Signal is a free and open source software application for Android, iOS, and Desktop that employs end-to-end encryption , allowing users to send end-to-end encrypted group, text, picture, and audio & video messages, and have encrypted phone conversations between Signal users.
Resources to get started:
Signal source code is open sourced here: [login to view URL]
Singal-Android client source code: [login to view URL]
Signal server installation guide: [login to view URL]