Kerberos

In Greek mythology, Kerberos, also called Cerberus, is the three-headed dog that guards the entrance to Hades.

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Kerberos is also a computer network authentication protocol designed for use on insecure networks (the Internet for example), based on the key distribution model of Needham and Schroeder. It allows individuals communicating over a network to prove their identity to each other while also preventing evesdropping or replay attacks, and provides for detection of modification and the prevention of unauthorized reading.

The protocol can be specified as follows in security protocol notation, where Alice is authenticating herself to Bob using a server S:

We see here that the security of the protocol relies heavily on Timestamps being reliable indicators of the freshness of a communication (see the BAN logic).

Kerberos is freely available from MIT, under copyright permissions similar those used for BSD. There is an RFC for Kerberos 5: RFC 1510.