Security is no longer optional in modern mobile apps. Users expect their data to be protected, and as developers, it’s our responsibility to implement authentication that goes beyond just email and password.
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In this article, we’ll build secure authentication in Flutter using modern and widely adopted techniques:
Table of contents
✅ OAuth2 (Google Login example)
✅ Biometrics (Fingerprint / Face ID)
✅ Two-Factor Authentication (OTP)
✅ JWT Tokens (Secure session handling)
Everything is explained step by step, in simple language, with working Flutter examples.
Why Simple Login Is Not Enough
A basic email + password system has multiple problems:
- Passwords can be leaked or reused
- Users forget passwords
- Brute-force attacks are common
- One compromised login can expose everything
That’s why modern apps use multiple layers of authentication:
- Social login (OAuth2)
- Token-based sessions (JWT)
- Device-level security (Biometrics)
- Extra verification (2FA)
Let’s build all of this in Flutter 👇
Prerequisites
Before starting, make sure you have:
- Flutter SDK installed
- A Firebase project set up
- Android/iOS app connected to Firebase
Dependencies
Add the required packages to your pubspec.yaml:
dependencies:
firebase_auth: ^4.16.0
google_sign_in: ^6.2.1
local_auth: ^2.1.7
flutter_secure_storage: ^9.0.0
Run: flutter pub get
1️⃣ OAuth2 Authentication (Google Login Example)
What is OAuth2?
OAuth2 allows users to log in using trusted providers like:
- GitHub
Your app never sees the user’s password. Instead, the provider verifies the user and gives your app a secure token.
This improves:
- Security
- User trust
- Signup speed
How Google Login Works
- User taps “Login with Google.”
- Google verifies identity
- Google returns an access token
- Firebase signs the user in
- Your app gets an authenticated user
✅ That’s it.
Now your user is authenticated securely using Google.
2️⃣ Biometric Authentication (Fingerprint / Face ID)
Why Biometrics?
Biometrics adds device-level security:
- No passwords
- Fast login
- Hard to fake
Common use cases:
- Unlock the app after login
- Approve payments
- Re-authenticate sensitive actions
Check & Authenticate with Biometrics
final auth = LocalAuthentication();
bool isAuthenticated = await auth.authenticate(
localizedReason: "Verify your identity",
options: const AuthenticationOptions(
biometricOnly: true,
),
);
if (isAuthenticated) {
print("Biometric authentication successful");
}
🔐 This uses:
- Fingerprint on Android
- Face ID / Touch ID on iOS
- Best Practice
- ✔ Use biometrics after initial login
- ❌ Never replace server-side authentication with biometrics
3️⃣ Two-Factor Authentication (2FA / OTP)
What is 2FA?
Two-Factor Authentication adds a second layer of security:
- Something you know (password)
- Something you have (OTP)
Even if credentials are stolen, the attacker can’t log in without the OTP.
Firebase Phone OTP Authentication
Firebase provides a built-in OTP system using SMS.
Send OTP
FirebaseAuth.instance.verifyPhoneNumber(
phoneNumber: "+91XXXXXXXXXX",
verificationCompleted: (credential) async {
await FirebaseAuth.instance.signInWithCredential(credential);
},
verificationFailed: (e) {
print(e.message);
},
codeSent: (verificationId, resendToken) {
print("OTP Sent");
},
codeAutoRetrievalTimeout: (verificationId) {},
);
Verify OTP
PhoneAuthCredential credential = PhoneAuthProvider.credential(
verificationId: verificationId,
smsCode: otp,
);
await FirebaseAuth.instance.signInWithCredential(credential);✅ Now the user is verified with 2FA.
4️⃣ JWT Authentication (Token-Based Sessions)
What is JWT?
JWT (JSON Web Token) is used to manage user sessions securely.
Flow:
- User logs in
- Server returns a JWT
- App stores the token securely
- Token is sent with API requests
JWTs are:
- Stateless
- Secure
- Widely used
Example JWT Token
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9…
Store JWT Securely
Never store tokens in SharedPreferences.
✅ Use flutter_secure_storage
final storage = FlutterSecureStorage();
await storage.write(
key: 'jwt_token',
value: token,
);
Read JWT Token
String? token = await storage.read(key: 'jwt_token');
This ensures:
- Encrypted storage
- Protection from reverse engineering
Bonus: JWT Refresh Token (Concept)
- JWTs usually expire.
Best practice:
- Short-lived access token
- Long-lived refresh token
- Automatically refresh tokens when expired
- This prevents:
- Forced logouts
- Token replay attacks
Final Security Checklist ✅
✔ OAuth2 for login
✔ Biometrics for fast re-auth
✔ 2FA for critical security
✔ JWT for session handling
✔ Secure storage for tokens
Conclusion
Building secure authentication in Flutter doesn’t have to be complicated.
By combining:
- OAuth2
- Biometrics
- 2FA
- JWT tokens
You can build apps that are:
- Secure
- User-friendly
- Production-ready
If you’re building a serious Flutter app, this stack is a must.
From Our Parent Company Aeologic
Aeologic Technologies is a leading AI-driven digital transformation company in India, helping businesses unlock growth with AI automation, IoT solutions, and custom web & mobile app development. We also specialize in AIDC solutions and technical manpower augmentation, offering end-to-end support from strategy and design to deployment and optimization.
Trusted across industries like manufacturing, healthcare, logistics, BFSI, and smart cities, Aeologic combines innovation with deep industry expertise to deliver future-ready solutions.
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