A program comprises a functional block. The functional block is encrypted, additional code is added to the program and a protected program is generated that is executable only in the presence of a predetermined license in a computer system which comprises a protection module for protection against unauthorized execution of the protected program. If the encrypted functional block is to be executed, the additional code is executed. If a license is present, the additional code and/or the protection module causes decryption of the functional block, allows execution of the decrypted functional block, and upon completion of execution of the functional block, causes removal of the decrypted functional block from the working memory.