Method for synchronization in encrypted communications using shared key

An encryption communication system in which a transmitting station encrypts data based on a shared key using a multi-valued signal and transmits the encrypted data, and in which a receiving station receives the encrypted data and decrypts the encrypted data based on the shared key,

• wherein the transmitting station comprises:
• first key generation means for generating a running key from the shared key;
• means for generating transmission data as an N-bit multi-valued signal in accordance with the running key that has been generated by the first key generation means;
• an optical modulator for outputting a signal whose optical intensity basis is changed by using the multi-valued signal that has been generated by the multi-valued signal generation means;
• and
• a logical processing unit for controlling the first key generation means to generate a multi-valued signal (multi-valued fixed pattern signal) having a predetermined fixed pattern;
• and
• wherein the receiving station comprises:
• second key generation means for generating the running key from the shared key;
• means for generating received data as an N-bit multi-valued signal in accordance with the running key that has been generated by the second generation means;
• and
• a logical processing unit for generating a bit discrimination threshold signal that allows for bit discrimination and has a fixed length, shifting bit by bit the phase of the bit discrimination threshold signal while monitoring bit discrimination with respect to the multi-valued fixed pattern signal that is to be received, and 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,
• wherein synchronization of the running key is performed in accordance with the phase that has been set by the logical processing unit between the transmitting station and the receiving station.

A receiving device which receives data that has been encrypted by using a running key generated from a shared key using a multi-valued signal and that has been transmitted from a transmitting device, and which decrypts the encrypted data using the running key, the receiving device comprising:

• a running key generator generating the running key from the shared key,
• wherein the shared key is the same as a shared key in the transmitting device;
• a D/A (digital-to-analog) modulator generating a bit discrimination threshold signal in accordance with the running key generated by the running key generator;
• a comparator comparing the bit discrimination threshold signal generated by the D/A modulator with a received signal,
• wherein the received data is discriminated into binary values using a result of the step of comparing;
• and
• a logical processing unit for generating a bit discrimination threshold signal that allows for bit discrimination and has a fixed length, shifting bit by bit the phase of the bit discrimination threshold signal, monitoring the bit discrimination threshold signal with respect to a fixed pattern signal included in the received data before generating the running key by the running key generator, and setting the phase of the bit discrimination threshold signal when the phase of the fixed pattern signal matches the phase of the bit discrimination threshold signal,
• wherein synchronization of the running key is performed in accordance with the phase that has been set by the logical processing unit between the receiving device and the transmitting device, and the received data is decrypted by the running key that has been synchronized.