  We examine security of a protocol on cryptographic key distribution via
classical noise proposed by Yuen and Kim (Phys. Lett. A 241 135 (1998)).
Theoretical and experimental analysis in terms of the secure key distribution
rate shows that secure key distribution is possible even if the eavesdropper
could receive more photons than the legitimate receiver, as long as the
signal-to-noise-ratio (SNR) of the receiver is better than -9dB of the
eavesdropper's SNR. Secure key distribution was demonstrated at the maximum
rate of 0.04 bit per sender's bit and transmission rate of 2 Mb/s in the
experiment employing conventional fiber optics. The present protocol has
advantages of the efficient key distribution and the simple implementation over
other quantum key distribution protocols. However, careful design and
management are necessary to keep the security of the crypto-system.
