Before establishing a connection with a Network Time Protocol (NTP) server, the Cisco ISE must authenticate using a bidirectional, cryptographically based authentication method that uses a FIPS-validated Advanced Encryption Standard (AES) cipher block algorithm to authenticate with the NTP server. This is required for compliance with C2C Step 1.

If the NTP server is not authenticated, an attacker can introduce a rogue NTP server. This rogue server can then be used to send incorrect time information to network devices, which will make log timestamps inaccurate and affect scheduled actions. NTP authentication is used to prevent this tampering by authenticating the time source.

Currently, AES block cipher algorithm is approved for use in DoD for both applying cryptographic protection (e.g., encryption) and removing or verifying the protection that was previously applied (e.g., decryption). NTP devices use MD5 authentication keys. The MD5 algorithm is not specified in either the FIPS or NIST recommendation. However, MD5 is preferred to no authentication at all.

The trusted-key statement permits authenticating NTP servers. The product must be configured to support separate keys for each NTP server. Severs should have a PKI device certificate involved for use in the device authentication process.