Nonces can be used in ________.

• A. All applications
• B. Client/server applications
• C. Time-insensitive applications
• D. Persistent applications

Answer: (B)

Nonces prevent replay attacks by providing a unique, single-use value that ties a specific transaction to that moment. In client-server authentication, the server issues a nonce as a challenge and the client must include that nonce in its response (often hashed with a secret). This makes the response valid only for that particular interaction, so an attacker who captures the response cannot reuse it to authenticate later. This freshness guarantee is essential in interactive client-server communications, where each login or request needs to be freshly verified. In time-insensitive or purely persistent contexts, there isn’t the same live challenge–response flow to guard against replay, so nonces aren’t typically required.