Question
Suppose Alice wants to communicate with Bob using symmetric key cryp- tography using a session key K S . 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 K A-KDC and K B-KDC . Design a scheme that uses the KDC to distribute K S 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, K A-KDC , K B-KDC , S, A, and B answer the following questions.
a. What is the second message?
b. What is the third message?
Answer
a)
Consider the above data.
The first message is K A-KDC (A, B). A message from the KDC to Alice and extracted R1 from that the message. Then the representation of second message is KB-KDC (A, K) and send it to Bob.
b)
The second message KB-KDC (A, K) is decrypt by Bob. Then extracts A and K. So, the third message is Bob receive the one time session key K and make the authenticating with Alice using K.