5.5 Challenge-response protocols

The working principle of cryptographic challenge-response protocols is illustrated in Figure 5.5. Claimant Bob proves his identity to verifier Alice by demonstrating that he knows a secret that, by design, is known only to Bob (and, potentially, Alice). However, unlike with basic password-based authentication, as shown in Figure 5.3, Bob does not reveal the secret on the communication channel.

Instead, Bob responds to a time-variant challenge. Computing the correct response requires both Bob’s secret and the challenge. The challenge, for example, a number, is chosen by verifier Alice randomly and secretly at the beginning of the protocol.

Figure 5.5: Working principle of entity authentication protocols

Why are challenge-response protocols more secure than passwords, and how do they overcome the drawbacks of password-based authentication we discussed previously?

First, the claimant never discloses the secret. Unlike with passwords...