Method and system for mobile terminals handing over between clear session and encrypted session communications

A system for clear session and scrambled session communication switch between mobile terminals comprising a scrambled session switch control module and an encryption module set in a mobile terminal, wherein

• the scrambled session switch control module is implemented by a processor in the mobile terminal, and initiates a scrambled session request to an opposite communication end when deciding to switch the current clear session communication to the scrambled session;
• said scrambled session switch control module starts a key negotiation process with the opposite communication end if receiving a scrambled session request response in a preset time sent by the opposite communication end, wherein the scrambled session request response indicates the session scrambled request is accepted by the opposite communication end;
• and then said scrambled session handover control module notifies the encryption module to switch to the scrambled session communication after receiving a scrambled session transferring response, or returns a second scrambled session transferring response after receiving a second scrambled session transferring request initiated by the opposite communication end while notifying the encryption module to switch to the scrambled session communication;
• the encryption module is implemented by the processor in the mobile terminal, and after switching to the scrambled session communication, said encryption module starts the scrambled session, performs an encryption operation on a sent voice/data according to a negotiated key and an encryption algorithm and performs a decryption operation on the received voice or data;
• the scrambled session switch control module is configured to use an end-to-end signaling message to perform the following signaling interaction with the opposite communication end:
• the scrambled session request, the scrambled session request response, the scrambled session transferring request, and the scrambled session transferring response, wherein the end-to-end signaling message includes a user information message, and carries a command type and/or parameter through a user-user field of the user information message in 3GPP protocol, wherein the command type comprises one of the scrambled session request, the scrambled session request response, the scrambled session transferring request, and the scrambled session transferring response.