• Kristjan Salev


How does the fingerprint door lock work?

Fingerprint door locks have been around for a while and are very convenient devices for opening your door without a traditional key. They were once quite expensive and used mainly in government institutions, banks, and places with a high level of security. By now, however, they are widespread and are used in many apartment buildings and private households. You can get one of these on Amazon for less than $100.

A fingerprint lock is a type of biometric lock that scans the user's fingerprint and compares it with a previously scanned and saved template that is authorized to open the lock. If the scanned fingerprint matches the saved template, the lock will open, but if the fingerprint does not match, it remains closed.

What is a biometric lock?

Biometric locks use certain characteristics of the human body. Each person has certain physiological characteristics that do not change over time. The most well-known is probably the fingerprint, which is unique for each person. There can also be other characteristics, such as a person's face, voice, DNA, iris, the retina in the eye, etc. These characteristics can be used to identify a person who is allowed access to, for example, computers or rooms with a high level of security. Even in smartphones, it is possible to use biometric identification based on the user's face or fingerprint in addition to the password.

Fingerprint minutiae matching

By placing your finger onto the door lock scanner, a digital template is created from your fingerprint. It contains all the required numerical data and can be saved to ensure access next time. In this way, it is possible to scan and save the fingerprint of everyone who is allowed to enter.

A closer look reveals that each fingerprint consists of a pattern of ridges and valleys. This pattern is very detailed and unique to each person.

Fingerprint ridges and valleys

As you can see from the image above, ridges can become discontinuous in many places. In some places, the ridge may come to an end or split into two ridges, while in some places it may be like a single isolated island. They are also the three main types of specific points on the fingerprint and are called the ridge ending, the bifurcation, and the dot. All of these different distinctive points on a fingerprint are called minutiae points, and nearly 150 different types have been identified.

Fingerprint minutiae points

Fingerprint scanners mainly use the minutia matching process and compare the minutia points of the scanned fingerprint with a previously saved template. It stores enough minutia points with their location and orientation to identify the user who is authorized to open the lock.

Now let's take a closer look at how the minutiae matching process works, and how it detects whether the fingerprint is authentic to open the lock. Although this whole process takes a fraction of a second, we can break it down into several parts.

1. Fingerprint scanning

When the fingerprint is scanned, a grayish image of the user's fingerprint is obtained. The darker lines in the image are ridges, the lighter areas are valleys. Image pixel values represent the intensity of each pixel and may ​​range from 0 to 255. 0 represents black, and 255 represents white.

2. Fingerprint image binarization

Binarizing an image essentially means converting the image to black and white. By finding some threshold value in the image, pixels below that value are converted to black (0), and pixels above that value are converted to white (255).

Door lock fingerprint scanning and binarization

3. Filtering a fingerprint image

After binarization, the fingerprint image is thinned. In a binarized image, the ridge lines are wide, and they are filtered down to a single pixel size. Thinning does not change the location or orientation of the minutiae points but helps to identify fingerprint features much more effectively.

Binarized and thinned fingerprint image

4. Minutiae extraction

Minutiae extraction is the process of determining the locations and orientation of the minutiae points in a thinned image. A certain number of minutiae points is extracted from the thinned image, and a map is created, showing the locations and orientations of these unique points. The illustration below shows two magnified minutiae points on a thinned image. At the first point, the ridge splits into two (bifurcation point), and at the second point, it ends (ending point).

Fingerprint minutiae extraction

5. Minutiae matching

A quality fingerprint image may contain up to 80 minutiae points, but a much smaller number of points is sufficient to identify a fingerprint (according to some experts, even 12 points are enough). The minutiae matching process uses an algorithm to compare the minutiae points of the entered fingerprint with the saved template data. Different algorithms are used for this, but they all compare the entered and saved data and calculate a matching score. If the matching score between two fingerprints is high, the door lock opens, and if it is low, the lock remains closed.

How secure are fingerprint door locks?

Generally, fingerprint locks are secure. While it is easy to copy a physical key, it is not so easy to copy someone's fingerprint. Besides, the normal key can be lost, but you cannot lose your fingerprint. Although biometric locks are generally more secure and easier to use than conventional locks, we can never build a 100% secure lock. And, of course, when someone tries to use brute force to open the door, it does not make much difference whether the door has a biometric or a regular lock.

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