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.
@article{arxiv.1402.2709,
title = {A directional coupler attack against the Kish key distribution system},
author = {Lachlan J. Gunn and Andrew Allison and Derek Abbott},
journal= {arXiv preprint arXiv:1402.2709},
year = {2014}
}