ios8 TouchID detection if fingerprint was added

Question

Im digging into Apple's Touch ID, more precisely the Local Authenticator. The documentation as of now is pretty sparse. Its mainly just this:

LAContext *myContext = [[LAContext alloc] init];
NSError *authError = nil;
NSString *myLocalizedReasonString = <#String explaining why app needs authentication#>;

if ([myContext canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics    error:&authError]) {
[myContext evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics
              localizedReason:myLocalizedReasonString
                        reply:^(BOOL success, NSError *error) {
        if (success) {
            // User authenticated successfully, take appropriate action
        } else {
            // User did not authenticate successfully, look at error and take appropriate action
        }
    }];
} else {
// Could not evaluate policy; look at authError and present an appropriate message to user
}

as taken from https://developer.apple.com/documentation/localauthentication

The idea of using your fingerprint for authentication is nice. But I can add fingerprints in the device if I know the passcode. And its very easy to get the passcode, like you sit in the train next to ur victim and watch him/her enter the passcode.

I want to use the fingerprint as a way of secure authentication but want to be able to detect if new fingerprints were added since the last time I requested the fingerprint.

Apple is doing this for the AppStore. If you want to authenticate a transaction in the AppStore and have added a new Fingerprint since your last transaction, the AppStore requests your AppleId-Password. This is sane behaviour, because the phone might have been taken by someone else who knows the passcode and added his own fingerprint to buy something expensive.

My Question: Can I detect if a new fingerprint was added since the last time that I used Local Authenticator?

Answer 1

This is now possible in iOS9. The property evaluatedPolicyDomainState has been added to LAContext.

If the fingerprint database is modified (fingers were added or removed), the data returned by evaluatedPolicyDomainState will change. The nature of the changes cannot be determined but by comparing data of evaluatedPolicyDomainState after different evaluatePolicy calls you can detect that the set of fingerprints has been modified.

Note that this property is set only when evaluatePolicy is called and a succesful Touch ID authentication was performed, or when canEvaluatePolicy succeeds for a biometric policy.