Five years ago, in a contribution to these pages, I suggested that the Supreme Court's oldest precedents and the original intent of the framers of the Constitution precluded the use of evidence produced under a grant of immunity against the producer, even though the material produced included documents that the producer had not been compelled to write. This implied that information concealed with a cryptographic key could not be used in a criminal prosecution against someone from whom the key had been obtained under a grant of immunity.
United States v. Hubbell: Encryption and the Discovery of Documents,
Rich. J.L. & Tech
Available at: http://scholarship.richmond.edu/jolt/vol7/iss4/3