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.

Practice

1. What is the main purpose of biometric authentication in a Flutter app?

easy

A. To send notifications to users

B. To change the app's theme colors automatically

C. To speed up app loading times

D. To secure the app using fingerprint or face recognition

Solution

1. Step 1: Understand biometric authentication

   Biometric authentication uses unique physical traits like fingerprint or face to secure access.

2. Step 2: Identify its purpose in apps

   It protects sensitive data by allowing only authorized users to enter.

3. Final Answer:

   To secure the app using fingerprint or face recognition -> Option D

4. Quick Check:

   Biometric authentication = Secure app access [OK]

Hint: Biometrics means using your body to unlock [OK]

Common Mistakes:

• Confusing biometric authentication with UI changes
• Thinking it improves app speed
• Assuming it sends notifications

2. Which Flutter package is commonly used to implement biometric authentication?

easy

A. local_auth

B. shared_preferences

C. provider

D. http

Solution

1. Step 1: Recall Flutter packages for biometrics

   The local_auth package provides access to fingerprint and face authentication.

2. Step 2: Compare with other packages

   http is for network calls, provider for state management, and shared_preferences for storing data.

3. Final Answer:

   local_auth -> Option A

4. Quick Check:

   Biometric package = local_auth [OK]

Hint: local_auth sounds like local device authentication [OK]

Common Mistakes:

• Choosing http for biometrics
• Confusing provider with authentication
• Picking shared_preferences for biometrics

3. What will the following Flutter code print if biometric authentication is available and successful?

final isAvailable = await auth.canCheckBiometrics;
if (isAvailable) {
  final didAuthenticate = await auth.authenticate(
    localizedReason: 'Please authenticate',
  );
  print(didAuthenticate);
} else {
  print('No biometrics');
}

medium

A. No biometrics

B. false

C. true

D. Exception error

Solution

1. Step 1: Check biometric availability

   The code checks if biometrics are available with canCheckBiometrics. If true, it proceeds.

2. Step 2: Authenticate and print result

   If authentication succeeds, didAuthenticate is true and printed.

3. Final Answer:

   true -> Option C

4. Quick Check:

   Available and success = true [OK]

Hint: Success prints true, failure prints false or message [OK]

Common Mistakes:

• Assuming it prints 'No biometrics' when available
• Expecting false on success
• Ignoring await causing errors

4. Identify the error in this Flutter biometric authentication snippet:

final didAuthenticate = await auth.authenticate(
  localizedReason: 'Scan fingerprint',
  useErrorDialogs: true,
  stickyAuth: true
);
if (didAuthenticate) {
  print('Access granted');
}

medium

A. No check if biometrics are available before authenticate

B. Incorrect parameter name localizedReason

C. Missing await keyword before authenticate

D. Missing else block after if

Solution

1. Step 1: Review biometric availability check

   The code calls authenticate without checking canCheckBiometrics first, which may cause errors on devices without biometrics.

2. Step 2: Validate other parts

   Await is present, parameter names are correct, and else block is optional.

3. Final Answer:

   No check if biometrics are available before authenticate -> Option A

4. Quick Check:

   Always check availability before authenticate [OK]

Hint: Check biometrics availability before authenticating [OK]

Common Mistakes:

• Skipping availability check
• Confusing parameter names
• Thinking else block is mandatory

5. You want to secure a Flutter app so it only allows access if biometric authentication succeeds. Which sequence of steps correctly implements this using local_auth? A) Call authenticate directly without checking availability. B) Check canCheckBiometrics, then call authenticate, then proceed if true. C) Skip biometric checks and use a password instead. D) Call authenticate and ignore its result.

hard

A. Call authenticate directly without checking availability

B. Check canCheckBiometrics, then authenticate, then proceed if true

C. Skip biometrics and use password only

D. Call authenticate and ignore its result

Solution

1. Step 1: Verify biometric availability

   Use canCheckBiometrics to ensure device supports biometrics before authenticating.

2. Step 2: Authenticate and check result

   Call authenticate and proceed only if it returns true, ensuring secure access.

3. Step 3: Avoid skipping checks or ignoring results

   Skipping availability or ignoring authentication result risks security.

4. Final Answer:

   Check canCheckBiometrics, then authenticate, then proceed if true -> Option B

5. Quick Check:

   Availability check + authenticate success = secure access [OK]

Hint: Always check availability before authenticating [OK]

Common Mistakes:

• Calling authenticate without availability check
• Ignoring authentication result
• Using password instead of biometrics when asked