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 apps, 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.

You can link Briar with a device running Briar Mailbox (even on Android: https://f-droid.org/packages/org.briarproject.mailbox/) to receive encrypted messages from your contacts while Briar is offline.