You make that sound so easy. What would count as evidence that you did _not_ possess a key capable of decrypting an arbitrary random-looking binary file? The key could be anything; the only real evidence that you _didn't_ have it is all in your head.
The requirement should be the other way around: they should have to prove that you did have the key, i.e. that you've decrypted the same file before, _and_ that the key is still in your possession. Even then, I would support your right to refuse to provide the key (without penalty), but then I've never been a fan of forced testimony, self-incriminating or otherwise.
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