A 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. It also provides for detection of modification and the prevention of unauthorized reading.
Kerberos is freely available from MIT, under copyright permissions similar those used for BSD. There is an RFC for Kerberos 5: RFC 1510.