I recently got my first "smart phone" (I've been a late adopter in that particular area of technology). One of the first things I noticed about it was that I could use my fingerprint, rather than a pesky pass code, to unlock it. Much more convenient, isn't it? A password can be forgotten, but it takes pretty severe physical trauma to lose one's fingerprint. If your hand gets cut off, your phone is the least of your worries, right?
Unfortunately, the convenience of "biometric" identification comes with a cost. When you take that route, at least two judges (first a Virginia circuit court judge and now a federal judge in California) have ruled, you can be forced to put your finger on the phone to unlock it.
This has serious and unfortunate implications for rights protected by the Fourth and Fifth Amendments to the US Constitution.
Fourth Amendment: Even when there's a valid search warrant for a premises -- or a phone -- actually executing the warrant is law enforcement's job, not yours. If the door is locked, they can break it down, but you don't have to unlock it for them. If they find your hidden compartment full of evidence, they find it. But you don't have to show them where it is, or even tell them that it exists. And that's how it should be.
Fifth Amendment: Giving the police access to your phone is no different than telling them about every call you made, every text you sent, every note you wrote, etc. It is testifying against yourself, which you cannot constitutionally be required to do.
The usual response from proponents of unlimited state power to such arguments is that the framers of the US Constitution couldn't possibly have imagined a future of "smart phones," unbreakable encryption, and so forth.
Maybe they're right. But what the framers COULD imagine was the possibility that the Constitution would require occasional amendments to keep up with changing times. Those who want to repeal the Fourth and Fifth Amendments have clear instructions for doing so. All they need is the support of two thirds of both houses of Congress and ratification by three quarters of the states' legislatures. A high bar, but not at all unclear.
Until and unless that happens -- and it won't -- resist much, obey little. And secure your phone with a long and complex pass code, not with your fingerprint.