That's a fair point, and the last sentence of my previous reply should be qualified: compelling someone to reveal the combination to a wall safe is not necessarily a violation of the Fifth Amendment. In this case, the court decided that the 'foregone conclusion' exception in Fisher v. United States, 425 U.S. 391 (1976)[1] applies. In Fisher, the court acknowledged that producing documents in response to a subpoena can be testimonial, because it 'tacitly concedes the existence [and possession] of the papers demanded.' But when the existence of the location of the documents are a foregone conclusion, the testimonial aspect of the production of documents – the tacit concession as to their existence – adds 'little or nothing to the sum total of the Government's information.'
As you suggested, compulsory key disclosure can be a violation of the Fifth Amendment, and here the court referred to an example of that in In re Grand Jury Subpoena Duces Tecum Dated Mar. 25, 2011, 670 F.3d 1335 (2012)[2]. The difference was that in In re Grand Jury Subpoena, there was no evidence that the TrueCrypt volumes contained any data, and no evidence that the suspect could actually decrypt them.
As you suggested, compulsory key disclosure can be a violation of the Fifth Amendment, and here the court referred to an example of that in In re Grand Jury Subpoena Duces Tecum Dated Mar. 25, 2011, 670 F.3d 1335 (2012)[2]. The difference was that in In re Grand Jury Subpoena, there was no evidence that the TrueCrypt volumes contained any data, and no evidence that the suspect could actually decrypt them.
[1]: https://supreme.justia.com/cases/federal/us/425/391/case.htm...
[2]: https://scholar.google.com.au/scholar_case?case=201586737907...