Prioritized-MVBA: A New Approach to Design an Optimal Asynchronous Byzantine Agreement Protocol

TL;DR

This work tackles the high communication cost of asynchronous MVBA by introducing pMVBA, a committee-based MVBA that dynamically selects a small subset of parties to broadcast proposals and uses ABBA for agreement. By incorporating a truncated Verifiable Consistent Broadcast (pVCBC), a Propose-Recommend flow, random permutation via threshold coin-tossing, and Sequential-ABBA, the protocol achieves optimal resilience up to $f<\frac{n}{3}$ with constant expected rounds and $O(n^2)$ message complexity. The authors provide formal proofs, efficiency analyses, and case studies, demonstrating that pMVBA reduces communication overhead from $O(n^3)$ to $O((l+λ)n^2)$ without sacrificing correctness or liveness. Implementation results in a gRPC-based simulator show significant throughput gains and lower latency compared to state-of-the-art MVBA protocols, highlighting practical impact for large-scale, asynchronous distributed systems. These improvements make pMVBA a scalable core primitive for atomic broadcast and fault-tolerant state machine replication in decentralized environments.

Abstract

The multi-valued byzantine agreement protocol (MVBA) in the authenticated setting has been widely used as a core to design atomic broadcast and fault-tolerant state machine replication protocols in asynchronous networks. Originating from the seminal work of Cachin et al. \cite{CACHIN01}, subsequent research endeavors have sought to optimize protocol efficiency in terms of communication complexity. Notable advancements following Cachin's contributions include: i) VABA \cite{BYZ17}, requiring multiple protocol instances to achieve agreement on a party's request, and ii) Dumbo-MVBA \cite{LU20}, employing a cryptographic asynchronous dispersal and recovery methods to manage communication complexity alongside additional computational and communication rounds overheads. Our objective is to devise an MVBA protocol that achieves agreement in each instance without extra computation and communication rounds while maintaining the optimal metrics. Central to our design approach is the introduction of the committee in the classic MVBA protocol, wherein a randomly selected subset of ($f+1$, where $n=3f+1$) parties get selected and simultaneously broadcast their requests (transactions) to gather verifiable proofs. Successive distributions of these proofs afford us the necessary properties to employ the asynchronous binary Byzantine agreement (ABBA) protocol for reaching an agreement on a selected party's requests. By integrating the committee and ABBA protocols, we devise the optimal MVBA protocol, termed pMVBA (Prioritized-MVBA). This protocol exhibits resilience to tolerate up to $\lfloor \frac{n}{3}\rfloor$ Byzantine failures, with an expected runtime of $O(1)$, optimal message complexity of $O(n^2)$, and optimal communication complexity $O((l+λ)n^2)$ .

Figures (13)

• Figure 1: Comparison of two protocols. We show the time complexity of each sub-protocol and the step where our protocol wins in terms of communication. The $tx$ represents the transaction, the $ts$ represents the threshold signature. $1,1,1,0$ represents the bit array and each color for the particular party and its messages.
• Figure 3: Committee Selection and pVCBC protocol
• Figure 4: Message flow without the presence of faulty nodes.
• Figure 5: Message flow in the presence of faulty nodes.
• Figure 6: All selected parties complete their pVCBC and broadcast their proposals. All parties are non-Byzantine; therefore, all of them recommend their received proposal. Every party's proposal reaches at least 7 parties.

Theorems & Definitions (8)

• Lemma 5.1
• proof
• Lemma 5.2
• proof
• Lemma 5.3
• proof
• Theorem 5.4
• proof