Nonlinearity Attack against the Kirchhoff-Law-Johnson-Noise (KLJN) Secure Key Exchange Protocol.

This paper introduces a new attack against the Kirchhoff-Law-Johnson-Noise (KLJN) secure key exchange scheme. The attack is based on the nonlinearity of the noise generators. We explore the effect of total distortion (TD) at the second order (D2), third order (D3), and a combination of the second and third orders (D2,3) on the security of the KLJN scheme. It is demonstrated that a as little as 1% results in a notable power flow along the information channel, which leads to a significant information leak. We also show that decreasing the effective temperature (that is, the wire voltage) and, in this way reducing nonlinearity, results in the KLJN scheme approaching perfect security.