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 designer's 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. | |
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