Notes on Recent Approaches Concerning the Kirchhoff-Law-Johnson-Noise-based Secure Key Exchange

We break the security of the circulator-based secure key exchange system in [Physics Letters A 373 (2009) 901-904] by showing that it violates the basic mechanism of security related to the second law of thermodynamics. Further, we show that the strong security leak appeared in the simulations is only an artifact and not caused by "multiple reflections". Since no wave modes exist at cable length of 5% of the shortest wavelength of the signal, no wave is present to reflect it. In the high wave impedance limit, the conditions used in the simulations are heavily unphysical (requiring cable diameters up to 28000 times greater than the measured size of the known universe) and the results are modeling artifacts due to the unphysical values. At the low cable impedance limit, the observed artifacts are due to violating the recommended (and tested) conditions by neglecting the cable capacitance restrictions and using about 100 times longer cable than recommended without cable capacitance compensation arrangement. Finally, we analyze the proposed method to increase security by dropping only high-risk bits. We point out the differences between different types of high-risk bits and show the shortage of this strategy for some simple key exchange protocols.