  Physical-layer key generation methods utilize the variations of the
communication channel to achieve a secure key agreement between two parties
with no prior security association. Their secrecy rate (bit generation rate)
depends heavily on the randomness of the channel, which may reduce
significantly in a stable environment. Existing methods seek to improve the
secrecy rate by injecting artificial noise into the channel. Unfortunately,
noise injection cannot alter the underlying channel state, which depends on the
multipath environment between the transmitter and receiver. Consequently, these
methods are known to leak key bits toward multi-antenna eavesdroppers, which is
capable of filtering the noise through the differential of multiple signal
receptions. This work demonstrates an improved approach to reinforce
physical-layer key generation schemes, e.g., channel randomization. The channel
randomization approach leverages a reconfigurable antenna to rapidly change the
channel state during transmission, and an angle-of-departure (AoD) based
channel estimation algorithm to cancel the changing effects for the intended
receiver. The combined result is a communication channel stable in the eyes of
the intended receiver but randomly changing from the viewpoint of the
eavesdropper. We augmented an existing physical-layer key generation protocol,
iJam, with the proposed approach and developed a full-fledged remote
instrumentation platform to demonstrate its performance. Our evaluations show
that augmentation does not affect the bit error rate (BER) of the intended
receiver during key establishment but reduces the eavesdropper's BER to the
level of random guessing, regardless of the number of antennas it equips.
