Suppose Alice wants to communicate with Bob using symmetric key cryptography using a session key KS. In Section 8.2, we learned how public-key cryptography can be used to distribute the session key from Alice to Bob. In this problem, we explore how the session key can be distributed-without public key cryptography-using a key distribution center (KDC). The KDC is a server that shares a unique secret symmetric key with each registered user. For Alice and Bob, denote these keys by KA-KDC and KB-KDC Design a scheme that uses the KDC to distribute Ks to Alice and Bob. Your scheme should use three messages to distribute the session key: a message from Alice to the KDC; a message from the KDC to Alice; and finally a message from Alice to Bob. The first message is K A.KDC (A, B). Using the notation, KA-KDC' KB-KDC' S, A, and B answer the following questions.
a. What is the second message?
b. What is the third message?