ntp-keygen - phpMan

ntp-keygen(1) ntp-keygen(1)

NAME
ntp-keygen - generate public and private keys

DESCRIPTION
This program generates cryptographic data files used by the NTPv4 authentication
and identification schemes. It generates MD5 key files used in symmetric key cryp-
tography. In addition, if the OpenSSL software library has been installed, it gen-
erates keys, certificate and identity files used in public key cryptography. These
files are used for cookie encryption, digital signature and challenge/response
identification algorithms compatible with the Internet standard security infras-
tructure.

All files are in PEM-encoded printable ASCII format, so they can be embedded as
MIME attachments in mail to other sites and certificate authorities. By default,
files are not encrypted. The -p password option specifies the write password and -q
password option the read password for previously encrypted files. The ntp-keygen
program prompts for the password if it reads an encrypted file and the password is
missing or incorrect. If an encrypted file is read successfully and no write pass-
word is specified, the read password is used as the write password by default.

The ntpd configuration command crypto pw password specifies the read password for
previously encrypted files. The daemon expires on the spot if the password is miss-
ing or incorrect. For convenience, if a file has been previously encrypted, the
default read password is the name of the host running the program. If the previous
write password is specified as the host name, these files can be read by that host
with no explicit password.

File names begin with the prefix ntpkey_ and end with the postfix _host-
name.filestamp, where hostname is the owner name, usually the string returned by
the Unix gethostname() routine, and filestamp is the NTP seconds when the file was
generated, in decimal digits. This both guarantees uniqueness and simplifies main-
tenance procedures, since all files can be quickly removed by a rm ntpkey* command
or all files generated at a specific time can be removed by a rm *filestamp com-
mand. To further reduce the risk of misconfiguration, the first two lines of a file
contain the file name and generation date and time as comments.

All files are installed by default in the keys directory /etc/ntp. The actual loca-
tion of the keys directory and each file can be overridden by configuration com-
mands, but this is not recommended. Normally, the files for each host are generated
by that host and used only by that host, although exceptions exist as noted later
on this page.

Normally, files containing private values, including the host key, sign key and
identification parameters, are permitted root read/write-only; while others con-
taining public values are permitted world readable. Alternatively, files containing
private values can be encrypted and these files permitted world readable, which
simplifies maintenance in shared file systems. Since uniqueness is insured by the
hostname and file name extensions, the files for a NFS server and dependent clients
can all be installed in the same shared directory.

The recommended practice is to keep the file name extensions when installing a file
and to install a soft link from the generic names specified elsewhere on this page
to the generated files. This allows new file generations to be activated simply by
changing the link. If a link is present, ntpd follows it to the file name to
extract the filestamp. If a link is not present, ntpd extracts the filestamp from
the file itself. This allows clients to verify that the file and generation times
are always current. The ntp-keygen program uses the same timestamp extension for
all files generated at one time, so each generation is distinct and can be readily
recognized in monitoring data.

RUNNING THE PROGRAM
The safest way to run the ntp-keygen program is logged in directly as root. The
recommended procedure is change to the keys directory, usually /etc/ntp, then run
the program. When run for the first time, or if all ntpkey files have been removed,
the program generates a RSA host key file and matching RSA-MD5 certificate file,
which is all that is necessary in many cases. The program also generates soft links
from the generic names to the respective files. If run again, the program uses the
same host key file, but generates a new certificate file and link.

The host key is used to encrypt the cookie when required and so must be RSA type.
By default, the host key is also the sign key used to encrypt signatures. When
necessary, a different sign key can be specified and this can be either RSA or DSA
type. By default, the message digest type is MD5, but any combination of sign key
type and message digest type supported by the OpenSSL library can be specified,
including those using the MD2, MD5, SHA, SHA1, MDC2 and RIPE160 message digest
algorithms. However, the scheme specified in the certificate must be compatible
with the sign key. Certificates using any digest algorithm are compatible with RSA
sign keys; however, only SHA and SHA1 certificates are compatible with DSA sign
keys.

Private/public key files and certificates are compatible with other OpenSSL appli-
cations and very likely other libraries as well. Certificates or certificate
requests derived from them should be compatible with extant industry practice,
although some users might find the interpretation of X509v3 extension fields some-
what liberal. However, the identification parameter files, although encoded as the
other files, are probably not compatible with anything other than Autokey.

Running the program as other than root and using the Unix su command to assume root
may not work properly, since by default the OpenSSL library looks for the random
seed file .rnd in the user home directory. However, there should be only one .rnd,
most conveniently in the root directory, so it is convenient to define the $RAND-
FILE environment variable used by the OpenSSL library as the path to /.rnd.

Installing the keys as root might not work in NFS-mounted shared file systems, as
NFS clients may not be able to write to the shared keys directory, even as root. In
this case, NFS clients can specify the files in another directory such as /etc
using the keysdir command. There is no need for one client to read the keys and
certificates of other clients or servers, as these data are obtained automatically
by the Autokey protocol.

Ordinarily, cryptographic files are generated by the host that uses them, but it is
possible for a trusted agent (TA) to generate these files for other hosts; however,
in such cases files should always be encrypted. The subject name and trusted name
default to the hostname of the host generating the files, but can be changed by
command line options. It is convenient to designate the owner name and trusted name
as the subject and issuer fields, respectively, of the certificate. The owner name
is also used for the host and sign key files, while the trusted name is used for
the identity files.

TRUSTED HOSTS AND GROUPS
Each cryptographic configuration involves selection of a signature scheme and iden-
tification scheme, called a cryptotype, as explained in the Authentication Options
page. The default cryptotype uses RSA encryption, MD5 message digest and TC identi-
fication. First, configure a NTP subnet including one or more low-stratum trusted
hosts from which all other hosts derive synchronization directly or indirectly.
Trusted hosts have trusted certificates; all other hosts have nontrusted certifi-
cates. These hosts will automatically and dynamically build authoritative certifi-
cate trails to one or more trusted hosts. A trusted group is the set of all hosts
that have, directly or indirectly, a certificate trail ending at a trusted host.
The trail is defined by static configuration file entries or dynamic means
described on the Automatic NTP Configuration Options page.

On each trusted host as root, change to the keys directory. To insure a fresh file-
set, remove all ntpkey files. Then run ntp-keygen -T to generate keys and a trusted
certificate. On all other hosts do the same, but leave off the -T flag to generate
keys and nontrusted certificates. When complete, start the NTP daemons beginning at
the lowest stratum and working up the tree. It may take some time for Autokey to
instantiate the certificate trails throughout the subnet, but setting up the envi-
ronment is completely automatic.

If it is necessary to use a different sign key or different digest/signature scheme
than the default, run ntp-keygen with the -S type option, where type is either RSA
or DSA. The most often need to do this is when a DSA-signed certificate is used.
If it is necessary to use a different certificate scheme than the default, run ntp-
keygen with the -c scheme option and selected scheme as needed. If ntp-keygen is
run again without these options, it generates a new certificate using the same
scheme and sign key.

After setting up the environment it is advisable to update certificates from time
to time, if only to extend the validity interval. Simply run ntp-keygen with the
same flags as before to generate new certificates using existing keys. However, if
the host or sign key is changed, ntpd should be restarted. When ntpd is restarted,
it loads any new files and restarts the protocol. Other dependent hosts will con-
tinue as usual until signatures are refreshed, at which time the protocol is
restarted.

IDENTITY SCHEMES
As mentioned on the Autonomous Authentication page, the default TC identity scheme
is vulnerable to a middleman attack. However, there are more secure identity
schemes available, including PC, IFF, GQ and MV described on the Identification
Schemes page. These schemes are based on a TA, one or more trusted hosts and some
number of nontrusted hosts. Trusted hosts prove identity using values provided by
the TA, while the remaining hosts prove identity using values provided by a trusted
host and certificate trails that end on that host. The name of a trusted host is
also the name of its sugroup and also the subject and issuer name on its trusted
certificate. The TA is not necessarily a trusted host in this sense, but often is.

In some schemes there are separate keys for servers and clients. A server can also
be a client of another server, but a client can never be a server for another
client. In general, trusted hosts and nontrusted hosts that operate as both server
and client have parameter files that contain both server and client keys. Hosts
that operate only as clients have key files that contain only client keys.

The PC scheme supports only one trusted host in the group. On trusted host alice
run ntp-keygen -P -p password to generate the host key file ntp-
key_RSAkey_alice.filestamp and trusted private certificate file ntpkey_RSA-
MD5_cert_alice.filestamp. Copy both files to all group hosts; they replace the
files which would be generated in other schemes. On each host bob install a soft
link from the generic name ntpkey_host_bob to the host key file and soft link ntp-
key_cert_bob to the private certificate file. Note the generic links are on bob,
but point to files generated by trusted host alice. In this scheme it is not possi-
ble to refresh either the keys or certificates without copying them to all other
hosts in the group.

For the IFF scheme proceed as in the TC scheme to generate keys and certificates
for all group hosts, then for every trusted host in the group, generate the IFF
parameter file. On trusted host alice run ntp-keygen -T -I -p password to produce
her parameter file ntpkey_IFFpar_alice.filestamp, which includes both server and
client keys. Copy this file to all group hosts that operate as both servers and
clients and install a soft link from the generic ntpkey_iff_alice to this file. If
there are no hosts restricted to operate only as clients, there is nothing further
to do. As the IFF scheme is independent of keys and certificates, these files can
be refreshed as needed.

If a rogue client has the parameter file, it could masquerade as a legitimate
server and present a middleman threat. To eliminate this threat, the client keys
can be extracted from the parameter file and distributed to all restricted clients.
After generating the parameter file, on alice run ntp-keygen -e and pipe the output
to a file or mail program. Copy or mail this file to all restricted clients. On
these clients install a soft link from the generic ntpkey_iff_alice to this file.
To further protect the integrity of the keys, each file can be encrypted with a
secret password.

For the GQ scheme proceed as in the TC scheme to generate keys and certificates for
all group hosts, then for every trusted host in the group, generate the IFF parame-
ter file. On trusted host alice run ntp-keygen -T -G -p password to produce her
parameter file ntpkey_GQpar_alice.filestamp, which includes both server and client
keys. Copy this file to all group hosts and install a soft link from the generic
ntpkey_gq_alice to this file. In addition, on each host bob install a soft link
from generic ntpkey_gq_bob to this file. As the GQ scheme updates the GQ parameters
file and certificate at the same time, keys and certificates can be regenerated as
needed.

For the MV scheme, proceed as in the TC scheme to generate keys and certificates
for all group hosts. For illustration assume trish is the TA, alice one of several
trusted hosts and bob one of her clients. On TA trish run ntp-keygen -V n -p pass-
word, where n is the number of revokable keys (typically 5) to produce the parame-
ter file ntpkeys_MVpar_trish.filestamp and client key files ntp-
keys_MVkeyd_trish.filestamp where d is the key number (0 < d < n). Copy the parame-
ter file to alice and install a soft link from the generic ntpkey_mv_alice to this
file. Copy one of the client key files to alice for later distribution to her
clients. It doesn’t matter which client key file goes to alice, since they all work
the same way. Alice copies the client key file to all of her cliens. On client bob
install a soft link from generic ntpkey_mvkey_bob to the client key file. As the MV
scheme is independent of keys and certificates, these files can be refreshed as
needed.

-d Enable debugging. This option displays the cryptographic data produced in
eye-friendly billboards.

-e Write the IFF client keys to the standard output. This is intended for
automatic key distribution by mail.

-G Generate parameters and keys for the GQ identification scheme, obsoleting
any that may exist.

-g Generate keys for the GQ identification scheme using the existing GQ param-
eters. If the GQ parameters do not yet exist, create them first.

-H Generate new host keys, obsoleting any that may exist.

-I Generate parameters for the IFF identification scheme, obsoleting any that
may exist.

-i name Set the suject name to name. This is used as the subject field in certifi-
cates and in the file name for host and sign keys.

-M Generate MD5 keys, obsoleting any that may exist.

-P Generate a private certificate. By default, the program generates public
certificates.

-p password
Encrypt generated files containing private data with password and the DES-
CBC algorithm.

-q Set the password for reading files to password.

-S [ RSA | DSA ]
Generate a new sign key of the designated type, obsoleting any that may
exist. By default, the program uses the host key as the sign key.

-s name Set the issuer name to name. This is used for the issuer field in certifi-
cates and in the file name for identity files.

-T Generate a trusted certificate. By default, the program generates a non-
trusted certificate.

-V nkeys
Generate parameters and keys for the Mu-Varadharajan (MV) identification
scheme.

RANDOM SEED FILE
All cryptographically sound key generation schemes must have means to randomize the
entropy seed used to initialize the internal pseudo-random number generator used by
the library routines. The OpenSSL library uses a designated random seed file for
this purpose. The file must be available when starting the NTP daemon and ntp-key-
gen program. If a site supports OpenSSL or its companion OpenSSH, it is very likely
that means to do this are already available.

It is important to understand that entropy must be evolved for each generation, for
otherwise the random number sequence would be predictable. Various means dependent
on external events, such as keystroke intervals, can be used to do this and some
systems have built-in entropy sources. Suitable means are described in the OpenSSL
software documentation, but are outside the scope of this page.

The entropy seed used by the OpenSSL library is contained in a file, usually called
.rnd, which must be available when starting the NTP daemon or the ntp-keygen pro-
gram. The NTP daemon will first look for the file using the path specified by the
randfile subcommand of the crypto configuration command. If not specified in this
way, or when starting the ntp-keygen program, the OpenSSL library will look for the
file using the path specified by the RANDFILE environment variable in the user home
directory, whether root or some other user. If the RANDFILE environment variable is
not present, the library will look for the .rnd file in the user home directory. If
the file is not available or cannot be written, the daemon exits with a message to
the system log and the program exits with a suitable error message.

CRYPTOGRAPHIC DATA FILES
All other file formats begin with two lines. The first contains the file name,
including the generated host name and filestamp. The second contains the datestamp
in conventional Unix date format. Lines beginning with # are considered comments
and ignored by the ntp-keygen program and ntpd daemon. Cryptographic values are
encoded first using ASN.1 rules, then encrypted if necessary, and finally written
PEM-encoded printable ASCII format preceded and followed by MIME content identifier
lines.

The format of the symmetric keys file is somewhat different than the other files in
the interest of backward compatibility. Since DES-CBC is deprecated in NTPv4, the
only key format of interest is MD5 alphanumeric strings. Following hte heard the
keys are entered one per line in the format

keyno type key

where keyno is a positive integer in the range 1-65,535, type is the string MD5
defining the key format and key is the key itself, which is a printable ASCII
string 16 characters or less in length. Each character is chosen from the 93
printable characters in the range 0x21 through 0x7f excluding space and the ’#’
character.

Note that the keys used by the ntpq and ntpdc programs are checked against pass-
words requested by the programs and entered by hand, so it is generally appropriate
to specify these keys in human readable ASCII format.

The ntp-keygen program generates a MD5 symmetric keys file ntpkey_MD5key_host-
name.filestamp. Since the file contains private shared keys, it should be visible
only to root and distributed by secure means to other subnet hosts. The NTP daemon
loads the file ntp.keys, so ntp-keygen installs a soft link from this name to the
generated file. Subsequently, similar soft links must be installed by manual or
automated means on the other subnet hosts. While this file is not used with the
Autokey Version 2 protocol, it is needed to authenticate some remote configuration
commands used by the ntpq and ntpdc utilities.

BUGS
It can take quite a while to generate some cryptographic values, from one to sev-
eral minutes with modern architectures such as UltraSPARC and up to tens of minutes
to an hour with older architectures such as SPARC IPC.

SEE ALSO
Primary source of documentation: /usr/share/doc/ntp-*/ntpd.html

AUTHOR
David L. Mills <mills AT udel.edu>

ntp-keygen(1)

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