(a) Consider the following encryption scheme to provide confidentiality. A wants to send a message M, a 64-bit message, to B, and wants to make sure BG can't read M. B generates a random 64-bit number R and the step (1) communication from B to A is R. A XORs M with R to get the ciphertext C and the step (2) communication from A to B is C. When B receives C, he XORs C with R to get M. 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. (b) 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 . 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? 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 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