The Double Ratchet: Security Notions, Proofs, and Modularization for the Signal Protocol
Signal is a famous secure messaging protocol used by billions of people, by virtue of many secure text messaging applications including Signal itself, WhatsApp, Facebook Messenger, Skype, and Google Allo. At its core it uses the concept of “double ratcheting,” where every message is encrypted and authenticated using a fresh symmetric key; it has many attractive properties, such as forward security, post-compromise security, and “immediate (no-delay) decryption,” which had never been achieved in combination by prior messaging protocols.