Most Android authentication tutorials stop at “getting the token”.
But in real production systems (especially fintech apps), that’s just the beginning.
I recently built a secure authentication system using:
• OAuth2 Authorization Code Flow with PKCE
• OpenID Connect (OIDC) via Keycloak
• AppAuth-Android for browser-based login
• Android KeyStore with AES-256-GCM encryption
• Jetpack DataStore for secure persistence
• Kotlin Flow + MVVM reactive architecture
🔐 Key Highlights
- Tokens are NEVER stored in plain text
- Hardware-backed encryption using Android KeyStore
- PKCE prevents authorization code interception attacks
- Chrome Custom Tabs instead of insecure WebViews
- Secure token refresh with race-condition handling
- Fully reactive authentication state management
🧠 Why this matters
Most mobile apps still store tokens in SharedPreferences or insecure storage.
That approach is not production-grade for fintech or secure systems.
This implementation follows real-world security standards used in production backend + mobile systems.
🏗 Architecture Overview
The system is built in 3 layers:
1. UI Layer
Handles login and observes authentication state using MVVM.
2. Auth Layer (AppAuth-Android)
Handles OAuth2 Authorization Code Flow with PKCE and browser-based login.
3. Security Layer
Encrypts and stores tokens using:
- Android KeyStore
- AES-256-GCM encryption
- Secure persistence with DataStore
⚡ Key Security Idea: PKCE
PKCE ensures that even if an attacker intercepts the authorization code, they cannot exchange it for tokens without the original code_verifier.
This makes OAuth2 safe for mobile public clients.
🔒 Secure Storage
Tokens are encrypted using:
- AES-256-GCM encryption
- Hardware-backed Android KeyStore
- Unique IV per encryption
This ensures tokens remain safe even if storage is compromised.
📌 Final Thoughts
Production authentication is not just “login and get token”.
It includes:
- Secure transport (OAuth2 + PKCE)
- Secure storage (KeyStore encryption)
- Safe architecture (MVVM + reactive flows)
- Robust lifecycle handling
🔗 Full Article + Code
Would love feedback from fellow developers!
