A CDE Definition

A continuously changing number used in combination with a secret key to encrypt data. Initialization vectors (IVs) are used to prevent a sequence of text that is identical to a previous sequence from producing the same exact ciphertext when encrypted. For example, packets have address fields that are generally fixed in location within the header of the packet. If attackers view the same encrypted data over and over, it provides them with clues to interpret their original values. See nonce.

(Number ONCE) An arbitrary number that is generated to provide a unique identification or for security purposes such as when logging in to a network (see initialization vector). The nonce is used only once and not repeated. Although random and pseudo-random numbers theoretically produce unique numbers, there is the possibility that the same number can be generated more than once. However, if a very large, true random number is used, the chances are extremely small. A perfect nonce is the time of day; for example, 12.53 seconds past 5:13pm on 1/18/2012 can only occur once.

Pronounced like the "nons" in "nonsense," nonce is actually an English word that means "for the present occasion or time."