How about puzzles for a metaphor? The police can seize my puzzles, but they can't compell me to tell them the solution, even if they can't solve it themselves.

How about puzzles for a metaphor? The police can seize my puzzles, but they can't compell me to tell them the solution, even if they can't solve it themselves. It does miss the effects (potentially incriminating content being decrypted), and maybe it's a bit too silly.