Abstract: An arrangement to protect software, particularly freely distributed application software, against utilization without permission of the copyright holder. By encrypting the software employing a key (k1) which is different from that key (k2) which is employed in the decryption, better protection is obtained against unauthorized utilization when the decryption key is kept secret to the user. Further improved security is achieved by additionally executing scrambling-descrambling of the communication between the computer in which the software is utilized and the external unit in which the decryption key is stored. Also, the external unit is arranged such that it returns to the host computer, the result from its processing of data received from the host, the result then being utilized in the further execution of the respective program.