The idea is straightforward. 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.
So why isn't this susceptible to a simple man in the middle attack?:
Eve cuts the wire between Alice and Bob (AB line) and insert her own
node that connects to Alice (AE line) and Bob (BE Line) individually.
Alice can't tell the difference between the AB line or the AE Line and
sets her resisters. Eve sets her resisters connected on the AE line to
random and deciphers the sequence that Alice used. Eve then Uses that
sequence on the BE Line. Bob can't tell the difference between the AB
line and the BE line, sets his resisters randomly and decodes the
message. (Eve can even send Bob a False message).
Seems like this method requires a 100% secure land line, which is
impractical.
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"It’s cheap to maintain Lies and expensive to maintain Trvth."
--KZK's Maxim
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