Patent ID: 7457954

Claim:
A communications system where a transmitter transmits, to a receiver, message data as a plurality of packets, wherein each of the packets includes a data region comprising: transmission object data that is one of division portions into which the message data is divided so that each of the division portions is contained within each of the packets; authentication information, which is designated from first information for authentication and is used for executing substitution detection to the transmission object data, the first information being independent of the transmission object data; and control information that at least indicates what kind of a constituent of the message data the transmission object data is, wherein the transmitter is configured to include a transmitter-end storage for storing an encryption key and the first information for authentication, arrange the data region including the transmission object data, the authentication information, and the control information in a predetermined order, and encrypt the arranged data region using the stored encryption key to thereby generate the data region encrypted, wherein, when the receiver receives each of the packets, the receiver includes a receiver-end storage for storing a decryption key and second information that is the same as the first information stored in the transmitter-end storage, decrypts the data region included in the packet received from the transmitter using the decryption key stored in the receiver-end storage, extracts, from the decrypted data region, the transmission object data, the authentication information, and the control information, executes, using the authentication information extracted, a substitution detection of whether the transmission object data received is normal data that is really sent from the transmitter by verifying the authentication information extracted using the second information stored in the receiver-end storage, determining that the transmission object data is normal data really sent from the transmitter when the authentication information extracted is successfully verified, and determining that the transmission object data is abnormal data really not sent from the transmitter when the authentication information extracted is not successfully verified, and restructures the message data from the transmission object data received based on the control information received, wherein, when all the message data is included in a single packet, the transmitter forms a packet including the data region so that the transmission object data is all the message data, and the control information indicates that the transmission object data itself is the message data, and wherein, when the receiver receives the packet from the transmitter, the receiver treats the transmission object data received as all the message data based on the control information received that indicates that the transmission object data itself is the message data.