Which technique is NOT listed as a method to thwart replay attacks in the material?

• A. Time stamps
• B. Sequence numbers
• C. Nonces
• D. Encryption

Answer: (D)

Replay attacks rely on reusing a valid message to gain access or repeat an action. To prevent them, protocols use freshness mechanisms such as time stamps, sequence numbers, and nonces. Time stamps attach the moment a message was created and the receiver checks that it’s recent enough and not reused. Sequence numbers provide a changing counter that the receiver tracks, so duplicates or out-of-order messages are rejected. Nonces are random values used once; a response must prove knowledge of the specific nonce issued for that session, making replay ineffective because old nonces won’t be accepted.

Encryption alone does not address freshness. An attacker can capture and resend an encrypted message, and without a freshness check, the system may accept it. So encryption by itself isn’t a replay-protection mechanism, whereas the other methods explicitly enforce that each message is unique and timely.