We describe our success in defeating the security of an RFID device known as a Digital Signature Transponder (DST). Manufactured by Texas Instruments, DST (and variant) devices help secure millions of SpeedPass TM payment transponders and automobile ignition keys. Our analysis of the DST involved three phases: 1. Reverse engineering: Starting from a rough published schematic, we determined the complete functional details of the cipher underpinning the challenge-response protocol in the DST. We accomplished this with only “oracle ” or “black-box ” access to an ordinary DST, that is, by experimental observation of responses output by the device. 2. Key cracking: The key length for the DST is only 40 bits. With an array of of sixteen FPGAs operating in parallel, we can recover a DST key in under an hour using two responses to arbitrary challenges. 3. Simulation: Given the key (and serial number) of a DST, we are able to simulate its RF output so as to spoof a reader. As validation of our results, we purchased gasoline at a service station and started an automobile using simulated DST devices. We accomplished all of these steps using inexpensive off-the-shelf equipment, and with minimal RF expertise. This suggests that an attacker with modest resources can emulate a target DST after brief short-range scanning or long-range eavesdropping across several authentication sessions. We conclude that the cryptographic protection afforded by the DST device is relatively weak.