QUOTE (kzt)
Any cryptosystem that can be broken by one piece of chosen plaintext is worthless. People find attacks that that require only several hundred million plaintexts to be pretty clever.
In the real world, yes. However, the assumption for playability is that "known plaintext attacks" generally can be made to work using whatever systems of math people use in the future. This makes public keys inherently unsafe, and means that direct access to any computer which is performing the encryption scheme will potentially render up the encryption key.
This is what we call "movie logic" - which is that if you show something on screen, it has to be small enough to be comprehensible for the viewers. So for example, finding a weakness in the system would in the "real world" be something involving combing through millions of bytes of metadata to reverse engineer the transforms that the computer would generate. But in the game world, this involves finding the actual e-mail and comparing it to the encrypted version. Because that's much cooler looking, and easier to explain to non-mathematicians.
People who aren't good at math can still understand the logic behind a known plaintext attack. You have the input, you have the output, you do "mathematics" to find the transform in the middle. This makes it ideal as the standard for what players are attempting to be able to do, because players who have no special computer knowledge can still imagine the kinds of things that their character might do in order to get to the weaknesses of the system. The logic behind counting processor cycles and extrapolating what reference frame shifts would be generated for specific messages is something which does not make sense to a lay person; making it very bad for the game. Remember that the ultimate goal is for complete lay people to describe in action words what the hacker is doing, both from the standpoint of the gamemaster and the players.
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Oh, and because I forgot to mention: we're still using the technomancer terminology of echoes and submersion.
-Frank