A new look at the classical key exchange system based on amplified Johnson noise

A classical key exchange system based on the amplified Johnson noise has been proposed and demonstrated recently. In this Letter, a modified system is presented. By using circulator and feedback, the new system can be compatible with traditional analog and digital communications systems using carrier waves. Results of circuit simulations, however, indicate that the signal transit time poses a serious risk. Because of delay and multiple reflections, there is always a difference between two parties communicating. The eavesdropper can exploit this difference. By choosing impedance and delay time, the difference can be reduced. Nevertheless, the system is far from being secure except when two parties are collocated.