Nonces

A nonce is a value used only once in cryptographic communications to ensure freshness and protect against replay attacks. The term is a contraction of "number used once" and can refer to a random, unpredictable value or a monotonically increasing counter, depending on the protocol.

In practice, nonces are combined with other data such as a shared key, username, timestamp, or message to produce a unique input for authentication, encryption, or integrity checks. A nonce is typically not secret; its value must be known to both communicating parties but should not be reused with the same key.

Common applications include challenge-response authentication, where a server issues a nonce and a client responds with a hash that includes the nonce; and in transport protocols like TLS, where nonces help derive session keys. In blockchain networks using proof-of-work, miners repeatedly modify a value called the nonce in the block header until the header hash meets a target.

Important distinctions: a nonce is not a secret, and its primary properties are uniqueness and, in some cases, unpredictability; a nonce is different from a salt (which prevents precomputation of hashes) and from an initialization vector (which ensures distinct ciphertexts).

Implementation considerations include choosing an appropriate generation method (cryptographically secure randomness or strict counters), ensuring freshness within the same key, and securely storing nonces when necessary to avoid reuse. Reuse with the same key can undermine security.