Bridges 30, 40 are interposed between a server 10 or a client 20 having two channels 2ch, one of the two channels making a LAN connection to either the server or the client and the other channel making the LAN connection to the internet. Each of bridges 30, 40 performs a LAN packet scramble, a scramble purpose encryption key management, a scramble purpose encryption key management, and a bridging system authentication. Furthermore, each of bridges 30, 40 performs a LAN packet scramble, a scramble purpose encryption key management, and a bridging system authentication for a packet derived from the server or the client. Furthermore, a composite authentication having a plurality of authentication purpose interfaces is carried out in each of the bridges. Furthermore, a composite authentication having a plurality of authentication purpose interfaces is carried out in each of the bridges. The bridge has a plurality of authentication purpose interfaces to perform a composite authentication. Each of the bridges performs a key management to manage and hold the common key in the authentication and performs a non-decryption file management which manages and holds a file information encrypting and transmitting a payload of an application communicated with a common key at an internal of a transmission side bridge.