Linux Instant Messaging Clients
Signal
Signal is an encrypted communications app for Android and iOS. A desktop version is also available for Linux, Windows, and macOS. It uses the Internet to send one-to-one and group messages, which can include files, voice notes, images and videos, and make one-to-one voice and video calls.
Signal uses standard cellular mobile numbers as identifiers (so you need to have the app on your phone), and uses end-to-end encryption to secure all communications to other Signal users.
Signal clients are published as free and open-source software under the GPLv3 license. The server code is published under the AGPLv3 license.
For Debian based systems you can use:
curl -s https://updates.signal.org/desktop/apt/keys.asc | sudo apt-key add -
echo "deb [arch=amd64] https://updates.signal.org/desktop/apt xenial main" | sudo tee -a /etc/apt/sources.list.d/signal-xenial.list
sudo apt update && sudo apt install -ysignal-desktop