Halpha: Asynchronous Leaderless Probabilistic Consensus with Near-Half Adversaries

Recent advances in Byzantine Fault-Tolerant State Machine Replication have led to practical protocols for partially synchronous or asynchronous networks by combining classical methods with modern cryptographic tools like verifiable random functions. Despite improved performance in throughput and latency, these protocols remain limited by the FLP impossibility and the Dwork-Lynch-Stockmeyer bound, tolerating at most ⌊(n − 1) /3⌋ adversaries. In contrast, Proof-of-Work (PoW) achieves up to ⌊(n − 1) /2⌋ fault tolerance under asynchrony, albeit with high energy costs and probabilistic finality. Protocols like Avalanche similarly exceed the ⌊(n − 1) /3⌋ bound by relaxing deterministic guarantees. In this paper, we propose Halpha, a leaderless consensus protocol that tolerates nearly half Byzantine faults with overwhelming probability. Unlike PoW and Avalanche, which provide probabilistic finality post-termination, Halpha relaxes liveness during proposal. It leverages a probabilistic multi-valued validated Byzantine agreement (P-MVBA) to propose transaction sets, ensuring each referenced transaction is seen by at least one honest node. While P-MVBA may halt, it progresses under the intermittent existence of a bounded-delay interval, and ensures consistency with overwhelming probability without synchrony. Halpha then runs multiple asynchronous binary agreement instances, using verifiable random functions to support randomized liveness under Byzantine conditions and achieves finality when synchrony arrives. Experiments show Halpha achieves safety with high probability and low-latency deterministic finality in normal cases.