A number that is used only one time and then discarded is called what?

A number used only once and then discarded is referred to as a nonce. Nonces are crucial in cryptographic protocols to ensure that old communications cannot be reused in replay attacks. By using a unique number for each session or transaction, systems can validate that a message or request is fresh and has not been reused maliciously.

For instance, when a nonce is included in a cryptographic process, it adds an additional layer of security by differentiating each interaction, making it very difficult for an attacker to intercept and reuse messages.

The other options, while related to cryptography, serve different purposes. A signature is used to validate authenticity and integrity of data, serving as a digital fingerprint. A session key typically refers to a symmetric key used for a single session of communication; it's reusable during that session but not afterward. A cipher is an algorithm for encoding or decoding messages, rather than a one-time use number.