Briar is a messaging app designed for activists,Briaralists, 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.
More info at https://briarproject.org/how-it-works.html
Article which lead me to Briar -