Yes. As a simple example, if you XORd your input text with a 1-byte key, the letter frequency of english text would be unchanged. So your most common byte in y

Yes. As a simple example, if you XORd your input text with a 1-byte key, the letter frequency of english text would be unchanged. So your most common byte in your output would be likely to be the encrypted char 'e'. You could then recover the key by XOR 'e' with that most common byte. More complex systems have more complex patterns, but that's what cryptopgraphers do - try and spot weaknesses in an algorithm. As always, this is a great read: http://chargen.matasano.com/chargen/2009/7/22/if-youre-typing-the-letters-a-e-s-into-your-code-youre-doing.html http://chargen.matasano.com/chargen/2009/7/22/if-youre-typin... Search for "Garbling a block to confuse an app" and read on from there for a real-world use of output patterns (used when you can partially control the input).