Signal
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Signal is a messaging app developed by Signal Messenger LLC and supported by a 501(c)(3) nonprofit. It offers instant messaging, voice, and video calling. Everything is end-to-end encrypted unless you choose to send as SMS.
Key Features:
- End-to-end encrypted messages and calls
- Independently audited
- Usernames option (communicate without sharing phone numbers)
- Phone number required to start
- Free and open-source
Protocol: Signal’s protocol is widely used by other apps including WhatsApp. Signal’s implementation is widely considered a strong reference for encrypted messaging.