This document describes the format of an NTP symmetric key file. For a description of the use of this type of file, see the "Authentication Support" page of the Web documentation.
ntpd(8) reads its keys from a file specified using the -k command line option or the keys statement in the configuration file. While key number 0 is fixed by the NTP standard (as 56 zero bits) and may not be changed, one or more keys numbered between 1 and 65534 may be arbitrarily set in the keys file.
The key file uses the same comment conventions as the configuration file. Key entries use a fixed format of the form
keyno type key
where keyno is a positive integer (between 1 and 65534), type is the message digest algorithm, and key is the key itself.
The file does not need to be sorted by keyno.
type can be any digest type supported by your OpenSSL package. Digests longer than 20 bytes will be trucnated.
You can probably get a list from man 1 dgst or openssl help. (As of Jan 2018, they lie. Be sure to try it. ntpd(8) will print an error on startup if a selected type isn’t supported.)
The following types are widely supported:
md5, sha1, ripemd160, sha224, sha256, sha384, sha512
FIPS 140-2, FIPS 180-4, and/or FIPS 202 may restrict your choices. If it matters to you, check with your lawyer. (Let us know if you find a good reference.)
The key may be printable ASCII excluding "#" or hex encoded. Keys longer than 20 characters are assumed to be hex. The max length of a (possibly de-hexified) key is 32 bytes. If you want to use an ASCII key longer than 20 bytes, you must hexify it.
Note that the keys used by the ntpq(1) programs are checked against passwords entered by hand, so it is generally appropriate to specify these keys in ASCII format. Or you can cut-paste a hex string from your password manager.
In order to use symmetric keys, the client side configuration file needs:
keys <path-to-client-keys-file> trustedkey <keyno> server ... key <keyno>
The server side needs:
keys <path-to-server-keys-file> trustedkey <keyno>
Note that the client and server key files must both contain identical copies of the line specified by keyno.
is a common location for the keys file
Reminder: You have to keep it secret.