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 65535 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 65535), type is the message digest or cipher algorithm, and key is the key itself.
The file does not need to be sorted by keyno.
type can be the name of any digest or cipher supported by your OpenSSL package. Digests or CMACs longer than 20 bytes will be truncated.
You can get a list from openssl list -digest-algorithms or openssl list -cipher-algorithms. (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 aes-128, aes-192, aes-256
Only the -cbc cipher modes are useful. The -cbc is appended to the type internally. Do not include it in type.
AES is an abbreviation for aes-128.
Note that MD5 was deprecated by RFC 8573 in June of 2019. AES-128 is currently preferred. The code still supports MD5 for backwards compatibility.
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 (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.
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.