Abstract:
A system and method of controlling ciphering of call information between a mobile communication terminal and a network initiates the transmission of a ciphering request from the terminal to the network. The network then transmits a ciphering authentication request message to the terminal and the terminal responds by transmitting a ciphering authentication response message to the network. The network then transmits a ciphering activation completion message to the terminal in accordance with the ciphering authentication response message. A system and method for controlling deactivation of ciphering of call information initiates transmission of a ciphering deactivation request from the mobile terminal to the network. The network then performs the ciphering deactivation and transmits a ciphering deactivation completion message to the terminal. Through these systems and methods, the user terminal controls ciphering and deciphering of call information instead of the network.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to ciphering data, and more particularly to a system and method for controlling the ciphering of voice, data, and/or other types of call information in a mobile communication system. 
   2. Background of the Related Art 
   In a GSM (General Systems for Mobile communication) mobile communication system, which performs 2-generation commercial services and a 2.5 generation GPRS (General Packet Radio Service) commercial services in the U.S.A or Europe, a user authentication process and a ciphering activation process are performed. These processes require registration procedures which allow the user of a mobile terminal to notify a network that the user will always use the terminal whenever the terminal is powered on. The registration procedures include an attach registration procedure, a location area update registration procedure, and a routing area update registration procedure. 
   In the course of performing registration procedures, the user authentication process and ciphering activation process are carried out when required by the network. More specifically, when a mobile communication terminal requests registration to the network, the network performs the user authentication process of confirming whether the terminal is an authenticated terminal. The ciphering activation process then determines whether data to be transferred between the terminal and network should be ciphered. 
   The user authentication and ciphering activation processes prevent wireless voice or data from being wiretapped or abused by a third party. Once the ciphering activation is set up between the mobile communication terminal and the network through the ciphering activation process, the call information is ciphered and then transferred therebetween. 
   Two methods may be used to determine whether ciphering should be carried out. In the first method, whether or not ciphering should be performed is determined in the course of registering the mobile communication terminal to the network after the terminal is powered on. In the second method, whether or not ciphering should be performed is determined when the mobile communication terminal wants to perform a specific service. The specific service includes a voice call for talking over the telephone, an SMS (Short Message Service) for transmitting a short message, an SS (Supplementary Service), a PDP (Packet Data Protocol) context activation service for transmitting a GPRS packet, etc. 
   Even when the data transfer procedures are not promised between the mobile communication terminal and the network (that is, even when the ciphering activation process is not carried out in registering the mobile communication terminal to the network and the data is transferred without ciphering), the ciphering activation process can be carried out by the network at the time of executing a service. In so doing, the data is ciphered and transferred from that time point. 
     FIG. 1  shows a general configuration of a mobile communication system which ciphers data transmitted using the GSM standard. This system comprises a mobile communication switching center  150  connected to a public switched telephone network (PSTN) and/or an integrated services digital network (ISDN)  160  for connection to a computer  180  through a general wire phone  170 , or for direct connection to the computer, a plurality of base station controllers  130 ,  140  connected to the mobile communication switching center  150 , a plurality of base stations  111 ,  121  respectively connected to the base station controllers  130 ,  140 , and a plurality of mobile communication terminals  110 ,  120  respectively connected to the base stations  111 ,  121  through wireless links. 
   The base stations  111 ,  121  convert signal formats for wireless and wire links between the mobile communication terminals  110 ,  120  and the mobile communication switching center  150 . The base stations also measure a transmitting/receiving electric field strength (signal intensity) of the terminals and transmit this information to the switching center  150 . 
   The base station controllers  130 ,  140  serve as connection means between the respective functions by elements in the plurality of base stations  111 ,  121  and cell operators. The base station controllers also perform functions such as operational management of the base stations, management of service conditions of hardware and software in the base stations, assignment and construction of resources with respect to call traffic, establishment of information on operations of the base stations, operation and monitoring of the base stations, monitoring of sub-equipments associated with failure, etc. 
   The switching center  150  provides a line switching service to the base station controllers  130 ,  140  and the mobile communication terminals  110 ,  120 , and performs a channel handoff as an additional function. In the mobile communication system described above, when any user transmits or receives a call using terminals  110 ,  120 , the terminal is connected to a respective one of the base stations  111 ,  121 . 
     FIG. 2  shows a flow of messages between a mobile communication terminal and a network for a related-art ciphering activation process, which may be performed in the GSM mobile communication system shown in  FIG. 1 . 
   In GSM and GPRS commercial services provided in Europe, when a mobile communication terminal  10  is powered on, the terminal requests registration to a network  20 . At that time, the terminal transmits a registration request message (Attach Request)  201  to a base station through a wireless link, and the base station transmits the registration request message  201  to a mobile communication switching center through a base station controller. 
   The network  20  receives the registration request message  201  from terminal and determines whether a ciphering activation process should be performed. When it is determined that the ciphering activation process should be performed, the network transmits a ciphering authentication request message  202  to the terminal  10 . This terminal then transmits a ciphering authentication response message  203 , suitable for the ciphering authentication request message  202  from the network, to the network. 
   When the network receives the ciphering authentication response message  203  from the terminal, the network transmits a registration completion message  204  indicative of completion of the registration procedure to the terminal, and thus the ciphering activation process between the terminal and network is finished. 
   When it is determined that the ciphering activation process should not be preformed by the network, the network does not transmit the ciphering authentication request message to terminal. Thus, data is transmitted between the terminal and network without being ciphered. 
     FIG. 3  shows the flow of messages for ciphering a voice call in accordance with a related-art process, which may be performed in the GSM system shown in  FIG. 1 . When terminal  10  transmits a connection management (CM) service request message  301  to the network  20  in order to transmit a voice call, the network determines whether or not a ciphering activation process should be preformed in response to the CM service request message  301 . When it is determined that the ciphering activation process should be performed, the network transmits a ciphering authentication request message  302  to the terminal. 
   The terminal transmits a ciphering authentication response message  303  suitable for the ciphering authentication request message  302  transmitted from the network  20 , and thus the ciphering activation process between the terminal and network is finished. 
   When the network receives the ciphering authentication response message  303  from the terminal, the network transmits a CM service registration completion message  304  indicative of completion of the registration procedure to the terminal. 
   The related-art processes described above with reference to  FIGS. 2 and 3  will be described again with reference to  FIG. 4 . When a GSM-type mobile communication terminal is powered on and is in a wait state  401 , the terminal transmits a registration request message associated with data or a CM service request message associated with a voice call to a network. The network receives the message,  402 , and determines whether a ciphering activation process should be performed or not,  403 . 
   When it is determined that the ciphering activation process should not be performed, the network transmits a registration/CM service completion message indicating that the ciphering activation process should not be performed to the mobile communication terminal,  404 . Thus, the procedure is finished without performing the ciphering activation,  405 . Then, all data to be transmitted after the procedure is transmitted without being ciphered. 
   When it is determined that the ciphering activation process should be performed,  403 , the network generates a RAND (RANDom number) value to compute/store an SRES (Signed RESponse) value,  406 . The network then transmits a ciphering authentication request message to the terminal,  407 . 
   The terminal receives the ciphering authentication request message from the network and transmits a ciphering authentication response message to the network. Then, the network receives the ciphering authentication response message from the terminal,  408 , and compares an SRES value transmitted from the terminal with the SRES value stored in the network to determine whether the two values are equal,  409 . 
   When it is determined that the two values are not equal,  409 , the network transmits a registration/CM service unavailable message indicating that the registration/CM service will not be used any more to the terminal,  410 , and then the procedure is finished,  411 . After the above procedure is finished, the transfer data cannot be subjected to the ciphering service. 
   When it is determined that the two values are equal, the network transmits a registration/CM service available message indicating that the registration/CM service will be used for the terminal  412 , and then the procedure is finished,  413 . After the above procedure is finished, the transfer data can be subjected to the ciphering service. All actions for the ciphering activation process are finished in the course of the voice call. 
   In the related-art methods described with reference to  FIGS. 2-4 , only the network determines whether the ciphering activation process should be performed or not. Therefore, when the ciphering activation process is not initially preformed or data is not specific data, there is a disadvantage in that the ciphering activation process cannot be performed even when a user wants to cipher and transmit and/or receive important data through the network. As a result, the user&#39;s important data is susceptible to being wiretapped or abused externally. Thus, since voice and data can be transmitted without being ciphered, there is a serious disadvantage that the user&#39;s important information can be revealed to a third party. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to solve one or more of the drawbacks of the related-art methods described above. 
   Another object of the present invention to provide a system and method which allows a mobile communication terminal to directly execute a ciphering activation process and a ciphering deactivation process of call information to be transferred between the terminal and a network in the mobile communication system. 
   In accordance with one embodiment of the present invention, a ciphering activation process is directly initiated and executed by the mobile communication terminal. However, the ciphering activation process between the mobile communication terminal and the network is not performed in an initial registration of the terminal to the network or in a starting point of a service. Instead, a user of the terminal who wants to cipher the voice call or data from a specific time point, requests execution of a predetermined ciphering activation process to the network. The network then performs one of two ciphering activation methods in response to the ciphering request at the specific time point transmitted from the mobile communication terminal. 
   In the first method, when the mobile communication terminal requests performance of the ciphering activation process, the network performs the ciphering activation process using a related-art method previously described. 
   In the second method, when the mobile communication terminal transmits a specific value at the same time the ciphering activation request is transmitted, the network computes a key value required for the ciphering activation process based on the specific value and then notifies the completion of the ciphering activation process. From that time, the mobile communication terminal and the network starts ciphering. 
   In addition, when a user has transmitted the data and thus does not want any more ciphering of data, the user terminal transmits a message indicating that the ciphering should stop to the network. The network then transmits a response to the message, and thus the ciphering process between the mobile communication terminal and the network is not performed any more. 
   A method of ciphering data to be transmitted between a mobile communication terminal and a network according to one embodiment of the present invention comprises: a step in which the mobile communication terminal makes a ciphering request of the data to the network; a step in which, when receiving the ciphering request message from the mobile communication terminal, the network transmits a ciphering authentication request message to the mobile communication terminal; a step in which the mobile communication terminal transmits a ciphering authentication response message to the network in response to the ciphering authentication request message; and a step in which the network transmits a ciphering activation completion message to the mobile communication terminal in accordance with the ciphering authentication response message transmitted from the mobile communication terminal. The ciphering authentication request message preferably includes a RAND value. 
   The ciphering request may be made by the mobile communication terminal even during transferring the data between the mobile communication terminal and the network. 
   The ciphering request may be made by the mobile communication terminal even when the data is not transferred between the mobile communication terminal and the network. 
   A method of ciphering a data to be transferred between a mobile communication terminal and a network according to another embodiment of the present invention comprises: a step in which the mobile communication terminal transmits a ciphering request message of the data including a specific value to the network; a step in which the network computes a key value required for a ciphering activation process on the basis of the specific value transmitted from the mobile communication terminal, and transmits a ciphering activation completion message indicative of completion of the ciphering activation process; and a step in which after the ciphering activation completion message is transmitted, the ciphering of the data to be transferred between the mobile communication terminal and the network. Here, the specific value uses an RAND value. 
   A method of ciphering a data to be transferred between a mobile communication terminal and a network according to another embodiment of the present invention comprises: a step in which the mobile communication terminal makes a ciphering request of the data to the network; a step in which the network receives the ciphering request message from the mobile communication terminal and determines whether an RAND value is included in the received ciphering request message or not; a step in which when it is determined that the RAND value is included in the received ciphering request message, the network generates a key value (Kc) required for the ciphering using the RAND value, and then transmits a ciphering activation completion message of the data to the mobile communication terminal; a step in which when it is determined that the RAND value is not included in the received ciphering request message, the network generates a RAND value, computes/stores an SRES value, and transmits a ciphering authentication request message to the mobile communication terminal, depending upon whether a ciphering activation should be performed or not; a step in which the mobile communication terminal transmits a ciphering authentication response message including the SRES value to the network, in response to the ciphering authentication request message transmitted from the network; and a step in which the network compares the SRES value transmitted from the mobile communication terminal with the SRES value stored in the network, and determines whether the ciphering of the data is available or not, depending upon whether the two values are equal to each other. 
   The ciphering authentication request message transmitted from the network to the mobile communication terminal includes the RAND value, and the ciphering authentication response message transmitted from the mobile communication terminal to the network includes the SRES value computed by the mobile communication terminal. 
   The present invention further comprises a step in which when it is determined that the received ciphering request message does not include the RAND value and the ciphering activation should not be performed, the network transmits a ciphering authentication unavailable message of the data to the mobile communication terminal. 
   Furthermore, a method of deactivating a ciphering of a data to be transferred between a mobile communication terminal and a network according to the present invention comprises: a step in which the mobile communication terminal makes a ciphering deactivation request of the data to the network; and a step in which the network performs the ciphering deactivation and transmits a ciphering deactivation completion message to the mobile communication terminal, in response to the ciphering deactivation request message transmitted from the mobile communication terminal. 
   The ciphering deactivation request may be made by the mobile communication terminal even during transferring the data between the mobile communication terminal and the network. 
   The ciphering deactivation request may be made by the mobile communication terminal even when the data is not transferred between the mobile communication terminal and the network. 
   A method of ciphering a voice call to be transmitted between a mobile communication terminal and a network according to another embodiment of the present invention comprises: a step in which the mobile communication terminal makes a ciphering request of the voice call to the network; a step in which, when receiving the ciphering request message from the mobile communication terminal, the network transmits a ciphering authentication request message to the mobile communication terminal; a step in which the mobile communication terminal transmits a ciphering authentication response message to the network in response to the ciphering authentication request message; and a step in which the network transmits a ciphering activation completion message to the mobile communication terminal in accordance with the ciphering authentication response message transmitted from the mobile communication terminal. The ciphering authentication request message preferably includes a RAND value. 
   The ciphering request may be made by the mobile communication terminal even during transferring the voice call between the mobile communication terminal and the network. 
   The ciphering request may be made by the mobile communication terminal even when the voice call is not transferred between the mobile communication terminal and the network. 
   A method of ciphering a voice call to be transferred between a mobile communication terminal and a network according to another embodiment of the present invention comprises: a step in which the mobile communication terminal transmits a ciphering request message of the voice call including a specific value to the network; a step in which the network computes a key value required for a ciphering activation process on the basis of the specific value transmitted from the mobile communication terminal, and transmits a ciphering activation completion message indicative of completion of the ciphering activation process; and a step in which after the ciphering activation completion message is transmitted, the ciphering of the voice call to be transferred between the mobile communication terminal and the network. Here, the specific value uses an RAND value. 
   A method of ciphering a voice call to be transferred between a mobile communication terminal and a network according to another embodiment of the present invention comprises: a step in which the mobile communication terminal makes a ciphering request of the voice call to the network; a step in which the network receives the ciphering request message from the mobile communication terminal and determines whether an RAND value is included in the received ciphering request message or not; a step in which when it is determined that the RAND value is included in the received ciphering request message, the network generates a key value (Kc) required for the ciphering using the RAND value, and then transmits a ciphering activation completion message of the voice call to the mobile communication terminal; a step in which when it is determined that the RAND value is not included in the received ciphering request message, the network generates a RAND value, computes/stores an SRES value, and transmits a ciphering authentication request message to the mobile communication terminal, depending upon whether a ciphering activation should be performed or not; a step in which the mobile communication terminal transmits a ciphering authentication response message including the SRES value to the network, in response to the ciphering authentication request message transmitted from the network; and a step in which the network compares the SRES value transmitted from the mobile communication terminal with the SRES value stored in the network, and determines whether the ciphering of the voice call is available or not, depending upon whether the two values are equal to each other. 
   The ciphering authentication request message transmitted from the network to the mobile communication terminal includes the RAND value, and the ciphering authentication response message transmitted from the mobile communication terminal to the network includes the SRES value computed by the mobile communication terminal. 
   The present invention further comprises a step in which when it is determined that the received ciphering request message does not include the RAND value and the ciphering activation should not be performed, the network transmits a ciphering authentication unavailable message of the voice call to the mobile communication terminal. 
   A method of deactivating a ciphering of a voice call to be transferred between a mobile communication terminal and a network according to another embodiment of the present invention comprises: a step in which the mobile communication terminal makes a ciphering deactivation request of the voice call to the network; and a step in which the network performs the ciphering deactivation and transmits a ciphering deactivation completion message to the mobile communication terminal, in response to the ciphering deactivation request message transmitted from the mobile communication terminal. 
   The ciphering deactivation request may be made by the mobile communication terminal even during transferring the voice call between the mobile communication terminal and the network. 
   The ciphering deactivation request may be made by the mobile communication terminal even when the voice call is not transferred between the mobile communication terminal and the network. 
   A method of ciphering a data and a voice call to be transmitted between a mobile communication terminal and a network according to another embodiment of the present invention comprises: a step in which the mobile communication terminal makes a ciphering request of the data and the voice call to the network; a step in which, when receiving the ciphering request message from the mobile communication terminal, the network transmits a ciphering authentication request message to the mobile communication terminal; a step in which the mobile communication terminal transmits a ciphering authentication response message to the network in response to the ciphering authentication request message; and a step in which the network transmits a ciphering activation completion message to the mobile communication terminal in accordance with the ciphering authentication response message transmitted from the mobile communication terminal. The ciphering authentication request message preferably includes a RAND value. 
   The ciphering request may be made by the mobile communication terminal even during transferring the data and the voice call between the mobile communication terminal and the network. 
   The ciphering request may be made by the mobile communication terminal even when the data and the voice call are not transferred between the mobile communication terminal and the network. 
   A method of ciphering a data and a voice call to be transferred between a mobile communication terminal and a network according to another embodiment of the present invention comprises: a step in which the mobile communication terminal transmits a ciphering request message of the data and the voice call including a specific value to the network; a step in which the network computes a key value required for a ciphering activation process on the basis of the specific value transmitted from the mobile communication terminal, and transmits a ciphering activation completion message indicative of completion of the ciphering activation process; and a step in which after the ciphering activation completion message is transmitted, the ciphering of the data and the voice call to be transferred between the mobile communication terminal and the network. Here, the specific value uses an RAND value. 
   A method of ciphering a data and a voice call to be transferred between a mobile communication terminal and a network according to another embodiment of the present invention comprises: a step in which the mobile communication terminal makes a ciphering request of the data and the voice call to the network; a step in which the network receives the ciphering request message from the mobile communication terminal and determines whether an RAND value is included in the received ciphering request message or not; a step in which when it is determined that the RAND value is included in the received ciphering request message, the network generates a key value (Kc) required for the ciphering using the RAND value, and then transmits a ciphering activation completion message of the data and the voice call to the mobile communication terminal; a step in which when it is determined that the RAND value is not included in the received ciphering request message, the network generates a RAND value, computes/stores an SRES value, and transmits a ciphering authentication request message to the mobile communication terminal, depending upon whether a ciphering activation should be performed or not; a step in which the mobile communication terminal transmits a ciphering authentication response message including the SRES value to the network, in response to the ciphering authentication request message transmitted from the network; and a step in which the network compares the SRES value transmitted from the mobile communication terminal with the SRES value stored in the network, and determines whether the ciphering of the data and the voice call are available or not, depending upon whether the two values are equal to each other. 
   The ciphering authentication request message transmitted from the network to the mobile communication terminal includes the RAND value, and the ciphering authentication response message transmitted from the mobile communication terminal to the network includes the SRES value computed by the mobile communication terminal. 
   The present invention further comprises a step in which when it is determined that the received ciphering request message does not include the RAND value and the ciphering activation should not be performed, the network transmits a ciphering authentication unavailable message of the data and the voice call to the mobile communication terminal. 
   A method of deactivating a ciphering of a data and a voice call to be transferred between a mobile communication terminal and a network according to another embodiment of the present invention comprises: a step in which the mobile communication terminal makes a ciphering deactivation request of the data and the voice call to the network; and a step in which the network performs the ciphering deactivation and transmits a ciphering deactivation completion message to the mobile communication terminal, in response to the ciphering deactivation request message transmitted from the mobile communication terminal. 
   The ciphering deactivation request may be made by the mobile communication terminal even during transferring the data and the voice call between the mobile communication terminal and the network. 
   The ciphering deactivation request may be made by the mobile communication terminal even when the data and the voice call are not transferred between the mobile communication terminal and the network. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view showing a general configuration of a GSM mobile communication system; 
       FIG. 2  is a flowchart of messages performed in a related-art ciphering activation process that may be implemented in the system of  FIG. 1 ; 
       FIG. 3  is a flowchart of messages performed in a related-art procedure for activating a voice call which may take place in the system of  FIG. 1 ; 
       FIG. 4  is a flowchart showing steps performed in a related-art ciphering activation process; 
       FIG. 5  is a flowchart of messages corresponding to one embodiment of a ciphering activation process performed in a mobile communication system according to the present invention; 
       FIG. 6  is a flowchart of messages corresponding to another embodiment of a ciphering activation process which uses a specific key value according to the present invention; 
       FIG. 7  is a flowchart of messages included in one embodiment of a ciphering deactivation process performed in a mobile communication system according to the present invention; and 
       FIGS. 8(A) and 8(B)  are flowcharts showing steps included in a ciphering activation process according to the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Although the embodiments of the present invention will in many cases be described as transmitting user data between a mobile communication terminal and a network, it is emphasized that the present invention is applicable to transferring other types of call information including but not limited to voice call messages transmitted by a user, SMS (Short Message Service) for transmitting short messages, SS (Supplementary Service), and PDP (Packet Data Protocol) context activation service information for transmitting GPRS packets. It is also noted that the GSM-implementation of the mobile communication system and mobile communication terminal described in the embodiments which follow is only illustrative of the present invention. The present invention is widely applicable to any type of mobile communication system and mobile communication terminal such as but not limited to a CDMA mobile communication system and terminal. 
     FIG. 5  shows flow of messages in a ciphering activation process performed in a GSM mobile communication system according to one embodiment of the present invention. For illustrative purposes, some reference numerals used in  FIG. 1  will be used in describing the flow of messages in  FIG. 5 . 
   When a user turns on his mobile communication terminal  10 , the terminal requests registration to the network  20 . 
   The terminal transmits a registration request message  501  to a base station  111  in the network through a wireless link, and the base station transmits a registration request message  501  to a mobile communication switching center  150  through base station controller  130 . 
   When the registration request message  501  is received from terminal  10 , the network determines whether ciphering activation should be performed. When it is determined that ciphering activation should be performed, the network transmits a ciphering authentication request message  502  to the terminal. 
   The terminal then transmits a ciphering authentication response message  503  to the network in response to the ciphering authentication request message  502 . 
   After the ciphering authentication response message  503  is transmitted from the terminal, the network transmits a registration completion message  504  to the terminal indicating completion of the registration procedure. Thus, an initial process for ciphering activation between the mobile communication terminal and network is finished. 
   When it is determined that ciphering activation should not be preformed, the network does not transmit the ciphering authentication request message to the mobile communication terminal. Consequently, the data is normally transferred between the terminal and network without being ciphered,  505 . 
   When the ciphering activation process between the terminal and network has not been formed in the initial registration procedure and it is required to cipher the data during transferring the data form the mobile communication terminal, the terminal transmits a ciphering request message during transferring data  506  to the network. At that time, the ciphering request message is transmitted to the network during transferring data  506  without containing a specific value (RAND: RANDom number). 
   When the network receives the ciphering request message during transferring data  506 , the network performs the ciphering activation process. 
   When the network finishes the ciphering activation process, the network transmits a ciphering authentication request message  507  to the terminal. 
   The terminal transmits a ciphering authentication response message  508  to the network in response to the ciphering authentication request message  507 . 
   When the ciphering authentication response message  508  is transmitted to the network, the network transmits a ciphering activation completion message during transferring data  509  indicative of completion of the ciphering activation process during transferring data to the mobile communication terminal. Thus, all the processes required for ciphering activation during transfer of the data are finished. After all the processes required for ciphering activation, all data to be transferred is ciphered and transmitted. 
     FIG. 6  shows flow of messages for ciphering activation process using a specific key value in the GSM mobile communication system according to another embodiment of the present invention. This flow includes a message  601  which requests ciphering of data during transfer of data from the mobile communication terminal  10  to the network  20 . This message contains a specific value (RAND: RANDom number) and is transmitted to the network  20 . 
   The network generates a key value required for the ciphering activation process based on the specific value RAND transmitted from the terminal, and then transmits a ciphering activation completion message during transferring data  602  which indicates completion of the ciphering activation process to the terminal. 
   After the ciphering activation completion message is transmitted, all the data to be transferred between the terminal and network is ciphered and transmitted. Here, the time point when the terminal transmits the ciphering request message during transfer of the data can be varied. For example, the ciphering request may be made during transfer of data between the mobile communication terminal and network, and the ciphering request may be made when no data is transferred between the terminal and network. 
   When a user does not want any more ciphering of data, even during transfer of the ciphered data between the terminal and network as shown in  FIG. 7 , the terminal of the user transmits a ciphering deactivation request message  701  indicating that the ciphering to the network should be stopped. 
   The ciphering deactivation request message transmitted to the network from the terminal is preferably included in the data to be transferred by the terminal. 
   When the network receives the ciphering deactivation request message  701  during transferring data from the terminal, the network transmits a ciphering deactivation completion message  702  corresponding to the ciphering deactivation request message  701  to the mobile communication terminal. As a result, the ciphering activation process is deactivated. 
     FIGS. 8(A) and 8(B)  are flowcharts showing steps included in the aforementioned ciphering activation process performed in a GSM mobile communication system according to the present invention. 
   When in a wait state or data-transferring state  801 , the network receives a ciphering request message from the GSM-type terminal,  802 , the network determines whether the RAND value is included in the ciphering request message transmitted from the terminal,  803 . 
   In the present invention, transmitting the ciphering request message from the terminal to the network during transfer of data means that the mobile communication terminal has already generated the key value (Kc) for ciphering activation based on the RAND value to be prepared for the ciphering activation. This means that the ciphering activation can be performed only if the network is prepared for ciphering activation. Therefore, in the present invention, the ciphering process of data during data transfer is carried out under the lead of the mobile communication terminal. 
   When it is determined,  803 , that the RAND value is included in the received ciphering request message, the network computes the key value required for the ciphering activation process using the RAND value,  804 , and transmits the ciphering activation completion message to the terminal. When the terminal receives the ciphering activation completion message,  805 , the ciphering activation process  806  is finished,  806 . After the ciphering activation process is finished,  806 , all data is ciphered and then transmitted. 
   When it is determined,  803 , that the RAND value is not included in the ciphering request message, the network determines whether ciphering should be performed for the data to be transferred,  807 . 
   When it is determined,  807 , that the ciphering should not be performed for the data to be transferred, the network transmits the ciphering unavailable message to the mobile communication terminal,  808 , and the procedure is finished,  809 . After the ciphering has become unavailable,  809 , all data continues to be transmitted without being ciphered. 
   When it is determined, in  807 , that the ciphering should be performed for the data to be transferred, the network generates the RAND value, computes/stores an SRES value, and then transmits a ciphering authentication request message to the mobile communication terminal,  811 . At that time, the RAND value is included in the ciphering authentication request message to be transmitted. 
   The terminal receives the ciphering authentication request message from the network and then transmits the ciphering authentication response message to the network. That is, the network receives the ciphering authentication response message from the terminal,  812 . Here, the ciphering authentication response message includes the SRES value computed by the mobile communication terminal. 
   The network compares the SRES value transmitted from the mobile communication terminal with the SRES value stored in the network to determine whether the two values are equal,  813 . 
   When it is determined,  813 , that the two values are not equal, the network transmits a ciphering authentication unavailable message, indicating that ciphering data to be transferred should not be performed any more, to the mobile communication terminal,  814 , and the procedure is finished,  815 . After the procedure is finished,  815 , the service of ciphering the data to be transferred is unavailable. 
   When it is determined,  815 , that the two values are equal, the network transmits a ciphering authentication available message indicating the possibility of ciphering the data to be transferred to the terminal,  816 , and then the procedure is finished,  817 . After the procedure is finished,  817 , the service of ciphering the data to be transferred is available. 
   As described above, according to the present invention, by improving the procedure for ciphering data in the GSM type mobile communication system, it is possible to more reliably transmit the data. In addition, since the mobile communication terminal can also determine whether the ciphering activation process should be preformed, a user can enjoy the ciphering of data or the ciphering deactivation of data, and thus the reliability of data transfer can be enhanced. 
   The present invention may also be implemented in the form of a computer-readable medium storing a computer program for performing any one or more of the methods described above. The computer-readable medium may be removable or permanent, magnetic or optical, or any other type of storage medium known. Examples include but are not limited to floppy disks or diskettes, compact disks, digital tape, hard disks, and digital memories. The program may be implemented in a form executable by a general-purpose processor such as a microprocessor or special-purpose processor such as an ASIC or any other type known. 
   The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.