A method in a portable data carrier for executing a cryptographic operation on security-relevant data, comprising a step of determining a remainder (r) of a dividend (a) modulo a divisor (b), wherein the remainder (r) is determined iteratively by means of a division device of the data carrier, by carrying out in each iteration a Montgomery multiplication with the divisor (b) as the modulus, and an additive linkage of an output value of the Montgomery multiplication with a coefficient (a i) derived from the dividend (a) and associated with the respective iteration, the Montgomery multiplication being carried out by means of a multiplication device of the data carrier, and the Montgomery multiplication of a subsequent iteration receiving a result of a preceding iteration as an input value, the step of determining a remainder (r) of a dividend (a) modulo a divisor (b) comprising the following steps: - decomposing the dividend (a) in the form of a sum, the number of summands of the sum corresponding to the number of iterations required for determining the remainder (r), and each summand of the sum being composed of a product comprising the coefficient (a i) associated with the respective iteration and a power of two (2 N * i; R i) associated with the iteration; - initializing the remainder (r); - carrying out the following substeps in each iteration, with the remainder (r) being respectively updated in each of the substeps: - carrying out a Montgomery multiplication with the divisor (b) as the modulus, the Montgomery multiplication receiving as input values the remainder (r) as well as a compensation factor (m; m1) and outputting an updated remainder (r) and the compensation factor (m; m1) being equal for each iteration; - additively linking the remainder (r) with the coefficient (a i) associated with the iteration and modularly reducing the result modulo the divisor (b) to determine an updated remainder (r). - decomposing the dividend (a) in the form of a sum, the number of summands of the sum corresponding to the number of iterations required for determining the remainder (r), and each summand of the sum being composed of a product comprising the coefficient (a i) associated with the respective iteration and a power of two (2 N * i; R i) associated with the iteration; - initializing the remainder (r); - carrying out the following substeps in each iteration, with the remainder (r) being respectively updated in each of the substeps: - carrying out a Montgomery multiplication with the divisor (b) as the modulus, the Montgomery multiplication receiving as input values the remainder (r) as well as a compensation factor (m; m1) and outputting an updated remainder (r) and the compensation factor (m; m1) being equal for each iteration; - additively linking the remainder (r) with the coefficient (a i) associated with the iteration and modularly reducing the result modulo the divisor (b) to determine an updated remainder (r). - carrying out a Montgomery multiplication with the divisor (b) as the modulus, the Montgomery multiplication receiving as input values the remainder (r) as well as a compensation factor (m; m1) and outputting an updated remainder (r) and the compensation factor (m; m1) being equal for each iteration; - additively linking the remainder (r) with the coefficient (a i) associated with the iteration and modularly reducing the result modulo the divisor (b) to determine an updated remainder (r).