Computing equipment may perform format-preserving encryption and decryption by partitioning an input string into first and second portions. The computing equipment may combine the first portion with the second portion after the second portion has been passed through a pseudorandom function using a format-preserving combination operation that requires inputs having a similar domain size. A data size adjustment engine may adjust the domain size of the output of the pseudorandom function to be similar to the domain size of the first string. In order to mitigate undesirable data overflows, the output of the pseudorandom function may be partitioned into a number of computationally simpler terms each having a modulo factor and a pre-computed constant value. The computing equipment may pre-compute the constant values in advance, thereby reducing the computational complexity required for performing the size adjustment operations without generating undesirable data overflows.