Ratcheting with Immediate Decryption

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.

The most significant impact of the renewal of Royal Holloway's CDT in Cyber Security will be the production of at least 30 further PhD-level graduates. In view of the strong industry involvement in both the taught and research elements of the programme, CDT graduates are "industry-ready": through industry placements, they have exposure to real-world cyber security problems and working environments; because of the breadth of our taught programme, they gain exposure to cyber security in all its forms; through involvement of our industrial partners at all stages of the programme, the students are regularly exposed to the language and culture of industry. At the same time, they will continue to benefit from generic skills training, equipping them with a broad set of skills that will be of use in their subsequent workplaces (whether in academia, industry or government). They will also engage in PhD-level research projects that will lead to them developing deep topic-specific knowledge as well as general analytical skills.

One of the longer-term impacts of CDT research, expressed directly through research outputs, is to provide mechanisms that help to enhance confidence and trust in the on-line society for ordinary citizens, leading in turn to quality of life enhancement. CDT research has the potential of directly impacting the security of deployed system, for example helping to make the Internet a more secure place to do business. Moreover the work on the socio-technical dimensions of security and privacy also gives us the means to influence government policy to the betterment of society at large. Through the training component of the CDT, and subsequent engagement with industry, our PhD students are exposed to the widest set of cyber security issues and forced to think beyond the technical boundaries of their research. In this way, our CDT is training a generation of cyber security researchers who are equipped - philosophically as well as technically - to cope with whatever cyber security threats the future may bring. The programme equip students with skills that will enable them to understand, represent and solve complex engineering questions, skills that will have an impact in UK industry and academic long beyond the lifetime of the CDT.