Expected Constant Round Byzantine Broadcast under Dishonest Majority

Byzantine Broadcast (BB) is a central question in distributed systems, and an important challenge is to understand its round complexity. Under the honest majority setting, it is long known that there exist randomized protocols that can achieve BB in expected constant rounds, regardless of the number of nodes n. However, whether we can match the expected constant round complexity in the corrupt majority setting — or more precisely, when f ≥ n/2 + ω(1) — remains unknown, where f denotes the number of corrupt nodes. In this paper, we are the first to resolve this long-standing question. We show how to achieve BB in expected O((n/(n − f))) rounds. In particular, even when 99% of the nodes are corrupt we can achieve expected constant rounds. Our results hold under both a static adversary and a weakly adaptive adversary who cannot perform “after-the-fact removal” of messages already sent by a node before it becomes corrupt. ∗junwan@mit.edu, Massachusetts Institute of Technology †hsxiao@mit.edu, Massachusetts Institute of Technology ‡runting@gmail.com, Cornell University §devadas@mit.edu, Massachusetts Institute of Technology