crypt(3) - perldoc - phpman

    crypt PLAINTEXT,SALT
            Creates a digest string exactly like the crypt(3) function in
            the C library (assuming that you actually have a version there
            that has not been extirpated as a potential munition).

            "crypt" is a one-way hash function. The PLAINTEXT and SALT are
            turned into a short string, called a digest, which is returned.
            The same PLAINTEXT and SALT will always return the same string,
            but there is no (known) way to get the original PLAINTEXT from
            the hash. Small changes in the PLAINTEXT or SALT will result in
            large changes in the digest.

            There is no decrypt function. This function isn't all that
            useful for cryptography (for that, look for Crypt modules on
            your nearby CPAN mirror) and the name "crypt" is a bit of a
            misnomer. Instead it is primarily used to check if two pieces of
            text are the same without having to transmit or store the text
            itself. An example is checking if a correct password is given.
            The digest of the password is stored, not the password itself.
            The user types in a password that is "crypt"'d with the same
            salt as the stored digest. If the two digests match, the
            password is correct.

            When verifying an existing digest string you should use the
            digest as the salt (like "crypt($plain, $digest) eq $digest").
            The SALT used to create the digest is visible as part of the
            digest. This ensures "crypt" will hash the new string with the
            same salt as the digest. This allows your code to work with the
            standard "crypt" and with more exotic implementations. In other
            words, assume nothing about the returned string itself nor about
            how many bytes of SALT may matter.

            Traditionally the result is a string of 13 bytes: two first
            bytes of the salt, followed by 11 bytes from the set
            "[./0-9A-Za-z]", and only the first eight bytes of PLAINTEXT
            mattered. But alternative hashing schemes (like MD5), higher
            level security schemes (like C2), and implementations on
            non-Unix platforms may produce different strings.

            When choosing a new salt create a random two character string
            whose characters come from the set "[./0-9A-Za-z]" (like "join
            '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]").
            This set of characters is just a recommendation; the characters
            allowed in the salt depend solely on your system's crypt
            library, and Perl can't restrict what salts "crypt" accepts.

            Here's an example that makes sure that whoever runs this program
            knows their password:

                my $pwd = (getpwuid($<))[1];

                system "stty -echo";
                print "Password: ";
                chomp(my $word = <STDIN>);
                print "\n";
                system "stty echo";

                if (crypt($word, $pwd) ne $pwd) {
                    die "Sorry...\n";
                } else {
                    print "ok\n";
                }

            Of course, typing in your own password to whoever asks you for
            it is unwise.

            The "crypt" function is unsuitable for hashing large quantities
            of data, not least of all because you can't get the information
            back. Look at the Digest module for more robust algorithms.

            If using "crypt" on a Unicode string (which *potentially* has
            characters with codepoints above 255), Perl tries to make sense
            of the situation by trying to downgrade (a copy of) the string
            back to an eight-bit byte string before calling "crypt" (on that
            copy). If that works, good. If not, "crypt" dies with "Wide
            character in crypt".

            Portability issues: "crypt" in perlport.