Implementing Biometric Authentication (FaceID/TouchID) in Flutter

Introduction

Biometric authentication offers a seamless and secure way to verify users via FaceID or TouchID. In Flutter, the local_auth plugin bridges native biometric APIs on iOS and Android, letting you integrate fingerprint and facial recognition with minimal effort. This tutorial walks you through setting up local_auth, checking device capabilities, implementing the authentication flow, and handling edge cases. By the end, you’ll have a reliable biometric sign-in mechanism in your Flutter app.

Setting Up the local_auth Plugin

First, add local_auth to your pubspec.yaml:

dependencies:
  flutter:
    sdk: flutter
  local_auth

Run flutter pub get. For iOS, enable FaceID/TouchID in Xcode under Signing & Capabilities → + Capability → Face ID. Add the following keys to Info.plist:

<key>NSFaceIDUsageDescription</key>
<string>Authenticate using Face ID</string

On Android, ensure your minSdkVersion is 23 or higher in android/app/build.gradle:

android {
  defaultConfig {
    minSdkVersion 23
  }
}

No further setup is required; local_auth handles the platform channels and permissions.

Checking Biometric Availability

Before prompting the user, verify the device supports biometrics and that the user has enrolled credentials:

import 'package:local_auth/local_auth.dart';

final LocalAuthentication auth = LocalAuthentication();

Future<bool> checkBiometrics() async {
  bool canCheck = await auth.canCheckBiometrics;
  if (!canCheck) return false;

  List<BiometricType> available = await auth.getAvailableBiometrics();
  return available.contains(BiometricType.face) ||
         available.contains(BiometricType.fingerprint);
}

canCheckBiometrics returns true if the hardware is present. getAvailableBiometrics lists enrolled modalities. Use these checks to conditionally render biometric login options in your UI.

Implementing Authentication Flow

Once availability is confirmed, invoke authenticate to trigger the native prompt:

Future<bool> authenticateUser() async {
  try {
    return await auth.authenticate(
      localizedReason: 'Please authenticate to access secure data',
      options: const AuthenticationOptions(
        stickyAuth: true,
        biometricOnly: true,
      ),
    );
  } on Exception catch (e) {
    print('Authentication error: $e');
    return false;
  }
}

stickyAuth keeps the activity alive if the app goes to background briefly. biometricOnly prevents fallback to device passcode. On success, you receive true, then grant access to protected resources or proceed with your sign-in flow.

Handling Edge Cases

Consider these scenarios:

• No enrolled biometrics: Prompt users to enroll under device settings, then gracefully fallback to PIN or password.

• User cancels prompt: Treat as authentication failure; you can allow multiple attempts or revert to manual login.

• Lockout after too many failures: Devices may disable biometrics temporarily. Listen for exceptions and direct users to alternative auth.

• Platform differences: Android may return different BiometricType enums; always check for both face and fingerprint.

Example fallback logic:

bool isAvailable = await checkBiometrics();
if (isAvailable) {
  bool authenticated = await authenticateUser();
  if (!authenticated) {
    // fallback to passcode or show error
  }
} else {
  // show manual login screen
}

This approach ensures your app remains accessible under all conditions.

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Conclusion

Integrating biometric authentication in Flutter with local_auth is straightforward yet powerful. By following setup steps, validating availability, and handling edge cases, you deliver a secure, user-friendly sign-in experience. Customize prompts and fallback mechanisms to fit your UX, and test on real devices to ensure reliability. With biometrics in place, you enhance both security and convenience in your mobile applications.