From his analysis:
[T]he issue for Courts in these cases is how a demand that a suspect or defendant produce a password is viewed under the law. If it is viewed as testimony, then it would be barred under the Fifth Amendment even if it were supported by a warrant signed by a Judge. If, however, it is merely viewed as giving law enforcement access to something that they have the legal right, due to a warrant, to search, then it is not testimonial and permissible as long as the Fourth Amendment is complied with.
The analogy that has been made here is one that compares the password to either the key to a strongbox or the combination to a safe. If it’s viewed as analogous to the key, then its a Fourth Amendment issue and the subject of the warrant must provide access as long as there is a valid warrant. If, however, it is the combination to a safe, then requiring the subject to provide access is testimonial and forcing a person to reveal it would be barred by the Fifth Amendment because the combination constitutes the “expression of the contents of an individual’s mind.”
The analogies are, admittedly, not perfect, but they have come to be adopted in large part due to the distinction that the case law makes between these two activities, and they are useful analogy in a situation where there is very little actual law to apply.
