You can authenticate time synchronization to ensure that the local time server obtains its time services only from known sources. NTP authentication adds a level of security to your NTP configuration. By default, network time synchronization is not authenticated.
If you select authentication, the switch uses the Message Digest 5 (MD5) or the Secure Hash Algorithm 1 (SHA1) algorithm to produce a message digest of the key. The message digest is created using the key and the message, but the key itself is not sent. Depending on which algorithm you select, the MD5 or SHA1 algorithm verifies the integrity of the communication, authenticates the origin, and checks for timeliness.
To authenticate the message, the client authentication key must match that of the time server. Therefore, you must securely distribute the authentication key in advance (the client administrator must obtain the key from the server administrator and configure it on the client).
While you can configure a server with many keys (identified by many key IDs), it is possible to declare only a subset of these as trusted. The time server uses this feature to share keys with a client that requires authenticated time and that trusts the server, but that is not trusted by the time server.