Answer Happy

In this question, you are asked to compare the security services that are provided by digital signatures (DS) and message authentication codes (MAC). Assume Oscar is able to observe all messages sent from Alice to Bob and vice versa. Search documents and filenames for text public one in case of DS. State whether and how (1) and (1) MAC protect against each attack. The value auth(x) is computed with a DS or a MAC algorithm, respectively. (a) (Message integrity) Alice sends a message x = "Transfer £1000 to Mark" in the clear and also sends auth(x) to Bob. Oscar intercepts the message and replaces "Mark" with "Oscar." Will Bob detect this? Explain your answer. (b) (Replay) Alice sends a message x="Transfer £1000 to Oscar" in the clear and also sends auth(x) to Bob. Oscar observes the message and signature and sends them 100 times to Bob. Will Bob detect this? Explain your answer. (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? Explain your answer. (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? Explain your answer.