1. A system for insuring operation of a computer controlled function, only in the presence of an authorized key in which such operation is protected by encrypted security message portions having a moving target portion, and where the function control includes pre-set errors therein to cause erroneous operation of the function unless said errors are nulled during execution of the function control by said computer, said computer having a communications port and said system including in combination: at least one security key means responsive to a unique key indentification code for connection to the communications port of said computer and including decryption circuit means therein uniquely encoded for decrypting said security message portions; means for connecting said key means to the communications port of said computer for transferring said identification code and signals including said encrypted security message portions from said computer to said key means and for transferring signals from said key means to said computer; circuit means in said key means enabled by said identification code for recognizing predetermined ones of said security message portions and, in response to such recognition, for modifying such predetermined ones of said message portions supplied to said key means from said computer and for reencrypting such modified message portions and supplying such reencrypted modified portions from said key means back to said computer for comparison and verification with computer generated modified message portions of said same predetermined ones of said message portions to cause said computer to store said modified security message portions outside said key means as replacements for previously stored such predetermined ones of said security message portions for use by said computer and said key means in a later cycle of operation; said key means having a target detecting means and counter means therein for producing a count each time said moving target portion is detected by said key means in signals supplied from said computer to said key means; said count being encrypted by said key means and stored outside said key means by said computer; and said key means includes means responsive to a predetermined count in said counter for initiating a comparison of the count in said key means and the count stored outside said key means whereupon failure of said comparison causes said erroneous operation to occur; and null signal producing means coupled with said recognizing means in said key means for generating null signals supplied through said connecting means to said computer for nulling said pre-set errors during operation thereof.