An Artificial Finger
The second most frequent manner in which fingerprints are currently mechanically scanned is the optical one. In this case the finger which is positioned above a prism or a diffracting grid is illuminated by light from color LEDs and photographed by a CCD or a CMOS camera. An alternative technique consists of placing the finger illuminated from below upon a light-conducting fiberglass surface that is directly linked to a CMOS chip element. Accordingly, during our tests we were unable by reverting to simple latent image activation to get the better of our candidate, Identixs Bio-Touch USB 200 - with systems of this kind to trigger the recognition procedure at all it is necessary that, prior to the CMOS camera taking the picture presented to it via a concave mirror, the light from the red LED source be reflected by an object on the scanners surface. For the first time we thus had to avail ourselves of an artificial finger. An intruder with even minor manual skills might, for example, with the aid of photo-sensitive lacquer fashion the image of a fingertip into a mould for a three-dimensional likeness of the fingertip in question. As these steps are obvious we felt free under laboratory conditions to take a somewhat simpler approach: We took small common tea-warming candles, removed their wicks, pressed fingertips into the warm wax and proceed to fill the troughs with commercially available silicone.Moreover, in the course of further experiments we also detected that even without the aid of an artificial finger it was possible to deceive the optical sensor. For we were again able to gain access with our tried-and-tested adhesive film technique. In this case, however, though it was not enough to simply place the film with the graphite pattern on the scanners surface, once a halogen lamp was made to shine on the scanner from a distance of about 30 centimeters, that too worked. Apparently, the intense back-lighting on the one hand enhanced the contrastive properties of the graphite powder on the scanners surface whilst on the other inducing a kind of snow blindness in the sensor. The G81-12000 keyboard made available to us by Cherry is likewise equipped with Identixs optical fingerprint scanner, hence its results vis-à-vis our attempts at deception were more or less identical.
The moment we placed the thus fashioned fingertips on the scanners surface BioTouchs resistance collapsed: The DFR-200 optical sensor accepted the silicon copies without hesitation, during authentication as well as during enrollment. The reverse of the deception also worked: When in possession of a silicon copy of a fingerprint of a registered person we were able to log on to the computer incognito.