Prefix Consensus For Censorship Resistant BFT

TL;DR

This paper introduces Prefix Consensus, a novel abstraction for censorship-resistant Byzantine Fault Tolerance (BFT), enabling leaderless, asynchronous consensus with strong guarantees against censorship and suspension.

Contribution

The paper proposes Prefix Consensus and Strong Prefix Consensus, providing tight bounds, leaderless protocols, and applications to censorship-resistant BFT state machine replication.

Findings

Achieves asynchronous consensus with tight round complexity bounds.

Enables leaderless, censorship-resistant BFT protocols with four-round decision.

Connects Prefix Consensus to optimal-latency graded and leaderless binary consensus.

Abstract

Despite broad use of BFT consensus in blockchains, censorship resistance is weak: leaders can exclude transactions, a growing concern for trading and DeFi. We address this by introducing a new abstraction and protocol stack. First, we introduce \emph{Prefix Consensus}, where parties input vectors and output $(v^{\sf low},v^{\sf high})$ that (i) extend the maximum common prefix of honest inputs and (ii) satisfy $v_i^{\sf low}\preceq v_j^{\sf high}$ for all honest $i,j$. Unlike classical consensus, no single output is required. We show Prefix Consensus is solvable asynchronously and give tight round-complexity bounds. We then define \emph{Strong Prefix Consensus}, requiring agreement on the \emph{high} output. Our protocol is leaderless and partially synchronous: one Prefix Consensus instance decides (possibly different) lows, and additional instances yield a unique safe-to-extend high, even if an adversary can suspend one party per round. We lift this to a leaderless, multi-proposer, censorship-resistant BFT SMR protocol: per slot, all parties broadcast proposals, deterministically rank them, and run one Strong Prefix Consensus on proposal hashes, committing honest proposals in \emph{four rounds}. A deterministic demotion rule updates the ranking when a party's proposal is excluded, implying that after GST at most $f$ slots can miss an honest proposal while progress remains leaderless under suspension and up to $f{-}1$ Byzantine faults. Finally, we connect Prefix Consensus to graded and binary/validated consensus: we obtain an optimal-latency graded consensus (3 message delays) and leaderless Binary/Validated Consensus with worst-case message complexity $O(n^3)$ and communication $O(n^4)$.