In security engineering, nonce is an abbreviation of "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 that the 401 authentication challenge response code is presented, and each client request has a unique sequence number, thus making replay attacks and dictionary attacks virtually impossible.