Microsoft RC4 Flaw One of the most important rules of stream ciphers is to never use the same keystream to encrypt two different documents. If someone does, you can break the encryption by XORing the two ciphertext streams together. The keystream drops out, and you end up with plaintext XORed with plaintext -- and you can easily recover the two plaintexts using letter frequency analysis and other basic techniques. It's an amateur crypto mistake. The easy way to prevent this attack is to use a unique initialization vector (IV) in addition to the key whenever you encrypt a document. Microsoft...