THE QUANTUM INTELLIGENCER

Optimistic, Signature-Free Reliable Broadcast Achieves 2-Step Termination Under Honest Conditions In Plain English: This research tackles the problem of getting computers in a network to agree on information when some might fail or act badly. Normally, this process takes several back-and-forth messages, slowing things down. The team created a smarter method that finishes in fewer steps—if most computers are behaving well. They also showed this faster approach can be used in other coordination tasks. This matters because it makes secure, decentralized systems (like blockchains) faster and more efficient, especially on devices with weak processors, without needing heavy math that could be broken by future quantum computers. Summary: This paper presents a novel optimistic, signature-free Reliable Broadcast (RBC) protocol that achieves 2-step termination under favorable network conditions, improving upon the standard 3-step bound in signature-free RBC protocols. The protocol operates in a Byzantine fault-tolerant setting with $ n $ parties and tolerates up to $ f < n/3 $ faulty nodes. It achieves early termination when at least $ \lceil \frac{n+2f-2}{2} \rceil $ non-broadcaster parties behave honestly—a condition the authors prove is tight via a matching lower bound. The core contribution is a latency-reduction technique that leverages optimistic execution without compromising security or fault tolerance. This optimization is generalized to other asynchronous primitives: Asynchronous Verifiable Secret Sharing (AVSS) and Asynchronous Verifiable Information Dispersal (AVID), enabling 2-step completion in similar honest-majority scenarios. To demonstrate practical impact, the authors integrate their RBC protocol into Sailfish++, a new post-quantum secure, DAG-based BFT consensus protocol. Under optimistic conditions, Sailfish++ achieves a commit latency of just 3 steps—on par with the best signature-based protocols. Experiments show that the proposed system significantly outperforms both existing post-quantum and signature-based BFT protocols, particularly on machines with limited CPU capacity. In contrast, signature-based alternatives require substantial computational power to match performance. The Rust implementation of Sailfish++ is open-sourced to support reproducibility and further research. This work thus advances both theoretical understanding—by establishing optimal conditions for early termination—and practical deployment—by delivering high-performance, quantum-resistant consensus suitable for resource-constrained environments. Key Points: - Introduces an optimistic, signature-free Reliable Broadcast (RBC) protocol achieving 2-step termination under certain honest-behavior conditions. - Maintains full $ f < n/3 $ Byzantine fault tolerance while improving latency. - Proves a tight lower bound: at least $ \lceil \frac{n+2f-2}{2} \rceil $ honest non-broadcaster parties are required for 2-step termination. - Generalizes the optimization to AVSS and AVID, enabling faster execution in asynchronous secret sharing and data dispersal. - Integrates the RBC protocol into Sailfish++, a new DAG-based, post-quantum BFT consensus protocol. - Sailfish++ achieves 3-step commit latency under optimism, matching top-tier signature-based protocols. - Experimental results show superior performance, especially on low-CPU machines, compared to signature-based alternatives. - Open-sourced Rust implementation enhances reproducibility and adoption. Notable Quotes: - "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." - "We also prove a matching lower bound on the number of honest parties required for 2-step termination." - "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." Data Points: - Standard RBC protocols require 3 steps for termination. - Proposed protocol achieves termination in 2 steps under optimistic conditions. - Fault tolerance: up to $ f < n/3 $ Byzantine faults. - Minimum honest non-broadcaster parties for 2-step termination: $ \lceil \frac{n+2f-2}{2} \rceil $. - Sailfish++ achieves 3-step commit latency under optimism. - Performance advantage observed even on machines with limited CPU resources. - Rust implementation is open-sourced. Controversial Claims: - The claim that 2-step termination is achievable in signature-free RBC under $ \lceil \frac{n+2f-2}{2} \rceil $ honest parties may challenge assumptions in prior works that treated 3 steps as inherent to signature-free models. - The assertion that signature-based protocols require high CPU capacity to match performance could be seen as downplaying optimizations in modern cryptographic libraries or hardware acceleration. - The integration into Sailfish++ achieving 3-step commit latency "matching the best signature-based protocols" implies parity in practical performance, which may depend on network conditions and workload assumptions not fully detailed in the abstract. Technical Terms: - Reliable Broadcast (RBC) - Byzantine fault tolerance - Signature-free protocols - Optimistic termination - Communication complexity - Asynchronous Verifiable Secret Sharing (AVSS) - Asynchronous Verifiable Information Dispersal (AVID) - DAG-based consensus - Post-quantum security - BFT (Byzantine Fault Tolerant) consensus - Commit latency - Lower bound proof - Fault tolerance threshold ($ f < n/3 $) - Non-broadcaster parties - Early termination - Computational efficiency —Ada H. Pemberley Dispatch from The Prepared E0