Let’s shake on it: Extracting secure shared keys from Wi-Fi CSI

A shared secret key is necessary for encrypted communications. Since Wi-Fi relies on OFDM, we suggest a method for generating such key by utilizing Wi-Fi’s channel state information (CSI). The channel is typically reciprocal but very sensitive to location: while (legitimate) Alice and Bob should observe the same CSI, an eavesdropper Eve shall observe uncorrelated CSI when positioned over 0.5 wavelength away from either node.We show that if endpoint Bob is shaken, sufficient diversity is induced in the CSI, thus rendering it a true source for randomness. We then show that the CSI among neighboring sub-carriers is correlated, so we select a small set of judiciously-spaced sub-carriers, and decide on a value based on a majority rule about each. Consequently, Alice and Bob observe a 5-15% bit mismatch rate (BMR) in the extracted bitstream while Eve experiences a BMR of around 50% even when placed 10cm away from Alice.We employ the cryptography-oriented definition of min-entropy to estimate the number of secure bits in the received bitstream, and employ the so-called Cascade algorithm of quantum-key-distribution to reconcile Alice and Bob’s bitstreams while quantifying the number of bits leaked by the algorithm. Accounting for both the min-entropy and the Cascade-based leakage we quantify the Secured Bit Generation Rate realized by our method.Extensive set of experiments have been conducted in an indoor environment revealing secure bit generation rate of 1.2–1.6 bits per packet, for endpoint distances ranging from 0.5m–9m; a (128-bit) secured shared-key is generated in 20sec of device shaking.