The first rule of an output-feedback mode stream cipher, any of them, is that you should never use the same key to encrypt two different messages. Repeat after me: NEVER USE THE SAME KEY TO ENCRYPT TWO DIFFERENT MESSAGES. If you do, you completely break the security of the system. Here’s why: if you have two ciphertext streams, A + K and B + K, and you subtract one from the other, you get (A + K) - (B + K) = A + K - B - K = A - B. That’s two plaintext streams combined with each other, and is very easy to break. Trust me on this one: you might not be able to recover A and B from A - B, but a professional cryptanalyst can. This is vitally important: never use the same key to encrypt two different messages.