Which mode uses feedback by XORing the previous ciphertext with the plaintext to turn a block cipher into a stream cipher?

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

Answer: (C)

Cipher Feedback mode creates a keystream by feeding the previous ciphertext block (or an IV for the first block) into the block cipher, and then XORs that keystream with the plaintext to produce the ciphertext. That feedback loop—where each ciphertext influences the next keystream—gives the block cipher a stream-like behavior, allowing encryption to proceed as a stream of bits or bytes rather than fixed-size blocks. Decryption uses the same keystream generation, with P = C XOR E_k(previous ciphertext). This self-feeding setup is what makes it the mode that turns a block cipher into a stream-like cipher.