C. Existing Authentication in SNMPv3
USM is implemented in the structure of authentication and encryption and VACM in the form of access control. The authentication process assures that the message is from a valid source, the management information was not altered in transit and that it was not delayed or replayed. When a message passed the authentication process, it is certain the message came from an authorized source.
To achieve authentication, each pair of SNMP entities must share a secret authentication key. In SNMPv3, there are two different secret keys, one for privacy (encryption key or privacy key, privKey) and another for authentication (authentication key, authKey). Authentication is supplied through implementing Hashed Message Authentication Code