To allow communication with encrypted data with no significant design change of hardware, by suppressing increase of processing load at communication nodes in an existing communication system.

In an in-vehicle LAN10, ECUs 1, 2, 3, and 4 are connected to an encryption document bus 7 through encryption plaintext conversion connectors 11, 12, 13, and 14. The encryption plaintext conversion connectors 11, 12, 13, and 14 encrypt the data transmitted from corresponding ECUs 1, 2, 3, and 4 by a prescribed encryption algorithm. They transmit the encrypted data to the encryption document bus 7, and decode the encrypted data from other encryption plaintext conversion connectors transmitted through the encryption document bus 7 to original data. The decoded data are transmitted to corresponding ECUs 1, 2, 3, and 4.

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