To build a secure instant messaging app, focus on end-to-end encryption (E2EE), secure authentication, data encryption, metadata protection, spam prevention, and regular security audits. Use Signal Protocol for encryption and open-source cryptography libraries like libsodium.

Tech Stack:
Frontend: React Native, Flutter, Swift, Kotlin
Backend: Node.js, Django, Golang
Database: PostgreSQL, Firebase
Real-time Messaging: WebSockets, MQTT, Firebase Cloud Messaging