Ratcheting with Immediate Decryption

棘轮协议（英语：Ratcheting protocol）是一种双方消息传输协议，在强腐败假设下提供安全性：如果参与者的协议状态（包含例如密钥）暴露给对手，则应尽可能保持机密性和真实性。特别是，必须提供前向保密（在损坏后，过去的消息仍然保密）和后妥协安全（在损坏后，参与者自动修复并重新获得安全性）。棘轮协议被用作大多数现代即时消息应用程序的组件，每天有数十亿用户。实际的棘轮协议，如Signal，保证立即解密：接收方恢复并立即发送加密的消息，即使密文到达顺序混乱，之前的密文丢失（暂时或永久）。这确保了会话在不可靠的通信网络中的继续，最终有望改善用户体验。大多数学术研究都考虑棘轮效应而不立即解密。这个项目将解决理论与实践之间的脱节。它将填补差距，在学术文献中提供安全的定义和可证明的安全建设，在立即解密设置。

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.