A security system with methodology for computing a machine independent unique identifier for an executable file across different machines is described. In response to a request to uniquely identify an executable file that has been installed on a given machine, portions of the executable file modified as a result of installation of the executable file on the given machine are identified. A machine independent unique identifier is determined by performing a calculation on the executable file. The calculation is performed by excluding at least the identified portions of the executable file modified as a result of installation of the executable file on the given machine.