  We point out that arguments for the security of Kish's noise-based
cryptographic protocol have relied on an unphysical no-wave limit, which if
taken seriously would prevent any correlation from developing between the
users. We introduce a noiseless version of the protocol, also having illusory
security in the no-wave limit, to show that noise and thermodynamics play no
essential role. Then we prove generally that classical electromagnetic
protocols cannot establish a secret key between two parties separated by a
spacetime region perfectly monitored by an eavesdropper. We note that the
original protocol of Kish is vulnerable to passive time-correlation attacks
even in the quasi-static limit. Finally we show that protocols of this type can
be secure in practice against an eavesdropper with noisy monitoring equipment.
In this case the security is a straightforward consequence of Maurer and Wolf's
discovery that key can be distilled by public discussion from correlated random
variables in a wide range of situations where the eavesdropper's noise is at
least partly independent from the users' noise.
