1. 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 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 (TS 41 ) with the divisor (b) as the modulus, and an additive linkage (TS 41 ) 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.