Explain the use of a nonce in a challenge-response protocol.
Try an answer before revealing the guidance below.
Key Concepts
- Nonce
- Freshness
- Challenge-response
- Replay attack
Answer Approach
- Define the fresh challenge.
- Explain how the claimant proves knowledge of a secret using it.
- Show why an old response cannot simply be replayed.
Full Answer
Answer status: Verified against study notes. Revision notes, not an official marking scheme.
A verifier sends a fresh, unpredictable nonce (a number used only once) to the claimant. The claimant computes a response tied to that nonce and a secret, such as encrypting it with a shared key or signing it with a private key. The verifier checks the response. Because the challenge is new for each run, replaying a previously captured response fails, which defeats playback attacks.
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