  The Kish key distribution system has been proposed as a class ical
alternative to quantum key distribution. The idealized Kish scheme elegantly
promise s secure key distribution by exploiting thermal noise in a transmission
line. However, we demonstrate that 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
directional wave measurements, and experimentally demonstrate its efficacy. Our
attack is based on causality: in a spatially distributed system, propagation is
needed for thermodynamic equilibration, and that leaks information.
