Reactivating Fingerprints I

By Peter-Michael Ziegler  |  Posted 2002-06-03 Print this article Print

Sleight of Finger
The most common method for distinguishing fingerprints is based on the so-called minutiae, the small details. The minutiae are interruptions to the lines upon the fingertips, such as endpoints, bifurcations, whorls or islets. To identify a human fingerprint unambiguously information about the type, position and orientation of at least ten to twelve of these minutiae is required. In the main, capacitive fingerprint scanners are used to get hold of these minute details -- above all because the CMOS chips used in them have for some time now been available at a fairly reasonable price. When a finger is placed on the device the scanners 65,000 pixels treat the surface of the skin as a capacitive pole. The capacitance of each miniature capacitor depends on whether a line or a trough is to be found above the measuring point in question. The device then converts these individual values into an 8-bit gray scale, extracts about 20 minutiae and proceeds to store these values in a reference file for future authentication purposes.
Even simple breathing will do the trick of outwitting a capacitive fingerprint scanner.
In Germany the best known among the desktop fingerprint scanners is Siemenss ID Mouse, which is equipped with Infineons capacitive FingerTIP sensor. In its current Professional V4.0 version the device can, moreover, be used as a optical USB scroll mouse. During the tests there was never a problem with installing the USB drivers and setting up the application software. Under normal conditions the enrollment as well as the subsequent authentication almost always went off quickly and without error. It was equally easy, though, to outwit the ID Mouse with simple tricks. Although this according to the manufacturers statements should have been impossible we were able several times to reactivate by simply breathing upon them traces of fat left by fingerprints upon the sensors surface, thereby overcoming the biometric protection of the system. We cupped our hands above the scanner and within the shell thus formed breathed gently upon the sensors surface. Meanwhile on the screen of the biometrically-protected computer we were able to see the contours of an old fingerprint slowly reemerge. A fingerprint on adhesive film may suffice as a biometric ID.


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