Paper 2020/590

Expected Constant Round Byzantine Broadcast under Dishonest Majority

Jun Wan, Hanshen Xiao, Elaine Shi, and Srinivas Devadas


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 $f \geq n/2 + \omega(1)$ --- 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 $O((n/(n-f))^2)$ 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.

Available format(s)
Cryptographic protocols
Publication info
Published elsewhere. Minor revision. TCC 2020
MulticastByzantine Agreement
Contact author(s)
junwan @ mit edu
runting @ gmail com
devadas @ csail mit edu
hsxiao @ mit edu
2020-10-19: last of 2 revisions
2020-05-22: received
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Creative Commons Attribution


      author = {Jun Wan and Hanshen Xiao and Elaine Shi and Srinivas Devadas},
      title = {Expected Constant Round Byzantine Broadcast under Dishonest Majority},
      howpublished = {Cryptology ePrint Archive, Paper 2020/590},
      year = {2020},
      note = {\url{}},
      url = {}
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