1. A device for calculating a result of a sum of a first operand and a second operand less than a modulus by means of a calculating unit formed to maximally process numbers whose magnitude is less than or equal to a product of the modulus and an integer larger than 1, comprising: a unit for storing the first operand in the calculating unit, wherein a magnitude of the first operand is less than or equal to the maximally processable number; a modified second operand calculator, wherein the modified second operand is equal to the second operand minus the modulus, so that the modified second operand is negative; a sum calculator of the first operand and the modified second operand or the first operand and a product of the modified second operand times a factor less than or equal to the integer, and for storing the sum in the calculating unit; and a unit for reducing the sum with respect to the modulus to get the result.