David A Wagner writes:

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Tough!? It's trivial for an active attacker, in the stream cipher case. He just xors some bits: no clever cryptanalysis needed. (Or did you mean it's tough to mount an active attack? I agree: that requires significant knowledge or motivation.) Rev. Mark Grant responds: Exactly.. once you have an active attack set up, it's trivial, but setting that up is quite hard.

But setting up an active attack is probably a hell of a lot easier than breaking 128-bit RC-4 (or even 64-bit). andrew