Signal

Signal is a simple, powerful, and secure messenger which uses state-of-the-art end-to-end encryption (powered by the open source Signal Protocol) keeps your conversations & videos secure.

Linux (Debian-based) Installation Instructions

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 signal-desktop

Like Telegram, you will need your mobile phone to have the Signal app installed.