Quantum Cryptography Outperformed By Classical Technique: Once again the secrecy is guaranteed by the laws of physics but instead of quantum mechanics, Kish and co say the second law of thermodynamics provides the necessary underwriting...
Alice wants to send Bob a message via an ordinary wire. At each end of the wire, there are two different resistors that correspond to a 0 or 1.
Alice encodes her message by connecting these two resistors to the wire in the required sequence.
Bob, on the other hand, connects his resistors to the wire at random.
The crucial part of this set up is that the actual current and voltage through the wire is random, ideally Johnson noise. The essential features of this noise are determined by the combination of resistors at each end. This noise is public--anybody can see or measure it.
Now here's the clever bit. Bob knows which resistor he connected to the wire and so can work out which resistor Alice must have connected.
But Eve, who is listening in to the publicly available noise, does not know which resistor was connected at each end and cannot work it out either because the laws of thermodynamics prevent the extraction of this information from this kind of signal.
What's more, any kind of active attack that might interrogate the resistors at each end always introduces energy into the system that Alice and Bob can easily spot. That allows them to guarantee the secrecy, even when they send only a single bit.