An apparatus and method of disabling an unauthorized copy of a computer program. A set of numbers having an uncommon mathematical property is defined. For authorized copies, one of these numbers is written to a pre-determined location in the computer program. Whenever a copy of the computer program is attempted to be run, the computer system is instructed by the computer program to check the number at that pre-determined location. A determination is then made as to whether that number has the uncommon mathematical property. The computer program is disabled if its number at that location does not have the uncommon mathematical property.