Fingerprint sensors: how the scanning technology works

Every person has a unique fingerprint. No two fingerprints are the same – not even identical twins. Smartphone manufacturers are also taking advantage of this by incorporating fingerprint sensors to unlock the device. It takes just seconds: place your finger on the sensor and the smartphone is unlocked. Fingerprint sensors have been a standard feature on smartphones for several years now. Various scanning technologies are used for this purpose.

• Optical method: This method uses optical sensors. The finger is placed on a glass plate. During this contact, only the papillary ridges (finger grooves) touch the glass. A light source illuminates the finger, which reflects the glass plate onto an image sensor. The papillary ridges are shown dark, the valleys between them light. This photo, taken in a matter of seconds, is compared with the data stored on the smartphone.
• Disadvantage: The optical method can theoretically be easily tricked with a fingerprint prosthesis and an image of the print. At the same time, the technology has developed steadily in recent years, including in terms of security.
• Capacitive sensor: This method relies on an electrical charge. The fingerprint scanner consists of a conductive silicon layer and a network of thousands of capacitor cells on a sensor chip. The finger is placed on the silicon layer, which changes the electrical charge at the points where the papillary ridges are located. Where no grooves are registered, the charge remains unchanged. This creates a digital image of the fingerprint.
• Fingerprint sensors that work with an electrical charge have become established in the majority of smartphone models. The advantage: the method is more difficult to deceive. Many capacitive sensors even use infrared sensors to test whether the material placed on it is dead or alive.
• Ultrasonic fingerprint sensor: For a few years now, Samsung in particular has been using sensors based on ultrasound technology. Ultrasonic waves are sent which are swallowed or reflected by the skin – depending on whether the waves hit grooves or valleys. This creates a 3D model of the finger. Advantage: The ultrasonic sensor offers a high level of security as it is difficult to deceive.

In terms of security, the individual technologies offer different standards. In general, however, it can be said that regardless of the technology used, a fingerprint sensor is secure for everyday use.

• The reason: the user’s stored fingerprint, which is used for matching, is not simply stored in the image folder on the smartphone. The sensitive data is stored on a Trusted Execution Environment (TEE), an isolated part of the main processor. Only authorized software has access to it.
• In addition, the sensor and TEE only communicate directly with each other. Malware that infects the operating system therefore has no access to this part.
• However, there are also weaknesses in the fingerprint sensor that should not be ignored. For example, only a partial fingerprint is scanned due to a lack of space. A fingerprint is unique for every person. However, this is not the case with partial prints.
• With the help of a universal finger, the scanners can therefore be outwitted. However, such a universal finger is very complex and expensive. In everyday life, smartphone users therefore do not have to worry about biometric identification being cracked or bypassed.