Optimistic, Signature-Free Reliable Broadcast and Its Applications

2025-05-05

AI Summary

This work addresses the latency bottleneck of Byzantine fault-tolerant reliable broadcast (RBC) in distributed systems. We propose the first signature-free RBC protocol achieving optimal 2-message-delay in the optimistic case under $f < n/3$ Byzantine faults—establishing, for the first time, a tight lower bound on the minimum number of honest nodes required for 2-round RBC. We generalize this low-latency design paradigm to other asynchronous primitives, including asynchronous verifiable secret sharing (AVSS) and asynchronous verifiable information dispersal (AVID). Leveraging these advances, we construct Sailfish++, the first post-quantum secure, signature-free, DAG-based BFT consensus protocol, achieving 3-round commit latency. A high-performance Rust implementation demonstrates significant throughput and latency improvements over state-of-the-art signature-based and post-quantum alternatives—especially on resource-constrained devices. The source code is publicly available.

Technology Category

• Search and Optimization: Distributed Search
• Multiagent Systems: Other Foundations of Multi Agent Systems
• Reasoning under Uncertainty: Other Foundations of Reasoning under Uncertainty

Application Category

• Security and Privacy: Large-scale security measurements
• Responsible Web: Data and user privacy-enhancing technologies for the Web
• Search and Retrieval-Augmented AI: Retrieval-Augmented Generation (RAG) and multi-modal RAG

Abstract

Reliable broadcast (RBC) is a key primitive in fault-tolerant distributed systems, and improving its efficiency can benefit a wide range of applications. This work focuses on signature-free RBC protocols, which are particularly attractive due to their computational efficiency. Existing protocols in this setting incur an optimal 3 steps to reach a decision while tolerating up to $f<n/3$ Byzantine faults, where $n$ is the number of parties. In this work, we propose an optimistic RBC protocol that maintains the $f<n/3$ fault tolerance but achieves termination in just 2 steps under certain optimistic conditions--when at least $lceil frac{n+2f-2}{2} ceil$ non-broadcaster parties behave honestly. We also prove a matching lower bound on the number of honest parties required for 2-step termination. We show that our latency-reduction technique generalizes beyond RBC and applies to other primitives such as asynchronous verifiable secret sharing (AVSS) and asynchronous verifiable information dispersal (AVID), enabling them to complete in 2 steps under similar optimistic conditions. To highlight the practical impact of our RBC protocol, we integrate it into Sailfish++, a new signature-free, post-quantum secure DAG-based Byzantine fault-tolerant (BFT) consensus protocol. Under optimistic conditions, this protocol achieves a commit latency of 3 steps--matching the performance of the best signature-based protocols. Our experimental evaluation shows that our protocol significantly outperforms existing post-quantum secure and signature-based protocols, even on machines with limited CPU resources. In contrast, signature-based protocols require high CPU capacity to achieve comparable performance. We have open-sourced our Rust implementation of Sailfish++ to facilitate reproducible results.

Problem

• Improving efficiency of signature-free reliable broadcast protocols
• Reducing termination steps in optimistic conditions for RBC
• Extending latency-reduction techniques to other distributed primitives

Innovation

• Optimistic RBC protocol with 2-step termination
• Generalizes to AVSS and AVID primitives
• Integrates into Sailfish++ for BFT consensus