
The fingerprint sensor is not responding?
This tutorial will help you troubleshoot and give you an insight into the security of the sensor!
Let’s start..
- 1 How does a fingerprint sensor work?
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2
Why is my finger not recognised?
- 2.1 Dirty hands or wet hands
- 2.2 Hit? – Inaccurate position
- 2.3 Who was it? – Wrong finger
- 2.4 Ouch – scars / wear and tear
- 2.5 Insufficient pressure – too much pressure
- 2.6 Spider-Man – rubber finger
- 2.7 Dirt on the sensor
- 2.8 Technology? Damaged sensor
- 2.9 Long live the software bug
- 2.10 The person in the background – remote wipe
- 3 How can I trick a fingerprint?
- 4 Intelligent sensors – false finger recognition
How does a fingerprint sensor work?

Every finger has a characteristic structure, the fingerprint, which a sensor can pick up with light photography or contact conductivity. The sensor scans over the finger and records the grooves (ridges) and intersections of the grooves (minutiae).
The software is particularly interested in the minutiae, which can be 3-way, 4-way crossings and the beginnings of grooves. The software does not save the image of the finger, but the position of these minutiae. When the comparison is renewed, the software searches for the minutiae and checks how similar they are to the stored reference.
Why is my finger not recognised?
Dirty hands or wet hands
The fingerprint sensor on your smartphone or laptop is a fine measuring device. The device tries to detect the groove structure on the main surface. If there is dirt or water between the sensor and your finger, the scanner will not work reliably.
Hit? – Inaccurate position
If you place your finger on the sensor, the sensor can no longer recognise the image of your finger that you have saved.
→ Place the finger in the centre
Who was it? – Wrong finger
Every finger is different. You can save several fingers in modern smartphones. This is particularly practical because you only have one hand free from time to time. Don’t just save your thumb or index finger, but ideally all ten fingers.
→ Think about which fingers you have registered with
Ouch – scars / wear and tear
It can happen when cooking. One second of not paying attention and you’ve cut your finger. Apart from the plaster, the sensor recognises a “new” groove, the cut. The smartphone rejects you. Not only obvious injury, but also wear and tear caused by friction can smooth the skin so much that the grooves disappear.
→ Always remember a password as an emergency solution in the first instance
Insufficient pressure – too much pressure
The sensors do not break immediately if you apply too much pressure. If too much pressure is applied, the software only recognises a flattened impression that cannot be evaluated. On the other hand, too little pressure can prevent the skin of the finger from completely covering the sensor surface. The sensor will no longer find many minutiae and will also reject you.
→ Carefully place your finger on the sensor so that it touches the sensor completely
Spider-Man – rubber finger
The skin is an elastic and stretchy marvel. Unfortunately, the groove image changes if you first place your finger on the sensor and then pull your finger back (Spider-Man finger). The sensor perceives an impression that is stretched and can no longer recognise the proportions of the minutiae.
→ Place your finger straight on the sensor from above and do not pull it backwards
Dirt on the sensor
Not only your fins can be dirty, but also the sensor. If the sensor is placed at an entrance where many people come and go, the sensor can quickly become dirty.
→ Clean the sensor with a non-dripping cloth
Technology? Damaged sensor
If too much pressure is applied, the hardware can suffer permanent damage. The sensor may reject you because it is broken and therefore no longer accepts a person or finger.
→ Repair the sensor
Long live the software bug
Only humans write the software. People make mistakes. So the software has bugs. If you search long enough, an error may occur.
→ Talk to the manufacturer about it
The person in the background – remote wipe
An admin / person has removed the access. You can no longer log in. The fingerprint is no longer accepted, even if the sensor recognises the finger.
→ Ask why you were deleted from the database
How can I trick a fingerprint?
Firstly: Your smartphone is covered in greasy fingerprints, especially the shiny smartphones made of glass, smooth plastic or ceramic. What could possibly go wrong?
The CCC hack – heat up the sensor

The easiest way to trick a sensor is to heat it up. The previous user has left a greasy film with their fingerprint. Bring this print to life by placing a plastic bag of warm water on the sensor. This method does not work with every grease film print and not with every sensor.
The classic film – gelatine dummy
To make a gelatine dummy, press your finger into a putty. Allow the impression to harden. Mix a thick gelatine paste and pour it into the mould. Allow the object to harden and use this to apply your mark. Professionals can make a finger fake using a similar method to the gelatine dummy.
The technology – photography
High-resolution cameras such as mobile phone cameras, DSLRs or DSLMs make fingerprints visible on objects. If the exposure and resolution are right, you can make a dummy directly from a photo of your finger.
Sherlock is coming! – Graphite powder
A little graphite from a pencil makes the fingerprint visible if it is well hidden. You can then transfer this fingerprint to a dummy using photography.
Even more bugs
Criminals can exploit software bugs to trick the sensor. Finger recognition is only as good as the implementation.
Intelligent sensors – false finger recognition
IT security is a game of cat and mouse. The criminals show a way in which anyone can hack the scanner and the specialists have to solve the problem.
The finger’s life can be recognised by measuring its temperature. If the finger is cold, this can be an indication of a dummy.
Other methods analyse the reflections of the skin or silicone. Light rays reflect differently on different materials. The typical silicone reflections reveal the dummy.
The latest (and most expensive) scanners examine the veins in the finger. An ultrasound scan determines whether veins are visible at all.

If the criminal is wearing a thimble with the wrong fingerprint, the software compares the vein structure with the reference. The scanner checks whether the blood flow is recognisable. This enables the scanner to distinguish a freshly chopped off finger from a finger with blood flow. Otherwise, criminals collect fingers like a key (ouch).