Production Android Auth: OAuth2, PKCE & KeyStore Security

Om Kanse Posted on May 31 Production Android Auth: OAuth2, PKCE & KeyStore Security # android # oauth # security # kotlin 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