Mobile Authentication & Security Architecture

Most Android engineers know how to implement login screens.

Very few know how to design authentication for 5M+ users at global scale.

This handbook is written from the perspective of a Senior Android Engineer, focused specifically on:

1. Top Company system design rounds
2. Security-focused mobile roles
3. Senior → Staff level interviews
4. Fintech-grade authentication architecture

What This Book Covers

Authentication Fundamentals (Beyond Basics)

1. Stateless vs session-based systems
2. JWT internals (HS256 vs RS256)
3. Access vs Refresh architecture
4. Expiry strategy & risk modeling

Production Android Implementation (Kotlin)

1. Secure token storage (EncryptedSharedPreferences + Keystore)
2. OkHttp interceptor architecture
3. Token refresh & concurrency handling (Mutex approach)
4. Logout & token revocation strategy
5. Clean architecture token layer design

Real production-ready Kotlin code included.

Advanced Security Architecture

1. Multi-Factor Authentication (risk-based design)
2. Biometric authentication (proper usage patterns)
3. Certificate pinning (MITM protection)
4. Trusted device registry
5. Device fingerprinting
6. New device alerts
7. Suspicious activity detection
8. Refresh token rotation strategy

This is the level expected in fintech & big tech security teams.