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 Needham-Schroeder.
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 K A S is a symmetric key known only to A and S K B S is a symmetric key known only to B and S N A and N B are nonces generated by A and B respectively K A B is a symmetric, generated key, which will be the session key of the session between A and B A → B : A , N A
Alice sends a message to Bob requesting communication.
B → S : B , { A , N A , N B } K B S
Bob sends a message to the Server encrypted under
K B S .
S → A : { B , K A B , N A , N B } K A S , { A , K A B } K B S
The Server sends to Alice a message containing the generated session key
K A B and a message to be forwarded to Bob.
A → B : { A , K A B } K B S , { N B } K A B
Alice forwards the message to Bob and verifies
N A has not changed. Bob will verify
N B has not changed when he receives the message.
