With the safe analogy, I swear there's precedent that, if the security is a physical key, then a court can compel the owner to produce it. But if the safe uses a combination, the court cannot compel its divulgence, since that would violate the fifth amendment protections against being forced to testify against oneself. Encryption "keys", and the passwords from which they are commonly derived, are much more akin to combinations than to physical keys.