Original URL: http://www.theregister.co.uk/2011/07/27/dead_hand_detector/

Fingerprint scans learn to spot chopped-off fingers

Also Gummi Bears, zombies, other common fakes

By Richard Chirgwin

Posted in Security, 27th July 2011 10:18 GMT

What do Gummi Bears and amputated fingers have in common? They’ve both been demonstrated as techniques for defeating fingerprint scanners. Now, a German company called Dermalog Identification Systems is using the way skin changes colour under pressure to block both the soft sweet and the dead hand of the zombie from accessing systems protected by fingerprint scans.

The problem is that if a scanner responds only to the image of the fingerprint, you don’t need a living print to be accepted. An impression of a print on a Gummi Bear, or if you’re of a more gruesome mindset, a finger removed from a user (either living or dead), can be scanned as a valid fingerprint.

However, if you’ve ever looked at what happens to skin if you put pressure on it, you’ll know that it changes colour. Dermalog has tagged that colour change as the characteristic that can separate the living from the dead.

When you press a finger against a surface – such as the scanner surface – blood is squeezed away from the surface. According to Dermalog’s research, published in Forensic Science International, that blanching is predictable and measurable. Living fingers absorb light at 550 nanometers when they first press the contact, and after the skin has “blanched”, at 1450nm.

As for that zombie you saw shambling down the street with a copy of the Communist Manifesto in one hand and Atlas Shrugged in the other: blood isn’t pumping any more, so there’s none in the capillaries to squeeze out; and without blanching, the characteristic changes in colour absorption won’t be observed.

To identify the telltale wavelengths, researchers say they tested “reflection and transmission spectra in the wavelength region from 400 to 1650nm” on both “living volunteers and corpses”. ®