His watch-sized gadget, which he calls MagSpoof, can store more than a hundred credit card numbers and emit an electromagnetic field that's strong enough to hit a credit card reader's sensor from close proximity, sending a signal that imitates a credit card being swiped. Kamkar's device also includes a button that implements his prediction algorithm; if a criminal using MagSpoof were to find that a credit card they tried to spoof had been cancelled, the device could immediately generate the victim's next card number.