Suppose we are using a three-message mutual authentication protocol, and Alice initiates contact with bob. Suppose we wish bob to be a stateless server, and therefore it is inconvenient to require him to remember the challenge he sent to Alice. Let's modify the exchange so that Alice sends the challenge back to Bob, along with the encrypted challenge.

So the protocol is:

 

otocol is: Alice I'm Alice R R, KAlice-Bob(R) Bob