Signal Protocol

The Signal Protocol (formerly known as the TextSecure Protocol) is a non-federated cryptographic protocol that provides end-to-end encryption for voice and instant messaging conversations. The protocol was developed by Open Whisper Systems in 2013 and was introduced in the open-source TextSecure app, which later became Signal. Several closed-source applications have implemented the protocol, such as WhatsApp, which is said to encrypt the conversations of "more than a billion people worldwide" or Google who provides end-to-end encryption by default to all RCS-based conversations between users of their Google Messages app for one-to-one conversations. Facebook Messenger also say they offer the protocol for optional "Secret Conversations", as did Skype for its "Private Conversations". The protocol combines the Double Ratchet Algorithm, prekeys (i.e., one-time ephemeral public keys that have been uploaded in advance to a central server), and key agreement protocols including the original triple elliptic-curve Diffie–Hellman (3-DH) handshake and the post-quantum PQXDH protocol. Current Signal specifications model message encryption as authenticated encryption with associated data (AEAD), with recommended constructions including SIV-style AEAD schemes or a composition of AES-256 in CBC mode with HMAC.