What is a fixed-size pseudorandom number that increases randomness in a symmetric cipher called?

The correct choice refers to the Initialization Vector (IV). An IV is a fixed-size pseudorandom number that is used in various symmetric encryption algorithms to enhance the randomness and unpredictability of the ciphertext. Its primary purpose is to ensure that the same plaintext encrypted multiple times will produce different ciphertexts, which is crucial for maintaining confidentiality.

When a symmetric cipher operates, especially in modes like CBC (Cipher Block Chaining), using an IV adds an additional layer of security by ensuring that identical plaintext blocks will not result in the same ciphertext when encrypted with the same key. This helps protect against certain types of attacks, such as replay attacks or pattern analysis, where an attacker might try to derive information based on repeated ciphertexts.

The IV should be unique and unpredictable for every encryption session. However, it doesn’t need to be kept secret, but it must be transmitted alongside the ciphertext so that the decryption process can correctly reconstruct the original plaintext.

Contextually, other options such as key, salt, and nonce serve different purposes in the realm of cryptography. A key is a piece of information used to encrypt and decrypt messages, while salt is often used in password hashing to prevent precomputed attacks like rainbow tables. A nonce, which stands for "number used