Patent ID: 7596322
Filing Date: 2009-09-29
Classification: H04L

Abstract:
1. A method of transmitting random numbers to be used for generating a cryptographic key from a transmitter of a sender's apparatus to a receiver of a recipient's apparatus via an optical fiber, the cryptographic key being applied to data transmission on a communication line connecting the sender's apparatus and the recipient's apparatus, the method comprising the steps of: starting generation of two-bit random numbers in synchronization with transmission clock in the transmitter, in response to an instruction from the sender's apparatus; generating a train of phase-modulated signal light modulated according to the value of the two-bit random numbers in synchronization with the transmission clock in the transmitter; transmitting the train of phase-modulated signal light at a single photon level and a trigger light indicative of beginning of random number transmission from the transmitter to the receiver via the optical fiber; storing the random numbers at each slot position and associated slot position numbers into a first memory in the transmitter; receiving the phase-modulated signal light in synchronization with the transmission clock and the trigger light in the receiver; starting generation of slot position numbers in the receiver in response to the trigger light; selecting time slots sporadically at random at an average rate lower than that of the transmission clock, and detecting random numbers at the selected time slots from the phase-modulated signal light; storing the random numbers detected at the sporadically selected time slots and the associated slot position numbers into a second memory in the receiver; and communicating between the sender's apparatus and the recipient's apparatus through the communication line to determine a group of secret random numbers to be used in a cipher system as a cryptographic key, from the random numbers stored in common in both the first and second memories.