Abstract: The present invention discloses a network address conversion system and the method thereof, which applies to products of VoIP and is compliant with Q.931 and H.245 protocols used by the videophone. The network address conversion system according to the present invention comprises a private network, a public network, a NAT router and a network server. The NAT router is used to convert the private IP address of the data packet from the private network into a public IP address for entering the public network, and to convert the public IP address of the data packet from the public network into a private IP address for entering the private network. The network server is connected to the public network to execute a public IP address query program, and then transmits the public IP address back to the node in the private network.

Abstract: A method of secure data exchange is applied to a system that includes a server and at least a client. After starting to first connect to the server, the client sends a reset message to the server using an initial key. Once receiving the message, the server verifies the received messages and also uses the initial key to decrypt them. If the verification of the messages is approved, the server generates a first key and sends a key exchange message, which includes the first key and is encrypted by the initial key to the client. Afterward, the client replaces the initial key with the first key in response to the received key exchange message, and meanwhile returns a key confirmation message. If the key confirmation message is approved, the server sends a downloading message to allow the client to retrieve corresponding information. After the information is successfully downloaded, the client sends a finish message to notify the server to await the coming of a next session.