However, in addition to that, the court notes that there are two reasons why the Fifth Amendment prevents compelled description. Besides the foregone conclusion doctrine, there is discussion such at 22: "the decryption and production of the hard drives would require the use of the contents of Doe’s mind and could not be fairly characterized as a physical act that would be nontestimonial in nature. We conclude that the decryption and production would be tantamount to testimony by Doe of his knowledge of the existence and location of potentially incriminating files; of his possession, control, and access to the encrypted portions of the drives; and of his capability to decrypt the files." This to me implies the court's belief that the act of production would be testimonial because it would imply that Doe possesed and had access to incriminating files. Since possessing and having access to child pronography is a crime, that alone--that act of him affirming that he had access to such files by providing a decryption key--would incriminate him. By implication, this would not be the case if possession of certain docuemnts was not itself a crime, yet these documents could be incriminating.
The court spends 2 paragraphs discussing this, but I think it is not inconceivable that this sort of argument could be applied to other cases where the foregone conclusion doctrine might otherwise be succesfully applied to compel decryption, since ostensibly any 1 of the court's two points could be used to prevent compelled decryption. I wonder if this conclusion is based solely on the nature of the crime alleged here, or would be applicable to other crimes where merely showing that you have possession to access to incriminating documents is not itself a crime, as in child pornography.