XRD: Scalable Messaging System with Cryptographic Privacy
It addresses metadata privacy for users of messaging systems, offering a scalable solution with cryptographic guarantees, though it builds on prior work in mix networks.
The paper tackles the problem of metadata leakage in private messaging by presenting XRD, a system that provides cryptographic privacy and scales easily with more servers, achieving 251 seconds latency for 2 million users with 100 servers, which is 12x faster than Atom and 3.7x faster than Pung.
Even as end-to-end encrypted communication becomes more popular, private messaging remains a challenging problem due to metadata leakages, such as who is communicating with whom. Most existing systems that hide communication metadata either (1) do not scale easily, (2) incur significant overheads, or (3) provide weaker guarantees than cryptographic privacy, such as differential privacy or heuristic privacy. This paper presents XRD (short for Crossroads), a metadata private messaging system that provides cryptographic privacy, while scaling easily to support more users by adding more servers. At a high level, XRD uses multiple mix networks in parallel with several techniques, including a novel technique we call aggregate hybrid shuffle. As a result, XRD can support 2 million users with 251 seconds of latency with 100 servers. This is 12x and 3.7x faster than Atom and Pung, respectively, which are prior scalable messaging systems with cryptographic privacy.