A communication system comprising: a key management server device (20a, 20b); and a router (10a, 10b), the key management server device (20a, 20b) comprising: a key distribution processing module configured to share a bit string by quantum key distribution; a control signal processing module configured to receive a control signal including key identification information and a key length, the key identification information identifying an encryption key generated from the bit string, and the key length indicating a length of the encryption key; a decryption key generation module configured to generate a decryption key corresponding to the encryption key; and a supply module configured to supply the decryption key to the router, and the router (10a, 10b) comprising: a packet reception processing module configured to receive a packet encrypted with the encryption key; and a decryption processing module configured to decrypt the packet by using the decryption key supplied from the key management server device without requesting the key management server device to generate the decryption key, characterized in that the decryption key generation module is configured to generate, upon receiving the control signal, the decryption key from the bit string based on the key identification information and the key length, without waiting for a request to generate the decryption key from the router. a key management server device (20a, 20b); and a router (10a, 10b), the key management server device (20a, 20b) comprising: a key distribution processing module configured to share a bit string by quantum key distribution; a control signal processing module configured to receive a control signal including key identification information and a key length, the key identification information identifying an encryption key generated from the bit string, and the key length indicating a length of the encryption key; a decryption key generation module configured to generate a decryption key corresponding to the encryption key; and a supply module configured to supply the decryption key to the router, and a key distribution processing module configured to share a bit string by quantum key distribution; a control signal processing module configured to receive a control signal including key identification information and a key length, the key identification information identifying an encryption key generated from the bit string, and the key length indicating a length of the encryption key; a decryption key generation module configured to generate a decryption key corresponding to the encryption key; and a supply module configured to supply the decryption key to the router, and the router (10a, 10b) comprising: a packet reception processing module configured to receive a packet encrypted with the encryption key; and a decryption processing module configured to decrypt the packet by using the decryption key supplied from the key management server device without requesting the key management server device to generate the decryption key, a packet reception processing module configured to receive a packet encrypted with the encryption key; and a decryption processing module configured to decrypt the packet by using the decryption key supplied from the key management server device without requesting the key management server device to generate the decryption key, characterized in that the decryption key generation module is configured to generate, upon receiving the control signal, the decryption key from the bit string based on the key identification information and the key length, without waiting for a request to generate the decryption key from the router.