An apparatus for calculating a modular multiplication includes an examiner for examining digits of the multiplier with a lookahead algorithm to obtain a multiplication shift value. In addition, a determinator and intermediate-result shift value are provided which determine a positive intermediate-result shift value. A calculator for calculating a multiplicand shift value as the difference between the intermediate-result shift value and the multiplication shift value. The intermediate result from the preceding iteration step as well as the multiplicand are then shifted by the corresponding shifting magnitudes to then perform a three-operands addition with the shifted values, if need be while considering lookahead parameters.