What is a number that is used only once and then discarded?

A nonce, short for "number used once," is specifically designed to be a unique value that is used in various cryptographic protocols and is discarded after a single use. This characteristic helps prevent replay attacks, where a malicious party might try to reuse valid data transmission to gain unauthorized access. By ensuring that a nonce is unique for each session or transaction, systems can confirm that each operation is distinct and has not already been executed.

In contrast, other terms like IV (Initialization Vector), salt, and chain refer to mechanisms that may be reused under certain conditions or serve different purposes within security protocols. An Initialization Vector is used in encryption algorithms to ensure that the same plaintext encrypted multiple times produces different ciphertexts, but it can be reused across sessions if it is random and not reconstructed. A salt is typically used in hashing functions to protect passwords by ensuring that identical passwords do not yield identical hashes, and it can be stored alongside the hashes. Chain, on the other hand, generally refers to cryptographic operations where elements are interdependent and thus cannot be treated as single-use.

Overall, the distinguishing property of a nonce being used only once before being discarded is what makes it the correct answer for this question.