Realize that this is a fairly narrow opinion, and, in my opinion, not a particularly well-reasoned one.

The issue here is child pornography: the would-be defendant was suspected of having child pornography on various hard drives which were encrypted. The court states that the actual contents of the hard drive are themselves not testimonial—that is, they are not covered by the Fifth Amendment and, if the government had access to the hard drives, they could present whatever was incriminating on them into evidence. So the issue is whether the act of producing the documents is a testimonial act and therefore covered by the Fifth Amendment

The court concludes that the act of production is a testimonial act because, one, the testimony was not a "foregone conclusion." This holding is based on a case called Fisher v. United States, in which the Supreme Court stated that it was not testimonial to hand over certain papers that might have incriminating evidence because conceding that documents existed, that you had control over the documents, or that they were in your possession was not incriminatory given the circumstances of that case. Under the "foregone conclusion" doctrine, the government knew of the existence and location of these papers so the production of the papers added nothing or little to the government's information. If the government did NOT know that documents existed, they could not compel a would-be defendant to reveal the documents.

Second, and most importantly, the court concluded that decrypting the documents would "use the contents of [the would-be defendant]'s mind" because "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." It's again important to note that this is a child pornography case: possession of child pornography is a crime, so if the would-be defendant here provided a decryption key, this would be tantamount to him admitting that he possessed the hard drive and had access to the files within it—that alone would constitute a crime if the files were found to be child pornography. This is therefore what the court later refers to as an "implied factual statement" and the Fifth Amendment protects this. Although the court also suggests that providing a decryption key might be like providing a combination (and therefore be admissible for Fifth Amendment protection on other grounds), it unfortunately devotes very little space to this discussion—and this seems to be the really big issue here.

The case therefore leaves several unanswered questions: this is a child pornography cases where mere possession alone is a crime: what if that wasn't the case? What if this was a murder case and the defendant had stored notes about his murder on the computer? What if the foregone conclusion doctrine wasn't applicable—would the conclusion here be the same (most of the opinion is actually devoted to this discussion, which is less broadly applicable because, if the police know of the existence of specific files on hard drive, this doctrine is inapplicable)?

Anyhow: it still is possible to get access to these documents if the government gives him sufficient immunity, as the court notes. This would be pretty important because if no one could ever access these documents (which presumably would be possible if the would-be defendant doesn't decrypt them) that would be an enormous problem for our justice system.

In conclusion: the applicability of this case to future cases is unclear, so, for those that want this result, I don't really think this is a "slam dunk." There will likely be many future cases further developing this doctrine. As such, right now, it's very difficult to discuss the merits of the court's holding on the "decryption is testimony" argument (which, in my mind, is the most important) in a general sense, since the reasoning here seems very specific to the facts of case.