Decorrelating secret bit extraction via channel hopping in body area networks

Recent research has demonstrated that two communicating parties can generate shared secret keys by exploiting characteristics of the wireless fading channel between them. These channel characteristics are symmetric, dependent on position and orientation, highly sensitive to motion, and cannot be deduced in detail by an eavesdropper. One problem with this approach, however, is that over small channel sampling intervals, successively sampled values are correlated in time, which therefore yields keys with reduced entropy. In this paper, we undertake experiments to determine the efficacy of using channel hopping to increase diversity and improve secret key entropy, in the context of body area networks. We conduct extensive experiments using off-the-shelf IEEE 802.15.4 devices, mounted on the human body, in a real indoor environment. Our experimental results show that: (i) channel hopping increases frequency diversity and effectively decorre-lates successive channel samples, significantly increasing entropy (at minimum approximately 20%) and thereby improving the strength of the secret key, (ii) the benefit can be maximized by devising a hopping strategy that takes into account the number of channels available, the spacing between them, and the activity of the user.