Nonce ("number used once") is, typically a randomly generated value (technically does not need to be a number) that's associated with a message in a cryptographic scheme and must be unique within some specified scope (such as a given time interval, or a session).[1]

Nonce typically used to prevent Replay attack.

For example, challenges used in challenge-response authentication protocols generally must not be repeated until authentication keys are changed, or there is a possibility of a Replay attack.

Using a Nonce as a challenge is a different requirement than a random challenge, because a Nonce may be predictable.

Nonce may be used for an Initialization Vector it MUST only to be non-repeating and the required randomness is derived internally.

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