Briar: secure messaging even without internet connectivity
From their website:
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.
Go to the Briar listing.