Why Dead Fingers (Usually) Can't Unlock a Phone

In March, two detectives went to a funeral home and asked to see a body. The reason? They wanted to unlock the man's phone and needed his fingerprints, according to news reports. And though the detectives were granted access to the man's body, they couldn't unlock the phone.

The man, Linus Phillip, was shot and killed by a police officer outside a Wawa convenience store in Largo, Florida. Detectives were looking for information to help them investigate Phillip's death as well as information about a separate investigation involving drugs, according to the Tampa Bay Times.

What the detectives did is legal, as Forbes reported, but it certainly raises ethical questions. And given that the detectives were unsuccessful, it resurfaces a science question as well: Do you need to be alive for your fingertips to unlock your phone?

In fact, it gets more difficult to unlock a phone using fingertips the longer a person has been dead, said Anil Jain, a professor of computer science and engineering who has spent years working on fingerprint recognition at Michigan State University and one of the authors of the "Handbook of Fingerprint Recognition" (Springer, 2009)

This is because, on most smartphones, fingerprint identification works through electrical conductance, Jain told Live Science.

We all have a little bit of electricity running through our bodies. When we place our fingertips on a fingerprint scanner, the ridges of our fingerprint touch the surface whereas the valleys don’t. Tiny capacitors — devices that store electrical charge — will store more charge coming from the finger if they sit under ridges than they would under valleys. The sensors will use these patterns to form a detailed image. But when a person dies, that flow of electricity ceases, and with it, any chance of interacting with the scanner.

Scientists are unsure how long exactly after a person dies the body loses this conductance. To figure that out, you would "need a lot of dead bodies, and you [would] have to take their fingerprints to unlock the phone every hour or so … somebody has to be in the morgue continuously," Jain said. "It's a pretty difficult experiment to do."

Not all fingerprint readers work through conductance, however. Older fingerprint readers, for example, use optical sensors that measure the changes in light between the ridges and valleys to form an image. But this method is easier to fool with photographs, since no electricity is required, making the systems easier to hack.

And the technology is ever-changing: Take, for example, the iPhone X, which doesn't have a separate button to read fingerprints, but rather unlocks when it detects your face.

Future phones have optical sensors beneath their screens, Jain said. It's unclear if these will be accessible with lifeless fingers, he added, but conductance wouldn't be an issue in such a case. Other new technology includes "ultrasonic" scanners that send ultrasonic waves into the finger to measure the stress patterns that result from ridges and valleys. How these will interact with lifeless fingers is still to be determined.

Still, all of these sensors will be affected by the fact that "the skin gets noisy over time," Jain said. "If there's no blood flow, then the fingers will start showing some kind of wrinkles or shrivel up." This will change the signals of the ridges and valleys, creating new ones and erasing older ones so that the sensor will detect something completely unlike the original fingerprint, he said.

What's more, most phones have some sort of buffer that would make it more difficult for someone to use your fingerprint to unlock your phone after your death, Jain added. That is, most phones require you to type in a passcode after a day or two of inactivity, and you typically get only a certain number of attempts, he said.

But if someone does manage to get around these hurdles, and the body is only recently deceased, it is certainly possible to unlock a phone using lifeless fingers, Jain said.