StealthDust: Secret Quorums for Faster Fractional Spending

With the goal of building a decentralized and fully parallel payment system, we address the Fractional Spending Problem using (k1, k2)-quorum systems - both introduced by Bazzi and Tucci-Piergiovanni (PODC 2024). Fractional spending enables payments without immediate validation of an entire quorum, as necessary in classical approaches. Multiple spending from a same fund can occur concurrently, with final settlement involving previously contacted quorums. To tolerate a rushing-adaptive adversary, the composition of these quorums must stay hidden until settlement succeeds. We propose a new abstraction called secret quorums - of independent interest - that fulfill this property and implement it through ring verifiable random functions. We then propose a new protocol called StealthDust, where secret quorums allow to reduce payment latency from five to three communications steps and improve settlment message complexity from O(n^3) to O(n^2) compared to the original protocol.