Meinberg LANTIME/GPS Operating Instructions Manual page 58

The ntp.keys file mentioned above holds a list of all keys and their respective ID
known by the server. This file should not be world-readable (only root should be able
to look into this) and it may look like this:
# ntp keys file (ntp.keys)
1 N 29233E0461ECD6AE
2 M RIrop8KPPvQvYotM
14 M sundial
15 A sundial
# the following 3 keys are identical
10 A SeCReT
10 N d3e54352e5548080
10 S a7cb86a4cba80101
The first column holds the key ID (used in the ntp.conf file), the second column
defines the format of the key, which is following in column three. There are four
different key formats: "A" means DES key with up to eight 7-bit ASCII characters,
where each character is standing for a key octet (this is used by Unix passwords, too).
"S" is a DES key written in hexadecimal notation, where the lowest bit (LSB) of each
octet is used as the odd parity bit. If the key format is specified as "N", it also consists
of a hexadecimal string, but in NTP standard format by using the highest bit (HSB) of
each octet used as the odd parity bit. A key defined as "M" is a MD5 key with up to
31 ASCII characters. The Lantime supports MD5 authentication only.
Please be aware of the following restrictions: No "#", "\t" (tab), "\n" (newline) and
"\0" (null) are allowed in a DES or MD5 ASCII key. The key ID 0 is reserved for
special purposes and should not appear in the keys file.
# des key in NTP format
# md5 key as an ASCII random string
# md5 key as an ASCII string
# des key as an ASCII string
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