A Novel Algorithm for Secret Key Generation in Passive Backscatter Communication Systems

The extreme asymmetry of passive backscatter communications systems such as passive Wi-Fi, while allowing significant reduction of node power consumption for communications, imposes severe resource limitations on implementing secure communications. Target applications for this technology are typically driven by the promise of low power consumption, up to four orders of magnitude lower than commercial Wi-Fi chipsets. Industry standard security approaches using encryption technology are problematic in this power regime, particularly as the potential low complexity and size of passive nodes will encourage application to high-density networks of very small, energy-poor devices. Generation of shared symmetric keys through reciprocal channel measurements, for example of received signal strength (RSS), is a natural approach in this situation. However previous work in this area has focused on the symmetric case where base station and nodes communicate at the same radio frequency. Backscatter communications uses two frequencies, typically a pilot beacon transmitted by a base station on one frequency, and response on a shifted frequency. This paper describes a protocol for RSS-based shared key generation for this architecture and reports the results of an experimental implementation using software radio emulation of backscatter communication.