A New Transient Attack on the Kish Key Distribution System

The Kish Key Distribution (KKD) system has been proposed as a classical alternative to quantum key distribution, making use of temperature-matched thermal noise. Previous analyses assume instant propagation of signals along the cable connecting the two users. We describe a new attack that takes an advantage of propagation delays. At the start of each bit period, the noise temperature will then be increased from zero to its final value. During this process, the noise temperature variation will take time to propagate along the line, resulting in a temperature mismatch. We analyze the information leak due to this effect and consider several potential mitigation schemes.