DIRNAME                             R_PUBLIC_KEY_ENCRYPT_ERROR
        GEN_DNS                                 R_PUBLIC_KEY_IS_NOT_RSA
        GEN_EDIPARTY                            R_READ_WRONG_PACKET_TYPE
        GEN_EMAIL                               R_SHORT_READ
        GEN_IPADD                               R_SSL_SESSION_ID_IS_DIFFERENT
        GEN_OTHERNAME                           R_UNABLE_TO_EXTRACT_PUBLIC_KEY
        GEN_RID                                 R_UNKNOWN_REMOTE_ERROR_TYPE
        GEN_URI                                 R_UNKNOWN_STATE
        GEN_X400                                R_X509_LIB
        LIBRESSL_VERSION_NUMBER                 SENT_SHUTDOWN
        MBSTRING_ASC                            SESSION_ASN1_VERSION
        MBSTRING_BMP                            SESS_CACHE_BOTH
        MBSTRING_FLAG                           SESS_CACHE_CLIENT
        MBSTRING_UNIV                           SESS_CACHE_NO_AUTO_CLEAR
        MBSTRING_UTF8                           SESS_CACHE_NO_INTERNAL
        MIN_RSA_MODULUS_LENGTH_IN_BYTES         SESS_CACHE_NO_INTERNAL_LOOKUP
        MODE_ACCEPT_MOVING_WRITE_BUFFER         SESS_CACHE_NO_INTERNAL_STORE
        MODE_AUTO_RETRY                         SESS_CACHE_OFF
        MODE_ENABLE_PARTIAL_WRITE               SESS_CACHE_SERVER
        MODE_RELEASE_BUFFERS                    SSL2_MT_CLIENT_CERTIFICATE
        NID_OCSP_sign                           SSL2_MT_CLIENT_FINISHED
        NID_SMIMECapabilities                   SSL2_MT_CLIENT_HELLO
        NID_X500                                SSL2_MT_CLIENT_MASTER_KEY
        NID_X509                                SSL2_MT_ERROR
        NID_ad_OCSP                             SSL2_MT_REQUEST_CERTIFICATE
        NID_ad_ca_issuers                       SSL2_MT_SERVER_FINISHED
        NID_algorithm                           SSL2_MT_SERVER_HELLO
        NID_authority_key_identifier            SSL2_MT_SERVER_VERIFY
        NID_basic_constraints                   SSL2_VERSION
        NID_bf_cbc                              SSL3_MT_CCS
        NID_bf_cfb64                            SSL3_MT_CERTIFICATE
        NID_bf_ecb                              SSL3_MT_CERTIFICATE_REQUEST
        NID_bf_ofb64                            SSL3_MT_CERTIFICATE_STATUS
        NID_cast5_cbc                           SSL3_MT_CERTIFICATE_URL
        NID_cast5_cfb64                         SSL3_MT_CERTIFICATE_VERIFY
        NID_cast5_ecb                           SSL3_MT_CHANGE_CIPHER_SPEC
        NID_cast5_ofb64                         SSL3_MT_CLIENT_HELLO
        NID_certBag                             SSL3_MT_CLIENT_KEY_EXCHANGE
        NID_certificate_policies                SSL3_MT_ENCRYPTED_EXTENSIONS
        NID_client_auth                         SSL3_MT_END_OF_EARLY_DATA
        NID_code_sign                           SSL3_MT_FINISHED
        NID_commonName                          SSL3_MT_HELLO_REQUEST
        NID_countryName                         SSL3_MT_KEY_UPDATE
        NID_crlBag                              SSL3_MT_MESSAGE_HASH
        NID_crl_distribution_points             SSL3_MT_NEWSESSION_TICKET
        NID_crl_number                          SSL3_MT_NEXT_PROTO
        NID_crl_reason                          SSL3_MT_SERVER_DONE
        NID_delta_crl                           SSL3_MT_SERVER_HELLO
        NID_des_cbc                             SSL3_MT_SERVER_KEY_EXCHANGE
        NID_des_cfb64                           SSL3_MT_SUPPLEMENTAL_DATA
        NID_des_ecb                             SSL3_RT_ALERT
        NID_des_ede                             SSL3_RT_APPLICATION_DATA
        NID_des_ede3                            SSL3_RT_CHANGE_CIPHER_SPEC
        NID_des_ede3_cbc                        SSL3_RT_HANDSHAKE
        NID_des_ede3_cfb64                      SSL3_RT_HEADER
        NID_des_ede3_ofb64                      SSL3_RT_INNER_CONTENT_TYPE
        NID_des_ede_cbc                         SSL3_VERSION
        NID_des_ede_cfb64                       SSLEAY_BUILT_ON
        NID_des_ede_ofb64                       SSLEAY_CFLAGS
        NID_des_ofb64                           SSLEAY_DIR
        NID_description                         SSLEAY_PLATFORM
        NID_desx_cbc                            SSLEAY_VERSION
        NID_dhKeyAgreement                      ST_ACCEPT
        NID_dnQualifier                         ST_BEFORE
        NID_dsa                                 ST_CONNECT
        NID_dsaWithSHA                          ST_INIT
        NID_dsaWithSHA1                         ST_OK
        NID_dsaWithSHA1_2                       ST_READ_BODY
        NID_dsa_2                               ST_READ_HEADER
        NID_email_protect                       TLS1_1_VERSION
        NID_ext_key_usage                       TLS1_2_VERSION
        NID_ext_req                             TLS1_3_VERSION
        NID_friendlyName                        TLS1_VERSION
        NID_givenName                           TLSEXT_STATUSTYPE_ocsp
        NID_hmacWithSHA1                        VERIFY_CLIENT_ONCE
        NID_id_ad                               VERIFY_FAIL_IF_NO_PEER_CERT
        NID_id_ce                               VERIFY_NONE
        NID_id_kp                               VERIFY_PEER
        NID_id_pbkdf2                           VERIFY_POST_HANDSHAKE
        NID_id_pe                               V_OCSP_CERTSTATUS_GOOD
        NID_id_pkix                             V_OCSP_CERTSTATUS_REVOKED
        NID_id_qt_cps                           V_OCSP_CERTSTATUS_UNKNOWN
        NID_id_qt_unotice                       WRITING
        NID_idea_cbc                            X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT
        NID_idea_cfb64                          X509_CHECK_FLAG_MULTI_LABEL_WILDCARDS
        NID_idea_ecb                            X509_CHECK_FLAG_NEVER_CHECK_SUBJECT
        NID_idea_ofb64                          X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS
        NID_info_access                         X509_CHECK_FLAG_NO_WILDCARDS
        NID_initials                            X509_CHECK_FLAG_SINGLE_LABEL_SUBDOMAINS
        NID_invalidity_date                     X509_FILETYPE_ASN1
        NID_issuer_alt_name                     X509_FILETYPE_DEFAULT
        NID_keyBag                              X509_FILETYPE_PEM
        NID_key_usage                           X509_LOOKUP
        NID_localKeyID                          X509_PURPOSE_ANY
        NID_localityName                        X509_PURPOSE_CRL_SIGN
        NID_md2                                 X509_PURPOSE_NS_SSL_SERVER
        NID_md2WithRSAEncryption                X509_PURPOSE_OCSP_HELPER
        NID_md5                                 X509_PURPOSE_SMIME_ENCRYPT
        NID_md5WithRSA                          X509_PURPOSE_SMIME_SIGN
        NID_md5WithRSAEncryption                X509_PURPOSE_SSL_CLIENT
        NID_md5_sha1                            X509_PURPOSE_SSL_SERVER
        NID_mdc2                                X509_PURPOSE_TIMESTAMP_SIGN
        NID_mdc2WithRSA                         X509_TRUST_COMPAT
        NID_ms_code_com                         X509_TRUST_EMAIL
        NID_ms_code_ind                         X509_TRUST_OBJECT_SIGN
        NID_ms_ctl_sign                         X509_TRUST_OCSP_REQUEST
        NID_ms_efs                              X509_TRUST_OCSP_SIGN
        NID_ms_ext_req                          X509_TRUST_SSL_CLIENT
        NID_ms_sgc                              X509_TRUST_SSL_SERVER
        NID_name                                X509_TRUST_TSA
        NID_netscape                            X509_V_ERR_AKID_ISSUER_SERIAL_MISMATCH
        NID_netscape_base_url                   X509_V_ERR_AKID_SKID_MISMATCH
        NID_netscape_ca_policy_url              X509_V_ERR_APPLICATION_VERIFICATION
        NID_netscape_ca_revocation_url          X509_V_ERR_CA_KEY_TOO_SMALL
        NID_netscape_cert_extension             X509_V_ERR_CA_MD_TOO_WEAK
        NID_netscape_cert_sequence              X509_V_ERR_CERT_CHAIN_TOO_LONG
        NID_netscape_cert_type                  X509_V_ERR_CERT_HAS_EXPIRED
        NID_netscape_comment                    X509_V_ERR_CERT_NOT_YET_VALID
        NID_netscape_data_type                  X509_V_ERR_CERT_REJECTED
        NID_netscape_renewal_url                X509_V_ERR_CERT_REVOKED
        NID_netscape_revocation_url             X509_V_ERR_CERT_SIGNATURE_FAILURE
        NID_netscape_ssl_server_name            X509_V_ERR_CERT_UNTRUSTED
        NID_ns_sgc                              X509_V_ERR_CRL_HAS_EXPIRED
        NID_organizationName                    X509_V_ERR_CRL_NOT_YET_VALID
        NID_organizationalUnitName              X509_V_ERR_CRL_PATH_VALIDATION_ERROR
        NID_pbeWithMD2AndDES_CBC                X509_V_ERR_CRL_SIGNATURE_FAILURE
        NID_pbeWithMD2AndRC2_CBC                X509_V_ERR_DANE_NO_MATCH
        NID_pbeWithMD5AndCast5_CBC              X509_V_ERR_DEPTH_ZERO_SELF_SIGNED_CERT
        NID_pbeWithMD5AndDES_CBC                X509_V_ERR_DIFFERENT_CRL_SCOPE
        NID_pbeWithMD5AndRC2_CBC                X509_V_ERR_EE_KEY_TOO_SMALL
        NID_pbeWithSHA1AndDES_CBC               X509_V_ERR_EMAIL_MISMATCH
        NID_pbeWithSHA1AndRC2_CBC               X509_V_ERR_ERROR_IN_CERT_NOT_AFTER_FIELD
        NID_pbe_WithSHA1And128BitRC2_CBC        X509_V_ERR_ERROR_IN_CERT_NOT_BEFORE_FIELD
        NID_pbe_WithSHA1And128BitRC4            X509_V_ERR_ERROR_IN_CRL_LAST_UPDATE_FIELD
        NID_pbe_WithSHA1And2_Key_TripleDES_CBC  X509_V_ERR_ERROR_IN_CRL_NEXT_UPDATE_FIELD
        NID_pbe_WithSHA1And3_Key_TripleDES_CBC  X509_V_ERR_EXCLUDED_VIOLATION
        NID_pbe_WithSHA1And40BitRC2_CBC         X509_V_ERR_HOSTNAME_MISMATCH
        NID_pbe_WithSHA1And40BitRC4             X509_V_ERR_INVALID_CA
        NID_pbes2                               X509_V_ERR_INVALID_CALL
        NID_pbmac1                              X509_V_ERR_INVALID_EXTENSION
        NID_pkcs                                X509_V_ERR_INVALID_NON_CA
        NID_pkcs3                               X509_V_ERR_INVALID_POLICY_EXTENSION
        NID_pkcs7                               X509_V_ERR_INVALID_PURPOSE
        NID_pkcs7_data                          X509_V_ERR_IP_ADDRESS_MISMATCH
        NID_pkcs7_digest                        X509_V_ERR_KEYUSAGE_NO_CERTSIGN
        NID_pkcs7_encrypted                     X509_V_ERR_KEYUSAGE_NO_CRL_SIGN
        NID_pkcs7_enveloped                     X509_V_ERR_KEYUSAGE_NO_DIGITAL_SIGNATURE
        NID_pkcs7_signed                        X509_V_ERR_NO_EXPLICIT_POLICY
        NID_pkcs7_signedAndEnveloped            X509_V_ERR_NO_VALID_SCTS
        NID_pkcs8ShroudedKeyBag                 X509_V_ERR_OCSP_CERT_UNKNOWN
        NID_pkcs9                               X509_V_ERR_OCSP_VERIFY_FAILED
        NID_pkcs9_challengePassword             X509_V_ERR_OCSP_VERIFY_NEEDED
        NID_pkcs9_contentType                   X509_V_ERR_OUT_OF_MEM
        NID_pkcs9_countersignature              X509_V_ERR_PATH_LENGTH_EXCEEDED
        NID_pkcs9_emailAddress                  X509_V_ERR_PATH_LOOP
        NID_pkcs9_extCertAttributes             X509_V_ERR_PERMITTED_VIOLATION
        NID_pkcs9_messageDigest                 X509_V_ERR_PROXY_CERTIFICATES_NOT_ALLOWED
        NID_pkcs9_signingTime                   X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED
        NID_pkcs9_unstructuredAddress           X509_V_ERR_PROXY_SUBJECT_NAME_VIOLATION
        NID_pkcs9_unstructuredName              X509_V_ERR_SELF_SIGNED_CERT_IN_CHAIN
        NID_private_key_usage_period            X509_V_ERR_STORE_LOOKUP
        NID_rc2_40_cbc                          X509_V_ERR_SUBJECT_ISSUER_MISMATCH
        NID_rc2_64_cbc                          X509_V_ERR_SUBTREE_MINMAX
        NID_rc2_cbc                             X509_V_ERR_SUITE_B_CANNOT_SIGN_P_384_WITH_P_256
        NID_rc2_cfb64                           X509_V_ERR_SUITE_B_INVALID_ALGORITHM
        NID_rc2_ecb                             X509_V_ERR_SUITE_B_INVALID_CURVE
        NID_rc2_ofb64                           X509_V_ERR_SUITE_B_INVALID_SIGNATURE_ALGORITHM
        NID_rc4                                 X509_V_ERR_SUITE_B_INVALID_VERSION
        NID_rc4_40                              X509_V_ERR_SUITE_B_LOS_NOT_ALLOWED
        NID_rc5_cbc                             X509_V_ERR_UNABLE_TO_DECODE_ISSUER_PUBLIC_KEY
        NID_rc5_cfb64                           X509_V_ERR_UNABLE_TO_DECRYPT_CERT_SIGNATURE
        NID_rc5_ecb                             X509_V_ERR_UNABLE_TO_DECRYPT_CRL_SIGNATURE
        NID_rc5_ofb64                           X509_V_ERR_UNABLE_TO_GET_CRL
        NID_ripemd160                           X509_V_ERR_UNABLE_TO_GET_CRL_ISSUER
        NID_ripemd160WithRSA                    X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT
        NID_rle_compression                     X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY
        NID_rsa                                 X509_V_ERR_UNABLE_TO_VERIFY_LEAF_SIGNATURE
        NID_rsaEncryption                       X509_V_ERR_UNHANDLED_CRITICAL_CRL_EXTENSION
        NID_rsadsi                              X509_V_ERR_UNHANDLED_CRITICAL_EXTENSION
        NID_safeContentsBag                     X509_V_ERR_UNNESTED_RESOURCE
        NID_sdsiCertificate                     X509_V_ERR_UNSPECIFIED
        NID_secretBag                           X509_V_ERR_UNSUPPORTED_CONSTRAINT_SYNTAX
        NID_serialNumber                        X509_V_ERR_UNSUPPORTED_CONSTRAINT_TYPE
        NID_server_auth                         X509_V_ERR_UNSUPPORTED_EXTENSION_FEATURE
        NID_sha                                 X509_V_ERR_UNSUPPORTED_NAME_SYNTAX
        NID_sha1                                X509_V_FLAG_ALLOW_PROXY_CERTS
        NID_sha1WithRSA                         X509_V_FLAG_CB_ISSUER_CHECK
        NID_sha1WithRSAEncryption               X509_V_FLAG_CHECK_SS_SIGNATURE
        NID_shaWithRSAEncryption                X509_V_FLAG_CRL_CHECK
        NID_stateOrProvinceName                 X509_V_FLAG_CRL_CHECK_ALL
        NID_subject_alt_name                    X509_V_FLAG_EXPLICIT_POLICY
        NID_subject_key_identifier              X509_V_FLAG_EXTENDED_CRL_SUPPORT
        NID_surname                             X509_V_FLAG_IGNORE_CRITICAL
        NID_sxnet                               X509_V_FLAG_INHIBIT_ANY
        NID_time_stamp                          X509_V_FLAG_INHIBIT_MAP
        NID_title                               X509_V_FLAG_LEGACY_VERIFY
        NID_undef                               X509_V_FLAG_NOTIFY_POLICY
        NID_uniqueIdentifier                    X509_V_FLAG_NO_ALT_CHAINS
        NID_x509Certificate                     X509_V_FLAG_NO_CHECK_TIME
        NID_x509Crl                             X509_V_FLAG_PARTIAL_CHAIN
        NID_zlib_compression                    X509_V_FLAG_POLICY_CHECK
        NOTHING                                 X509_V_FLAG_POLICY_MASK
        OCSP_RESPONSE_STATUS_INTERNALERROR      X509_V_FLAG_SUITEB_128_LOS
        OCSP_RESPONSE_STATUS_MALFORMEDREQUEST   X509_V_FLAG_SUITEB_128_LOS_ONLY
        OCSP_RESPONSE_STATUS_SIGREQUIRED        X509_V_FLAG_SUITEB_192_LOS
        OCSP_RESPONSE_STATUS_SUCCESSFUL         X509_V_FLAG_TRUSTED_FIRST
        OCSP_RESPONSE_STATUS_TRYLATER           X509_V_FLAG_USE_CHECK_TIME
        OCSP_RESPONSE_STATUS_UNAUTHORIZED       X509_V_FLAG_USE_DELTAS
        OPENSSL_BUILT_ON                        X509_V_FLAG_X509_STRICT
        OPENSSL_CFLAGS                          X509_V_OK
        OPENSSL_CPU_INFO                        XN_FLAG_COMPAT
        OPENSSL_DIR                             XN_FLAG_DN_REV
        OPENSSL_ENGINES_DIR                     XN_FLAG_DUMP_UNKNOWN_FIELDS
        OPENSSL_FULL_VERSION_STRING             XN_FLAG_FN_ALIGN
        OPENSSL_INFO_CONFIG_DIR                 XN_FLAG_FN_LN
        OPENSSL_INFO_CPU_SETTINGS               XN_FLAG_FN_MASK
        OPENSSL_INFO_DIR_FILENAME_SEPARATOR     XN_FLAG_FN_NONE
        OPENSSL_INFO_DSO_EXTENSION              XN_FLAG_FN_OID
        OPENSSL_INFO_ENGINES_DIR                XN_FLAG_FN_SN
        OPENSSL_INFO_LIST_SEPARATOR             XN_FLAG_MULTILINE
        OPENSSL_INFO_MODULES_DIR                XN_FLAG_ONELINE
        OPENSSL_INFO_SEED_SOURCE                XN_FLAG_RFC2253
        OPENSSL_MODULES_DIR                     XN_FLAG_SEP_COMMA_PLUS
        OPENSSL_PLATFORM                        XN_FLAG_SEP_CPLUS_SPC
        OPENSSL_VERSION                         XN_FLAG_SEP_MASK
        OPENSSL_VERSION_MAJOR                   XN_FLAG_SEP_MULTILINE
        OPENSSL_VERSION_MINOR                   XN_FLAG_SEP_SPLUS_SPC
        OPENSSL_VERSION_NUMBER                  XN_FLAG_SPC_EQ
        OPENSSL_VERSION_PATCH

  INTERNAL ONLY functions (do not use these)
    The following functions are not intended for use from outside of Net::SSLeay module. They might
    be removed, renamed or changed without prior notice in future version.

    Simply DO NOT USE THEM!

    *   hello

    *   blength

    *   constant

EXAMPLES
    One very good example to look at is the implementation of "sslcat()" in the "SSLeay.pm" file.

    The following is a simple SSLeay client (with too little error checking :-(

        #!/usr/bin/perl
        use Socket;
        use Net::SSLeay qw(die_now die_if_ssl_error) ;
        Net::SSLeay::load_error_strings();
        Net::SSLeay::SSLeay_add_ssl_algorithms();
        Net::SSLeay::randomize();

        ($dest_serv, $port, $msg) = @ARGV;      # Read command line
        $port = getservbyname ($port, 'tcp') unless $port =~ /^\d+$/;
        $dest_ip = gethostbyname ($dest_serv);
        $dest_serv_params  = sockaddr_in($port, $dest_ip);

        socket  (S, &AF_INET, &SOCK_STREAM, 0)  or die "socket: $!";
        connect (S, $dest_serv_params)          or die "connect: $!";
        select  (S); $| = 1; select (STDOUT);   # Eliminate STDIO buffering

        # The network connection is now open, lets fire up SSL

        $ctx = Net::SSLeay::CTX_new() or die_now("Failed to create SSL_CTX $!");
        Net::SSLeay::CTX_set_options($ctx, &Net::SSLeay::OP_ALL)
             or die_if_ssl_error("ssl ctx set options");
        $ssl = Net::SSLeay::new($ctx) or die_now("Failed to create SSL $!");
        Net::SSLeay::set_fd($ssl, fileno(S));   # Must use fileno
        $res = Net::SSLeay::connect($ssl) and die_if_ssl_error("ssl connect");
        print "Cipher `" . Net::SSLeay::get_cipher($ssl) . "'\n";

        # Exchange data

        $res = Net::SSLeay::write($ssl, $msg);  # Perl knows how long $msg is
        die_if_ssl_error("ssl write");
        CORE::shutdown S, 1;  # Half close --> No more output, sends EOF to server
        $got = Net::SSLeay::read($ssl);         # Perl returns undef on failure
        die_if_ssl_error("ssl read");
        print $got;

        Net::SSLeay::free ($ssl);               # Tear down connection
        Net::SSLeay::CTX_free ($ctx);
        close S;

    The following is a simple SSLeay echo server (non forking):

        #!/usr/bin/perl -w
        use Socket;
        use Net::SSLeay qw(die_now die_if_ssl_error);
        Net::SSLeay::load_error_strings();
        Net::SSLeay::SSLeay_add_ssl_algorithms();
        Net::SSLeay::randomize();

        $our_ip = "\0\0\0\0"; # Bind to all interfaces
        $port = 1235;
        $sockaddr_template = 'S n a4 x8';
        $our_serv_params = pack ($sockaddr_template, &AF_INET, $port, $our_ip);

        socket (S, &AF_INET, &SOCK_STREAM, 0)  or die "socket: $!";
        bind (S, $our_serv_params)             or die "bind:   $!";
        listen (S, 5)                          or die "listen: $!";
        $ctx = Net::SSLeay::CTX_new ()         or die_now("CTX_new ($ctx): $!");
        Net::SSLeay::CTX_set_options($ctx, &Net::SSLeay::OP_ALL)
             or die_if_ssl_error("ssl ctx set options");

        # Following will ask password unless private key is not encrypted
        Net::SSLeay::CTX_use_RSAPrivateKey_file ($ctx, 'plain-rsa.pem',
                                                 &Net::SSLeay::FILETYPE_PEM);
        die_if_ssl_error("private key");
        Net::SSLeay::CTX_use_certificate_file ($ctx, 'plain-cert.pem',
                                               &Net::SSLeay::FILETYPE_PEM);
        die_if_ssl_error("certificate");

        while (1) {
            print "Accepting connections...\n";
            ($addr = accept (NS, S))           or die "accept: $!";
            select (NS); $| = 1; select (STDOUT);  # Piping hot!

            ($af,$client_port,$client_ip) = unpack($sockaddr_template,$addr);
            @inetaddr = unpack('C4',$client_ip);
            print "$af connection from " .
            join ('.', @inetaddr) . ":$client_port\n";

            # We now have a network connection, lets fire up SSLeay...

            $ssl = Net::SSLeay::new($ctx)      or die_now("SSL_new ($ssl): $!");
            Net::SSLeay::set_fd($ssl, fileno(NS));

            $err = Net::SSLeay::accept($ssl) and die_if_ssl_error('ssl accept');
            print "Cipher `" . Net::SSLeay::get_cipher($ssl) . "'\n";

            # Connected. Exchange some data.

            $got = Net::SSLeay::read($ssl);     # Returns undef on fail
            die_if_ssl_error("ssl read");
            print "Got `$got' (" . length ($got) . " chars)\n";

            Net::SSLeay::write ($ssl, uc ($got)) or die "write: $!";
            die_if_ssl_error("ssl write");

            Net::SSLeay::free ($ssl);           # Tear down connection
            close NS;
        }

    Yet another echo server. This one runs from "/etc/inetd.conf" so it avoids all the socket code
    overhead. Only caveat is opening an rsa key file - it had better be without any encryption or
    else it will not know where to ask for the password. Note how "STDIN" and "STDOUT" are wired to
    SSL.

        #!/usr/bin/perl
        # /etc/inetd.conf
        #    ssltst stream tcp nowait root /path/to/server.pl server.pl
        # /etc/services
        #    ssltst         1234/tcp

        use Net::SSLeay qw(die_now die_if_ssl_error);
        Net::SSLeay::load_error_strings();
        Net::SSLeay::SSLeay_add_ssl_algorithms();
        Net::SSLeay::randomize();

        chdir '/key/dir' or die "chdir: $!";
        $| = 1;  # Piping hot!
        open LOG, ">>/dev/console" or die "Can't open log file $!";
        select LOG; print "server.pl started\n";

        $ctx = Net::SSLeay::CTX_new()     or die_now "CTX_new ($ctx) ($!)";
        $ssl = Net::SSLeay::new($ctx)     or die_now "new ($ssl) ($!)";
        Net::SSLeay::set_options($ssl, &Net::SSLeay::OP_ALL)
             and die_if_ssl_error("ssl set options");

        # We get already open network connection from inetd, now we just
        # need to attach SSLeay to STDIN and STDOUT
        Net::SSLeay::set_rfd($ssl, fileno(STDIN));
        Net::SSLeay::set_wfd($ssl, fileno(STDOUT));

        Net::SSLeay::use_RSAPrivateKey_file ($ssl, 'plain-rsa.pem',
                                             Net::SSLeay::FILETYPE_PEM);
        die_if_ssl_error("private key");
        Net::SSLeay::use_certificate_file ($ssl, 'plain-cert.pem',
                                           Net::SSLeay::FILETYPE_PEM);
        die_if_ssl_error("certificate");

        Net::SSLeay::accept($ssl) and die_if_ssl_err("ssl accept: $!");
        print "Cipher `" . Net::SSLeay::get_cipher($ssl) . "'\n";

        $got = Net::SSLeay::read($ssl);
        die_if_ssl_error("ssl read");
        print "Got `$got' (" . length ($got) . " chars)\n";

        Net::SSLeay::write ($ssl, uc($got)) or die "write: $!";
        die_if_ssl_error("ssl write");

        Net::SSLeay::free ($ssl);         # Tear down the connection
        Net::SSLeay::CTX_free ($ctx);
        close LOG;

    There are also a number of example/test programs in the examples directory:

        sslecho.pl   -  A simple server, not unlike the one above
        minicli.pl   -  Implements a client using low level SSLeay routines
        sslcat.pl    -  Demonstrates using high level sslcat utility function
        get_page.pl  -  Is a utility for getting html pages from secure servers
        callback.pl  -  Demonstrates certificate verification and callback usage
        stdio_bulk.pl       - Does SSL over Unix pipes
        ssl-inetd-serv.pl   - SSL server that can be invoked from inetd.conf
        httpd-proxy-snif.pl - Utility that allows you to see how a browser
                              sends https request to given server and what reply
                              it gets back (very educative :-)
        makecert.pl  -  Creates a self signed cert (does not use this module)

INSTALLATION
    See README and README.* in the distribution directory for installation guidance on a variety of
    platforms.

LIMITATIONS
    "Net::SSLeay::read()" uses an internal buffer of 32KB, thus no single read will return more. In
    practice one read returns much less, usually as much as fits in one network packet. To work
    around this, you should use a loop like this:

        $reply = '';
        while ($got = Net::SSLeay::read($ssl)) {
            last if print_errs('SSL_read');
            $reply .= $got;
        }

    Although there is no built-in limit in "Net::SSLeay::write()", the network packet size
    limitation applies here as well, thus use:

        $written = 0;

        while ($written < length($message)) {
            $written += Net::SSLeay::write($ssl, substr($message, $written));
            last if print_errs('SSL_write');
        }

    Or alternatively you can just use the following convenience functions:

        Net::SSLeay::ssl_write_all($ssl, $message) or die "ssl write failure";
        $got = Net::SSLeay::ssl_read_all($ssl) or die "ssl read failure";

KNOWN BUGS AND CAVEATS
    LibreSSL versions in the 3.1 - 3.3 series contain a TLS 1.3 implementation that is not fully
    compatible with the libssl API, but is still advertised during protocol auto-negotiation. If you
    encounter problems or unexpected behaviour with SSL or SSL_CTX objects whose protocol version
    was automatically negotiated and libssl is provided by any of these versions of LibreSSL, it
    could be because the peers negotiated to use TLS 1.3 - try setting the maximum protocol version
    to TLS 1.2 (via "Net::SSLeay::set_max_proto_version()" or
    "Net::SSLeay::CTX_set_max_proto_version()") before establishing the connection. The first stable
    LibreSSL version with a fully libssl-compatible TLS 1.3 implementation is 3.4.1.

    An OpenSSL bug CVE-2015-0290 "OpenSSL Multiblock Corrupted Pointer Issue" can cause POST
    requests of over 90kB to fail or crash. This bug is reported to be fixed in OpenSSL 1.0.2a.

    Autoloader emits a

        Argument "xxx" isn't numeric in entersub at blib/lib/Net/SSLeay.pm'

    warning if die_if_ssl_error is made autoloadable. If you figure out why, drop me a line.

    Callback set using "SSL_set_verify()" does not appear to work. This may well be an openssl
    problem (e.g. see "ssl/ssl_lib.c" line 1029). Try using "SSL_CTX_set_verify()" instead and do
    not be surprised if even this stops working in future versions.

    Callback and certificate verification stuff is generally too little tested.

    Random numbers are not initialized randomly enough, especially if you do not have "/dev/random"
    and/or "/dev/urandom" (such as in Solaris platforms - but it's been suggested that cryptorand
    daemon from the SUNski package solves this). In this case you should investigate third party
    software that can emulate these devices, e.g. by way of a named pipe to some program.

    Another gotcha with random number initialization is randomness depletion. This phenomenon, which
    has been extensively discussed in OpenSSL, Apache-SSL, and Apache-mod_ssl forums, can cause your
    script to block if you use "/dev/random" or to operate insecurely if you use "/dev/urandom".
    What happens is that when too much randomness is drawn from the operating system's randomness
    pool then randomness can temporarily be unavailable. "/dev/random" solves this problem by
    waiting until enough randomness can be gathered - and this can take a long time since blocking
    reduces activity in the machine and less activity provides less random events: a vicious circle.
    "/dev/urandom" solves this dilemma more pragmatically by simply returning predictable "random"
    numbers. Some" /dev/urandom" emulation software however actually seems to implement
    "/dev/random" semantics. Caveat emptor.

    I've been pointed to two such daemons by Mik Firestone <mik@@speed.stdio._com> who has used them
    on Solaris 8:

    1   Entropy Gathering Daemon (EGD) at <http://www.lothar.com/tech/crypto/>

    2   Pseudo-random number generating daemon (PRNGD) at
        <http://www.aet.tu-cottbus.de/personen/jaenicke/postfix_tls/prngd.html>

    If you are using the low level API functions to communicate with other SSL implementations, you
    would do well to call

        Net::SSLeay::CTX_set_options($ctx, &Net::SSLeay::OP_ALL)
             or die_if_ssl_error("ssl ctx set options");

    to cope with some well know bugs in some other SSL implementations. The high level API functions
    always set all known compatibility options.

    Sometimes "sslcat()" (and the high level HTTPS functions that build on it) is too fast in
    signaling the EOF to legacy HTTPS servers. This causes the server to return empty page. To work
    around this problem you can set the global variable

        $Net::SSLeay::slowly = 1;   # Add sleep so broken servers can keep up

    HTTP/1.1 is not supported. Specifically this module does not know to issue or serve multiple
    http requests per connection. This is a serious shortcoming, but using the SSL session cache on
    your server helps to alleviate the CPU load somewhat.

    As of version 1.09 many newer OpenSSL auxiliary functions were added (from
    "REM_AUTOMATICALLY_GENERATED_1_09" onwards in "SSLeay.xs"). Unfortunately I have not had any
    opportunity to test these. Some of them are trivial enough that I believe they "just work", but
    others have rather complex interfaces with function pointers and all. In these cases you should
    proceed wit great caution.

    This module defaults to using OpenSSL automatic protocol negotiation code for automatically
    detecting the version of the SSL/TLS protocol that the other end talks. With most web servers
    this works just fine, but once in a while I get complaints from people that the module does not
    work with some web servers. Usually this can be solved by explicitly setting the protocol
    version, e.g.

       $Net::SSLeay::ssl_version = 2;  # Insist on SSLv2
       $Net::SSLeay::ssl_version = 3;  # Insist on SSLv3
       $Net::SSLeay::ssl_version = 10; # Insist on TLSv1
       $Net::SSLeay::ssl_version = 11; # Insist on TLSv1.1
       $Net::SSLeay::ssl_version = 12; # Insist on TLSv1.2
       $Net::SSLeay::ssl_version = 13; # Insist on TLSv1.3

    Although the autonegotiation is nice to have, the SSL standards do not formally specify any such
    mechanism. Most of the world has accepted the SSLeay/OpenSSL way of doing it as the de facto
    standard. But for the few that think differently, you have to explicitly speak the correct
    version. This is not really a bug, but rather a deficiency in the standards. If a site refuses
    to respond or sends back some nonsensical error codes (at the SSL handshake level), try this
    option before mailing me.

    On some systems, OpenSSL may be compiled without support for SSLv2. If this is the case,
    Net::SSLeay will warn if ssl_version has been set to 2.

    The high level API returns the certificate of the peer, thus allowing one to check what
    certificate was supplied. However, you will only be able to check the certificate after the
    fact, i.e. you already sent your form data by the time you find out that you did not trust them,
    oops.

    So, while being able to know the certificate after the fact is surely useful, the security
    minded would still choose to do the connection and certificate verification first and only then
    exchange data with the site. Currently none of the high level API functions do this, thus you
    would have to program it using the low level API. A good place to start is to see how the
    "Net::SSLeay::http_cat()" function is implemented.

    The high level API functions use a global file handle "SSLCAT_S" internally. This really should
    not be a problem because there is no way to interleave the high level API functions, unless you
    use threads (but threads are not very well supported in perl anyway). However, you may run into
    problems if you call undocumented internal functions in an interleaved fashion. The best
    solution is to "require Net::SSLeay" in one thread after all the threads have been created.

DIAGNOSTICS
    Random number generator not seeded!!!
        (W) This warning indicates that "randomize()" was not able to read "/dev/random" or
        "/dev/urandom", possibly because your system does not have them or they are differently
        named. You can still use SSL, but the encryption will not be as strong.

    open_tcp_connection: destination host not found:`server' (port 123) ($!)
        Name lookup for host named "server" failed.

    open_tcp_connection: failed `server', 123 ($!)
        The name was resolved, but establishing the TCP connection failed.

    msg 123: 1 - error:140770F8:SSL routines:SSL23_GET_SERVER_HELLO:unknown proto
        SSLeay error string. The first number (123) is the PID, the second number (1) indicates the
        position of the error message in SSLeay error stack. You often see a pile of these messages
        as errors cascade.

    msg 123: 1 - error:02001002::lib(2) :func(1) :reason(2)
        The same as above, but you didn't call load_error_strings() so SSLeay couldn't verbosely
        explain the error. You can still find out what it means with this command:

            /usr/local/ssl/bin/ssleay errstr 02001002

    Password is being asked for private key
        This is normal behaviour if your private key is encrypted. Either you have to supply the
        password or you have to use an unencrypted private key. Scan OpenSSL.org for the FAQ that
        explains how to do this (or just study examples/makecert.pl which is used during "make test"
        to do just that).

SECURITY
    You can mitigate some of the security vulnerabilities that might be present in your SSL/TLS
    application:

  BEAST Attack
    http://blogs.cisco.com/security/beat-the-beast-with-tls/
    https://community.qualys.com/blogs/securitylabs/2011/10/17/mitigating-the-beast-attack-on-tls
    http://blog.zoller.lu/2011/09/beast-summary-tls-cbc-countermeasures.html

    The BEAST attack relies on a weakness in the way CBC mode is used in SSL/TLS. In OpenSSL
    versions 0.9.6d and later, the protocol-level mitigation is enabled by default, thus making it
    not vulnerable to the BEAST attack.

    Solutions:

    *   Compile with OpenSSL versions 0.9.6d or later, which enables SSL_OP_ALL by default

    *   Ensure SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS is not enabled (its not enabled by default)

    *   Don't support SSLv2, SSLv3

    *   Actively control the ciphers your server supports with set_cipher_list:

    Net::SSLeay::set_cipher_list($ssl, 'RC4-SHA:HIGH:!ADH');

  Session Resumption
    http://www.openssl.org/docs/ssl/SSL_CTX_set_options.html

    The SSL Labs vulnerability test on your SSL server might report in red:

    Session resumption No (IDs assigned but not accepted)

    This report is not really bug or a vulnerability, since the server will not accept session
    resumption requests. However, you can prevent this noise in the report by disabling the session
    cache altogether: Net::SSLeay::CTX_set_session_cache_mode($ssl_ctx,
    Net::SSLeay::SESS_CACHE_OFF()); Use 0 if you don't have SESS_CACHE_OFF constant.

  Secure Renegotiation and DoS Attack
    https://community.qualys.com/blogs/securitylabs/2011/10/31/tls-renegotiation-and-denial-of-servi
    ce-attacks

    This is not a "security flaw," it is more of a DoS vulnerability.

    Solutions:

    *   Do not support SSLv2

    *   Do not set the SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION option

    *   Compile with OpenSSL 0.9.8m or later

BUGS
    If you encounter a problem with this module that you believe is a bug, please create a new issue
    <https://github.com/radiator-software/p5-net-ssleay/issues/new> in the Net-SSLeay GitHub
    repository. Please make sure your bug report includes the following information:

    *   the code you are trying to run;

    *   your operating system name and version;

    *   the output of "perl -V";

    *   the version of OpenSSL or LibreSSL you are using.

AUTHOR
    Originally written by Sampo Kellomäki.

    Maintained by Florian Ragwitz between November 2005 and January 2010.

    Maintained by Mike McCauley between November 2005 and June 2018.

    Maintained by Chris Novakovic, Tuure Vartiainen and Heikki Vatiainen since June 2018.

COPYRIGHT
    Copyright (c) 1996-2003 Sampo Kellomäki <sampo AT iki.fi>

    Copyright (c) 2005-2010 Florian Ragwitz <rafl AT debian.org>

    Copyright (c) 2005-2018 Mike McCauley <mikem AT airspayce.com>

    Copyright (c) 2018- Chris Novakovic <chris AT chrisn.uk>

    Copyright (c) 2018- Tuure Vartiainen <vartiait AT radiatorsoftware.com>

    Copyright (c) 2018- Heikki Vatiainen <hvn AT radiatorsoftware.com>

    All rights reserved.

LICENSE
    This module is released under the terms of the Artistic License 2.0. For details, see the
    "LICENSE" file distributed with Net-SSLeay's source code.

SEE ALSO
      Net::SSLeay::Handle                      - File handle interface
      ./examples                               - Example servers and a clients
      <http://www.openssl.org/>                - OpenSSL source, documentation, etc
      openssl-users-request AT openssl.org        - General OpenSSL mailing list
      <http://www.ietf.org/rfc/rfc2246.txt>    - TLS 1.0 specification
      <http://www.w3c.org>                     - HTTP specifications
      <http://www.ietf.org/rfc/rfc2617.txt>    - How to send password
      <http://www.lothar.com/tech/crypto/>     - Entropy Gathering Daemon (EGD)
      <http://www.aet.tu-cottbus.de/personen/jaenicke/postfix_tls/prngd.html>
                               - pseudo-random number generating daemon (PRNGD)
      perl(1)
      perlref(1)
      perllol(1)
      perldoc ~openssl/doc/ssl/SSL_CTX_set_verify.pod


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