What distinguishes cipher block chaining (CBC) mode in a cryptographic system?

• A. It encrypts data block-by-block independently
• B. It uses the previous ciphertext block to XOR with the next plaintext block
• C. It uses a streaming keystream
• D. It requires no initialization vector

Answer: (B)

CBC mode ties each block’s encryption to the result of the previous block. Specifically, before encrypting a plaintext block, CBC XORs it with the previous ciphertext block, then runs that through the block cipher. This creates a chain where the output depends on everything that came before, and the first block is seeded with an initialization vector (IV). Because of this chaining, identical plaintext blocks produce different ciphertext blocks at different positions, which helps hide patterns in the data and prevents simple block repetition from leaking information.

This isn’t block-by-block independent encryption—that would be a mode like ECB. It also doesn’t rely on a streaming keystream, which is used by certain other modes. Additionally, CBC requires an initialization vector to start the chain, so saying that no IV is used isn’t accurate.