Cryptographic key sharing system

A cryptographic key sharing system comprising a differential phase shift keying quantum key transmitter and a differential phase shift keying quantum key receiver that share a cryptographic key therebetween using an optical signal propagating through a line-of-sight (LOS) communication channel,

• wherein
• the differential phase shift keying quantum key transmitter is equipped with
• an optical transmitter that encodes random bits into the optical signal using differential phase shift keying and transmits the encoded optical signal at a predetermined strength via the LOS communication channel, and
• a first key distillator that generates a cryptographic key from a sequence of the random bits outputted by the optical transmitter using key distillation processing via an authenticated public communication channel;
• and
• the differential phase shift keying quantum key receiver is equipped with:
• an optical receiver that receives the optical signal from the optical transmitter via the LOS communication channel and decodes the received optical signal into the random bits, and
• a second key distillator that generates a cryptographic key from a sequence of the random bits outputted by the optical receiver using the key distillation processing via the authenticated public communication channel, and
• wherein
• the differential phase shift keying quantum key transmitter further includes a first communication channel state monitor that measures atmospheric fluctuations in the LOS communication channel, and
• the differential phase shift keying quantum key receiver further includes:
• a second communication channel state monitor that measures the atmospheric fluctuations in the LOS communication channel;
• and
• a communication channel state estimator that estimates an intensity of leakage light leaking outside the LOS communication channel on a basis of the atmospheric fluctuations measured by the first communication channel state monitor and the second communication channel state monitor, and wherein
• the optical transmitter sets an intensity of the optical signal to be transmitted based on the intensity of the leakage light estimated by the communication channel state estimator,
• the first key distillator adjusts a compression ratio for generating the cryptographic key from the sequence of the random bits outputted by the optical transmitter based on the intensity of the leaked light,
• the second key distillator adjusts a compression ratio for generating the cryptographic key from the sequence of the random bits outputted by the optical receiver based on the intensity of the leaked light.