We have found a security risk in the Liu's cypher based on random signals andfeedback, when it utilizes a large class of noises for communication in itsmost secure state, the steady state. For the vulnerability to exist, the noisemust have a spectrum which can be transformed to white-like noise by linearfiltering. For the cracking, we utilize the natural properties of power densityspectra and autocorrelation functions. We introduce and demonstrate the methodfor Lorentzian spectra. Some of the implications of the results concern thetransient operation during changing bits, where the modulation products ofnoise cannot be band-limited therefore the cypher is vulnerable. We propose theapplication of line filters to provide a proper spectral shape and to improvethe security.
展开▼
