Abstract: In a method of and apparatus for limiting program execution to only an authorized data processing system, a proprietary program, together with first and second authorization codes, is stored on a magnetic disc or other storage medium. The first and second authorization codes are read. A hardware module containing a pseudorandom number generator unique to the authorized system receives the first authorization code as a key. The resultant number generated by the number generator, which is a function of the key and particular pseudorandom generator algorithm, is compared with the second authorization code in direct or encrypted form. An execution enable signal is generated in response to a positive comparison to enable the stored program to be executed.