Problem 14.6 on pp 448-449 of the textbook (Cryptography-and-Network-Security-Principles-and-Practice-6th-Edition)

14.6 Consider the following protocol, designed to let A and B decide on a fresh, shared session key K=AB. We assume that they already share a long-term key KAB. 1. AS B:A, NA.

2. B S A:E(KAB, [NA, K=AB])

3. AS B:E(K=AB, NA)

a. We first try to understand the protocol designers reasoning:

Why would A and B believe after the protocol ran that they share K=AB with the other party?

Why would they believe that this shared key is fresh?

In both cases, you should explain both the reasons of both A and B, so your answer should complete the sentences

A believes that she shares K=AB with B since

B believes that he shares K=AB with A since

A believes that K=AB is fresh since

B believes that K=AB is fresh since

b. Assume now that A starts a run of this protocol with B. However, the connection is intercepted by the adversary C. Show how C can start a new run of the protocol using reflection, causing A to believe that she has agreed on a fresh key with B (in spite of the fact that she has only been communicating with C). Thus, in particular, the belief in (a) is false.

c. Propose a modification of the protocol that prevents this attack.