Signal
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Signal is a mobile app developed by Signal Messenger LLC. Supported by a 501c3 nonprofit organization. The app provides instant messaging, as well as voice, and video calling. All communications are end-to-end encrypted unless you choose to send as SMS.
Key Features:
- Protocol is the gold standard for encryption
- Has been independently audited
- Now supports User Names for communications without exchanging phone numbers
- Requires phone number but easy to begin with
- Completely free and open-source
Security: Its protocol is used by many other messaging apps including WhatsApp, but Signal remains the most secure implementation.