  We have found a security risk in the Liu's cypher based on random signals and
feedback, when it utilizes a large class of noises for communication in its
most secure state, the steady 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 implications of the results concern the
transient operation during changing bits, where the modulation products of
noise cannot be band-limited therefore the cypher is vulnerable. We propose the
application of line filters to provide a proper spectral shape and to improve
the security.
