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Subcubic Coin Tossing in Asynchrony Without PKI

Authors Mose Mizrahi , Roger Wattenhofer

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LIPIcs.DISC.2026.39.pdf
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Subject Classification

ACM Subject Classification
  • Theory of computation → Communication complexity
  • Theory of computation → Distributed algorithms
  • Theory of computation → Cryptographic protocols
  • Security and privacy → Distributed systems security
Keywords
  • Byzantine Agreement
  • Common Coin
  • Leader Election
  • Adaptive Adversary

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Abstract

We consider an asynchronous network of n parties, up to t of which are byzantine. We study common coin tossing, a task where the parties try to agree on an unpredictable random value, with some chance of failure due to the byzantine parties' influence. Coin tossing is a well-known and often-studied task due to its use in byzantine agreement.
We present a committee-based method to transform strong (rarely failing) binary common coins into weaker ones which asymptotically require less communication. For any k > 2 and ε > 0, we can transform a strong binary coin that costs Õ(n^k) bits of communication into a weak binary coin that costs Õ(ε^{-2k} n^{3 - 2/k}) bits. This latter coin tolerates ε n fewer byzantine parties than the strong coin it is based on, and it fails with an arbitrarily small constant probability.
With our method, we obtain a secure-channel-based perfectly secure common coin for t ≤ (1/4 - ε)n faults that costs Õ(n^{2.5}) bits, as well as a common coin based on cryptographic hashing for t ≤ (1/3 - ε)n faults that costs Õ(n^{7/3}κ) bits. These are to our knowledge the first PKI-free asynchronous common coins that cost o(n³) bits but nevertheless succeed with at least constant probability against t = Θ(n) (strongly) adaptive byzantine faults.

Cite As Get BibTex

Mose Mizrahi and Roger Wattenhofer. Subcubic Coin Tossing in Asynchrony Without PKI. In 40th International Symposium on Distributed Computing (DISC 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 397, pp. 39:1-39:24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026) https://doi.org/10.4230/LIPIcs.DISC.2026.39

Author Details

Mose Mizrahi
  • ETH Zurich, Switzerland
Roger Wattenhofer
  • ETH Zurich, Switzerland

Acknowledgements

We thank Ittai Abraham and Gilad Asharov for their assistance regarding [Abraham et al., 2024].

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