[Resource Topic] 2020/972: Optimized Binary GCD for Modular Inversion

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Title:
Optimized Binary GCD for Modular Inversion

Authors: Thomas Pornin

Abstract:

In this short note, we describe a practical optimization of the well-known extended binary GCD algorithm, for the purpose of computing modular inverses. The method is conceptually simple and is applicable to all odd moduli (including non-prime moduli). When implemented for inversion in the field of integers modulo the prime 2^{255}-19, on a recent x86 CPU (Coffee Lake core), we compute the inverse in 6253 cycles, with a fully constant-time implementation.

ePrint: https://eprint.iacr.org/2020/972

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