What value is used to ensure unique ciphertexts by varying per encryption?

• A. Nonce
• B. Initialization Vector (IV)
• C. Pass-the-Hash
• D. Digital Signature

Answer: (B)

Varying the input that seeds the encryption with the same key is what prevents identical plaintexts from producing identical ciphertexts. That varying value is the initialization vector. In typical symmetric modes, the IV is combined with the plaintext (or influence the first block) so that encrypting the same message twice under the same key yields different ciphertexts. The IV is usually transmitted with the ciphertext so the recipient can decrypt it. A nonce is similar in concept but is used differently in certain modes, so the standard mechanism that ensures unique ciphertexts per encryption with a fixed key is the initialization vector. Pass-the-Hash and Digital Signature serve authentication and integrity functions, not the randomness of the ciphertext.