Method and device for executing a decrypting mechanism through calculating a standardized modular exponentiation for thwarting timing attacks

A method for executing a decrypting modular exponentiation modulo M, by digit-wise calculating modular and looped multiplications X*Y mod M, according to a non-Montgomery procedure, with M a temporally steady but instance-wise non-uniform modulus, said method involving an iterative series of steps organized in a loop hierarchy wherein one or two first multiplications are executed to produce a first result, and the loop hierarchy is associated to a reduction of a size of the first result by one or more second multiplications to produce a second result,

• said method furthermore taking a measure for keeping a final result of such step below a predetermined multiplicity of said modulus,
• said method postpones a subtraction of the modulus, if needed, as pertaining to said measure to a terminal phase of the modular exponentiation, as being conditional to choosing one or more preprocessing parameters figuring in said method whilst maintaining overall temporal performance of said method.