Briar

Briar is a messaging app designed for activists, journalists, and anyone else
who needs a safe, easy and robust way to communicate. Unlike traditional
messaging tools such as email, Twitter or Telegram, Briar doesn't rely on a
central server - messages are synchronized directly between the users' devices.
If the Internet's down, Briar can sync via Bluetooth or Wi-Fi, keeping the
information flowing in a crisis. If the Internet's up, Briar can sync via the
Tor network, protecting users and their relationships from surveillance.

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