Here again we see two scenarios involving Alice, Bob, and Trudy. In the left scenario, a blue arrow labeled "I am Alice, password" points rightward from Alice to Bob. A second blue arrow points from Alice to a tape recorder positioned next to Trudy. A blue arrow labeled "OK" points leftward from Bob to Trudy.

In the right scenario, a blue arrow labeled "I am Alice, password" points rightward from Trudy's tape recorder to Bob. A blue arrow labeled "OK" points from Bob to Trudy.