A classical key distribution system based on Johnson (like) noise - How   secure?
  We present a comprehensive analysis of the Johnson (like) noise based classical key-distribution scheme presented by Kish [1]. We suggest two passive attack strategies that enable an adversary to gain complete knowledge of the exchanged key. The first approach exploits the transient response of the voltage difference on the transmission line after the resistors are switched and the second one exploits the finite impedance of the wire connecting the two parties. 
Jacob Scheuer, Amnon Yariv