Authentication and session key establishment require a protocol to assure the timeliness and originality of messages. Timestamps or nonces are used to guarantee the freshness of the messages.We saw two authentication protocols that use public-key ciphers, one that required synchronized clocks and one that did not. Needham–Schroeder is a protocol for authenticating two participants who each share a master symmetric-key cipher key with a Key Distribution Center. Kerberos is an authentication system based on the Needham–Schroeder protocol and specialized for client/server environments. The Diffie–Hellman key agreement protocol establishes a session key without predistributed keys and authentication.