A directional coupler attack against the Kish key distribution system

The Kish key distribution system has been proposed as a classical alternative to quantum key distribution. The idealised Kish scheme elegantly promises secure key distribution by exploiting thermal noise in a transmission line. However, it is vulnerable to nonidealities in its components, such as the finite resistance of the transmission line connecting its endpoints. We introduce a novel attack against this nonideality using a directional coupler, and experimentally demonstrate its efficacy.