A method and a circuit for protecting a digital quantity over a first number of bits, in an algorithm executing at least one modular exponentiation of data by the quantity, the steps including at least one squaring up and at least one multiplication and implementing, for each bit of the quantity, different calculation steps according to the state of the bit, a same number of multiplications being performed whatever the state of the bit and all the calculation steps using a multiplication being taken into account to calculate a final result.