Secure Chat and File Sharing App

I'm looking for a secure communication and file-sharing chatting app, similar to Reddy (Reddy.team). The solution should include a website and mobile application for Android and desktop application for Windows.

Essential Features:
- File Sharing
- End-to-End Encryption
- Chat Functionality
- Single Sign-On (SSO) for user authentication (no registration option)

Ideal Skills and Experience:
- Experience in developing secure communication apps
- Proficiency in implementing End-to-End Encryption
- Familiarity with Android and Windows app development
- Web development skills to create a corresponding website

Please provide examples of similar work done. Looking forward to your proposals!