Re-Defining Secure Distance for CSI-based Key Generation Protocols

Channel-State-Information (CSI)-based Key Generation (KG) is an attractive technique for generating symmetric cryptographic keys due to low memory and processing power requirements. CSI-based KG protocols also promise high key rates for vehicular communication channels due to the rich entropy inherited in such dynamic channels. However, as a relatively new field, physical layer security is often limited to idealistic scenarios. The assumption that the channels decorrelate at a half-wavelength distance brings secrecy vulnerabilities that are quantified and brought to attention. This paper convinces that, even in rich scattering environments, the distance of half-wavelength is not perfectly secure, and it may be disastrous under non-idealistic antenna gain patterns. Aiming to bring CSI-based KG a step closer to practical implementation, this paper drops one of the most common idealistic assumptions in the field of PLS and redefines secure distance.