Briar

Briar is a messaging app for activists, journalists, and anyone who needs a safe way to communicate. It does not rely on a central server, so messages sync directly between users' devices instead of through a hosted service.

When the Internet is down, Briar can sync over Bluetooth or Wi-Fi, keeping information flowing in a crisis. When the Internet is available, it can sync over the Tor network to help protect users and their relationships from surveillance.

Because there is no central account or phone number, the contact list and message history live only on the devices that hold them. Briar also publishes reproducible builds so the published APK can be checked against its public source code.