ntp.keys(5) on FreeBSD
The text b21f56f4e59a, as FreeBSD 4.11-RELEASE, FreeBSD 4.10-RELEASE, FreeBSD 4.9-RELEASE, FreeBSD 4.8-RELEASE shipped it.
NTP daemon key file format
The text b21f56f4e59a, as FreeBSD 4.11-RELEASE, FreeBSD 4.10-RELEASE, FreeBSD 4.9-RELEASE, FreeBSD 4.8-RELEASE shipped it.
NTP daemon key file format