Yahalom (protocol)
Introduction '''Yahalom''' is an authentication and secure key-sharing protocol designed for use on an insecure network such as the Internet. Yahalom uses a trusted arbitrator to distribute a shared key between two people. This protocol can be considered as an improved version of Wide Mouth Frog protocol (with additional protection against man-in-the-middle attack), but less secure than the Needham–Schroeder protocol.
Protocol description If Alice (A) initiates the communication to Bob (B) with S is a server trusted by both parties, the protocol can be specified as follows using security protocol notation:* A and B are identities of Alice and Bob respectively*  is a symmetric key known only to A and S*  is a symmetric key known only to B and S*  and  are nonces generated by A and B respectively*  is a symmetric, generated key, which will be the session key of the session between A and B:Alice sends a message to Bob requesting communication. :Bob sends a message to the Server encrypted under . :The Server sends to Alice a message containing the generated session key  and a message to be forwarded to Bob. :Alice forwards the message to Bob and verifies  has not changed. Bob will verify  has not changed when he receives the message.
BAN-Yahalom Burrows􏰂, Abadi􏰂 and Needham proposed a variant of this protocol in their 1989 paper as follows:In 1994, Paul Syverson demonstrated two attacks on this protocol.
See also * Kerberos protocol* Otway–Rees protocol* Neuman–Stubblebine protocol
References ** M. Burrows, M. Abadi, R. Needham  A Logic of Authentication, Research Report 39, Digital Equipment Corp. Systems Research Center, Feb. 1989* M. Burrows, M. Abadi, R. Needham  A Logic of Authentication. ACM Transactions on Computer Systems, v. 8, n. 1, Feb. 1990, pp. 18—36