In Figure 4, Bart is pretending to be Bob so he can steal Alice’s identity and confidence. This is referred to as a man-in-the-middle attack if the “attacker” (Bart) makes a connection with the victims (Alice and Bob) and relays messages between them while “eavesdropping” on their communication.

How can Alice be confident that she is having a private “conversation” with Bob and not an imposter?

The answer is that she and Bob have a mutual friend, Cathy, who authenticates Bob’s credentials and warns Alice when the person to whom she is connected is not Bob.