A method of protecting a modular exponentiation calculation on a first number and an exponent, modulo a first modulo, executed by an electronic circuit using a first register or memory location and a second register or memory location, successively including, for each bit of the exponent: generating a random number; performing a modular multiplication of the content of the first register or memory location by that of the second register or memory location, and placing the result in one of the first and second registers or memory locations selected according to the state of the bit of the exponent; performing a modular squaring of the content of one of the first and second registers or memory locations selected according to the state of the exponent, and placing the result in this selected register or memory location, the multiplication and squaring operations being performed modulo the product of the first modulo by said random number.