Otway-Rees

Otway-Rees is a computer network authentication protocol designed for use on insecure networks (the Internet for example). 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:

One problem with this protocol is that A and B do not know if each other have a copy of the key. Another problem is that although the server tells B that A used a nonce, B doesn't know if this was a replay of an old message.

See also: Kerberos, Otway-Rees, Wide Mouth Frog.