Patent ID: 6895092

Claim:
A cryptographic key distribution method at a cryptographic key distribution apparatus in which a sender transmits signal light to a receiver through an optical transmission channel, wherein: said sender, comprising the steps of: setting light intensity and a modulation index of outputting signal light so that the signal to noise ratio (SNR) of an eavesdropper is smaller than 2 dB even when said eavesdropper eavesdrops at the sending end by using any receiving equipment and also the SNR of said receiver at the receiving end is larger than −10 dB; and transmitting signal light modulated by modulation signals that a random number sequence was coded, and said receiver, comprising the steps of: receiving said signal light of said random number sequence transmitted from said sender; calculating the probability distributions by using the frequency (occurrence times) distributions of said received signal light having fluctuation caused by that noise was added; judging whether said eavesdropper exists or not based on changes of said probability distributions, and also setting a discrimination threshold value so that the error rate of said receiver is 5% or less; discriminating the bit value of each bit in said random number sequence based on said discrimination threshold value; and informing said sender of the positions of bits that said receiver could discriminated, and taking out only the bit sequence having no errors in the discrimination, and sharing said bit sequence having no errors with said sender, and wherein: said shared bit sequence is made to the cryptographic key.