Apparatus for calculating a result of a modular multiplication of a first operand and a second operand with regard to a modulus, each having a length of 2 n bits, the operands and the modulus are split into sub-operands of half the length and are fed to controller controlling MMD unit for performing a MultModDiv operation in accordance with a predetermined step sequence with corresponding input operands and MMD moduli to obtain integer quotient values and residual values with regard to the MMD modulus at an output. The combiner is operable to combine integer quotient values and residual values from predetermined steps of the step sequence to obtain the result.