What is a nonce in cryptography?

• A. A value used once to prevent reuse attacks
• B. A fixed secret key
• C. A bit pattern used for padding
• D. A counter that never resets

Answer: (A)

Nonces provide freshness by using a value that is used exactly once in a given cryptographic operation. This uniqueness prevents replay attacks, where an attacker resends a captured message hoping it will be accepted as new. Nonces are usually not kept secret; they’re included with the ciphertext or agreed during a protocol. They can be random, time-based, or derived from a counter, as long as each use under the same key is distinct. In modes like CTR or GCM, reusing a nonce with the same key can expose all the plaintext, so ensuring a unique nonce per encryption is critical. That’s why a nonce is best understood as a value used once to prevent reuse attacks. The other options describe a fixed secret key, a padding pattern, or a counter that never resets, none of which capture the essential role of a nonce.