Consider the following challenge-response protocol for A to convince B that he is indeed A. Here we are assuming that once a virtual circuit has been set up, the BG cannot alter messages in the middle. So the idea is that at the start when the virtual circuit is being set up, A has to convince B that he is indeed A. A and B share a secret value S. B sends A a nonce N. A takes the first 64 bits of N and treats this as a key K for DES, and returns C = EK (S) back to B. B checks whether DK (C) = S and if it is, accepts that A is indeed A (because he knows the secret S).

i. Suppose this protocol was being used only once i.e. A has to convince B of who he is only one time. Once A has done this, this protocol is never going to be used again. Is this a good scheme? Give a YES/NO answer. If you said NO, explain your answer i.e. explain what you think is the single biggest weakness of the scheme.