Validated Asynchronous Byzantine Agreement with Optimal Resilience and Asymptotically Optimal Time and Word Communication (1811.01332v1)

Abstract: We provide a new protocol for Validated Asynchronous Byzantine Agreement. Validated (multi-valued) Asynchronous Byzantine Agreement is a key building block in constructing Atomic Broadcast and fault-tolerant state machine replication in the asynchronous setting. Our protocol can withstand the optimal number $f<n/3$ of Byzantine failures and reaches agreement in the asymptotically optimal expected $O(1)$ running time. Honest parties in our protocol send only an expected $O(n^2)$ messages where each message contains a value and a constant number of signatures. Hence our total expected communication is $O(n^2)$ words. The best previous result of Cachin et al. from 2001 solves Validated Byzantine Agreement with optimal resilience and $O(1)$ expected time but with $O(n^3)$ expected word communication. Our work addresses an open question of Cachin et al. from 2001 and improves the expected word communication from $O(n^3)$ to the asymptotically optimal $O(n^2)$.

• The paper introduces a new Validated Asynchronous Byzantine Agreement (VABA) protocol that reduces expected word communication complexity from O(n^3) to an asymptotically optimal O(n^2).
• This efficient protocol achieves optimal resilience, tolerating f < n/3 adaptive Byzantine failures, with an expected running time of O(1).
• The advancements have significant implications for distributed systems, including blockchain technologies, by improving the reliability and efficiency of consensus mechanisms.

Overview of "Validated Asynchronous Byzantine Agreement with Optimal Resilience and Asymptotically Optimal Time and Word Communication"

This paper proposes a new protocol for Validated Asynchronous Byzantine Agreement (VABA), addressing an open question from Cachin et al., 2001, by reducing the expected word communication complexity from $O(n^3)$ to $O(n^2)$, achieving asymptotically optimal performance. The authors present a protocol with optimal resilience and an expected running time of $O(1)$, maintaining robustness against an adaptive adversary controlling up to $f < n/3$ parties.

Protocol Characteristics

The developed VABA protocol is crucial for implementing Atomic Broadcast and fault-tolerant state machine replication in asynchronous systems, two significant needs in distributed computing. The protocol demonstrates optimal resilience by efficiently handling up to $n=3f+1$ parties where f is the maximum number of Byzantine failures, adhering to standard definitions and modern cryptographic models.

Key Contributions

1. Asymptotically Optimal Word Communication: Addressing the limitations found in the solution by Cachin et al., the new protocol significantly reduces communication complexity to $O(n^2)$ while retaining optimal resilience and time, aligning with recent lower bounds of asynchronous agreement against adaptive adversaries, which necessitate $\Omega(n^2)$ message communication.
2. Efficient Protocol Design: Utilizing modular, leader-based thread parallelization, the new protocol allows for significant improvements in total expected communication volume, all while preserving critical aspects such as agreement validity and cryptographic security against adversaries that adaptively target honest parties.
3. Leader Election and View-Change Protocol: Inspired by synchronous models and partial-synchrony strategies, the authors introduce a mechanism to elect a random leader before executing the core view-change process, promoting rapid convergence to secure agreements quickly even in asynchronous environments.

Implications and Future Directions

The results have theoretical implications for the development and deployment of blockchain-based systems and distributed ledger technologies, enhancing reliability and efficiency by minimizing communication overhead. The practical consequences extend to other fields, aiming to decentralize and secure voting protocols, consensus mechanisms, and resource distribution without relying on synchronous communication.

The paper opens the door for further investigation into protocols with weak termination properties in asynchronous adaptive settings. Specifically, paper whether $\Omega(n^2)$ remains a lower bound and explore alternatives that might yield linear communication complexity.

Conclusion

Overall, this protocol presents an essential advancement to the landscape of asynchronous agreement methodologies. It significantly reconciles cryptographic and computational frameworks, establishing a clear path toward faster, more efficient consensus, and setting the foundation for future developments in distributed systems and secure communications.