  This paper develops a novel framework for sharing secret keys using the
well-known Automatic Repeat reQuest (ARQ) protocol. The proposed key sharing
protocol does not assume any prior knowledge about the channel state
information (CSI), but, harnesses the available opportunistic secrecy gains
using only the one bit feedback, in the form of ACK/NACK. The distribution of
key bits among multiple ARQ epochs, in our approach, allows for mitigating the
secrecy outage phenomenon observed in earlier works. We characterize the
information theoretic limits of the proposed scheme, under different
assumptions on the channel spatial and temporal correlation function, and
develop low complexity explicit implementations. Our analysis reveals a novel
role of "dumb antennas" in overcoming the negative impact of spatial
correlation, between the legitimate and eavesdropper channels, on the
achievable secrecy rates. We further develop an adaptive rate allocation policy
which achieves higher secrecy rates by exploiting the channel temporal
correlation. Finally, our theoretical claims are validated by numerical results
that establish the achievability of non-zero secrecy rates even when the
eavesdropper channel is less noisy, on the average, than the legitimate
channel.
