Symmetric Keys are an encryption means that can be used with NTP for authentication purposes.
SecureSync supports authenticated NTP packets using an MD5 authenticator. This feature does not encrypt the time packets, but attaches an authenticator, which consists of a key identifier and an MD5 message digest, to the end of each packet. This can be used to guarantee that NTP packets came from a valid NTP client or server, and that they were not tampered with during transmission. The Symmetric Keys tab allows NTP to be configured to use MD5 authentication.