Data communication system, center device, host device, and secret information sharing program

A data communication system (1) is provided with: a center device (2) that distributes update data to a host device; and a host device (4) that installs the update data downloaded from the center device in an electronic control device to be reprogrammed. Key distribution of the central device and the host device adopts Diffie-Hellman key sharing (DHE) or elliptic curve Diffie-Hellman key sharing (ECDHE) to share random secret information. The center device encrypts an encryption key for encrypting update data on the basis of the shared secret information, and distributes the encrypted encryption key to the host device. 数据通信系统(1)具备将更新数据分发给主机装置的中心装置(2)、和将从中心装置下载的更新数据安装于重编对象的电子控制装置的主机装置(4)。中心装置以及主机装置的密钥配送采用迪菲-赫尔曼密钥共享(DHE)或者椭圆曲线迪菲-赫尔曼密钥共享(ECDHE)共享随机的秘密信息。中心装置基于该共享的秘密信息对用于加密更新数据的加密密钥进行加密，并将该已加密的加密密钥分发到主机装置。