My understanding is that if you are in possession of a key to a safe then law enforcement or the court can compel you to produce the key. But if the safe has a combination, then the court cannot make you give up the combination as that may be self incriminating. Not to mention the practical aspects of trying to compel someone to give up a secret. What if they claim to have forgotten the combination? What if they actually _did_ forget?

I imagine the situation with electronics will be similar. If a key to, say, an encrypted volume is stored on a flash drive then they may compel someone to produce it (analogous to a safe key). But if the drive is protected by a password, then the government cannot compel someone to provide the secret.

I think that's why the article mentions finger prints and faceid. Those are the 'keys' to your phone.