What value is used only once to prevent replay attacks?

• A. Initialization Vector (IV)
• B. Replay Attack
• C. Pass-the-Hash
• D. Nonce

Answer: (D)

A nonce is a value that is used only once to guarantee freshness in a cryptographic exchange. By generating a new, unpredictable value for each session or message, the system ensures that any intercepted communication cannot be reused later. If someone captures a valid message and tries to replay it, the recipient will see the old nonce and reject the replay because the nonce has already been used or doesn’t match the current challenge.

This idea is different from an initialization vector, which randomizes encryption to produce different ciphertexts for the same plaintext but isn’t inherently about preventing replays. Pass-the-Hash refers to stealing and reusing credential hashes, a different kind of attack, and a replay attack is the threat itself, not the mechanism that stops it. Nonce specifically provides the freshness needed to stop replayed messages from being accepted.