Ratcheting with Immediate Decryption

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A ratcheting protocol is a two-party message transmission protocol that offers security under strong corruption assumptions: If the protocol state (containing e.g. secret keys) of a participant is exposed to the adversary, confidentiality and authenticity are to be maintained as much as possible. In particular, forward secrecy (after corruption, past messages remain confidential) and post-compromise security (after corruption, participants auto-heal and regain security) have to be be provided. Ratcheting protocols are used as components in most modern instant messaging apps, with billions of users per day.Practical ratcheting protocols like Signal guarantee immediate decryption: Receivers recover and deliver the messages wrapped in ciphertexts immediately when they become available, even if ciphertexts arrive out-of-order and preceding ciphertexts are missing (temporarily or permanently). This ensures the continuation of sessions in unreliable communication networks, ultimately promising an improved user experience.Most academic treatments consider ratcheting without immediate decryption. This project will address the disconnect between theory and practice. It will fill the gap in the academic literature by providing security definitions and a provably secure construction in the immediate decryption setting.