On the Round Complexity of Randomized Byzantine Agreement

Ran Cohen, Iftach Haitner, Nikolaos Makriyannis, Matan Orland, Alex Samorodnitsky

Research output: Contribution to journalArticlepeer-review

Abstract

We prove lower bounds on the round complexity of randomized Byzantine agreement (BA) protocols, bounding the halting probability of such protocols after one and two rounds. In particular, we prove that: 1. BA protocols resilient against n/3 [resp., n/4] corruptions terminate (under attack) at the end of the first round with probability at most o(1) [resp., 1 / 2 + o(1)]. 2. BA protocols resilient against a fraction of corruptions greater than 1/4 terminate at the end of the second round with probability at most 1 - Θ (1). 3. For a large class of protocols (including all BA protocols used in practice) and under a plausible combinatorial conjecture, BA protocols resilient against a fraction of corruptions greater than 1/3 [resp., 1/4] terminate at the end of the second round with probability at most o(1) [resp., 1 / 2 + o(1)]. The above bounds hold even when the parties use a trusted setup phase, e.g., a public-key infrastructure (PKI). The third bound essentially matches the recent protocol of Micali (ITCS’17) that tolerates up to n/3 corruptions and terminates at the end of the third round with constant probability.

Original languageEnglish
Article number10
JournalJournal of Cryptology
Volume35
Issue number2
DOIs
StatePublished - Apr 2022

Keywords

  • Byzantine agreement
  • Lower bound
  • Round complexity

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