And depending on the situation, the key-holder will decide whether to give them a key that destroys the real data, or that doesn't (and hides it).

Don't even bother trying to destroy the original. Chances are they will make a backup of everything before attempting anything. And destroying something is likely to be very obvious, and very damning. Best bet is to hide a second thing, which is kind of incriminating, but not much. And hide it less well that the real thing. People who think they understood the trick won't look any further, because they think they understood it. So, be sure to wail and complain when they discover the fake data, and not bear a knowing smile :) -- Vincent Penquerc'h