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  • To clarify where you were not sure what I was talking about "how much data it takes before the key runs out": I was really not knowing where to go, but I understand that if I have 1KB of data and a 1KB key, then it is impossible to decrypt. Only by re-use of the key is it possible to decrypt. However, I missed the point. If I used up a 1KB key, there is no way to pass a new one except by re-using the last one. A way to resolve this might be to do some weird stuff with key rotation, but then again, it may not be worth while. Also, I don't yet understand what IV and Nonce are. Commented Jul 19, 2011 at 15:54
  • By impossible to decrypt I mean, by brute force you could find a possible decrypted copy, but with a little more brute force you would find another possible copy, repeatedly. The only clue that you would have found the right one is the length of the file. And knowing the content is, say, 1019 bytes long is not going to give you much of a clue as to which possibility is correct. Commented Jul 19, 2011 at 16:02
  • Your first describes a one-time pad. A one time pad has no cryptographic weakness, and is often described as unbreakable. For a one-time pad to work the key must be as long or longer than the data. Additionally the key, or any significant piece of the key must never be reused. In order to use a one-time pad, both send and receiver must have identical copies of a set of keys. If they run out of keys they must arange to meet and create a new set. The requirement to generate a large number of keys and then meet securely to synchronize the keys means one-time pads are infrequently used. Commented Jul 19, 2011 at 16:26
  • Most encryption systems use small (when compared to the data) keys. In one-time pad the algorithm is trivial, in more common encryption the algorithms are complex. The algorithm allows us to take a fixed size key and use it to encrypt an arbitrarly long piece of data. So, in more common encryption, the length of the data doesn't prevent encryption. Also more brute force is not meaningful. Brute force means the process of trying every possible key. There are no more keys than every possible key. Commented Jul 19, 2011 at 16:34