Secure conversion between boolean and arithmetic masking of any order

Abstract

An effective countermeasure against side-channel attacks is to mask all sensitive intermediate variables with one (or more) random value(s). When a cryptographic algorithm involves both arithmetic and Boolean operations, it is necessary to convert from arithmetic masking to Boolean masking and vice versa. At CHES 2001, Goubin introduced two algorithms for secure conversion between arithmetic and Boolean masks, but his approach can only be applied to first-order masking. In this paper, we present and evaluate new conversion algorithms that are secure against attacks of any order. To convert masks of a size of k bits securely against attacks of order n, the proposed algorithms have a time complexity of O(n k) in both directions and are proven to be secure in the Ishai, Sahai, and Wagner (ISW) framework for private circuits. We evaluate our algorithms using HMAC-SHA-1 as example and report the execution times we achieved on a 32-bit AVR microcontroller.