  A shared secret key is necessary for encrypted communications. Since Wi-Fi
relies on OFDM, we suggest a method to generate such a key by utilizing Wi-Fi's
channel state information (CSI). CSI is typically reciprocal but very sensitive
to location: While the legitimate Alice and Bob observe the same CSI, an
eavesdropper Eve observes an uncorrelated CSI when positioned over 0.5
wavelength away. We show that if endpoint Bob is shaken, sufficient diversity
is induced in the CSI so that it can serve as a source of true randomness. Then
we show that the CSI among neighboring sub-carriers is correlated, so we select
a small set of judiciously-spaced sub-carriers, and use a majority rule around
each. We demonstrate that Alice and Bob observe a 5-15\% bit mismatch rate
(BMR) in the extracted bitstream while Eve observes a BMR of around 50\% even
when placed within 10cm of Alice. We employ the cryptography-oriented
definition of min-entropy to estimate the number of secure bits within the
bitstream, and use the 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
leakage we quantify the Secured Bit Generation Rate of our method.
  We conducted extensive tests in an indoor environment. Our system exhibits a
secure bit generation rate of 1.2--1.6 %secure bits per packet, at distances
ranging from 0.5m--9m, and can generate a secure shared 128-bit key with 20sec
of device shaking.
