Byzantine consensus is $$\Theta (n^2)$$ : the Dolev-Reischuk bound is tight even in partial synchrony!

The Dolev-Reischuk bound says that any deterministic Byzantine consensus protocol has (at least) quadratic (in the number of processes) communication complexity in the worst case: given a system with n processes and at most \(f < n / 3\) failures, any solution to Byzantine consensus exchanges \(\Omega \big (n^2\big )\) words, where a word contains a constant number of values and signatures. While it has been shown that the bound is tight in synchronous environments, it is still unknown whether a consensus protocol with quadratic communication complexity can be obtained in partial synchrony where the network alternates between (1) asynchronous periods, with unbounded message delays, and (2) synchronous periods, with \(\delta \)-bounded message delays. Until now, the most efficient known solutions for Byzantine consensus in partially synchronous settings had cubic communication complexity (e.g., HotStuff, binary DBFT). This paper closes the existing gap by introducing SQuad, a partially synchronous Byzantine consensus protocol with \(O\big (n^2\big )\) worst-case communication complexity. In addition, SQuad is optimally-resilient (tolerating up to \(f < n / 3\) failures) and achieves \(O(f \cdot \delta )\) worst-case latency complexity. The key technical contribution underlying SQuad lies in the way we solve view synchronization, the problem of bringing all correct processes to the same view with a correct leader for sufficiently long. Concretely, we present RareSync, a view synchronization protocol with \(O\big (n^2\big )\) communication complexity and \(O(f \cdot \delta )\) latency complexity, which we utilize in order to obtain SQuad.