The Massachusetts High Court recently ruled that a suspect can be compelled to decrypt disks, files, and devices which have been seized by law enforcement. The crux of the question before the court was whether compelling the password for decryption is forbidden by the Fifth Amendment protection against self incrimination.
The analogy one most often sees is to being compelled to provide the combination to a safe, the contents of which are subject to a search warrant. That is well settled law, you can be compelled to do so.
The court said:
We now conclude that the answer to the reported question is, "Yes, where the defendant's compelled decryption would not communicate facts of a testimonial nature to the Commonwealth beyond what the defendant already had admitted to investigators." Accordingly, we reverse the judge's denial of the Commonwealth's motion to compel decryption.
In this case, there was nothing testimonial about decrypting the files because the defendant has already admitted to owning the computers and devices, and to being able to decrypt them.
The much more interesting situation will come in a case where the defendants say they never had, or have forgotten, the password. One can not be compelled to do something impossible, but generally the proof of the impossibility falls on the defendant. In this case one would have to prove a negative. How could you prove that you don’t have the password? The only thing that can be proved is that you do, and that only by doing so.
This ruling is only binding in the sate of Massachusetts, but is likely to be influential in cases in other areas.