1. Explain why private-public keys cannot be used in creating a MAC.
2. The nonce in Figure 31.22 is to prevent a replay of the third message. Eve cannot replay the third message and pretend that it is a new request from Alice, because when Bob receives the response, the value of RB is not valid anymore. This means that we can eliminate the first and the second message if we add a timestamp to the diagram. Show a new version of Figure 31.22 using a timestamp.
