Cracking the Liu key exchange protocol in its most secure state with Lorentzian spectra

We fully crack the Liu's cypher based on random signals and feedback [P.-L. Liu, Physics Letters A 373 (2009) 3207-3211], when it utilizes a large class of noises for communication in its most secure state. For the vulnerability to exist, the noise must have a spectrum which can be transformed to white-like noise by linear filtering. For the cracking, we utilize the natural properties of power density spectra and autocorrelation functions. We introduce and demonstrate the method for Lorentzian spectra. Some of the new implications for the transient security, where the cypher is most vulnerable, are the necessity to use line filters to provide satisfactory spectral shape during changing bits.