Error-Free Near-Optimal Validated Agreement
arxiv(2024)
摘要
Byzantine agreement enables n processes to agree on a common L-bit value,
despite t > 0 arbitrary failures. A long line of work has been dedicated to
improving the worst-case bit complexity of Byzantine agreement in synchrony.
This has culminated in COOL, an error-free (deterministically secure against a
computationally unbounded adversary) algorithm that achieves a near-optimal bit
complexity of O(nL + n^2 log n). COOL satisfies strong validity: if all correct
processes propose the same value, only that value can be decided. Thus,
whenever correct processes do not a priori agree, COOL might decide on
"bottom", thus limiting its application in today's state machine replication
(SMR) and blockchain protocols. In this work, we focus on the aforementioned
limitation. Can we design an error-free near-optimal Byzantine agreement
algorithm applicable in today's SMR and blockchain protocols? Can we design an
error-free near-optimal agreement algorithm with external validity (a.k.a.
validated agreement) stipulating that only values valid according to a
predetermined predicate can be decided?
This paper answers the question affirmatively. Namely, we present EXT, an
error-free synchronous Byzantine agreement algorithm that satisfies external
(along with strong) validity while exchanging O(n log n L + n^2 log n) bits in
the worst case. Importantly, EXT is optimally resilient (tolerates t < n / 3
failures) and terminates in optimal O(n) rounds. Perhaps surprisingly, we
construct EXT by exploiting existing concepts: (1) the recursive framework
proposed by Berman, Garay and Perry and Coan and Welch and recently restated by
Momose and Ren, (2) the aforementioned COOL algorithm introduced by Chen, and
(3) the data dissemination primitive introduced by Das, Xiang and Ren.
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