Communication complexity of byzantine agreement, revisited

Ittai Abraham, T. H.Hubert Chan, Danny Dolev, Kartik Nayak, Rafael Pass, Ling Ren, Elaine Shi

Research output: Contribution to journalArticlepeer-review


As Byzantine Agreement (BA) protocols find application in large-scale decentralized cryptocurrencies, an increasingly important problem is to design BA protocols with improved communication complexity. A few existing works have shown how to achieve subquadratic BA under an adaptive adversary. Intriguingly, they all make a common relaxation about the adaptivity of the attacker, that is, if an honest node sends a message and then gets corrupted in some round, the adversary cannot erase the message that was already sent—henceforth we say that such an adversary cannot perform “after-the-fact removal”. By contrast, many (super-)quadratic BA protocols in the literature can tolerate after-the-fact removal. In this paper, we first prove that disallowing after-the-fact removal is necessary for achieving subquadratic-communication BA. Next, we show new subquadratic binary BA constructions (of course, assuming no after-the-fact removal) that achieve near-optimal resilience and expected constant rounds under standard cryptographic assumptions and a public-key infrastructure (PKI) in both synchronous and partially synchronous settings. In comparison, all known subquadratic protocols make additional strong assumptions such as random oracles or the ability of honest nodes to erase secrets from memory, and even with these strong assumptions, no prior work can achieve the above properties. Lastly, we show that some setup assumption is necessary for achieving subquadratic multicast-based BA.

Original languageEnglish (US)
Pages (from-to)3-28
Number of pages26
JournalDistributed Computing
Issue number1
StatePublished - Mar 2023


  • Byzantine agreement
  • Communication complexity
  • Lower bounds
  • Subquadratic

ASJC Scopus subject areas

  • Theoretical Computer Science
  • Hardware and Architecture
  • Computer Networks and Communications
  • Computational Theory and Mathematics


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