You can create an authentication key for the purpose of authenticating an external NTP server .
You use the ntp authentication-key command to configure a key identifier and secret key strings, which are added to the database. The key is shared by the client (switch) and an external NTP server. You then associate the key to an NTP server by using the ntp server command.
For more information, refer to the ntp authentication-key and ntp server commands in the Network OS Command Reference.