complex , time varying codes are used in an access regulating system that includes three types of synchronized apparatuses . the first apparatus is a central computer , or other suitable means that issues a set of time - dependent code segments to a plurality of the second , linking , type of apparatus . the codes are relayed by the linking apparatus to the third , access granting , apparatus that grants access only if there is a match between the code from the linking apparatus and a code stored in the access granting apparatus .. the time - varying code segments are chosen so that after the expiation of a predetermined interval none of the code segments in the linking apparatus match code segments in the access - granting apparatus .