Use Signal Private Messenger if you want something that is open source, audited and well trusted. Alternatively, Briar is an option, though it is not as polished and has a different feature set.
Ring is a free and universal communication platform which preserves the users' privacy and freedoms.
Official GNU package
Released under GPLv3 licence
Supported by an active community
OpenDHT protocol
Decentralized communication
Peer-to-peer discovery and connection
End-to-end encryption with authentification
Identity managed by X.509 certificates
Based on RSA/AES/DTLS/SRTP technologies
