Waving a Smart card in front of a radio frequency identification (RFID) reader can provide access to buildings, pay for subway rides, and even initiate credit-card transactions. With more than a billion units sold, the NXP Mifare Classic RFID tag is the most commonly used smart-card chip; it can be found in the London subway system's Oyster card, Australia's SmartRider, and the Boston subway's ­Charlie Card. Security researcher Karsten Nohl, who recently got his PhD in computer science from the University of Virginia, and ­"Starbug," a member of a Berlin hacker group called the Chaos Computer Club, hacked into a Mifare Classic's hardware to gain insight into its cryptographic algorithms. After analyzing the chip, Nohl questioned its security in a series of presentations at recent conferences, including Black Hat in Las Vegas.

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