Describe in detail a man-in-the-middle attack on the Diffie-Hellman key-exchange protocol whereby the adversary ends up sharing a key k_A with Alice and a different key k_B with Bob, and Alice and Bob cannot detect that anything has gone wrong.

What happens if Alice and Bob try to detect the presence of a man-in-the-middle adversary by sending each other (encrypted) questions that only the other party would know how to answer?