Absolutely Secure Communications by Johnson-like Noise and Kirchhoff's Laws

We survey the most important results and some recent developments about the secure key exchange protocol where the security is based on the Second Law of Thermodynamics and the robustness of classical physical information. We conclude that a classical physical system offers a higher level of control and security during the communication. We also mention some recent attempts inspired by this communicator to create other systems where Alice and Bob do not form an organic single system and/or the Second Law is irrelevant. It seems philosophically that they cannot be unconditionally secure, however it is yet an open question how to crack them; how can they be best used for conditionally secure communications, and what are the practical implications.Comment: Invited paper for the Journal of the Society of Instrument and Control Engineers (SICE), Japan, for the special issue on "Fluctuations and Noise