Secret key agreement using asymmetry in channel state knowledge

Abstract

We study secret-key agreement protocols over a wiretap channel controlled by a state parameter. The secret-key capacity is established when the wiretap channel is discrete and memoryless, the sender and receiver are both revealed the underlying state parameter, and no public discussion is allowed. An optimal coding scheme involves a two step approach — (i) design a wiretap codebook assuming that the state parameter is also known to the eavesdropper (ii) generate an additional secret key by exploiting the uncertainty of the state parameter at the eavesdropper. When unlimited public discussion is allowed between the legitimate terminals, we provide an upper bound on the secret-key capacity and establish its tightness when the channel outputs of the legitimate receiver and eavesdropper satisfy a conditional independence property. Numerical results for an on-off fading model suggest that the proposed coding schemes significantly outperform naive schemes that either disregard the contribution of the common state sequence or the contribution of the underlying channel.