[Resource Topic] 2016/1039: A Fiat-Shamir Implementation Note

Welcome to the resource topic for 2016/1039

Title:
A Fiat-Shamir Implementation Note

Authors: Simon Cogliani, Rémi Géraud, David Naccache

Abstract:

In the Micali-Shamir paper improving the efficiency of the original Fiat-Shamir protocol, the authors state that “(…) not all of the v_i's will be quadratic residues mod n. We overcome this technical difficulty with an appropriate perturbation technique (…)” This perturbation technique is made more explicit in the associated patent application: “Each entity is allowed to modify the standard v_j which are QNRs. A particularly simple way to achieve this is to pick a modulus n=pq where p=3 \bmod 8 and q=7 \bmod 8, since then exactly one of v_j,-v_j,2v_j,-2v_j is a QR mod n for any v_j. The appropriate variant of each v_j can be (…) deduced by the verifier himself during the verification of given signatures.” In this short note we clarify the way in which the verifier can infer by himself the appropriate variant of each v_j during verification.

ePrint: https://eprint.iacr.org/2016/1039

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