Computer Enabled Biometric Devices: A Fingerprint Scanner Hardware Overview

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1 Computer System Fundamentals, Prof. Hill, 1/20/2007 Computer Enabled Biometric Devices: A Fingerprint Scanner Hardware Overview Submitted by: Alex Getty Getty 1

2 With the current mindset of a consumer edging more and more towards security with threats of identity theft and terrorism looming on the horizon, there is a world in need of a savior of data security. Identity theft was marked at the fastest growing crime in the nation for the year 2005 which is perhaps something to get excited about (Holzman 1). The savior may yet be very close at hand, your fingertips to be precise. Biometrics are becoming a large part of computer security by means of physical authentication in the corporate, government, and private sectors. With the growing use of these technologies it is necessary to understand the implementation and execution of these products. Biometric devices come in a wide variety implementations, each of which recognizes a unique pattern in the human body which is inherently random enough to make the chances of multiples near impossible. Vein patterns, Irises, Retinas, Fingerprints, voice prints, and palm prints are all relatively unique. Fingerprints are formed much like the hills and valleys of the earth by wind and weather. The way fingerprints are created is differs only in that they are formed from the inherently random pattern of a combination of the position of the fetus in the womb, the density and makeup of the amniotic fluid surrounding the baby, as well as innumerable other environmental factors. Even identical twins do not share the same fingerprint, making this quite possibly the best unique human identifiers and subsequently the one of the best authentication methods. Fingerprint scanners are the most cost effective implementation of biometrics in the work place and at home. Their compact size and easy installation offer the most ease of use with the most security provided. Fingerprint scanners analyze the ridges and valleys in fingerprints in order to determine a match against a fingerprint that has been Getty 2

3 enrolled or added to the authentication system. A fingerprint is matched using software that checks for minutiae within the fingerprint. The software often checks for patterns in the fingerprints and not a full match. Because fingerprint scanners most often do not get a perfect image of the print they must do their analysis from point to point measurements. Two points are taken and then the distance is measured, the software then checks for features between the points such as bifurcations (where one ridge of a fingerprint splits into two ridges) or the ending or beginning of ridges and which direction they go in. The algorithms in the software check the new image compared to the image on record and make comparisons in the minutiae until a sufficient match percentage is reached. Most manufacturers keep their match percentage secret but it would seem that the average is close to percent on up. Fingerprint scanners generally differ in how they acquire the image to do the analysis. The scanners are usually classified as either an optical scanner or a capacitance scanner. An optical scanner works with a charge coupled device (CCD) at its core which is the same type of light sensor array found in digital cameras. A CCD us an array of photosensitive diodes that react by generating an electrical signal in reaction to light photons that it is exposed to. These diodes, called photosites, record pixels on an image. They are generally concerned with whether the area above the photosite is light or dark. The scanner normally has its own light source of one or a cluster of LEDs which light up the platen for this purpose. The electronic signal from the photosites then goes through an analog-to-digital converter to get the information to the analysis software but only after the scanner processor makes sure the CCD captured a clear image of the fingerprint. The CCD output is normally an inverted image of the fingerprint, or the darker areas Getty 3

4 represent the ridges and the light areas represent the valleys between the ridges. The image only ever makes it to the analysis software after intense scrutiny by the onboard scanner processor, making sure the image is crisp, properly exposed, and as close to smudge free as the software limits allow (Beal 1). A capacitance scanner works in a different fashion. While the capacitance scanner also checks for light and dark pixels, it works through capacitors using electrical current. The diagram shows a capacitive sensor which contains a series of cells, each of which contain two small conductor plates that are less than the width of the ridge of a fingerprint across. The capacitive cells are then covered in a layer of insulating material. Capacitive sensor array used in capacitive fingerprint scanners, diagram is only partial and shows one inverting operational amplifier engaged (ridges) while the other is disengaged (valleys) The sensor output is then fed into an integrator which is an electrical circuit built around an inverting operational amplifier, details of which are beyond the scope of this paper. Getty 4

5 The inverting amplifier alters a supply voltage, alteration of which is based on the relative voltage of two inputs, the inverting terminal and the non-inverting terminal. (Harris 4) The non-inverting terminal is connected to ground where the inverting terminal is connected to a reference voltage supply and a feedback loop. The feedback loop includes the conductor plates. The two conducting plates form a crude capacitor, along with the surface of the finger which acts as a third conducting plate when pressed against the insulating layer (Harris 4). The distance from the conductors to the finger surface changes the total capacitance of the capacitor. The capacitor under a ridge should have a higher capacitance than one under a valley because of this property. When the scanner puts together all of the signals from the capacitors it can put together an accurate picture of what the fingerprint looks like. The main advantage of a capacitance scanner over an optical scanner is that it requires an actual fingerprint shape as opposed to a pattern of light and dark which is required by an optical scanner, thus making the scanner harder to trick and subsequently smaller due to the lack of a CCD or LED array inside the unit. Two phase authentication is growing in the corporate sector and is categorized by one thing you have, usually an RSA card, and one thing you know, usually your password. Fingerprint scanners can be implemented in one phase, or multi-phase depending on the intended layer of security. The home user may wish to implement only one phase, logging into a computer account, account, or anything else simply by touching his or her finger on a platen. In a corporate environment it may be desirable to have a fingerprint read along with a password authentication. Since most scanners interface with the computer via USB and some older or industrial applications interface Getty 5

6 via RS-232 there should be little to no performance tradeoff for the extra security the hardware adds. The technology is relatively cheap, most often under $200 for a nice unit and is easy to deploy in the corporate or private computing environment with little software setup involved. CCD based units are often cheaper than Capacitance based units but can be tricked more easily by using a picture or facsimile of a fingerprint. There have been proofs of concepts that have arisen with fooling a CCD based scanner at least one in five times using a gelatin based fingerprint positive from a silly putty negative. (Rutter 1) The threat of an organization being compromised by Jell-O is relatively minimal however. Even so, one would need to get a fingerprint into a silly putty mold in order to get the negative. There are pros and cons to the hardware itself as well as the two major implementations of it, so deciding comes down to the end user in terms of price, size, and security. It s nothing that a paper can decide for you. A fingerprint scanner can be a great investment for your security. For the small cost of a USB unit, the security of the workstation or server is increased many-fold. The enrollment systems that come with the devices are easy to install and configure on Windows and Linux making the rollout little more than plugging in the unit, installing the software, and placing the finger on the platen or capacitor array. The technology of a human s random fingerprint has been around for centuries, and it s doubtful that it will change in the near future. This fact alone promises return on your investment in a fingerprint scanner, whether you choose a CCD based scanner or a capacitance scanner. Getty 6

7 Works Cited Beal, Vangie. "How Fingerprint Scanners Work." Webopedia. 18 Nov Jan <http://www.webopedia.com/didyouknow/computer_science/2004/ fingerprint.asp>. "Biometric security for protecting IT assets." Privaris Jan <http://www.privaris.com/biometric_computer.html>. Harris, Tom. "How Fingerprint Scanners Work." How Stuff Works. 24 Oct Jan <http://computer.howstuffworks.com/fingerprint-scanner.htm>. Holzman, Carey. "Recipe: Biometrics for Systems Builders." TechBuilder. 6 Sept Jan <http://www.techbuilder.org/recipes/ >. Rutter, Daniel. "Thinking Putty defeats Biometric Fingerprint Scanners!" Crazy Aaron's Puttyworld. Dan's Data. 22 Jan <http://www.puttyworld.com/thinputdeffi.html>. Getty 7