(a) Consider the following challenge-response protocol for A to convince B that lie 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 calculates C to be first 64 bits of N XORed with the first 64 bits of S. A returns C back to B. B checks whether C= first 64 bits of N XORed with the first 64 bits of S. If it is, B 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?

A. Give a YES/NO answer.

B. If you said NO, explain your answer i.e. explain what you think is the single biggest weakness of the scheme.

ii. Now suppose this protocol was being repeated many times i.e. A has to convince B of who he is repeatedly, and each time follows the above protocol, with B generating a different nonce each time. Is tills a good scheme?

A. Give a YES/NO answer.

B. If you said NO. explain your answer i.e. explain what you think is the single biggest weakness of the scheme.

(b) consider the following encryption scheme to provide confidentiality. A wants to send a message M to B. A calculates X, the ASCII representation of today's date. A then XORs M with X to get the ciphertext C. when B receives C, he XORs C with X to get M. you can assume that the number of bits in M is no more than the number of bits in X. Is this a good scheme?

i. Give a YES/ NO answer.

ii. If you said NO, explain your answer i.e. explain what you think is the single biggest weakness of the scheme.