> I would presume that the defendant would merely have to give evidence that he or she did not possess the key to "raise an issue".
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.