Simple Is COOL: Graded Dispersal and Its Applications for Byzantine Fault Tolerance

Authors Ittai Abraham , Gilad Asharov , Anirudh Chandramouli



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Ittai Abraham
  • Intel Labs, Petah Tikva, Israel
Gilad Asharov
  • Bar-Ilan University, Ramat-Gan, Israel
Anirudh Chandramouli
  • Bar-Ilan University, Ramat-Gan, Israel

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Ittai Abraham, Gilad Asharov, and Anirudh Chandramouli. Simple Is COOL: Graded Dispersal and Its Applications for Byzantine Fault Tolerance. In 16th Innovations in Theoretical Computer Science Conference (ITCS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 325, pp. 1:1-1:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025) https://doi.org/10.4230/LIPIcs.ITCS.2025.1

Abstract

The COOL protocol of Chen (DISC'21) is a major advance that enables perfect security for various tasks (in particular, Byzantine Agreement in Synchrony and Reliable Broadcast in Asynchrony). For an input of size L bits, its communication complexity is O(nL+n² log n), which is optimal up to a log n factor. Unfortunately, Chen’s analysis is rather intricate and complex.
Our main contribution is a simple analysis of a new variant of COOL based on elementary counting arguments. Our main consistency proof takes less than two pages (instead of over 20 pages), making the COOL protocol much more accessible. In addition, the simple analysis allows us to improve the protocol by reducing one round of communication and reducing the communication complexity by 40%.
In addition, we suggest a new way of extracting the core properties of COOL as a new primitive, which we call Graded Dispersal. We show how Graded Dispersal can then be used to obtain efficient solutions for Byzantine Agreement, Verifiable Information Dispersal, Gradecast, and Reliable Broadcast (in both Synchrony and Asynchrony, where appropriate). Our improvement of COOL directly applies here, and we improve the state-of-the-art in all those primitives by reducing at least one round and 40% communication.

Subject Classification

ACM Subject Classification
  • Security and privacy
  • Security and privacy → Cryptography
  • Security and privacy → Information-theoretic techniques
  • Security and privacy → Distributed systems security
Keywords
  • Byzantine Agreement
  • Broadcast

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References

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