A method and system for allowing communication devices to synchronously manage shared information are provided. A sender sends single-photon pulses modulated with original random numbers to a receiver and also sends frame pulses by using ordinary optical pulses. Bit comparison and basis reconciliation are performed by the frame which is defined by the frame pulses, whereby sifted keys, which are aggregated as a file, are generated by the sender and the receiver individually. The sifted keys are subjected to error correction, privacy amplification, and file sharing processing by the file, whereby common cryptographic keys are synchronously stored in the sender and the receiver individually. The generated cryptographic keys are managed as encryption keys and decryption keys separately. A newly generated key is preferentially placed in the encryption keys or decryption keys that have a smaller stored amount.