Method for synchronization in encrypted communications using shared key

The disclosed is a method for synchronization of the running key that is generated from a shared key and that is used for encryption and decryption in communications encrypted with the shared key using a multi-valued signal. In the method for synchronization, the transmitting node transmits a signal that is formed of a multi-valued signal and that has a predetermined fixed pattern before transmitting data encrypted with the shared key. The receiving node generates a bit discrimination threshold signal that allows for bit discrimination and that has a fixed length, shifts bit by bit the phase of the bit discrimination threshold signal while monitoring bit discrimination with respect to a fixed pattern signal that is to be received, and sets 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. Then, synchronization of the running key is performed in accordance with the phase that has been set between the transmitting node and the receiving node.