Signal

Signal is a free, open-source encrypted messaging service that provides end-to-end encrypted voice calls, video calls, text messaging, and file sharing. Developed by the Signal Foundation (a non-profit founded in 2018 by Brian Acton, co-founder of WhatsApp, with a $50 million initial funding) and Signal Messenger (led by Moxie Marlinspike, a renowned cryptographer), Signal is widely regarded as the gold standard for secure communication, endorsed by privacy advocates including Edward Snowden and the Electronic Frontier Foundation. The Signal Protocol (the cryptographic foundation) implements the Double Ratchet algorithm combining X3DH (Extended Triple Diffie-Hellman) for initial key agreement and the Double Ratchet for ongoing message encryption, providing forward secrecy (past messages remain secure even if keys are compromised later) and post-compromise security (future messages become secure again after a compromise). Each message is encrypted with AES-256-CBC authenticated with HMAC-SHA256. Key features include: sealed sender (encrypted sender identity to prevent metadata analysis), disappearing messages (auto-delete after configurable time from 30 seconds to 1 week), group messaging with sender keys for efficient group encryption, voice and video calls with end-to-end encryption using SRTP over DTLS, link previews with privacy-preserving fetching, stickers, message reactions and replies, view-once media, typing indicators and read receipts (both optional), username-based contact without phone number sharing (added 2024), and multi-device support via linked devices with independent keys. The Signal Protocol is also used by WhatsApp (over 2 billion users) and Facebook Messenger's Secret Conversations. Signal's server-side code is also open-source, enabling independent verification of the privacy claims.