cars - 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'.

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.

In the UK, the term nonce is a slang word used to refer to a paedophile, and thus as an insult. The origin of this word is unknown, but several explanations have been put forth. One explanation is that it is derived from the word nancy, a derogatory term referring to homosexual males. Another holds that the word is an acronym derived from Her Majesty's Prison Service in the UK. Possible acronyms include Not Of Normal Criminal Ethos, used to mean an offender whose motivation was different from the purely acquisitive, and Not On Normal Courtyard Exercise, for prisoners that were not to be mixed with the general population for their own safety. Supposed etymologies containing acronyms should be held suspect as they are frequently cases of backronyms.