Patent ID: 8284937

Claim:
A method for synchronization of a running key used for encryption data to be transmitted and decryption data received, the method comprising: generating, by a running key generator, the running key from a shared key, the shared key being previously stored in a transmitting node; generating transmission data as an N-bit multi-valued signal in accordance with the running key that has been generated by the running key generator; outputting a signal having an optical intensity basis that is changed by using the multi-valued signal; controlling the running key generator to generate a multi-valued fixed pattern signal having a predetermined fixed pattern; transmitting, from the transmitting node, the multi-valued fixed pattern signal having the predetermined fixed pattern before transmitting the multi-valued signal, which is encrypted by using the running key; receiving, in a receiving node, the multi-valued fixed pattern signal transmitted from the transmitting node before receiving the multi-valued signal encrypted by using the running key; generating a bit discrimination threshold signal that allows for bit discrimination and that has a fixed length; shifting bit by bit the phase of the bit discrimination threshold signal while monitoring the bit discrimination with respect to the multi-valued fixed pattern signal that is to be received; setting the phase of the bit discrimination threshold signal when the phase of the multi-valued fixed pattern signal matches the phase of the bit discrimination threshold signal; and performing synchronization of the running key between the receiving node and the transmitting node in accordance with the phase that has been set, wherein the synchronized running key is generated from a shared key previously stored in the receiving node, wherein the shared key is the same as a shared key of the transmitting node, and wherein the received multi-valued signal is decrypted by the synchronized running key.