Blinding of modular exponentiation

The present invention proposes a method for executing a blinded modular exponentiation, based on a window method with a window size of k bits so using 2 k pre-calculated variables (Y i = X i mod N for i = 0 to 2 k -1), on input data X of n bits to obtain output data S of n bits, S = X d mod N, where d is the exponent of size m bits and N is the modulus of n bits, comprising the steps of: — blinding the pre-calculated variables by a blinding value B 1 being a pseudo-random variable of the size of the modulus (n bits) and lower than the modulus (Y i =Y ¡ × B 1 mod N for i = 0 to 2 k -1) — executing the modular exponentiation with the blinded pre-calculated variables, to obtain an intermediate result (A), — unblinding the intermediate result by a unblinding value C 1 =(B 1 g ) -1 mod N where g equals the concatenation of m/k times the value "1" coded on k bits, to obtain the output data S.