Revisiting hBFT: Speculative Byzantine Fault Tolerance with Minimum Cost

Abstract: FaB Paxos[5] sets a lower bound of 5f + 1 replicas for any two-step consensus protocols tolerating f byzantine failures. Yet, hBFT[3] promises a two-step consensus protocol with only 3f + 1 replicas. As a result, it violates safety property of a consensus protocol. In this note, we review the lower bound set by FaB Paxos and present a simple execution scenario that produces a safety violation in hBFT. To demonstrate the scenario, we require a relatively simple setup with only 4 replicas and one view-change.