Fill in the blank with the appropriate term.
A _________ is a random or pseudo random number issued in an authentication protocol to ensure that old communications are not used in replay attacks.

A.
nonce

Explanation:
In security engineering, a nonce is an abbreviation of 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. To ensure that a nonce is used only once, it should be time-variant.