  We deterministically crack the secure, statistical key exchange protocol
based on feedback proposed by Pao-Lo Liu [ J. Lightwave Techology 27 (2009) pp.
5230-34]. The crack is ultimate and absolute because it works under idealized
conditions, and produces much higher data visibility for the eavesdropper than
the protocol provides for Alice and Bob. Even with the most idealistic driving
noise spectrum stated by Liu, during the most secure phase of the protocol, far
away from the transients, where the system is already in its most secure
steady-state, the eavesdropper has 100% success rate in identifying the key
bits, at the same time when Alice and Bob have less than 100% success rate
while using the Liu protocol. No statistics is needed, Eve can extract the
secure bit from two samples of the signal in the two direction. Thus the
Liu-protocol offers no security against the attack described in this paper.
