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.