  This paper develops a novel framework for sharing secret keys using the
Automatic Repeat reQuest (ARQ) protocol. We first characterize the underlying
information theoretic limits, under different assumptions on the channel
spatial and temporal correlation function. Our analysis reveals a novel role of
"dumb antennas" in overcoming the negative impact of spatial correlation on the
achievable secrecy rates. We further develop an adaptive rate allocation
policy, which achieves higher secrecy rates in temporally correlated channels,
and explicit constructions for ARQ secrecy coding that enjoy low implementation
complexity. Building on this theoretical foundation, we propose a unified
framework for ARQ-based secrecy in Wi-Fi networks. By exploiting the existing
ARQ mechanism in the IEEE 802.11 standard, we develop security overlays that
offer strong security guarantees at the expense of only minor modifications in
the medium access layer. Our numerical results establish the achievability of
non-zero secrecy rates even when the eavesdropper channel is less noisy, on the
average, than the legitimate channel, while our linux-based prototype
demonstrates the efficiency of our ARQ overlays in mitigating all known,
passive and active, Wi-Fi attacks at the expense of a minimal increase in the
link setup time and a small loss in throughput.
