Briar is a genuine FOSS SHTF encrypted messaging system. There are versions for 64-bit Linux and Android phones, no versions for Apple or Windows (from what I can see).
https://nixfaq.org/2021/01/briar-epitom ... ngers.html
Briar is a secure messaging program that was originally designed for android phones. It uses encryption coupled with using the Tor network to do instant messaging over Tor. It is also capable of creating its own mesh network between users using bluetooth if your towers/network is down. That means you can use it without a sim card if desired.
The Briar Team is now developing a 64-bit Linux desktop version. The Linux version is currently considered beta, an install of the current beta .deb version expires 6 months after release date. I believe the Briar Team is close to a finished Linux product that does not expire.
I tested the briar-desktop-ubuntu-18.04.deb version on FossaPup64.
Briar installed to /opt/briar-desktop/ but did not create a menu entry but it did put a .desktop file at /opt/briar-desktop/lib/briar-desktop-Briar.desktop. The program will launch by mouse clicking that .desktop file or you can drag that briar-desktop-Briar.desktop file to your desktop to launch the program from there.
The official Briar-Desktop page.
https://code.briarproject.org/briar/briar-desktop/
The links to the binaries for download are on the lower portion of that page, as well as additional information.
To make use of Briar you will need at least two installs using Linux, Android, or any combination of both.