3. In this problem we will compare the security services that are provided by digital signatures (DS) and message authentication codes (MAC). We assume that Oscar is able to observe all messages sent from Alice to Bob and vice versa. Oscar has no knowledge of any keys but the public one in form of DS. Answer whether and how (i)DS and (ii) MAC protect against each of the following attacks and explain your answers. The value auth(x) is computed with a DS or a MAC algorithm, respectively. Here, x isjust the message (without additional information) (20 points) (a) (Message integrity) Alice sends a message x -"Transfer $1000 to Mark" in the clear and also sends authx) to Bob. Oscar intercepts the message and replaces Mark" with "Oscar." Will Bob detect this e ind sends (b Replay Alice sends a message x = Transfer $1000 to Oscar in the clear and also sends auth x to Boo. Oscar observes the message and i them 100 times to Bob. Will Bob detect this? a (c) (Sender authentication with cheating third party) Oscar claims that he sent some message x with a valid auth(x) to Bob but Alice claims the same. Can Bob clear the question in either case? (d) (Authentication with Bob cheating) Bob claims that he received a message x with a valid signature auth(x) from Alice (e.g., "Transfer $1000 from Alice to Bob") but Alice claims she has never sent it. Can Alice clear this question in either case