I don't agree that testifying is the same as confessing. To testify is to provide a testimony (under oath), which is broader than an actual confession. The reasoning behind the 5th amendment is that nobody should be put in a position where they either have to lie, incriminate themselves, or be held in contempt of court.

If the government can't compel someone to say when, where and how they disposed of the bodies, then they also can't make someone explain how they encrypted some files.

In case they can confirm the existence of files some other way I guess you could make the case that the government can force someone to produce those files. Of course this won't tell them anything they didn't know already (which is kind of the point). Making someone produce files that may or may not exist is the same as making them testify that those files exist and that they have access to them, which I would argue falls under the 5th amendment.