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 64bit (Debian-based) Installation Instructions

wget -O- https://updates.signal.org/desktop/apt/keys.asc | gpg --dearmor > signal-desktop-keyring.gpg

cat signal-desktop-keyring.gpg | sudo tee -a /usr/share/keyrings/signal-desktop-keyring.gpg > /dev/null

echo 'deb [arch=amd64 signed-by=/usr/share/keyrings/signal-desktop-keyring.gpg] 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.