24 Secure Channel, Key Distribution, and Certificates

In the previous chapter, we were considering how Bob can determine whether he's really communicating with Alice. Although Alice initiated the communication by sending a message to Bob, she has the mirror-image problem: she doesn't (yet) know that she's really talking with Bob. She knows that she's sending messages to Bob's address, but let's be paranoid for a minute. What if an attacker is intercepting those messages and sending back the attacker's version of the replies? Is there a way to rule out that possibility?