Communication system and communication method

A communication method that is used in a communication system including a management device configured to store first information in a management code formed by combining the first information and second information in the management device, the management device being configured to manage the stored first information to update the stored first information once, each time the second information is updated predetermined times, and a reception device and a transmission device communicatably connected to the management device over a communication network, the communication method comprising:

• transmitting, by the management device, a first communication message including the first information;
• receiving the first communication message, by the reception device and the transmission device, and holding, by the reception device and the transmission device, the first information included in the received first communication message each time the first communication message transmitted by the management device is received;
• managing, by the transmission device, the second information in the management code, generating a first authenticator from communication data and the management code formed by combining the held first information with the managed second information, generating and transmitting a second communication message including the communication data, the managed second information, and the generated first authenticator and not including the held first information, and updating a value of the managed second information after the generation of the second communication message;
• receiving, by the reception device, the second communication message transmitted by the transmission device, and authenticating the received second communication message based on a comparison between the first authenticator included in the received second communication message and a regenerated authenticator regenerated based on the received second communication message;
• and
• reconstructing, by the reception device, the management code by combining the held first information with the second information acquired from the first communication message transmitted by the transmission device, and acquiring the regenerated authenticator from the communication data acquired from the second communication message transmitted by the transmission device and the reconstructed management code.