I really like the idea of using biometrics to add extra security, but have always hated the idea that simply touching the fingerprint sensor would unlock your entire phone, so in my version of LineageOS the touch to unlock feature is disabled but I still use second factor biometrics for the security of various apps. Effectively the android unlock policy is Fingerprint OR PIN/Pattern/Password and I simply want that OR to become an AND.

The problem

The idea of using two factor authentication (2FA) was pioneered by GrapheneOS but since I like the smallness of the Pixel 3 that’s not available to me (plus it only seems to work with pin and fingerprint and my preferred unlock is pattern). However, since I build my own LineageOS anyway (so I can sign and secure boot it) I thought I’d look into adding the feature … porting from GrapheneOS should be easy, right? In fact, when looking in the GrapheneOS code for frameworks/base, there are about nine commits adding the feature:

a7a19bf8fb98 add second factor to fingerprint unlock
5dd0e04f82cd add second factor UI
9cc17fd97296 add second factor to FingerprintService
c92a23473f3f add second factor to LockPattern classes
c504b05c933a add second factor to TrustManagerService
0aa7b9ec8408 add second factor to AdaptiveAuthService
62bbdf359687 add second factor to LockSettingsStateListener
7429cc13f971 add second factor to LockSettingsService
6e2d499a37a2 add second factor to DevicePolicyManagerService

And a diffstat of over 3,000 lines … which seems a bit much for changing an OR to an AND. Of course, the reason it’s so huge is because they didn’t change the OR, they implemented an entirely new bouncer (bouncer being the android term in the code for authorisation gateway) that did pin and fingerprint in addition to the other three bouncers doing pattern, pin and password. So not only would I have to port 3,000 lines of code, but if I want a bouncer doing fingerprint and pattern, I’d have to write it. I mean colour me lazy but that seems way too much work for such an apparently simple change.

Creating a new 2FA unlock

So is it actually easy? The rest of this post documents my quest to find out. Android code itself isn’t always easy to read: being Java it’s object oriented, but the curse of object orientation is that immediately after you’ve written the code, you realise you got the object model wrong and it needs to be refactored … then you realise the same thing after the first refactor and so on until you either go insane or give up. Even worse when many people write the code they all end up with slightly different views of what the object model should be. The result is what you see in Android today: model inconsistency and redundancy which get in the way when you try to understand the code flow simply by reading it. One stroke of luck was that there is actually only a single method all of the unlock types other than fingerprint go through KeyguardSecurityContainerController.showNextSecurityScreenOrFinish() with fingerprint unlocking going via a listener to the KeyguardUpdateMonitorCallback.onBiometricAuthenticated(). And, thanks to already disabling fingerprint only unlock, I know that if I simply stop triggering the latter event, it’s enough to disable fingerprint only unlock and all remaining authentication goes through the former callback. So to implement the required AND function, I just have to do this and check that a fingerprint authentication is also present in showNext.. (handily signalled by KeyguardUpdateMonitor.userUnlockedWithBiometric()). The latter being set fairly late in the sequence that does the onBiometricAuthenticated() callback (so I have to cut it off after this to prevent fingerprint only unlock). As part of the Android redundancy, there’s already a check for fingerprint unlock as its own segment of a big if/else statement in the showNext.. code; it’s probably a vestige from a different fingerprint unlock mechanism but I disabled it when the user enables 2FA just in case. There’s also an insanely complex set of listeners for updating the messages on the lockscreen to guide the user through unlocking, which I decided not to change (if you enable 2FA, you need to know how to use it). Finally, I diverted the code that would call the onBiometricAuthenticated() and instead routed it to onBiometricDetected() which triggers the LockScreen bouncer to pop up, so now you wake your phone, touch the fingerprint to the back, when authenticated, it pops up the bouncer and you enter your pin/pattern/password … neat (and simple)!

Well, not so fast. While the code above works perfectly if the lockscreen is listening for fingerprints, there are two cases where it doesn’t: if the phone is in lockdown or on first boot (because the Android way of not allowing fingerprint only authentication for those cases is not to listen for it). At this stage, my test phone is actually unusable because I can never supply the required fingerprint for 2FA unlocking. Fortunately a rooted adb can update the 2FA in the secure settings service: simply run sqlite3 on /data/system/locksettings.db and flip user_2fa from 1 to 0.

The fingerprint listener is started in KeyguardUpdateMonitor, but it has a fairly huge set of conditions in updateFingerprintListeningState() which is also overloaded by doing detection as well as authentication. In the end it’s not as difficult as it looks: shouldListenForFingerprint needs to be true and runDetect needs to be false. However, even then it doesn’t actually work (although debugging confirms it’s trying to start the fingerprint listening service); after a lot more debugging it turns out that the biometric server process, which runs fingerprint detection and authentication, also has a redundant check for whether the phone is encrypted or in lockdown and refuses to start if it is, which also now needs to return false for 2FA and bingo, it works in all circumstances.

Conclusion

The final diffstat for all of this is

5 files changed, 55 insertions(+), 3 deletions(-)

So I’d say that is way simpler than the GrapheneOS one. All that remains is to add a switch for the setting (under the fingerprint settings) in packages/apps/Settings and it’s done. If you’re brave enough to try this for yourself you can go to my github account and get both the frameworks and settings commits (if you don’t want fingerprint unlock disable when 2FA isn’t selected, you’ll have to remove the head commit in frameworks). I suppose I should also add I’ve up-ported all of my other security stuff and am now on Android-15 (LineageOS-22.2).