https://supreme.justia.com/cases/federal/us/487/201/case.htm...
"""
JUSTICE STEVENS, dissenting.
A defendant can be compelled to produce material evidence that is incriminating. Fingerprints, blood samples, voice exemplars, handwriting specimens, or other items of physical evidence may be extracted from a defendant against his will. But can he be compelled to use his mind to assist the prosecution in convicting him of a crime? I think not. He may in some cases be forced to surrender a key to a strongbox containing incriminating documents, but I do not believe he can be compelled to reveal the combination to his wall safe -- by word or deed.
"""