Patent ID: 8805913

Claim:
A method, implemented by an electronic circuit, for evaluating a function of a finite field of characteristic p into itself, for an element x of the field, said element x representing data masked by a masking operation, said function being non-linear with respect to said masking operation, the method comprising: an evaluation, for said element x, of a polynomial formed by a plurality of monomials, wherein the polynomial is formulated using successive decompositions of a polynomial interpolating said function, thereby reducing the degree of the polynomial used to evaluate the function wherein the evaluation of the polynomial thus formulated comprises the following steps: evaluating monomials the degree of which is an integer power of the characteristic p by means of successive raisings of the element x to the power p; obtaining monomials the degree of which is different from an integer power of the characteristic p on the basis of the evaluated monomials, the degree of which is an integer power of the characteristic p, and by means of at least one multiplication of the element x by one of said evaluated monomials the degree of which is an integer power of the characteristic p, wherein said evaluating the function maintains the masking.