Which mode generates a keystream independent of plaintext and ciphertext?

• A. CTR Mode
• B. Initialization Vector (IV)
• C. OFB Mode
• D. Hash Function

Answer: (C)

In OFB mode, the block cipher is used as a keystream generator. You start with an initialization vector, encrypt that value with the secret key to produce the first keystream block, and then feed that keystream block back as the input to the block cipher to generate the next keystream block, and so on. The crucial point is that the keystream depends only on the key and the IV; it does not depend on the plaintext being encrypted or on any ciphertext produced so far. The actual encryption is simply XORing the plaintext with this keystream.

That’s why this mode is considered to generate a keystream independent of both plaintext and ciphertext. If you used the same key and IV again, you’d get the same keystream, which is why using a fresh IV for each message is essential. For contrast, CTR mode also produces a keystream that doesn’t depend on the plaintext or ciphertext, but it does so by encrypting an incrementing counter rather than feeding back the previous keystream block. The hash function option isn’t used to create a keystream in this encryption scheme.