Kerberos

Kerberos; also spelled Cerberus. n. The watch dog of Hades, whose duty it was to guard the entrance--against whom or what does not clearly appear; . . . it is known to have had three heads. . .

-Ambrose Bierce, The Enlarged Devil's Dictionary

Kerberos is a network authentication system for use on physically insecure networks, based on the key distribution model presented by Needham and Schroeder. It allows entities communicating over networks to prove their identity to each other while preventing eavsdropping or replay attacks. It also provides for data stream integrity (detection of modification) and secrecy (preventing unauthorized reading) using cryptography systems such as DES.

Kerberos works by providing principals (users or services) with tickets that they can use to identify themselves to other principals and secret cryptographic keys for secure communication with other principals. A ticket is a sequence of a few hundred bytes. These ticket can then be embedded in virtually any other network protocol, thereby allowing the processes implementing that protocol to be sure about the identity of the principals involved.