Encrypted Messaging Module Development

I am building a multi-service super app and the first piece I want to nail down is a rock-solid messaging component. For this initial milestone I only need plain text chat, but it must ship with true end-to-end encryption from day one—think Signal-level security, forward secrecy, perfect message authenticity, and tamper-proof delivery receipts.

I am open to whichever stack or protocol you prefer (Signal protocol, Matrix Olm, double-ratchet, etc.) as long as it is open-standard, fully audited, and can be licensed for commercial use without hidden fees. The code base should be clean, well-documented, and easy to integrate later with other services in the app, such as payments or ride-hailing.

Deliverables expected:
• A working Android and iOS messaging module (React Native, Flutter, or native—your choice) with secure account creation, key management, and encrypted one-to-one chat.
• REST or GraphQL endpoints plus concise API documentation so my backend team can plug it into our existing user service.
• Unit and integration tests that cover message encryption/decryption and key rotation.
• A short integration guide and one hand-off call so we can compile and run the feature locally.

When you reply, please confirm which encryption library or protocol you plan to use, how you will handle key storage on device, and your estimated timeline for a functional demo.