Which block cipher mode chains blocks together using XOR and an IV?

• A. CFB Mode
• B. AES
• C. CBC Mode
• D. ECB Mode

Answer: (C)

In this mode, each plaintext block is tied to the previous ciphertext block through XOR, with an initialization vector used for the first block. Specifically, the plaintext block is XORed with the previous ciphertext (or the IV for the first block) and then encrypted to produce the new ciphertext block. This creates a chain so that the encryption of each block depends on all earlier blocks, providing diffusion across the message. The IV ensures the first block is uniquely randomized even if the same message is encrypted multiple times.

This is different from other approaches: some modes encrypt blocks independently (no chaining), and others use the cipher's output as a keystream in a way that doesn’t match the plain XOR-with-previous-ciphertext-before-encryption description. Using an algorithm like AES is just the cipher itself, while the mode determines how blocks are chained and combined with IVs.