NSF-DFG: SaTC: CORE: Small: Interoperable Encrypted Messaging

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Billions of people use encrypted messaging apps every day. These apps generally do not interoperate - users must all have accounts on the same app to communicate. Users view this as a major pain point; increasingly, policymakers and industry groups do as well. In Europe, the Digital Markets Act (DMA) will require interoperability; in the US, similar legislation has been considered and Apple and Google are jointly designing a protocol that will allow encrypted text messaging between iPhone and Android devices. However, making encrypted messaging interoperable is challenging - existing apps were not designed to interoperate and the security of the few preliminary proposals for doing so is unclear; this is in part because interoperability enables new kinds of attacks that are not well-understood. More fundamentally, the formal cryptographic work on encrypted messaging does not consider interoperability, and it is not easy to generalize. This project’s novelties are building a practice-oriented foundation for the design of interoperable encrypted messaging systems. By both examining existing proposals and building new theory and protocols, the project’s broader significance and importance are to retain the strong privacy guarantees of existing encrypted messaging apps into the interoperable future. The project contributions are along three lines. First, the project analyzes existing proposals for interoperable encrypted messaging - specifically the proposals of Apple/Google, Meta, and the ongoing design of the IETF’s interoperability working group. It evaluates the handling of core security issues like identity management and the encryption protocol itself. Using the lessons learned from this evaluation the project develops novel cryptographic protocols that reduce metadata exposure and provide cross-platform key transparency. In parallel, the project investigates the group messaging setting; to resolve challenges like interoperable metadata management it designs protocols that distribute group metadata among multiple servers to ensure privacy. Finally, the last effort extends from text-based messaging protocols to encrypted interoperable voice and video calls. The project adopts the provable-security tools and insights from other areas of the research to understand them. The unique nature of the US-German collaboration allows the research to have broad impact to both US- and EU-based technologists and policymakers. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.