In CBC mode what requirement for the IV is emphasized by guidance?

• A. The IV must be unpredictable and unique for each message
• B. The IV must be secret
• C. The IV must be equal to the key length
• D. The IV can be reused across messages

Answer: (A)

In CBC mode, the Initialization Vector is XORed with the first plaintext block before encryption, so it must be unpredictable and unique for each message. Unpredictability prevents an attacker from guessing the IV and learning anything about the plaintext, while uniqueness per message ensures that even identical plaintext blocks produce different ciphertext when encrypted with the same key. If the IV is reused across messages, or if it’s predictable, two messages with the same first plaintext block could yield the same first ciphertext block, revealing patterns and enabling certain attacks. The IV does not need to be secret and is commonly sent with the ciphertext, but it should be random and unique for every encryption under the same key to maintain security.