The invention provides a method of cryptographically transforming data text c. In the field of cryptography there is a need to reduce the time taken to cryptographically transform data text while maintaining the low memory requirements associated with conventional square-and-multiply modular exponentiation. The method of cryptographically transforming data text c comprises the step of generating an integer representation m of the data text c according to m = c d where d is a predetermined exponent. The step of generating the integer representation m includes generating a sequence of intermediate numbers, each intermediate number being based on two or fewer earlier numbers in the sequence. Generating a sequence of intermediate numbers includes retrieving a pre-stored instruction to determine which two or fewer earlier numbers in the sequence a given intermediate number is based on and the functional manipulation of the or each earlier number required to generate the given intermediate number.