High-end vehicles are often equipped with a Passive Keyless Entry and Start (PKES) system. These PKES systems allow to unlock and start the vehicle based on the physical proximity of a paired key fob; no user interaction is required.
Researchers have already shown these systems to be particularly vulnerable to relay attacks [1, 2]. In this type of attack two adversaries relay the short-range communication over a long-range communication channel. Recent news reports and home security videos have shown that relay attacks are frequently used to steal luxury vehicles. Distance bounding mechanisms are gradually being deployed to preclude relay attacks. The goal of our research was to evaluate the resistance of a modern-day PKES system to attacks other than relay attacks. We have completely reverse engineered the PKES system used in the Tesla Model S. Our research shows that this system is using the outdated proprietary DST40 cipher. The Passive Keyless Entry and Start (PKES) system as introduced earlier allows to both unlock and start the car if the key fob is in proximity. In the remainder of this article we provide a simplified explanation of how the Tesla Model S PKES system works and why it is insecure. A more technical and thorough explanation of our research will be released soon as a paper. HOW DOES IT WORK? The PKES system we analyzed uses a simple challenge response protocol as is shown in the figure below. The car uses the Low Frequency (LF) band at 134.2 kHz for transmission. The key fob on the other hand transmits in the Ultra High Frequency (UHF) band at 433.92 MHz in Europe. During normal operation the car periodically advertises its identifier (denoted ‘wake’ in the figure below). The key will receive the car’s identifier, if it is the expected car identifier the key fob will reply, signaling it is ready to receive a challenge. In the next step the car will transmit a random challenge to the key fob. The key fob computes a response and transmits it. After receiving the key fob’s response, the car must verify it before unlocking the doors. The same challenge response protocol is repeated to start the car. SECURITY WEAKNESSES The simple challenge-response protocol described earlier does have some issues. For example, the lack of mutual authentication allows anyone who knows the car’s identifier to get responses from a key fob. This identifier is broadcasted by the vehicle in the wake messages and can be recorded by anyone. Read more on: FAST, FURIOUS AND INSECURE: PASSIVE KEYLESS ENTRY AND START IN MODERN SUPERCARS Related Article: How to open many keypad-access doors
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