Biometric authentication in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Biometric Authentication Button Behavior

What will happen when the user taps the biometric authentication button in this Flutter widget?

Flutter

ElevatedButton(
  onPressed: () async {
    final isAvailable = await auth.canCheckBiometrics;
    if (isAvailable) {
      final authenticated = await auth.authenticate(
        localizedReason: 'Please authenticate to proceed',
        options: const AuthenticationOptions(biometricOnly: true),
      );
      if (authenticated) {
        print('Access granted');
      } else {
        print('Access denied');
      }
    } else {
      print('Biometrics not available');
    }
  },
  child: const Text('Authenticate'),
)

A It throws a runtime error because authenticate is not awaited.

B It always prints 'Access granted' without checking biometrics.

C If biometrics are available, it prompts the user to authenticate and prints 'Access granted' on success.

D It prints 'Biometrics not available' even if biometrics are enabled.

💡 Hint

Check the flow: canCheckBiometrics is checked before authenticate is called.

The code first checks if biometrics are available. If yes, it calls authenticate and prints 'Access granted' if authentication succeeds.

❓ lifecycle

intermediate

Handling Biometric Authentication State

In a Flutter app using biometric authentication, which lifecycle method is best to check if biometrics are available when the app resumes from background?

A initState()

B didChangeAppLifecycleState(AppLifecycleState.resumed)

C dispose()

D build()

💡 Hint

Think about when the app returns to the foreground.

Checking biometric availability when the app resumes ensures the status is current after backgrounding.

📝 Syntax

advanced

Correct Usage of authenticate() Options

Which option correctly uses the authenticate() method from local_auth package to restrict authentication to biometrics only?

Flutter

final authenticated = await auth.authenticate(
  localizedReason: 'Authenticate',
  options: ???
);

A AuthenticationOptions(biometricOnly: true)

B AuthenticationOptions(biometricOnly: false)

C AuthenticationOptions(stickyAuth: true)

D AuthenticationOptions(useErrorDialogs: false)

💡 Hint

Look for the option that limits authentication to biometrics.

Setting biometricOnly to true restricts authentication to biometrics only.

🔧 Debug

advanced

Debugging Biometric Authentication Failure

A Flutter app using local_auth package always prints 'Biometrics not available' even on devices with biometrics enabled. What is the most likely cause?

A The app forgot to add the required permissions in AndroidManifest.xml or Info.plist.

B The authenticate() method was called without awaiting.

C The localizedReason string is empty.

D The app is running on an emulator that supports biometrics.

💡 Hint

Check platform-specific setup for biometric permissions.

Without proper permissions, canCheckBiometrics returns false, causing the app to think biometrics are unavailable.

🧠 Conceptual

expert

Security Implications of Biometric Authentication

Which statement best describes a security consideration when using biometric authentication in mobile apps?

A Biometric authentication guarantees 100% protection against unauthorized access.

B Biometric authentication replaces the need for any fallback authentication method.

C Biometric data is stored and accessible by the app for custom processing.

D Biometric authentication should be combined with fallback methods like PIN for better security.

💡 Hint

Think about what happens if biometrics fail or are unavailable.

Combining biometrics with fallback methods like PIN improves security and usability.