Nonce

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 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 nancy, a derogatory term referring to homosexual males. Several websites have been set up claiming to give details of such people, however whether the information is reliable or not is another matter.