13/11/2025 - 13 בנובמבר 2025

WHO: Dr. Eylon Yogev, Bar-Ilan University

WHEN: Thursday, November 13rd 2025 at 12:00

WHERE: BUILDING 503 (Computer Science) SEMINAR ROOM 226

Title: Towards a White-Box Secure Fiat-Shamir Transformation.

Abstract: The Fiat–Shamir transformation is a fundamental cryptographic technique widely used to convert public-coin interactive protocols into non-interactive ones. This transformation is crucial in both theoretical and practical applications, particularly in the construction of succinct non-interactive arguments (SNARKs). While its security is well-established in the random oracle model, practical implementations replace the random oracle with a concrete hash function, where security is merely assumed to carry over.

A growing body of work has given theoretical examples of protocols that remain secure under the Fiat–Shamir transformation in the random oracle model but become insecure when instantiated with any white-box implementation of the hash function. Recent research has shown how these attacks can be applied to natural cryptographic schemes, including real-world systems. These attacks rely on a general diagonalization technique, where the protocol exploits its access to the white-box implementation of the hash function. These attacks cast serious doubt on the security of cryptographic systems deployed in practice today, leaving their soundness uncertain.

We propose a new Fiat–Shamir transformation (XFS) that aims to defend against a broad family of attacks. Our approach is designed to be practical, with minimal impact on the efficiency of the prover and verifier and on the proof length. At a high level, our transformation combines the standard Fiat–Shamir technique with a new type of proof-of-work that we construct.

We hope that our transformation will help preserve the security of systems relying on the Fiat–Shamir transformation.

Joint work with Gal Arnon.