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The Martini Synch Device pairing is a significant problem for a large class of increasingly popular resource-constrained wireless protocols such as Bluetooth. The objective of pairing is to establish a secure wireless communication channel between two specific devices without a public-key infrastructure, a secure near-field communication channel, or electrical contact. We use a surprising user-device interaction as a solution to this problem. By adding an accelerometer, a device can sense its motion in a Cartesian space relative to the inertial space. The idea is to have two devices in a fixed, relative position to each other. Then, the joint object is moved randomly in 3D for several seconds. The unique motion generates approximately the same distinct signal at the accelerometers. The difference between the signals in the two inertially conjoined sensors should be relatively small under normal motion induced manually. The objective is to derive a deterministic key at both sides with maximized entropy that will be used as a private key for symmetric encryption. Currently, our prototype produces between 10–15 bits of entropy per second of usual manual motion using off-the-shelf components. We have deployed two simple algorithms that perform the task: one based upon matching syndromes in an error correction system [1], and another based upon a simple joint-quantizer [2]. We also developed a simple bump detector to initiate the key generation session [2]. The technology was first presented to the public in March of 2006 at the Microsoft Research TechFest (see the custom Bluetooth devices we built in a photo below). The following patent application covers some of the ideas. Subsequently, it has been demoed to NIST and at the 2006 Microsoft Faculty Summit. The idea has captured the interest of many bloggers. A similar idea has been proposed before but without a constructive algorithm in [3] and worked out later, elsewhere, and independently [4]. Learn more about our system from [1,2].
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