What is a fixed-size pseudorandom number that is fed into a symmetric cipher to increase randomness?

The correct answer is that the fixed-size pseudorandom number fed into a symmetric cipher to increase randomness is an Initialization Vector (IV). The IV plays a crucial role in ensuring that the same plaintext encrypted multiple times with the same key produces different ciphertexts. This randomness enhances security by preventing attackers from being able to make inferences based on repeated patterns within encrypted data.

In symmetric encryption algorithms, particularly block ciphers like AES in certain modes of operation (e.g., Cipher Block Chaining mode), the IV adds an additional layer of security. By introducing an IV, even if the same key is used for encryption, the output will vary due to the influence of the IV. This ensures that identical plaintext blocks will yield different ciphertext blocks, helping to mitigate vulnerabilities associated with predictable encryption patterns.

Other options, while they serve important functions in cryptography, do not fit this specific description. Salt, for instance, is used primarily in hashing algorithms to defend against pre-computed hash attacks but does not directly contribute to the randomness within a symmetric cipher. The key is the primary component for the encryption and decryption process itself but does not serve as an additional random input like an IV does. "Chain" generally refers to the chaining process in certain modes of operation