Patent Publication Number: US-2016249215-A1

Title: Communication control method, authentication server, and user terminal

Description:
TECHNICAL FIELD 
     The present invention relates to a communication control method for confirming that an authentication server is valid for a user terminal between different communication systems, an authentication server therefore, and a user terminal therefore. 
     BACKGROUND ART 
     In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a cellular communication system, it is planned to consider a technology capable of strengthening cooperation between a cellular communication system and a Wireless LAN system (see Non Patent Document 1). 
     For example, a method may be considered where a traffic exchanged between a user terminal and a cellular base station is transitioned (that is, offloaded) to a Wireless LAN system so as to disperse a traffic load in the cellular base station to the Wireless LAN system. 
     PRIOR ART DOCUMENT 
     Non-Patent Document 
     Non Patent Document 1: 3GPP contribution RP-1201455 
     SUMMARY OF THE INVENTION 
     In order to perform an effective offload between a cellular communication system and a Wireless LAN system, it is necessary that authentication servers in these systems work together so as to authenticate a user terminal for which the offload is executed. 
     However, when the authentication server in the other system lacks reliability as a valid authentication server for the user terminal, if the authentication servers work together, information on the user terminal may be leaked, resulting in a user experiencing a possible disadvantage. 
     This problem may occur between the cellular communication system and the Wireless LAN system, and in addition, a similar problem may also occur between other radio communication systems. 
     Therefore, the present invention provides a communication control method with which it is possible to secure reliability that an authentication server in another system is a valid authentication server for a user terminal between authentication servers in different radio communication systems, and provides also an authentication server therefore and a user terminal therefore. 
     MEANS OF SOLVING THE PROBLEMS 
     A communication control method according to an embodiment comprises: a first transmission step of transmitting, by a first authentication server in a first communication system, a first encryption key, to a user terminal; a second transmission step of transmitting, by the user terminal, first terminal information on the user terminal in the first communication system to a second authentication server in a second communication system; a third transmission step of transmitting, by the second authentication server, the first terminal information encrypted by using the first encryption key, to the first authentication server; and a determination step of determining, by the first authentication server, that the second authentication server is a valid authentication server for the user terminal, when the first authentication server has acquired the first terminal information by using a first decryption key capable of decrypting information encrypted by the first encryption key. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a system configuration diagram according to an embodiment. 
         FIG. 2  is a block diagram of a UE (user terminal) according to the embodiment. 
         FIG. 3  is a block diagram of an eNB (cellular base station) according to the embodiment. 
         FIG. 4  is a block diagram of an AP (access point) according to the embodiment. 
         FIG. 5  is a protocol stack diagram of a radio interface in an LTE system. 
         FIG. 6  is a diagram for describing an operation environment according to the embodiment. 
         FIG. 7  is a diagram for describing an operation sequence of setting an authentication target according to the embodiment. 
         FIG. 8  is a diagram for describing an operation sequence of updating an authentication target according to the embodiment. 
         FIG. 9  is a diagram for describing an operation sequence of setting an authentication target according to a modification of the embodiment. 
         FIG. 10  is a diagram for describing an operation sequence of setting the authentication target according to the modification of the embodiment. 
         FIG. 11  is a diagram for describing an operation sequence of setting the authentication target according to the modification of the embodiment. 
         FIG. 12  is a diagram for describing an operation sequence of setting the authentication target according to the modification of the embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENT 
     [Overview of Embodiment] 
     A communication control method according to an embodiment comprises: a first transmission step of transmitting, by a first authentication server in a first communication system, a first encryption key, to a user terminal; a second transmission step of transmitting, by the user terminal, first terminal information on the user terminal in the first communication system to a second authentication server in a second communication system; a third transmission step of transmitting, by the second authentication server, the first terminal information encrypted by using the first encryption key, to the first authentication server; and a determination step of determining, by the first authentication server, that the second authentication server is a valid authentication server for the user terminal, when the first authentication server has acquired the first terminal information by using a first decryption key capable of decrypting information encrypted by the first encryption key. 
     Here, the “valid authentication server for the user terminal” means a authentication server configured to allow the user terminal to connect a communication system. Alternatively, the “valid authentication server for the user terminal” may mean a authentication server controlled by a communication operator that make an agreement of a communication system with a user of the user terminal. 
     In the embodiment, in the second transmission step, the user terminal transmits, together with the first terminal information, the first encryption key received from the first authentication server, to the second authentication server. The communication control method further comprises: a step of encrypting, by the second authentication server, the first terminal information by using the first encryption key received from the user terminal. 
     In the embodiment, a communication operator that manages the first authentication server is different from a communication operator that manages the second authentication server. 
     The communication control method according to the embodiment further comprises: a request step of transmitting, by the user terminal, a request to the first authentication server before the first transmission step, the request being for causing to set the valid authentication server in the second communication system to the first authentication server. The request includes a user encryption key used for encryption of the first encryption key. In the first transmission step, when receiving the request from the user terminal, the first authentication server transmits the first encryption key encrypted by using the user encryption key to the user terminal. 
     In the embodiment, in the request step, when being connected both to the first communication system and the second communication system, the user terminal transmits the request to the first authentication server. 
     In the embodiment of a modification, in the request step, when being connected only to the first communication system, the user terminal transmits the request to the first authentication server. In the second transmission step, when performing a connection to the second communication system, the user terminal transmits the first terminal information. 
     In the embodiment, in the second transmission step, the user terminal transmits, together with the first terminal information, the first encryption key received from the first authentication server, to the second authentication server. In the third transmission step, the second authentication server transmits authentication setting information to the first authentication server together with the first terminal information, the authentication setting information being used for authentication of the user terminal in the second authentication server and being encrypted by using the first encryption key. The communication control method further comprises the steps of: transmitting, by the first authentication server, instead of the user terminal, the authentication setting information to the second authentication server before transitioning a traffic of the user terminal from the first communication system to the second communication system; and authenticating, by the second authentication server, the user terminal, on the basis of the authentication setting information received from the first authentication server. 
     The communication control method according to the embodiment further comprises the steps of: transmitting, by the second authentication server, a second encryption key to the user terminal; transmitting, by the user terminal, second terminal information on the user terminal in the second communication system received from the second authentication server, to the first authentication server; transmitting, by the first authentication server, the second terminal information encrypted by using the second encryption key, to the second authentication server; and determining, by the second authentication server, that the first authentication server is the valid authentication server, when the second authentication server has acquired the second terminal information by using a second decryption key capable of decrypting information encrypted by using the second encryption key. In the third transmission step, the second authentication server transmits the first terminal information encrypted by using the first encryption key to the first authentication server, as a response to the second terminal information from the first authentication server when determining that the first authentication server is the valid authentication server. 
     The communication control method according to another embodiment further comprises the steps of: setting, by the first authentication server, the second authentication server to the valid authentication server for the user terminal; and cancelling, by the first authentication server, setting of the second authentication server, when a frequency of a traffic transition of the user terminal between the first communication system and the second communication system is less than a threshold value. 
     An authentication server according to an embodiment is an authentication server configured to authenticate a connection of a user terminal to a first communication system. The authentication server comprises: a transmitter configured to transmit a first encryption key to the user terminal; a receiver configured to receive encrypted information in which information on the user terminal in the first communication system is encrypted from another authentication server in a second communication system; and a controller configured to determine that the another authentication server is, for the user terminal, a valid authentication server, when the controller acquires information on the user terminal in the first communication system by decrypting the encrypted information by using a first decryption key capable of decrypting information encrypted by the first encryption key. 
     A user terminal according to an embodiment is a user terminal capable of being used in a first communication system and a second communication system. The user terminal comprises: a receiver configured to receive a first encryption key from a first authentication server in the first communication system; and a transmitter configured to transmit, when receiving the first encryption key, terminal information on the user terminal in the first communication system to a second authentication server in the second communication system. The first encryption key is used for encryption of the terminal information. 
     Embodiment 
     Hereinafter, with reference to the accompanying drawings, the following description will be provided for embodiment in a case where a cellular communication system (an LTE system) configured in compliance with 3GPP standards is allowed to cooperate with a Wireless LAN (WLAN) system. 
     (System Configuration) 
       FIG. 1  is a configuration diagram of a system according to the embodiment. As illustrated in  FIG. 1 , the cellular communication system includes a plurality of UEs (User Equipments)  100 , E-UTRAN (Evolved Universal Terrestrial Radio Access Network)  10 , and EPC (Evolved Packet Core)  20 . The E-UTRAN  10  corresponds to a radio access network. The EPC  20  corresponds to a core network. 
     The UE  100  is a mobile radio communication device and performs radio communication with a cell with which a connection is established. The UE  100  corresponds to the user terminal. The UE  100  is a terminal (a dual terminal) supporting communication methods of both cellular communication and WLAN communication. 
     The E-UTRAN  10  includes a plurality of eNBs  200  (evolved Node-Bs). The eNB  200  corresponds to a cellular base station. The eNB  200  manages one or a plurality of cells (large cell(s)) and performs radio communication with the UE  100  having established a connection (RRC connection) with the cell of the eNB  200 . 
     It is noted that the “cell” is used as a term indicating a minimum unit of a radio communication area, and is also used as a term indicating a function of performing radio communication with the UE  100 . 
     The eNB  200 , for example, has a radio resource management (RRM) function, a routing function of user data, and a measurement control function for mobility control and scheduling. 
     In addition, the eNBs  200  are connected mutually via an X2 interface. Furthermore, the eNB  200  is connected to the MME/S-GW  500  included in the EPC  20  via an Si interface. 
     The EPC  20  includes a plurality of MMEs (Mobility Management Entities)/S-GWs (Serving-Gateways)  500 . The MME is a network node for performing various mobility controls, for example, for the UE  100  and corresponds to a control station. The S-GW is a network node that performs transfer control of user data and corresponds to a mobile switching center. 
     The WLAN system includes a WLAN AP (hereinafter, referred to as an “AP”)  300 . The WLAN system is configured in compliance with IEEE 802.11 standards, for example. The AP  300  performs communication with the UEs  100  in a frequency band (WLAN frequency band) different from a cellular frequency band. The AP  300  is connected to the EPC  20  via a router or the like. 
     It is noted that the eNB  200  and the AP  300  may be individually located, and in addition, may be collocated. As one mode of the “Collocated”, the eNB  200  and the AP  300  may be directly connected to each other through any interface of an operator. 
     The EPC  20  further includes a cellular authentication server  600  that performs a network authentication of the UE  100  in the cellular communication system and a WLAN authentication server  700  that performs a network authentication of the UE  100  in the WLAN system. 
     The cellular authentication server  600  includes a network interface, a memory, and a processor. The memory and the processor configure a controller. The network interface configures a transmitter and a receiver. The network interface and the processor execute various types of processes and various types of communication protocols described later. It is noted that the WLAN authentication server  700  also has configuration similar to that of the cellular authentication server  600 . 
     When succeeding the network authentication by the cellular authentication server  600 , the UE  100  becomes connectable to the cellular communication system. Further, when succeeding the network authentication by the WLAN authentication server  700 , the UE  100  becomes connectable to the WLAN system. 
     Next, the configurations of the UE  100 , the eNB  200 , and the AP  300  will be described. 
       FIG. 2  is a block diagram of the UE  100 . As illustrated in  FIG. 2 , the UE  100  has antennas  101  and  102 , a cellular transceiver  111 , a WLAN transceiver  112 , a user interface  120 , a GNSS (Global Navigation Satellite System) receiver  130 , a battery  140 , a memory  150 , and a processor  160 . The memory  150  and the processor  160  constitute a controller. The UE  100  may not have the GNSS receiver  130 . Furthermore, the memory  150  may be integrally formed with the processor  160 , and this set (that is, a chipset) may be called a processor  160 ′. 
     The antenna  101  and the cellular transceiver  111  are used for transmitting and receiving cellular radio signals. The cellular transceiver  111  converts a baseband signal output by the processor  160  to a cellular radio signal, and transmits it from the antenna  101 . The cellular transceiver  111  also converts a cellular radio signal received by the antenna  101  to a baseband signal, and outputs it to the processor  160 . 
     The antenna  102  and the WLAN transceiver  112  are used for transmitting and receiving WLAN radio signals. The WLAN transceiver  112  converts a baseband signal output by the processor  160  to a WLAN radio signal, and transmits it from the antenna  102 . The WLAN transceiver  112  also converts a WLAN radio signal received by the antenna  102  to a baseband signal, and outputs it to the processor  160 . 
     A MAC address (hereinafter, referred to as a “WLAN MAC-ID”) is allocated to the WLAN transceiver  112  as an identifier of the UE  100  in the WLAN system. A WLAN radio signal transmitted and received by the WLAN transceiver  112  includes the WLAN MAC-ID. 
     The user interface  120  is an interface with a user carrying the UE  100 , and includes, for example, a display, a microphone, a speaker, various buttons and the like. The user interface  120  receives an input from a user and outputs a signal indicating the content of the input to the processor  160 . The GNSS receiver  130  receives a GNSS signal in order to obtain location information indicating a geographical location of the UE  100 , and outputs the received signal to the processor  160 . The battery  140  accumulates a power to be supplied to each block of the UE  100 . 
     The memory  150  stores a program to be executed by the processor  160  and information to be used for a process by the processor  160 . The processor  160  includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like of the baseband signal, and a CPU that performs various processes by executing the program stored in the memory  150 . The processor  160  may further include a codec that performs encoding and decoding of sound and video signals. The processor  160  implements various processes and various communication protocols described later. 
       FIG. 3  is a block diagram of the eNB  200 . As illustrated in  FIG. 3 , the eNB  200  has an antenna  201 , a cellular transceiver  210 , a network interface  220 , a memory  230 , and a processor  240 . The memory  230  and the processor  240  constitute a controller. Note that, the memory  230  may be integrally formed with the processor  240 , and this set (that is, a chipset) may be called a processor. 
     The antenna  201  and the cellular transceiver  210  are used to transmit and receive a radio signal. The cellular transceiver  210  converts a baseband signal output by the processor  240  to a cellular radio signal, and transmits it from the antenna  201 . The cellular transceiver  210  also converts a cellular radio signal received by the antenna  201  to a baseband signal, and outputs it to the processor  240 . 
     The network interface  220  is connected to a neighboring eNB  200  via the X2 interface and is connected to the MME/S-GW  500  via the Si interface. Further, the network interface  220  is used in communication with the AP  300  via the EPC  20 . 
     The memory  230  stores a program to be executed by the processor  240  and information to be used for a process by the processor  240 . The processor  240  includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like of the baseband signal and a CPU that performs various processes by executing the program stored in the memory  230 . The processor  240  implements various processes and various communication protocols described later. 
       FIG. 4  is a block diagram of the AP  300 . As illustrated in  FIG. 3 , the AP  300  has an antenna  301 , a WLAN transceiver  311 , a network interface  320 , a memory  330 , and a processor  340 . The memory  330  and the processor  340  constitute a controller. Note that, the memory  330  may be integrally formed with the processor  340 , and this set (that is, a chipset) may be called a processor. 
     The antenna  301  and the WLAN transceiver  311  are used to transmit and receive a WLAN radio signal. The WLAN transceiver  311  converts a baseband signal output by the processor  340  to a WLAN radio signal, and transmits it from the antenna  301 . The WLAN transceiver  311  also converts a WLAN radio signal received by the antenna  301  to a baseband signal, and outputs it to the processor  340 . 
     The network interface  420  is connected to the EPC  20  via a router etc. Further, the network interface  320  is used in communication with the eNB  200  via the EPC  20 . 
     The memory  330  stores a program to be executed by the processor  340  and information to be used for a process by the processor  340 . The processor  340  includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like of the baseband signal and a CPU that performs various processes by executing the program stored in the memory  330 . 
       FIG. 5  is a protocol stack diagram of a radio interface in the cellular communication system. As illustrated in  FIG. 5 , the radio interface protocol is classified into a layer 1 to a layer 3 of an OSI reference model, wherein the layer 1 is a physical (PHY) layer. The layer 2 includes a MAC (Medium Access Control) layer, an RLC (Radio Link Control) layer, and a PDCP (Packet Data Convergence Protocol) layer. The layer 3 includes an RRC (Radio Resource Control) layer. 
     The physical layer performs encoding and decoding, modulation and demodulation, antenna mapping and demapping, and resource mapping and demapping. Between the physical layer of the UE  100  and the physical layer of the eNB  200 , data is transmitted through the physical channel. 
     The MAC layer performs preferential control of data, and a retransmission process and the like by hybrid ARQ (HARQ). Between the MAC layer of the UE  100  and the MAC layer of the eNB  200 , data is transmitted via a transport channel. The MAC layer of the eNB  200  includes a scheduler for selecting a transport format of an uplink and a downlink (a transport block size, a modulation and coding scheme and the like) and a resource block to be assigned. 
     The RLC layer transmits data to an RLC layer of a reception side by using the functions of the MAC layer and the physical layer. Between the RLC layer of the UE  100  and the RLC layer of the eNB  200 , data is transmitted via a logical channel. 
     The PDCP layer performs header compression and decompression, and encryption and decryption. 
     The RRC layer is defined only in a control plane. Between the RRC layer of the UE  100  and the RRC layer of the eNB  200 , a control message (an RRC message) for various types of setting is transmitted. The RRC layer controls the logical channel, the transport channel, and the physical channel in response to establishment, re-establishment, and release of a radio bearer. When a connection (an RRC connection) is established between the RRC of the UE  100  and the RRC of the eNB  200 , the UE  100  is in a connected state (RRC connected state), and when the RRC connection is not established, the UE  100  is in an idle state (RRC idle state). 
     A NAS (Non-Access Stratum) layer positioned above the RRC layer performs session management and mobility management, for example. 
     (Operation According to Embodiment) 
     Next, an operation according to the embodiment will be described. 
     (1) Setting of Authentication Target 
     An operation sequence of setting an authentication target according to the present embodiment will be described by using  FIG. 6  and  FIG. 7 .  FIG. 6  is a diagram for describing an operation environment according to the embodiment.  FIG. 7  is a diagram for describing an operation sequence of setting the authentication target according to the embodiment. 
     Description proceeds with an assumption that in the present embodiment, the UE  100  succeeds the network authentication by the cellular authentication server  600  and thus is connectable to the cellular communication system. In addition, description proceeds with an assumption that the UE  100  succeeds the network authentication by the WLAN authentication server  700  and thus is connectable to the WLAN communication system. That is, the UE  100  is connected both to the cellular communication system and the WLAN communication system. Therefore, as shown in  FIG. 6 , the UE  100  performs the communication with the cellular authentication server  600  by way of the eNB  200 , and performs the communication with the WLAN authentication server  700  by way of the AP  300 . Further, the cellular authentication server  600  and the WLAN authentication server  700  perform the communication via a network. 
     It is noted that in the present embodiment, a communication operator that manages the cellular authentication server  600  is different from a communication operator that manages the WLAN authentication server. 
     As shown in  FIG. 7 , in step S 101 , the UE  100  determines to cause the authentication servers (the cellular authentication server  600  and the WLAN authentication server  700 ) to perform setting of the authentication target for pre-authentication between the cellular communication system and the WLAN communication system. Specifically, the UE  100  determines to cause the cellular authentication server  600  to set the WLAN authentication server  700  to the authentication target (that is, a valid authentication server) for the UE  100  in the WLAN communication system, and determines to cause the WLAN authentication server  700  to set the cellular authentication server  600  to the authentication target for the UE  100  in the cellular communication system. 
     For example, when a frequency of a traffic transition between the cellular communication system and the WLAN communication system exceeds a threshold value, the UE  100  determines to cause the authentication server to perform the setting for pre-authentication. Alternatively, a user of the UE  100  selects a communication system (that is, a radio service) in which the pre-authentication for performing offload between different communication systems is permitted. The UE  100  determines to cause the authentication server to perform the setting for the pre-authentication between the selected communication systems. 
     The UE  100  generates a public key  1  and a secret key  1 . The secret key  1  is a key capable of decrypting information encrypted by the public key  1 . Further, the UE  100  generates a public key  2  and a secret key  2 . The secret key  2  is a key capable of decrypting information encrypted by the public key  2 . 
     In step S 102 , the UE  100  transmits a public key transmission request for causing the authentication server of the UE  100  in a different communication system to set to the authentication target, to the cellular authentication server  600 . The cellular authentication server  600  receives the public key transmission request. 
     The public key transmission request is to request a public key generated by the authentication server. The public key is a key for confirming that the authentication server is valid for the UE  100 . In the present embodiment, the public key transmission request in step S 102  includes the public key  1 . 
     In the present embodiment, when connecting both to the cellular communication system and the WLAN communication system, the UE  100  transmits the public key transmission request. 
     In step S 103 , the cellular authentication server  600  generates a public key  3  and a secret key  3 , in response to reception of the public key transmission request. 
     The cellular authentication server  600  uses the public key  1  included in the public key transmission request to encrypt the generated public key  3 . 
     In step S 104 , the cellular authentication server  600  transmits the public key  3  encrypted by using the public key  1 , to the UE  100 . The UE  100  receives the encrypted public key  3 . 
     In step S 105 , the UE  100  uses the secret key  1  to decrypt the encrypted public key  3 . Thus, the UE  100  acquires the public key  3 . 
     In step S 106 , similarly to step S 102 , the UE  100  transmits the public key transmission request to the WLAN authentication server  700 . The WLAN authentication server  700  receives the public key transmission request. Similarly to step S 102 , the public key transmission request includes the public key  2 . 
     In step S 107 , similarly to step S 103 , the WLAN authentication server  700  generates a public key  4  and a secret key  4 , in response to reception of the public key transmission request. 
     The WLAN authentication server  700  uses the public key  2  included in the public key transmission request to encrypt the generated public key  4 . 
     In step S 108 , similarly to step S 104 , the WLAN authentication server  700  transmits the public key  4  encrypted by using the public key  2 , to the UE  100 . The UE  100  receives the encrypted public key  4 . 
     In step S 109 , similarly to step S 105 , the UE  100  uses the secret key  2  to decrypt the encrypted public key  4 . Thus, the UE  100  acquires the public key  4 . 
     In step S 110 , the UE  100  transmits a first setting request to the cellular authentication server  600 . Specifically, the UE  100  uses the public key  3  to encrypt the first setting request, and transmits the encrypted first setting request to the cellular authentication server  600 . The cellular authentication server  600  receives the first setting request. 
     The first setting request is a request to cause to set the authentication server of the UE  100  in a different communication system to the authentication target, and is a request transmitted from the UE  100  to the authentication server. 
     In the present embodiment, the first setting request includes the public key (public key  4 ) acquired from the authentication server in another communication system, the information (WLAN authentication server information) on the authentication server in the other communication system, and the information (WLAN terminal information) on the UE  100  in the other communication system. 
     In the present embodiment, the WLAN authentication server information includes a destination (address) of the WLAN authentication server  700 . Further, the WLAN terminal information includes an identifier (WLAN MAC-ID) of the UE  100  in the WLAN communication system. 
     In step S 111 , similarly to step S 110 , the UE  100  transmits the first setting request to the WLAN authentication server  700 . The UE  100  uses the public key  4  to encrypt the first setting request, and transmits the encrypted first setting request to the WLAN authentication server  700 . The WLAN authentication server  700  receives the first setting request. 
     Here, the first setting request in step S 111  includes the public key  3 , the cellular authentication server information, and the cellular terminal information. The cellular authentication server information includes a destination (address) of the cellular authentication server  600 . The cellular terminal information includes an identifier (UE-ID) of the UE  100  in the cellular communication system. 
     In step S 112 , the cellular authentication server  600  uses the secret key  3  to decrypt the encrypted first setting request. As a result, the cellular authentication server  600  acquires the public key  4 , the WLAN authentication server information, and the WLAN terminal information included in the first setting request. 
     In step S 113 , the WLAN authentication server  700  uses the secret key  4  to decrypt the encrypted first setting request. As a result, the WLAN authentication server  700  acquires the public key  3 , the cellular authentication server information, and the cellular terminal information included in the first setting request. 
     In step S 114 , the cellular authentication server  600  transmits a second setting request to the WLAN authentication server  700 . Specifically, the cellular authentication server  600  uses the public key  4  acquired from the UE  100  to encrypt the second setting request, and transmits the encrypted second setting request to the WLAN authentication server  700 . The WLAN authentication server  700  receives the second setting request. 
     The second setting request is a request to cause to set the authentication server of the UE  100  in a different communication system to the authentication target, and is a request transmitted from the authentication server to another authentication server. 
     In the present embodiment, the second setting request includes the information (WLAN terminal information) on the UE  100  in the communication system of a transmission target and the information (cellular authentication server information) on the authentication server. 
     Here, the terminal information includes an identifier (WLAN MACID) of the UE  100  in the communication system. Further, the cellular authentication server information includes authentication setting information used for authentication of the UE  100  in the cellular authentication server  600  (network authentication of the UE  100 ). Therefore, when the WLAN authentication server  700 , instead of the UE  100 , transmits the authentication setting information to the cellular authentication server  600  before the offload, the cellular authentication server  600  is capable of authenticating the UE  100 , on the basis of the authentication setting information received from the WLAN authentication server  700 . As a result, it is possible to omit the transmission of the authentication setting information from the UE  100  and perform a smooth offload. 
     It is noted that the second setting request may include the WLAN authentication server information acquired from the UE  100 . 
     On the other hand, the WLAN authentication server  700  uses the secret key  4  to decrypt the encrypted second setting request. As a result, the WLAN authentication server  700  acquires the WLAN terminal information and the cellular authentication server information. When acquiring from the cellular authentication server  600  an identifier of the UE  100  in the WLAN communication system, where the identifier is information not normally known by the cellular authentication server  600  that is the authentication server in a different communication system, the WLAN authentication server  700  determines that the cellular authentication server  600  is, for the UE  100 , a valid authentication server. 
     In step S 115 , the WLAN authentication server  700  transmits the response (second setting response) to the second setting request, to the cellular authentication server  600 . Specifically, the cellular authentication server  600  uses the public key  3  acquired from the UE  100  to encrypt the second setting response, and transmits the encrypted second setting response to the cellular authentication server  600 . The cellular authentication server  600  receives the second setting response. 
     The second setting response is a response to the second setting request. Further, similarly to the second setting request, the second setting response is also a request to cause to set the authentication server of the UE  100  in a different communication system to the authentication target, and is a request transmitted from the authentication server to another authentication server. 
     In the present embodiment, the second setting response includes the cellular terminal information, the WLAN authentication server information, and the response to the second setting request. The response may include information indicating that the WLAN authentication server  700  determines that the cellular authentication server  600  is, for the UE  100 , the valid authentication server. 
     The cellular authentication server  600  uses the secret key  3  to decrypt the encrypted second setting response. As a result, the cellular authentication server  600  acquires the cellular terminal information, the WLAN authentication server information, and the response to the second setting request. When acquiring an identifier of the UE  100  in the cellular communication system, where the identifier is information not normally known by the WLAN authentication server  700  that is the authentication server in a different communication system, the cellular authentication server  600  determines that the WLAN authentication server  700  is, for the UE  100 , the valid authentication server. 
     In step S 116 , the cellular authentication server  600  transmits a response to the second setting response to the WLAN authentication server  700 . Specifically, the cellular authentication server  600  uses the public key  4  to encrypt the response to the second setting response, and transmits the encrypted second setting response to the WLAN authentication server  700 . The WLAN authentication server  700  receives the response to the second setting response. The response may include information indicating that the cellular authentication server  600  determines that the WLAN authentication server  700  is, for the UE  100 , the valid authentication server. 
     The WLAN authentication server  700  uses the secret key  4  to decrypt the encrypted response. As a result, the WLAN authentication server  700  acquires the response. 
     In step S 117 , the cellular authentication server  600  sets the WLAN authentication server  700  to the (pre-)authentication target of the UE  100 . 
     In step S 118 , the WLAN authentication server  700  sets the cellular authentication server  600  to the (pre-)authentication target of the UE  100 . 
     It is noted that the cellular authentication server  600  and the WLAN authentication server  700  may report, to the UE  100 , that the both servers  600  and  700  mutually set themselves to the authentication target of the UE  100 . 
     Thereafter, when it is determined that the traffic of the UE  100  is transitioned from the cellular communication system to the WLAN communication system, the cellular authentication server  600  is capable of requesting the network authentication of the UE  100 , to the WLAN authentication server  700  to which the authentication target of the UE  100  in the WLAN communication system is set. Further, likewise, the WLAN authentication server  700  is also capable of requesting the network authentication of the UE  100 , to the cellular authentication server  600 . 
     (2) Update of Authentication Target 
     Next, an operation sequence of updating an authentication target according to the present embodiment will be described by using  FIG. 8 .  FIG. 8  is a diagram for describing an operation sequence of updating an authentication target according to the embodiment. 
     As shown in  FIG. 8 , in step S 201 , the cellular authentication server  600  generates a public key  5  and a secret key  5 . For example, when a predetermined period passes since the public key  3  is generated, the cellular authentication server  600  generates the public key  5  and the secret key  5 . 
     In step S 202 , the cellular authentication server  600  transmits an update request to update the authentication target, to the WLAN authentication server  700 . Specifically, the cellular authentication server  600  uses the public key  4  to encrypt the update request, and transmits the encrypted update request to the WLAN authentication server  700 . The WLAN authentication server  700  receives the update request. 
     The update request includes the WLAN terminal information, the cellular authentication server information, and the public key  5 . The WLAN terminal information and the cellular authentication server information are information similar to the above-described second setting request. 
     The WLAN authentication server  700  uses the secret key  4  to decrypt the update request. As a result, the WLAN authentication server  700  acquires the WLAN terminal information, the cellular authentication server information, and the public key  5 . 
     In step S 203 , the WLAN authentication server  700  generates a public key  6  and a secret key  6 . When a predetermined time passes since the public key  4  is generated, the WLAN authentication server  700  may generate the public key  6  and the secret key  6 , and may generate the public key  6  and the secret key  6  in response to reception of the update request from the cellular authentication server  600 . 
     In step S 204 , the WLAN authentication server  700  transmits a response to the update request. Specifically, the WLAN authentication server  700  uses the public key  3  to encrypt the response, and transmits the encrypted response to the cellular authentication server  600 . The cellular authentication server  600  receives the response. 
     The update response includes the cellular terminal information, the WLAN authentication server information, the public key  6 , and the response to the update request. The cellular terminal information and the WLAN authentication server information are information similar to the above-described second setting response. The response may include information indicating that the WLAN authentication server  700  determines that the cellular authentication server  600  is, for the UE  100 , the valid authentication server. 
     The cellular authentication server  600  uses the secret key  3  to decrypt the update response. As a result, the cellular authentication server  600  acquires the cellular terminal information, the WLAN authentication server information, the public key  6 , and the response. 
     In step S 205 , the cellular authentication server  600  transmits a response to the update response, to the WLAN authentication server. Specifically, the cellular authentication server  600  uses the public key  4  to encrypt the response to the update response, and transmits the response to the encrypted update response, to the WLAN authentication server  700 . 
     The WLAN authentication server  700  uses the secret key  4  to decrypt the encrypted response. As a result, the WLAN authentication server  700  acquires the response. The response may include information indicating that the cellular authentication server  600  determines that the WLAN authentication server  700  is, for the UE  100 , the valid authentication server. 
     In step S 206 , the cellular authentication server  600  updates by setting the WLAN authentication server  700  to the authentication target of the UE  100 . Further, the cellular authentication server  600  abandons the public key  4  and holds the public key  6  instead of the public key  4 . 
     In step S 207 , similarly to step S 206 , the WLAN authentication server  700  updates by setting the cellular authentication server  600  to the authentication target of the UE  100 . Further, the WLAN authentication server  700  abandons the public key  3  and holds the public key  5  instead of the public key  3 . 
     (3) Summary 
     In the present embodiment, the cellular authentication server  600  transmits the public key  3  to the UE  100 . The UE  100  transmits the public key  3  and the cellular terminal information to the WLAN authentication server  700 . The WLAN authentication server  700  uses the public key  3  to encrypt the cellular terminal information. The WLAN authentication server  700  transmits the encrypted cellular terminal information to the cellular authentication server  600 . When being capable of acquiring the cellular terminal information by using the secret key  3 , the cellular authentication server  600  determines that the WLAN authentication server  700  is, for the UE  100 , the valid authentication server. As a result, when receiving, from the WLAN authentication server  700 , the public key  3  that is the information not known by the WLAN authentication server  700  and the cellular terminal information, the cellular authentication server  600  is also capable of regarding that the UE  100  guarantees that the WLAN authentication server  700  is, for the UE  100 , the valid authentication server. Further, in the same way, when receiving from the cellular authentication server  600  the public key  4  that is the information not known by the cellular authentication server  600  and the WLAN terminal information, the WLAN authentication server  700  is capable of regarding that the UE  100  guarantees that the cellular authentication server  600  is, for the UE  100 , the valid authentication server. Therefore, between the cellular authentication server  600  and the WLAN authentication server  700 , it is possible to confirm the authentication server is valid for the UE  100 , it is possible to ensure the reliability that between the different communication systems, the cellular authentication server  600  and the WLAN authentication server  700  are, for the UE  100 , the valid authentication server. 
     In the present embodiment, a communication operator that manages the cellular authentication server  600  is different from a communication operator that manages the WLAN authentication server  700 . The communication operators are different, and thus, even when an inquiry destination to inquire the authentication server of the UE  100  in the other communication system is not known, or even when there is no answer to the inquiry, it is possible to confirm, according to the above embodiment, the valid authentication server. 
     In the present embodiment, before receiving the public key  3  from the cellular authentication server  600 , the UE  100  transmits the public key transmission request to the cellular authentication server  600 . The public key request includes the public key  1  used for encrypting the public key  3 . When receiving the public key transmission request from the UE  100 , the cellular authentication server  600  transmits the public key  3  encrypted by using the public key  1 , to the UE  100 . As a result, a possibility that another authentication server acquires the public key  3  transmitted to the UE  100  decreases, and thus, it is possible to improve the reliability that the authentication server is, for the UE  100 , the valid authentication server. 
     In the present embodiment, when connecting both to the cellular communication system and the WLAN communication system, the UE  100  transmits the public key transmission request, to the cellular authentication server  600 . As a result, it is possible to transmit the public key  3  to the WLAN authentication server  700  immediately after acquiring the public key  3  from the cellular authentication server  600 , and thus, it is possible to transmit the public key  3  within an expiration even when the public key  3  has the expiration. 
     In the present embodiment, the WLAN authentication server  700  is capable of transmitting, together with the cellular terminal information, the authentication setting information that is encrypted by using the public key  3  and is used for authenticating the UE  100  in the WLAN authentication server  700 , to the cellular authentication server  600 . The cellular authentication server  600 , instead of the UE  100 , is capable of transmitting, before the traffic of the UE  100  is offloaded from the cellular communication system to the WLAN communication system, the authentication setting information to the WLAN authentication server  700 . The WLAN authentication server  700  is capable of performing the network authentication of the UE  100  on the basis of the authentication setting information received from the cellular authentication server  600 . As a result, the pre-authentication is performed before the offload, and thus, it is possible to execute a smooth offload. 
     In the present embodiment, when determining that the cellular authentication server  600  is the valid authentication server, the WLAN authentication server  700  is capable of transmitting, as the response to the WLAN terminal information (second setting request), the encrypted cellular terminal information, to the cellular authentication server  600 . As a result, the WLAN authentication server  700  is capable of preventing transmission of the WLAN terminal information to the authentication server that is not the valid authentication server. 
     It is noted that an operation for only either one of the cellular authentication server  600  and the WLAN authentication server  700  is described and the other operation is not described, where appropriate; however, it is naturally possible to obtain a similar operation and publication also in the other authentication server. The same applies, below. 
     Modification of Embodiment 
     Next, an operation according to a modification of the embodiment will be described. 
     (1) Setting of Authentication Target 
     An operation sequence of setting an authentication target according to a modification of the present embodiment will be described by using  FIG. 9  to  FIG. 12 .  FIG. 9  to  FIG. 12  are diagrams for describing an operation sequence of setting the authentication target according to the modification of the embodiment. It is noted that a description will be provided while focusing on a portion different from the above-described embodiment, and a description of a similar portion will be omitted, where necessary. 
     In the above-described embodiment, the UE  100  is connected both to the cellular communication system and the WLAN communication system. In the modification, the UE  100  is connected only to one of the cellular communication system and the WLAN communication system. Specifically, as shown in  FIG. 9 , the UE  100  is connected to the cellular communication system. 
     In step S 301 , the UE  100  determines whether or not a communication system to which the UE  100  is connected is a communication system in which the pre-authentication is permitted. For example, a user of the UE  100  selects a communication system in which the pre-authentication for performing offload between different communication systems is permitted. Alternatively, the user of the UE  100  (previously) registers the communication system in which the pre-authentication is permitted. When the communication system to which the UE  100  is connected is selected as the communication system in which the pre-authentication is permitted (or is registered), the UE  100  determines that the communication system to which the UE  100  is connected is the communication system in which the pre-authentication is permitted, and executes a process of step S 302 . 
     It is noted that when the communication system to which the UE  100  is connected is not the communication system in which the pre-authentication is permitted, the UE  100  is capable of ending the process. 
     In the present modification, description proceeds with an assumption that the cellular communication system is the communication system in which the pre-authentication is permitted. 
     Steps S 302  to S 305  correspond to steps S 102  to S 105  in  FIG. 7 . 
     Next, as shown in  FIG. 10 , description proceeds with an assumption that the UE  100  is disconnected with the cellular communication system and is connected to the WLAN communication system. 
     In step S 306 , similarly to step S 301 , the UE  100  determines whether or not a communication system to which the UE  100  is connected is a communication system in which the pre-authentication is permitted. In the present modification, description proceeds with an assumption that the WLAN communication system is the communication system in which the pre-authentication is permitted. 
     Steps S 307  to S 310  correspond to steps S 106  to S 109  in  FIG. 7 . 
     As shown in  FIG. 11 , in step S 311 , the UE  100  determines whether or not, between different communication systems, the respective authentication servers set the authentication server of the partner system to the authentication target. Specifically, the UE  100  determines whether or not the authentication server (WLAN authentication server  700 ) in the WLAN communication system connected thereto sets the cellular authentication server  600  to the authentication target. 
     For example, when not transmitting to the WLAN authentication server  700  the public key  3  from the cellular authentication server  600  that is the authentication server in the cellular communication system in which the pre-authentication is permitted, the UE  100  determines that the WLAN authentication server  700  does not yet set the authentication target in the cellular communication system. Alternatively, the UE  100  acquires, from the authentication server, authentication target information indicating the set authentication target, and on the basis of the authentication target information, makes a determination. In the present modification, description proceeds with an assumption that the UE  100  determines that the WLAN authentication server  700  does not yet set the authentication target. 
     It is noted that when determining that the WLAN authentication server  700  sets the authentication target in the cellular communication system, the UE  100  is capable of ending the process. 
     Steps S 312  and S 313  correspond to steps S 111  and S 113  in  FIG. 7 . 
     Next, as shown in  FIG. 12 , description proceeds with an assumption that the UE  100  is disconnected with the cellular communication system and is connected to the WLAN communication system. 
     As shown in  FIG. 12 , in step S 314 , similarly to step S 311 , the UE  100  determines whether or not the authentication server (cellular authentication server  600 ) in the cellular communication system connected thereto sets the WLAN authentication server  700  to the authentication target. In the present modification, description proceeds with an assumption that the UE  100  determines that the cellular authentication server  600  does not yet set the authentication target. 
     Steps S 315  to S 321  correspond to steps S 110 , S 112 , and S 114  to S 118  in  FIG. 7 . 
     It is noted that when determining that the cellular authentication server  600  sets the authentication target in the WLAN communication system, the UE  100  is capable of ending the process. 
     (2) Summary 
     In the present modification, when connecting only to the cellular communication system, the UE  100  transmits the public key transmission request, to the cellular authentication server  600 . When performing the connection to the WLAN communication system, the UE  100  transmits the public key  3  acquired from the cellular authentication server  600 , to the WLAN authentication server  700 . As a result, even when the UE  100  is not connected to the both communication systems (that is, connected only to either one communication system), it is capable of confirming that the cellular authentication server  600  is a valid authentication server for the UE  100 . 
     Other Embodiments 
     Thus, the present invention has been described with the embodiments. However, it should not be understood that those descriptions and drawings constituting a part of this disclosure limit the present invention. From this disclosure, a variety of alternate embodiments, examples, and applicable techniques will become apparent to one skilled in the art. 
     For example, in the above-described embodiment, both the cellular authentication server  600  and the WLAN authentication server  700  use a public key and a secret key to confirm that each of the authentication servers is a valid authentication server for each other; however, this is not limiting. Only either one authentication server (the cellular authentication server  600 , for example) may use a public key and a secret key to confirm that the other authentication server (the WLAN authentication server  700 , for example) is valid for the one respectively. When the cellular authentication server  600  may use a public key and a secret key to confirm that the WLAN authentication server  700  is the valid authentication server, the cellular authentication server  600  may transmit information acquired through decrypting, to the WLAN authentication server  700 , whereby the WLAN authentication server  700  may confirm that the cellular authentication server  600  is the valid authentication server. 
     Further, in the above-described embodiment, the public key transmission request includes the public key generated by the UE  100 ; however, this is not limiting. When the security is ensured in the communication with the cellular authentication server  600 , the UE  100  may transmit the public key transmission request not including the public key  1 , to the cellular authentication server  600 . Likewise, the UE  100  may transmit the public key transmission request not including the public key  2 , to the WLAN authentication server  700 . 
     Further, in the above-described embodiment, a pair of keys (a public key and a secret key) are used where the information encrypted by either one key (the public key, for example) is decrypted by the other key (the secret key) corresponding to the either one key; however, a common key in which a key used for encryption and a key used for decryption are identical may be used. 
     Further, in the above-described embodiment, in the update of the authentication target, information for updating the authentication target is exchanged (the update request, the update response, and the response) between the cellular authentication server  600  and the WLAN authentication server  700 , without passing through the UE  100 ; however, this is not limiting. For example, similarly to the “setting of the authentication target” according to the above-described embodiment, the update of the authentication target may be performed. 
     For example, the cellular authentication server  600  may generate a new public key  7  instead of the public key  3 . Likewise, the WLAN authentication server  700  may generate a new public key  8  instead of the public key  4 . The cellular authentication server  600  and the WLAN authentication server  700  may use the public key  7  and the public key  8  to confirm that the authentication server is valid, similarly to the above-described embodiment, and then, may update the authentication target. 
     Further, in the above-described embodiment, the cellular authentication server  600  may cancel the setting of the authentication target under a predetermined condition. Description proceeds with the cellular authentication server  600  as an example, below. 
     For example, when a frequency of a traffic transition (that is, an offload frequency) of the UE  100  between the cellular communication system and the WLAN communication system is less than a threshold value (the number of times of the offloads after an elapse of a predetermined period is zero, for example), the cellular authentication server  600  may cancel to set the WLAN authentication server  700  to the authentication target of the UE  100 . As a result, only the authentication target having a large offload frequencies is capable of maintaining the setting (registration), and thus, the cellular authentication server  600  is capable of effectively utilizing a memory provided in the cellular authentication server  600 . 
     It is noted that when there is a cancel request for the authentication target setting from the UE  100  or the authentication server (WLAN authentication server  700 ) in the other communication system, the setting of the authentication target may be cancelled. The cellular authentication server  600  may transmit and/or receive the cancel request when the information is exchanged (the update request, the update response, and the response) for updating the authentication target. Further, when the resource of the memory falls below a threshold value, the cellular authentication server  600  may cancel the setting of the authentication target. 
     In the above-described embodiment, as the WLAN terminal information included in the second setting request, the cellular authentication server  600  may transmit a message in which a WLAN MAC-ID is described rather than a WLAN MAC-ID. After the secret key  4  is used to decrypt the encrypted message, when the WLAN MAC-ID described in the message and the WLAN MAC-ID of the UE  100  to which the public key  4  is transmitted match, the WLAN authentication server  700  may determine that the cellular authentication server  600  is, for the UE  100 , the valid authentication server. 
     Further, the cellular authentication server  600  may store one public key so as to be related with one UE  100 , and so as to be related with a plurality of UEs  100 . 
     It is noted that in the above, a portion where only the operation of the cellular authentication server  600  is described may be replaced by the operation of the WLAN authentication server  700 . 
     Further, in the above-described embodiment, the communication operation that manages the cellular authentication server  600  and the communication operator that manages the WLAN authentication server are different; however, the communication operators that manage the cellular authentication server  600  and the WLAN authentication server may be the same. 
     In the above-described embodiment, the cellular authentication server  600  (the authentication server in either one communication system) acquires from the WLAN authentication server  700  the identifier of the UE  100  in the cellular communication system, as the information not usually known by the WLAN authentication server  700  (the authentication server in the other communication system) to determine that the WLAN authentication server  700  is, for the UE  100 , the valid authentication server; however, this is not limiting. The information may be any information related with the UE  100  such as a cell identifier (Cell ID) of a cell in which the UE  100  exists and a temporary identifier (C-RNT) temporarily assigned by a cell (eNB  200 ) to the UE  100 . Further, the information may be information that may be capable of being used in only either one communication system and related with the UE  100 , and may be information that may be used only in a communication system operated by either one communication operator (operator) and related with the UE  100 . 
     In the above-described embodiment, either one authentication server uses the public key of the other authentication server to encrypt terminal information of the other authentication server; however, this is not limiting. The UE  100  may use the public key of the other authentication server to encrypt the terminal information of the other authentication server, and may transmit the encrypted terminal information to the one authentication server. As a result, the one authentication server does not need to encrypt the terminal information of the other authentication server to enable transmission of the encrypted terminal information to the other authentication server, and thus, the UE  100  may omit the transmission of the public key of the authentication server. 
     In the above-described embodiments, as one example of cellular communication system, the LTE system is described; however, the present invention is not limited to the LTE system, and the present invention may be applied to systems other than the LTE system. Further, the present invention may be applied to a combination of the cellular communication system and the WLAN communication system, and in addition, applied to a combination of other communication systems. 
     It is noted that the entire content of Japanese Patent Application No. 2013-224470 (filed on Oct. 29, 2013) is incorporated in the present specification by reference. 
     INDUSTRIAL APPLICABILITY 
     Thus, the communication control method, the authentication server, and the user terminal according to the present invention, with which it is possible to secure the reliability that an authentication server in another system is, for the user terminal, a valid authentication server between authentication servers in different radio communication systems, are useful in a mobile communication field.