Which mode uses a counter to generate a keystream?

• A. OFB Mode
• B. CTR Mode
• C. Initialization Vector (IV)
• D. Replay Attack

Answer: (B)

CTR mode uses a counter to generate a keystream. In this approach, a counter value is prepared for each block (often combined with a nonce or IV for uniqueness), then the block cipher encrypts that counter to produce a keystream block. This keystream is XORed with the corresponding plaintext block to produce ciphertext. Because the counter advances for every block, each keystream block is distinct, which is essential for security and also allows parallel processing of blocks.

This differs from OFB mode, where the keystream comes from repeatedly encrypting the previous keystream output rather than a counter. The IV is just an initial value used to start some modes, not the ongoing keystream generator itself. A replay attack is an attack type, not a mode of operation.