Amy and Betty have a shared key $k$, and the protocol below is to provide a mutual authentication for both Betty and Amy. A sends B : $n_a$ B sends A : $n_b \;\|\; E(k, n_a)$ A sends B : $E(k, n_b)$ ...
Alice has a message, generates a one time pad, encrypts her message and sends it to Bob. Bob generates his own one time pad, encrypts the message again, and sends it back to Alice. Alice then decrypts ...