Weaker Assumptions for Asymmetric Trust

2025-09-11

AI Summary

In asymmetric-trust distributed systems, participants freely specify heterogeneous trust assumptions—modeled by asymmetric quorum systems—but reliable broadcast and consensus are impossible under classical consistency and availability definitions. Existing solutions rely on overly strong global assumptions, undermining the flexibility advantage of asymmetric trust. Method: We introduce the first theoretical framework that formally captures the essential challenges inherent in asymmetry. Our approach designs generic algorithms grounded in an asymmetric quorum model, jointly analyzing consistency and availability without requiring global strong assumptions. Contribution/Results: We present the first provably correct reliable broadcast and consensus protocols under weak, decentralized trust assumptions. Our methodology rigorously accommodates trust heterogeneity while guaranteeing primitive correctness, offering broad applicability and extensibility to other fundamental distributed problems under asymmetric trust.

Technology Category

• Data Mining & Knowledge Management: Representing, Reasoning, and Using Provenance, Trust
• Constraint Satisfaction and Optimization: Distributed CSP/Optimization
• Multiagent Systems: Multiagent Systems under Uncertainty

Application Category

• Security and Privacy: Data transparency and provenance
• Systems and Infrastructure for Web, Mobile and WoT: Decentralized Web and Fediverse systems
• Semantics and Knowledge: Provenance, trust, security and privacy, and ethical issues in managing semantic data

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.

Problem

• Characterizing asymmetric problems with weaker assumptions than existing solutions
• Enabling reliable broadcast and consensus in asymmetric trust systems
• Overcoming overly restrictive assumptions that eliminate asymmetric trust benefits

Innovation

• Weaker assumptions for asymmetric trust systems
• New approach to characterize asymmetric problems
• General algorithms for broadcast and consensus