Abstract: A system which enables a protected program to run only a selected plurality of computers includes a respective unique key Ki for each computer of the plurality, the key being triple encrypted in the form E.sub.FK [E.sub.Ki [E.sub.FK [Ki]]]. A respective module is coupled to each computer of the plurality. A checker program in each computer responds to a request to use the protected program by performing a single decryption procedure E.sub.FK.sup.-1 on the triple encrypted key and sends the result to the module as a message. The module performs a single decryption procedure E.sub.Ki.sup.-1 on the message and sends that result back to the computer. The checker program receives the module's result and performs another single decryption procedure E.sub.FK.sup.-1 on it to obtain key Ki. Then the checker program uses key Ki to decrypt an identifier, and proceeds with the execution of the protected program only if it is identified by the decrypted identifier.