Abstract: A computer based function control system is particularly suited for use as a software security device on the highly popular personal computers or a micro-processor driven function. The system includes an encrypted security message uniquely encoded at predetermined locations within the software or function program. The software or function program includes pre-set errors in it to cause failure of execution of the function or software program unless the errors are nulled during operation of the function or software program. A separate electronic key for retrieving, recognizing, decrypting, encrypting, and producing the null signals is connected to the communications port of the computer from which the key draws its power as well as the security message passed from the computer to the key and back to the computer. There is interchange of moving target and validation information between the computer software and the electronic key.