Biometric authentication in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is biometric authentication in mobile apps?

Biometric authentication uses unique body features like fingerprints or face to verify a user’s identity instead of passwords.

beginner

Which Flutter package is commonly used for biometric authentication?

The local_auth package helps Flutter apps access fingerprint and face recognition features.

intermediate

What is the first step before using biometric authentication in Flutter?

Check if the device supports biometrics and if the user has enrolled biometrics like fingerprint or face ID.

intermediate

Why should biometric authentication be combined with fallback options?

Because biometrics might fail or not be available, fallback like PIN or password ensures user can still access the app.

advanced

How does Flutter handle biometric authentication UI prompts?

Flutter uses native system dialogs for biometric prompts via local_auth, so UI looks familiar and secure.

Which biometric types are commonly supported on mobile devices?

A Fingerprint and Face recognition

B Voice and Retina only

C Password and PIN

D Pattern and Gesture only

Fingerprint and face recognition are the most common biometric methods on mobile devices.

What Flutter package do you use to implement biometric authentication?

A shared_preferences

B http

C local_auth

D provider

The local_auth package provides biometric authentication support in Flutter.

Before authenticating, what should your app check?

A If device supports biometrics and user has enrolled them

B If the user has internet connection

C If the app has camera permission

D If the battery is above 50%

Checking device support and enrollment ensures biometric authentication can proceed.

Why is fallback authentication important?

A Because it looks nicer

B Because biometrics can fail or be unavailable

C Because it speeds up login

D Because it uses less battery

Fallback options like PIN or password help users access the app if biometrics fail.

How does Flutter show biometric prompts to users?

A Using notifications

B Using custom Flutter widgets only

C Using web popups

D Using native system dialogs

Flutter uses native dialogs for biometric prompts to keep security and user familiarity.

Explain how to implement biometric authentication in a Flutter app.

Think about the steps from checking device to authenticating user.

Why is biometric authentication considered more secure than passwords?

Consider what makes biometrics special compared to traditional passwords.

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