How does a fingerprint unlock your phone?
The short answer
Your phone saves a set of numbers describing your ridges, not a picture of your finger, and it opens only when a fresh scan's numbers are close enough.
Phone screen: shine the torch to look inside
The full explanation
Your phone keeps no photo of your fingerprint. So how does a touch open it? A sensor feels the ridges of your fingertip.
Then the phone turns that pattern into numbers, like each ridge's direction and width. Only those numbers are saved, in a protected chip. Next touch, it makes fresh numbers and opens if they're close.
Apple says a stranger's finger fools Touch ID one time in fifty thousand. A four digit passcode is guessed one time in ten thousand.
Sources
Insidely's promise: every fact in this episode comes from these sources.
- No stored fingerprint images, only a mathematical representation; 1 in 50,000 match chance with one enrolled finger; 1 in 10,000 for a 4-digit passcode; five failed tries then passcode needed: Apple Support, About Touch ID advanced security technology, https://support.apple.com/en-us/105095
- The stored data is a mathematical representation of ridge direction and width, not images of the finger: Apple patent text, https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/11409410
- Capacitive sensors detect how ridges change the charge in tiny circuits; ultrasonic sensors use sound pulses: Android Police, https://www.androidpolice.com/optical-capacitive-and-ultrasonic-fingerprint-readers-how-do-they-work/
- Template is encrypted and handled by the Secure Enclave: Apple Support, https://support.apple.com/guide/security/biometric-security-sec067eb0c9e/web
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