What is the purpose of an initialization vector (IV)?

• A. To ensure different ciphertext for identical plaintext with the same key
• B. To increase speed
• C. To compress the plaintext
• D. To provide message authentication

Answer: (A)

The purpose of an initialization vector is to introduce randomness so that encrypting the same plaintext with the same key yields different ciphertext each time. In modes like CBC, the IV is combined with the first plaintext block before encryption, so changing the IV changes the first ciphertext block and, by extension, the rest of the ciphertext. This prevents pattern analysis and leakage from repeated messages. The IV itself is not a secret; it is typically transmitted with the ciphertext, but it must be unique (and in some modes unpredictable) for every encryption to maintain security. It does not affect speed, it does not compress data, and it does not provide message authentication; authentication comes from separate mechanisms like MACs or AEAD schemes.