Paper 2023/397

Extended Abstract: HotStuff-2: Optimal Two-Phase Responsive BFT

Abstract

In this paper, we observe that it is possible to solve partially-synchronous BFT and simultaneously achieves $O(n^2)$ worst-case communication, optimistically linear communication, a two-phase commit regime within a view, and optimistic responsiveness. Prior work falls short in achieving one or more of these properties, e.g., the most closely related work, HotStuff, requires a three-phase view while achieving all other properties. We demonstrate that these properties are achievable through a two-phase HotStuff variant named HotStuff-2. The quest for two-phase HotStuff variants that achieve all the above desirable properties has been long, producing a series of results that are yet sub-optimal and, at the same time, are based on somewhat heavy hammers. HotStuff-2 demonstrates that none of these are necessary: HotStuff-2 is remarkably simple, adding no substantive complexity to the original HotStuff protocol. The main takeaway is that two phases are enough for BFT after all.