https://stackoverflow.com/questions/22601046
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RussianHow does Android's screen locker work?
The password works like a traditional password. Its digested and stored. The data security is a little weak (see the bug reports below).
The pattern locker turns the pattern into a string, and then it works like a traditional password. The data security is a little weak (see the bug reports below).
The face unlocker is based on facial recognition. It falls back to passwords if detection fails. I don't know anything about the recognizer.
sstendal's answer below provides a link to using Yubikeys and One-Tme Passwords (OTP) over NFC to unlock your Android phone. Nikolay Elenkov's blog rocks, so you'll almost certainly learn something.
For the password and pattern locker source code, see https://android.googlesource.com/platform/frameworks/base/+/HEAD/core/java/com/android/internal/widget/LockPatternUtils.java.
For face recognition source code, see https://android.googlesource.com/platform/frameworks/base/+/HEAD/core/java/com/android/internal/widget/FaceUnlockView.java. Also see https://android.googlesource.com/platform/frameworks/base/+/99f2f5e/packages/Keyguard/src/com/android/keyguard/FaceUnlock.java.
As of Android 4.4, the unlocker (called KeyGuard) was moved to a separate component. I believe its source is at https://android.googlesource.com/platform/frameworks/base/+/99f2f5e/packages/Keyguard/src/com/android/keyguard.
You can use ADB to reset the relevant fields in the system's SQLite database. Here's the Google Search from VenomVendor below.
The implementation has some rough edges. I know there are some bug reports on the subject. For example:
