Dubbed MagSpoof, the quarter-sized computer can be waved in front of a magnetic stripe card reader to initiate a transaction. On a Web page promoting his research, Kamkar wrote that “if I were to obtain your Amex card and you called it in as lost or stolen, the moment you get a new card, I (would) know your new credit card number.” Kamkar, whose research has revealed privacy issues in Google and Apple smartphones, also said he manipulated the magnetic stripe data on the back of his old chip card to fool retailers into believing it should be swiped, not plugged.