Yeah you're right. According to this CNET article from 2007, it has been decided at least to the Federal District level, that investigators are unable to compel encryption keys at this time.
edit: maybe not the case, here's an update to the same case from 2009 where, although he was free to keep the key secret, he was ordered to produce the contents of the drive.
The second was a case where guards had already seen the incriminating content. Maybe defendants can still plead the 5th when the contents are unknown.
In any case, if you can show that you weren't the one who encrypted the document, I don't see how you could be prosecuted for not doing the impossible.
In any case, if you can show that you weren't the one who encrypted the document, I don't see how you could be prosecuted for not doing the impossible.