Payne conceded that “the use of biometrics to open an electronic device is akin to providing a physical key to a safe” but argued it is still a testimonial act because it “simultaneously confirm[s] ownership and authentication of its contents,” the court said. “However, Payne was never compelled to acknowledge the existence of any incriminating information. He merely had to provide access to a source of potential information.”

If you can be compelled to hand over a key to a safe, I can see how that translates to putting your thumb on the scanner.