What is a fingerprint scanner

Fingerprint sensors: this is how the technology works

A swipe of the finger unlocks the smartphone or confirms payment in the app store. Fingerprint sensors in cell phones have a long tradition as a biometric security function. But how does fingerprint scanner technology work? What methods do smartphones use to recognize your fingerprints and how securely is this data processed?

More than a snapshot?

With optical sensors, the finger is placed on a glass plate that serves as a prism. The papillary ridges (elevations) of the fingerprint come into contact with the glass. The valleys (grooves) in between are filled with air and do not touch the glass plate. A light source then illuminates the finger so that the prism reflects the light rays onto an image sensor.

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The reflection is disturbed at the points where the rays hit the elevations. The image sensor interprets the reflected rays and creates a two-dimensional photo of the impression. The problem: Even if this method can generate images with high resolutions, the system can be tricked relatively easily with prostheses and an image of the impression. In addition, this method is slow compared to the other methods and has therefore not been used for a long time with smartphones. However, manufacturers such as the Chinese company Vivo recently rediscovered optical sensors because they can be installed under OLED displays in a slightly modified form.

Difference in charge

Capacitive sensors are currently the most commonly installed technology in smartphones. They consist of small capacitors in which an electrical charge is stored. Here you place your finger on a conductive surface that is connected to the charge stores. The papillary ridges touch the surface and thereby slightly change the electrical charge of the capacitors.

The air in the grooves of the footprint isolates, so the charge on the capacitors below them changes less. The pattern of the print is calculated based on the charge difference between the capacitors. Since the changes in charge of different materials differ, this method is more difficult to outsmart than the optical one. But here, too, researchers have already made it, albeit with a lot of time.

Sampling by sound wave

The latest sensors on the market use ultrasound. They send a high-frequency sound impulse against the finger, which is absorbed and reflected back differently by the grooves, papillary ridges and pores. The result is a three-dimensional image of the print, which increases security even further thanks to the wealth of detail. A sensor developed by Qualcomm even detects the blood circulation in the tissue using ultrasound. It can also be installed under displays and metal, which should give manufacturers even more design freedom in the future.

For sure

No matter what technology is in the phone, your fingerprint is not simply an image in the device's memory. Just like passwords, manufacturers secure and process all data for your imprint in a Trusted Execution Environment (TEE). This is an isolated part of the main processor that only authorized software can access.

In addition, the sensor and TEE only communicate directly with each other. As of today, malicious programs that may have infected the system cannot access any information. In addition, the imprint is not on the system as a photo: the software analyzes the distribution and position of the minutiae (image below) among each other and saves them as a code - which, by the way, cannot be used to assemble an entire imprint.

When comparing, the entire print is also not compared. Only a certain number of minutiae has to match the reference print. The problem here is that smartphone scanners often only take a partial fingerprint due to their size. Again, these sections are not as unique as the entire finger.

Researchers have already succeeded in developing a universal master impression from 8200 partial impressions using software. This contained enough matches to unlock 65 percent of the tested phones. Security will increase, especially when ultrasonic sensors will sit under the display in the future and scan the entire finger. If you don't want to take any risks today, you can rely on a complex password.

Timeline: The development of fingerprint sensors

  • 2004
    The South Korean company Pantech launched the GI 100, the first cell phone with an integrated fingerprint sensor.
  • 2011
    The Motorola Atrix was the first pure smartphone with a fingerprint sensor. At that time, this also served as a start button.
  • 2013
    Apple did not invent it, but made it suitable for the masses with the iPhone 5s. With the Touch ID you saved the password entry in the App Store and iTunes.
  • 2014
    The Samsung Galaxy S5 was the first smartphone that could be used to authorize payments with a fingerprint. In the case of PayPal.
  • 2016
    Although it was only available in China, we don't want to ignore it: The Xiaomi Mi 5s was the first smartphone to have an ultrasonic sensor.
  • 06/2017
    At the MWC in Shanghai Qualcomm and Vivo presented a first prototype with an ultrasonic sensor in the display. The basis was a Vivo X Play 6.
  • 01/2018
    It was also Vivo who launched the X 20 Plus UD, the first series phone with a sensor under the display. However, this still works optically.
  • 03/2018
    With the Huawei Mate 10 RS Porsche Design, a phone with an optical fingerprint sensor under the display is available for the first time in Europe.
  • 05/2018
    Ultrasound technology conquers the mass market with the Honor 10. The sensor is under the front glass, but not under the display.
  • ?/2018
    Ultrasonic sensors under the display will be the next big thing. Potential candidates would be the Huawei Mate 20 or Samsung Galaxy S10
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