Consensus Beyond Thresholds: Generalized Byzantine Quorums Made Live
Orestis Alpos, Christian Cachin

TL;DR
This paper introduces the first implementation of Byzantine fault-tolerant consensus with generalized quorums, enabling more flexible trust assumptions beyond simple thresholds, and evaluates their performance in practical blockchain settings.
Contribution
It formalizes trust assumptions using monotone Boolean formulas and span programs, and integrates these into HotStuff BFT consensus, demonstrating practical viability and performance trade-offs.
Findings
MBF specifications do not slow down HotStuff significantly
MSP expressions impact latency and throughput
Generalized quorums enable more flexible trust models
Abstract
Existing Byzantine fault-tolerant (BFT) consensus protocols address only threshold failures, where the participating nodes fail independently of each other, each one fails equally likely, and the protocol's guarantees follow from a simple bound on the number of faulty nodes. With the widespread deployment of Byzantine consensus in blockchains and distributed ledgers today, however, more sophisticated trust assumptions are needed. This paper presents the first implementation of BFT consensus with generalized quorums. It starts from a number of generalized trust structures motivated by practice and explores methods to specify and implement them efficiently. In particular, it expresses the trust assumption by a monotone Boolean formula (MBF) with threshold operators and by a monotone span program (MSP), a linear-algebraic model for computation. An implementation of HotStuff BFT…
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