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Signal Protocol
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<p>The Signal Protocol (formerly known as the TextSecure Protocol) is a non-<a href="/facts/Federation_(information_technology)/3SUojUub">federated</a> <a href="/facts/Cryptographic_protocol/0WuyTkFo">cryptographic protocol</a> that provides <a href="/facts/End-to-end_encryption/e1MwRzEU">end-to-end encryption</a> for voice and <a href="/facts/Instant_messaging/8lwwDyjK">instant messaging</a> conversations. The protocol was developed by <a href="/facts/Open_Whisper_Systems/kmBSmmDn">Open Whisper Systems</a> in 2013 and was introduced in the <a href="/facts/Open-source_software/HzTaB8E9">open-source</a> <a href="/facts/TextSecure/RChBQbrK">TextSecure</a> app, which later became <a href="/facts/Signal_(messaging_app)/1d9mpcMH">Signal</a>. Several <a href="/facts/Proprietary_software/9zcskxYL">closed-source</a> applications have implemented the protocol, such as <a href="/facts/WhatsApp/VMlSeFb6">WhatsApp</a>, which is said to encrypt the conversations of "more than a billion people worldwide" or <a href="/facts/Google/GT9Sugza">Google</a> who provides end-to-end encryption by default to all <a href="/facts/Rich_Communication_Services/sI5UAdvy">RCS</a>-based conversations between users of their <a href="/facts/Google_Messages/boNLMWMK">Google Messages</a> app for one-to-one conversations. <a href="/facts/Facebook_Messenger/NUz3VX8d">Facebook Messenger</a> also say they offer the protocol for optional "Secret Conversations", as did <a href="/facts/Skype/qdKbuh23">Skype</a> for its "Private Conversations". </p><p>The protocol combines the <a href="/facts/Double_Ratchet_Algorithm/1tDRUFvU">Double Ratchet Algorithm</a>, prekeys (i.e., one-time ephemeral public keys that have been uploaded in advance to a central server), and a triple <a href="/facts/Elliptic-curve_Diffie%25E2%2580%2593Hellman/zy3IJCeK">elliptic-curve Diffie–Hellman</a> (3-DH) handshake, and uses <a href="/facts/Curve25519/wLr6FMkV">Curve25519</a>, <a href="/facts/AES-256/HdXGBY2Q">AES-256</a>, and <a href="/facts/HMAC-SHA256/Qy9WD4V9">HMAC-SHA256</a> as <a href="/facts/Cryptographic_primitive/fy6vq5YM">primitives</a>. </p>