Abstract: The communications between terminals are encrypted, and even a gateway device is permitted to analyze the encrypted communications, thereby to incarnate censorship for the prevention of information leakage. A gateway device 3 prepares a public key and a secret key in a pair on the basis of a public key encryption system, and distributes the public key to terminals 2a-2c which are managed by the gateway device 3. The terminals 2a-2c subject a communication packet to a prior-art common key encryption, and they thereafter encrypt a common key with the public key and bestow the encrypted common key on the packet. The gateway device 3 decrypts the common key by using the secret key, censors the packet and returns the packet into a prior-art encrypted packet format, and it thereafter transfers the packet to a network 1c where opposite terminals 2d-2f exist.

Abstract: The communications between terminals are encrypted, and even a gateway device is permitted to analyze the encrypted communications, thereby to incarnate censorship for the prevention of information leakage. A gateway device 3 prepares a public key and a secret key in a pair on the basis of a public key encryption system, and distributes the public key to terminals 2a-2c which are managed by the gateway device 3. The terminals 2a-2c subject a communication packet to a prior-art common key encryption, and they thereafter encrypt a common key with the public key and bestow the encrypted common key on the packet. The gateway device 3 decrypts the common key by using the secret key, censors the packet and returns the packet into a prior-art encrypted packet format, and it thereafter transfers the packet to a network 1c where opposite terminals 2d-2f exist.