In another embodiment {HEAD ~ the BCU} 103 may associate {TAIL ~ two or more unique “active” key pairs}—K1 pv/K1 pb, K2 pv/K2 pb, etc.—with an authorized user 101 of the PID 100, which key pairs—K1 pv/K1 pb, K2 pv/K2 pb, etc—may be updated by the BCU 103 from time to time, thereby replacing the previous key pairs 