Sublinear-Round Byzantine Agreement Under Corrupt Majority

T. H.Hubert Chan*, Rafael Pass, Elaine Shi

*Corresponding author for this work

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review


Although Byzantine Agreement (BA) has been studied for three decades, perhaps somewhat surprisingly, there still exist significant gaps in our understanding regarding its round complexity. A long-standing open question is the following: can we achieve BA with sublinear round complexity under corrupt majority? Due to the beautiful works by Garay et al. (FOCS’07) and Fitzi and Nielsen (DISC’09), we have partial and affirmative answers to this question albeit for the narrow regime where f is the number of corrupt nodes and n is the total number of nodes. So far, no positive result is known about the setting even for static corruption! In this paper, we make progress along this somewhat stagnant front. We show that there exists a corrupt-majority BA protocol that terminates in rounds in the worst case, satisfies consistency with probability at least, and tolerates fraction of corrupt nodes. Our protocol secures against an adversary that can corrupt nodes adaptively during the protocol execution but cannot perform “after-the-fact” removal of honest messages that have already been sent prior to corruption. Our upper bound is optimal up to a logarithmic factor in light of the elegant lower bound by Garay et al. (FOCS’07).

Original languageEnglish
Title of host publicationPublic-Key Cryptography – PKC 2020 - 23rd IACR International Conference on Practice and Theory of Public-Key Cryptography, Proceedings
EditorsAggelos Kiayias, Markulf Kohlweiss, Petros Wallden, Vassilis Zikas
Number of pages20
ISBN (Print)9783030453879
StatePublished - 2020
Externally publishedYes
Event23rd IACR International Conference on the Practice and Theory of Public-Key Cryptography, PKC 2020 - Edinburgh, United Kingdom
Duration: 4 May 20207 May 2020

Publication series

NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
Volume12111 LNCS
ISSN (Print)0302-9743
ISSN (Electronic)1611-3349


Conference23rd IACR International Conference on the Practice and Theory of Public-Key Cryptography, PKC 2020
Country/TerritoryUnited Kingdom


  • Byzantine agreement
  • Corrupt majority
  • Sublinear round complexity


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