Patent ID: 8582770

Claim:
A cryptographic communication system comprising: a transmitter including a first random number generator, an encoder for error-correcting coding, a second random number generator, a third random number generator, a first light source, a first modulator, equipment for privacy amplification, an encrypter and an optical transmitter having a second light source and a second modulator; a receiver including a first optical detector, a decoder for decoding an error-correcting code, equipment for privacy amplification, a second optical detector and a decrypter; and an optical transmission channel connecting the transmitter and the receiver to each other, wherein the transmitter and the receiver share a first seed key made of random numbers in advance, the first seed key provides a shared basis to the transmitter and the receiver, in the transmitter, an output of the first random number generator is set as a random number signal and is error-correcting coded in the encoder, an output of the second random number generator is set as a dummy random number, an output of the third random number generator determines a random basis for transmission of the random number signal and the dummy random number, the random number signal is used as a signal when the random basis and the shared basis coincide with each other; and the dummy random number is used as the signal when the random basis and the shared basis do not coincide with each other, the first modulator forms first signal light by superimposing the signal with the random basis on output light from the first light source, in the formation of the first signal light, the output light from the first light source is accompanied with fluctuations, or fluctuations are superimposed thereon in the first light source or the first modulator, the first signal light is outputted to the optical transmission channel, the equipment for privacy amplification generates a secret key by reducing the number of bits of the random number signal which is the output of the first random number generator, the encrypter encrypts a transmission signal by use of the secret key, and the optical transmitter outputs second signal light modulated with the encrypted transmission signal, and in the receiver, the first optical detector receives the first signal light, a judgment is made on values of the random basis and the signal value, then the received random basis is checked with the shared basis, and if the random basis coincides with the shared basis, the signal is decoded to a random number signal in the decoder, the equipment for privacy amplification extracts the secret key by reducing the number of bits, the second optical detector receives the second signal light, and the decrypter decrypts the transmission signal from the signal received with the second optical receiver, by use of the extracted secret key.