  This paper develops a novel framework for sharing secret keys using existing
Automatic Repeat reQuest (ARQ) protocols. Our approach exploits the multi-path
nature of the wireless environment to hide the key from passive eavesdroppers.
The proposed framework does not assume the availability of any prior channel
state information (CSI) and exploits only the one bit ACK/NACK feedback from
the legitimate receiver. Compared with earlier approaches, the main innovation
lies in the distribution of key bits among multiple ARQ frames. Interestingly,
this idea allows for achieving a positive secrecy rate even when the
eavesdropper experiences more favorable channel conditions, on average, than
the legitimate receiver. In the sequel, we characterize the information
theoretic limits of the proposed schemes, develop low complexity explicit
implementations, and conclude with numerical results that validate our
theoretical claims.
