Paper 2026/931

Fair Multiparty Coin Tossing from Minimal Assumptions

Miranda Christ, Columbia University

Yevgeniy Dodis, New York University

Rachit Garg, New York University

Abstract

Coin flipping in the presence of a dishonest majority is a fundamental cryptographic primitive whose requirements lack a clean characterization. Recent work (Bonneau et al., Eurocrypt 2025) showed a lower bound that fair dishonest-majority coin-flipping implies delay functions. However, until now known upper bounds exhibited a significant gap: All existing protocols rely on assumptions that we do not know how to instantiate in the plain model. In this work, we close this gap. Specifically, we show that fair $n$-party coin flipping in the presence of up to $n-1$ malicious corruptions follows from the minimal assumption of delay functions. This completes the equivalence between delay functions and fair dishonest-majority coin-flipping protocols.

@misc{cryptoeprint:2026/931,
      author = {Marshall Ball and Miranda Christ and Yevgeniy Dodis and Rachit Garg},
      title = {Fair Multiparty Coin Tossing from Minimal Assumptions},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/931},
      year = {2026},
      url = {https://eprint.iacr.org/2026/931}
}