Cracking the Liu key exchange protocol in its most secure state

We fully crack the Liu's cypher based on random signals and feedback [P.-L. Liu, Physics Letters A 373 (2009) 3207-3211], in its most secure state. We utilize the natural properties of power density spectra and autocorrelation functions. Our method to extract information by Eve is much more efficient than the originally proposed way of key exchange by Liu. Therefore, Alice and Bob must use this new method to communicate, otherwise Eve has more information than they do, and that means that Eve has exactly the same amount of information as they have. The Liu key exchange protocol has zero security against this attack.