Which mode turns a block cipher into a stream cipher?

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

Answer: (C)

Cipher Feedback mode uses the block cipher to generate a keystream and XORs that keystream with the plaintext, effectively turning the block cipher into a stream cipher. It does this by maintaining a shifting register that starts with an initialization vector. You encrypt the contents of that register with the block cipher to produce a keystream block, take the leftmost s bits of that keystream, and XOR them with the next s bits of plaintext to produce ciphertext. The register is then updated by shifting in the produced ciphertext, and the process repeats for the next segment. Because you can operate on small segments (bits or bytes) rather than fixed blocks, the cipher behaves like a stream cipher and doesn’t require padding. Other modes operate on whole blocks and don’t provide this continuous keystream generation with feedback, so they don’t turn the block cipher into a stream cipher.