What term is best described as a fixed-size, random stream added to a block cipher to increase randomness?

The term that is best described as a fixed-size, random stream added to a block cipher to increase randomness is the Initialization Vector (IV). An Initialization Vector is crucial in cryptographic algorithms, especially in modes of operation for block ciphers such as Cipher Block Chaining (CBC). It acts as a random input that, when combined with the plaintext, ensures that the same plaintext block will encrypt to a different ciphertext block each time, thus enhancing the security by preventing patterns from appearing in the encrypted data.

Using an IV helps in ensuring that identical plaintexts will yield different ciphertexts, which in turn protects against certain types of cryptographic attacks, such as replay or chosen-plaintext attacks. Its randomness is essential for creating a unique encryption instance even when the same key is used, thereby significantly bolstering overall data security.

In contrast, other terms like a random number generator is a tool used to produce random values, but it does not specifically refer to the context of adding randomness to block ciphers. Salt is typically used in hashing to prevent rainbow table attacks, and a nonce refers to a number used once to ensure freshness in communication protocols but is not necessarily tied to block ciphers in the same way as an Initialization Vector.