Weaker Assumptions for Asymmetric Trust
Ignacio Amores-Sesar, Christian Cachin, Simon Holmgaard Kamp, Juan Villacis
TL;DR
This work tackles distributed systems with asymmetric trust by introducing a depth-based framework that weakens traditional guild-based assumptions. It develops depth-aware primitives—RB[$d$], CC[$d',d$], BGA[$d',d$]—and a depth-based consensus C[$10$], showing reliable broadcast and consensus can be achieved under substantially weaker conditions than prior guild-reliant solutions. The authors demonstrate that guild-based advantages can be replicated or bypassed by deriving symmetric fail-prone systems in guild contexts, and they establish concrete depth thresholds (e.g., RB[$3$], C[$10$]) to guarantee progress. The significance lies in enabling more flexible, scalable asymmetric protocols applicable to core problems in distributed systems and potentially blockchain-like environments.
Abstract
In distributed systems with asymmetric trust, each participant is free to make its own trust assumptions about others, captured by an asymmetric quorum system. This contrasts with ordinary, symmetric quorum systems and threshold models, where trust assumptions are uniformly shared among participants. Fundamental problems like reliable broadcast and consensus are unsolvable in the asymmetric model if quorum systems satisfy only the classical properties of consistency and availability. Existing approaches overcome this by introducing stronger assumptions. We show that some of these assumptions are overly restrictive, so much so that they effectively eliminate the benefits of asymmetric trust. To address this, we propose a new approach to characterize asymmetric problems and, building upon it, present algorithms for reliable broadcast and consensus that require weaker assumptions than previous solutions. Our methods are general and can be extended to other core problems in systems with asymmetric trust.