Among the more powerful Constitutional protections we have is the Fifth Amendment right to refuse to incriminate ourselves. You cannot be made to testify at a criminal trial or during an investigation. If you ask for an attorney as soon as police place you in custody, you essentially never have to say another word to police or prosecutors during the criminal process.
This right also encompasses providing information that would allow police to uncover additional information. You don’t have to give up passwords or assist in the decryption of personal information. So you cannot be required to provide law enforcement with a password or code because that password is something you know.
A recent case suggests that is not the case if your phone is unlocked by your fingerprint. Police had a warrant for a woman and she had an iPhone. A judge ordered her to provide her fingerprint to allow the police access to the contents of the phone.
This is allowed under existing Fifth Amendment precedent because a fingerprint is not “testimonial.” It does not say anything about anything you know. You leave them on glasses in restaurants and door handles in numerous public places.
Because they do not unlock the contents of your mind, they have been seen as being less incriminating, and courts consider them akin to a physical object, such as a key.
The problem for many is that because of the incredibly rich nature of the material contained on your phone, having access to that device is potentially far more incriminating than accessing paper records locked in a safe in your home.
Source: theatlantic.com, “Police Can Force You to Use Your Fingerprint to Unlock Your Phone,” Kaveh Waddell, May 3, 2016