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How fingerprint scanners work

My Thumb is My Passport: Using Biometrics
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A fingerprint scanner can store an image of your fingerprint and match the basic patterns that make up a fingerprint (arch, loop and whorl), but storing the image could be a security risk, and the 500 ppi most sensors use results in large image files. Instead, most systems store a mathematical representation of the characteristics in the fingerprint, called minutiae, based on where the ridges in the fingerprint start and stop, split or join together.

Storing the details of a fingerprint in the system is known as registering or enrolling. You scan the same finger three or four times (or more if you’re not getting a good scan), so that the software can calculate minutiae for enough areas of your finger to be sure of recognizing it. Start by pressing the first joint of your finger or thumb against the scanner, press firmly but not too hard, keep your finger straight and move it smoothly. If you share your PC, everyone can scan their own fingerprints for access.

fingerprint scanner

Scan several fingers in case you cut yourself.

As soon as your finger is enrolled, you can use it to log on to Windows, or to unlock your screensaver when you come back to your PC. With some software, you need to put in the details of the Web sites you use, but most tools will ask you whenever you type in a username and password whether you want to store them and log into the site automatically in the future. You can also log in to password-protected applications. Eikon’s Digital Privacy Manager lets you assign shortcuts for different applications to your fingers, so you can swipe once to log in and again to open a program.

With a built-in fingerprint reader, think about where it’s placed before you decide which fingers to scan (assuming you’re not doing all ten of them). If it’s below the trackpad, you can move either thumb over it comfortably, but if it’s at the base of the screen it will probably be easier to reach with one hand than the other. If the reader is on the side of the screen, you’ll get better results if you move your finger or thumb gently and stabilize the screen with your other hand-if the screen moves, your fingerprint may not be recognized.

You’ll also need to wipe the sensor clean from time to time (or press on a piece of adhesive tape then peel it off). It’s more obvious on a scanner that you press your finger onto-like the scanners you use at the airport when you come back from a foreign country-where you can sometimes see latent fingerprints made by the oil that comes out of your skin. Even on a scanner that you swipe your finger over, though, the oil can make it harder to get your fingerprint recognized.

This is more of a problem for optical fingerprint scanners like the Microsoft Fingerprint Reader; they have a light source and the same kind of sensor as a digital camera, so the surface has to be clean (and so does your finger). Other systems, including sensors made by UPEK, are like a touchscreen or a trackpad. While the outer layer of your skin, the epidermis, is non-conductive because the skin cells are dead, the layer beneath - called the dermis - is made up of living cells, and these do conduct electrical charges. When you put your finger onto the sensor, it can measure the difference in charge between the ridges of your fingerprint (which are all skin) and the valleys between the ridges (where there is some skin and some air).

Active capacitance sensors apply a tiny voltage to your skin before taking these measurements; they’re often used in touch scanners rather than swipe scanners, and they’re more expensive. That’s why the Microsoft Fingerprint Reader is less than $40, while the Silex Combo Mini Fingerprint Reader is $150. There isn’t room on a notebook for a touch scanner, and a swipe scanner can also be used for scrolling through Web pages and documents if the driver supports this; it works well on Motion Computing tablet PCs. When you swipe your finger, you’re also cleaning the sensor each time. Many notebook manufacturers use AuthenTec’s swipe fingerprint sensors, which send an RF current through the skin to directly read the ridge and valley pattern in the dermis.

fingerprint sensor tablet

A fingerprint sensor is particularly useful on a tablet PC, especially if you can also use it to scroll through pages.

If the fingerprint reader doesn’t recognize you when it should (false rejection) it’s irritating, but you can scan your finger again or use your password. If it recognizes the wrong person or a fake fingerprint (false acceptance) that’s a much bigger problem. Active fingerprint sensors offer better security than optical sensors like the Microsoft model, because they should be reading the fingerprint pattern in your dermis, so the contours on a fake fingerprint made just from a fingerprint you’ve left on a smooth surface wouldn’t trick them (and the finger of a dead person wouldn’t be recognized either).

A fake finger made out of dental mould or Play-Doh will fool many sensors, but it needs to be made from a mould of your finger. Either procedure takes time and know-how. Anyone with the time and access to make a fake fingerprint can just as easily take your hard drive out and read it in another computer anyway. If you’re worried about protecting your information if your PC is stolen, you need to add extra security by using whole-disk encryption software like PGP or BitLocker that makes the drive unreadable in another PC. To be really safe, don’t register your most confidential passwords-like the login for your online banking-with the fingerprint software.

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