If you’re arrested or a suspect in a crime, the police can’t force you to remember the combination to your safe, or the passcode for your iPhone. But what if that phone can be unlocked with biometric data like a fingerprint? Does the ready access to this information give law enforcement an easy way to open secure devices, or would that be a violation of your constitutional rights?

The U.S. Supreme Court made it clear in 2014 that police must have a warrant to search the contents of a mobile device, but does that warrant give them the authority to compel you to use your thumb to actually unlock the device?

The answer to that will ultimately hinge on how courts view fingerprints. Is using your finger to unlock the device no different than being ordered to turn over a key, or is putting your fingertip on that button tantamount to testifying against yourself in violation of the Fifth Amendment?

Some courts have already chimed in on the matter, with a Virginia state court ruling in Oct. 2014 that while police can’t force suspects to reveal the passcode to their phones, compelling the use of a fingerprint is acceptable because it “does not require the witness to divulge anything through his mental process.”

More recently, the L.A. Times writes of a U.S. Magistrate Judge who signed off on a warrant compelling a woman to provide her fingerprint to unlock a phone seized at her boyfriend’s apartment.

However, University of Dayton law professor Susan Brenner tells the Times that she believes this is a clear violation of the Fifth Amendment’s prohibition against self-incrimination.

“By showing you opened the phone, you showed that you have control over it,” she explains. “It’s the same as if she went home and pulled out paper documents — she’s produced it.”

Yet, as noted in the Virginia ruling, the Supreme Court has held in 1976’s Fisher v. United States that “The Fifth Amendment protects against compelled self-incrimination, not the disclosure of private information,” and that compelling a suspect to turn over documents or other evidence that may ultimately be incriminating is not the same as testifying against yourself.

While a passcode might be more old-fashioned and seemingly less secure than a biometric lock, there is court precedent giving passcodes more legal protections than fingerprints.

In 2010, in U.S. v. Kirschner, a federal court in Michigan held that compelling a defendant to provide a passcode qualifies as testimony because it requires the defendant to “communicate knowledge, unlike the production of a handwriting sample or a voice exemplar,” which — like fingerprints — have long been excluded from Fifth Amendment protections.

“This is why I tell my criminal procedure students that they have more protections if they use a passcode rather than fingerprint to guard entry to their phones,” University of Washington law professor Mary Fan explains to Ars Technica. “While I don’t conduct crimes on my cell phone, I still decline to use my fingerprint out of an abundance of caution!”

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