• Nonce means 'for the present time' or 'for a single occasion or purpose', although the word is not often found in general use. A dictionary may note nonce words, those for which there is only a single textual instance.

• In security engineering, a nonce is a 'number used once'. It is often a random or pseudo-random number issued in an authentication protocol to ensure that old communications cannot be reused in 'replay attacks'. For instance, nonces are used in HTTP digest access authentication to calculate an MD5 digest of the password. The nonces are different each time the 401 authentication challenge response code is presented, thus making the replay attack virtually impossible. Some also refer to Initialization Vectors as nonces for the above reasons. In order to ensure that a nonce is used only once it should be time-variant (including a suitably granular timestamp in its value), or generated with enough random bits to ensure a probabalistically insignificant chance of repeating a previously generated value.

• In the UK, the term nonce is a slang word used to refer to a sex offender, especially child sexual abusers, and thus as an insult. Folk etymologies for the origins of this word state that the word derives from Not Of Normal Criminal Ethos, used to mean an offender whose motivation was different from the purely acquisitive, or possibly Not On Normal Courtyard Exercise, for prisoners that were not to be mixed with the general population for their own safety. As with most cases, these folk etymologies are examples of backronyms and should not be taken seriously. The actual origin of this word is unknown, but it is probable that it is derived from a combination of nancy, a derogatory term referring to homosexual males, and British prisoner slang for prisoners who have committed a "nonsense crime", as opposed to a crime such as theft. Several websites have been set up claiming to give details of such people, however whether the information is reliable or not is another matter. Although nonce in the UK is traditionally a term for a sex offender, it is also sometimes used as a general term of abuse.

External links

• Sam Ruby Blogging on Nonce with an implementation
• HTTP Authentication: Basic and Digest Access Authentication
• Robust Explicit Congestion Notification (ECN) Signaling with Nonces
• UMAC: Message Authentication Code using Universal Hashing
• The Sweeney dictionary