Tox

Tox is a peer-to-peer instant-messaging and video-calling protocol that offers end-to-end encryption. The stated goal of the project is to provide secure yet easily accessible communication for everyone. A reference implementation of the protocol is published as free and open-source software under the terms of the GNU GPL-3.0-or-later.

Excerpt from Wikipedia: Tox (protocol). Content available under CC-BY-SA 4.0.