The nonces are different each time the 401 authentication challenge response code is presented, thus making replay attacks virtually impossible. For instance, nonces are used in HTTP digest access authentication to calculate an MD5 digest of the password. Usage Authentication Īuthentication protocols may use nonces to ensure that old communications cannot be reused in replay attacks. Some authors define pseudo-randomness (or unpredictability) as a requirement for a nonce. To ensure that a nonce is used only once, it should be time-variant (including a suitably fine-grained timestamp in its value), or generated with enough random bits to ensure a probabilistically insignificant chance of repeating a previously generated value. The addition of a client nonce (" cnonce") helps to improve the security in some ways as implemented in digest access authentication. Many nonces also include a timestamp to ensure exact timeliness, though this requires clock synchronisation between organisations. They are often random or pseudo-random numbers. A nonce is an arbitrary number used only once in a cryptographic communication, in the spirit of a nonce word.
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