IJam with channel randomization

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.