MAC stands for Message Authentication Code, which provides a method to be certain that messages between the ATM and the host are authentic and unmodified, thus preventing man-in-the-middle attacks and bogus hosts or ATMs. The first step is to contact your Host/Processor to verify if they support MACing. The Host/Processor will provide MAC Master Keys that must be loaded at the ATM just like PIN Master Keys are loaded. A Key Download is performed at the ATM and MAC Working Keys are sent from the Host/Processor to the ATM. The Host/Processor can then enable MACing.