Reaching Univalency with Subquadratic Communication

Abstract

The Dolev-Reischuk lower bound establishes that any deterministic Byzantine Agreement (BA) protocol for $n$ processors tolerating $f$ faults requires $Ω(f^2+n)$ messages. But what exactly does this quadratic cost pay for? Even the minimal requirement that every correct processor \emph{receive at least one message} already necessitates $Ω(f^2 + n)$ messages. This raises a fundamental question: is the Dolev-Reischuk bound about the difficulty of \emph{reaching univalency} -- the point at which the protocol's outcome is determined -- or merely about \emph{disseminating} the outcome to all processors afterward? We resolve this question by showing that reaching univalency does \emph{not} require quadratic communication. Specifically, we introduce $ε$-BA, a relaxation allowing an $ε$-fraction of correct processors to output incorrectly, and prove it can be solved deterministically with $O(n \log n)$ communication complexity when $f < n(1/3 - ε)$. Crucially, any $ε$-BA protocol can serve as the first phase of a full BA protocol: after $ε$-BA, a single all-to-all exchange and majority vote completes BA. Since the outcome is already determined after $ε$-BA, this demonstrates that the quadratic cost in Dolev-Reischuk stems entirely from dissemination, rather than from reaching univalency. We also define Extractable BA for authenticated settings, capturing when processors collectively hold enough signed messages to determine the agreed value, and show it can be solved with communication complexity $O(f \log f)$.