5.3 Message authentication versus entity authentication
What happens if message authentication or entity authentication fails? We can answer this question by looking at Figure 5.2 again. There are two ways in which Eve can manipulate messages sent from Bob to Alice. Eve’s first option, illustrated on the left-hand side in Figure 5.2, is to break the message authentication of the communication between Alice and Bob by suppressing the original messages, changing them, or replacing them with messages of her own. Eve’s other option, shown on the right-hand side in Figure 5.2, is simply to replace Bob in the communication and receive Alice’s messages instead of Bob, or send Alice any message of Eve’s choice, thereby breaking entity authentication.
The difference between entity authentication and message authentication is further illustrated by the notorious email-based phishing attacks you are all familiar with. In this case, an attacker sends you...