Move Over, Quantum Cryptography: Classical Physics Can Be Unbreakable Too

A ridiculouos idea, if you're an electrical engineer, for many reasons:

(1) The noise on the wire, for reasonable values of resistors and bandwidth, is down in the low microvolts. If the cable is unshielded, it's going to pick up several microvolts of radio signals per foot. Even if it's really well shielded, we're still talking microvolts per kilometer.

(2) Eve can put a probe signal on the wire, it just has to be random noise. Alice and Bob have no way of proving that a small spike of random noise, only half a standard deviation above the average, isn't perfectly fine Johnson noise coming from the other end. Eve knows the amplitude of the noise she is putting on the wire, so she can subtract that amount, and the difference reveals the values of the resistors.

(3) For any moderately long wire, in the kilometer range, there is a time delay, allowing Eve to inject short bursts of noise and get the resistor info from each end coming back, spread out in time.

(4) Bell Labs proposed this idea, the part about injecting noise inn from both ends, back around 1955.

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