A Fiat–Shamir Transformation From Duplex Sponges

Alessandro Chiesa; Michele Orrù

Paper 2025/536

A Fiat–Shamir Transformation From Duplex Sponges

Alessandro Chiesa, École Polytechnique Fédérale de Lausanne

Michele Orrù

, French National Centre for Scientific Research

Abstract

We analyze a variant of the Fiat–Shamir transformation based on an ideal permutation. The transformation relies on the popular duplex sponge paradigm, and minimizes the number of calls to the permutation (given the information to absorb/squeeze). This closely models deployed variants of the Fiat–Shamir transformation, and our analysis provides concrete security bounds to guide security parameters in practice. Our results contribute to the ongoing community-wide effort to achieve rigorous, and ultimately formally verified, security proofs for deployed cryptographic proofs. Along the way, we find that indifferentiability (a property proven for many modes of operation, including the duplex sponge) is ill-suited for establishing the knowledge soundness and zero knowledge of a non-interactive argument. We additionally contribute spongefish, an open-source Rust library implementing our Fiat–Shamir transformation. The library is interoperable across multiple cryptographic frameworks, and works with any choice of permutation. The library comes equipped with Keccak and Poseidon permutations, as well as several "codecs" for re-mapping prover and verifier messages to the permutation's domain.

Metadata

Available format(s): PDF
Category: Cryptographic protocols
Publication info: A major revision of an IACR publication in TCC 2025
DOI: 10.1007/978-3-032-12287-2_16
Keywords: Fiat–Shamir duplex sponge
Contact author(s): alessandro chiesa @ epfl ch
m @ orru net
History: 2026-03-27: last of 3 revisions; 2025-03-22: received; See all versions
Short URL: https://ia.cr/2025/536
License: CC BY

BibTeX

@misc{cryptoeprint:2025/536,
      author = {Alessandro Chiesa and Michele Orrù},
      title = {A Fiat–Shamir Transformation From Duplex Sponges},
      howpublished = {Cryptology {ePrint} Archive, Paper 2025/536},
      year = {2025},
      doi = {10.1007/978-3-032-12287-2_16},
      url = {https://eprint.iacr.org/2025/536}
}