Abstract:
The present disclosure relates to a pre-5th-Generation (5G) or 5G communication system to be provided for supporting higher data rates Beyond 4th-Generation (4G) communication system such as Long Term Evolution (LTE). The present disclosure further relates to a method and apparatus for managing a security key in a communication system are provided. The method includes transmitting a first key request message including an identifier (ID) of an originating user equipment (UE) to a server through a mobility management entity (MME), receiving a key response message including security parameters and a secret key of the originating UE from the server, determining a security key based on the security parameters and the secret key of the originating UE by the originating UE, and communicating with a terminating UE based on the security key by the originating UE, while a connection between the originating UE and the terminating UE is maintained.

Description:
PRIORITY 
       [0001]    This application is a National Phase Entry of PCT International Application No. PCT/KR2014/010979, which was filed on Nov. 14, 2014, and claims a priority to Indian Patent Application No. 1298/KOL/2013, which was filed on Nov. 14, 2013, the contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a method and apparatus for performing Device-to-Device (D2D) communication in a D2D Communication system. 
       BACKGROUND 
       [0003]    To meet the demand for wireless data traffic having increased since deployment of 4G (4th-Generation) communication systems, efforts have been made to develop an improved 5G (5th-Generation) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’. 
         [0004]    The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems. 
         [0005]    In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like. 
         [0006]    In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed. 
         [0007]    Meanwhile, a main function of a User Equipment (UE) in a D2D communication system is to participate in direct one-to-one communication with another UE via a radio interface. However, a passive attacker may intercept data packets during exchange of the data packets between the UEs. In this case, the passive attacker may acquire the original contents of the exchanged data packets. On the other hand, an active attacker may modify the exchanged data packets without being noticed by either of the UEs. 
         [0008]    Accordingly, there is a need for ciphering, replay protection, and integrity protection for D2D communication. To enable such a secure communication, security keys are needed. 
       SUMMARY 
       [0009]    It is therefore an object of the invention is to propose disclosure devised to solve the problem a method and apparatus for managing and establishing security key(s) in a D2D communication system. 
         [0010]    Another object of the invention is to propose a method and apparatus for generating security keys and exchanging the security keys between User Equipments (UEs) in D2D communication system. 
         [0011]    In an aspect of the present disclosure, there is provided a method for managing a security key in a Device-to-Device (D2D) communication system, the method comprising transmitting a key request message including an Identifier (ID) of an originating User Equipment (UE) to a server through a Mobility Management Entity (MME) by the originating UE, receiving a key response message including security parameters from among the security parameters and a security key transmitted by the server by the originating UE, the security key being derived using a secret key of the originating UE, deriving the security key using the security parameters and the secret key of the originating UE by the originating UE, and communicating with a terminating UE using the security key by the originating UE, while a connection between the originating UE and the terminating UE is maintained. 
         [0012]    In another aspect of the present disclosure, there is provided a method for managing a security key in a D2D communication system, the method comprising the steps of transmitting a key request message including an ID of a transmitting UE to a server through an MME by the transmitting UE, receiving a key response message including security parameters from among the security parameters and a first security key transmitted by the server by the transmitting UE, the first security key being derived using a secret key of the transmitting UE, deriving the first security key using the security parameters and the secret key of the transmitting UE by the transmitting UE, transmitting a first D2D key request message including an ID of a receiving UE through the MME by the transmitting UE, receiving a first D2D key response message including the security parameters and a second security key transmitted by the server by the transmitting UE, the second security key being derived using a secret key of the receiving key, and communicating with the receiving UE using the first and second security keys by the transmitting UE, while a connection between the transmitting UE and the receiving UE is maintained. 
         [0013]    In still another aspect of the present disclosure, there is provided a method for managing a security key in a D2D communication system, the method comprising generating a first security key for a connection to a receiving UE and transmitting a key information message including an ID of a transmitting UE, the first security key, and a first key index for the transmitting UE to a server through an MME by the transmitting UE, transmitting a first key request message including the ID of the transmitting UE and the first key index to the receiving UE by the transmitting UE, if the MME authenticates the key information message, receiving a first key response message including an ID of the receiving UE and a second key index for the receiving UE from the receiving UE and transmitting a second key request message including the ID of the receiving UE and the second key index to the MME by the transmitting UE, receiving a second key response message including a second security key derived using a secret key of the receiving UE from the MME by the transmitting UE, and communicating with the receiving UE using the first and second security keys by the transmitting UE, while the connection between the transmitting UE and the receiving UE is maintained. 
         [0014]    In yet another aspect of the present disclosure, there is provided a method for managing a security key in a D2D communication system, the method comprising transmitting a D2D key request message including an ID of an originating UE to a server through an MME by a terminating UE, receiving a D2D key response message including security parameters and a security key transmitted by the server by the terminating UE, the security key being derived using a secret key of the originating UE, transmitting a key response message including the security parameters to the originating UE by the terminating UE, and communicating with the originating UE using the security key by the terminating UE, while a connection between the originating UE and the terminating UE is maintained. 
         [0015]    In a further aspect of the present disclosure, there is provided a method for managing a security key in a D2D communication system, comprising transmitting a D2D key request message including an ID of a transmitting UE to a server through an MME by a receiving UE, receiving a D2D key response message including security parameters and a first security key transmitted by the server by the receiving UE, the first security key being derived using a secret key of the transmitting UE, transmitting a key response message including the security parameters to the transmitting UE by the receiving UE, deriving a second security key using the security parameters included in a key response ACKnowledgment (ACK) message received from the transmitting UE and a secret key of the receiving UE by the receiving UE, and communicating with the transmitting UE using the first and second security keys by the receiving UE, while a connection between the transmitting UE and the receiving UE is maintained. 
         [0016]    In a still further aspect of the present disclosure, there is provided a method for managing a security key in a D2D communication system, comprising receiving a key request message including an ID of a transmitting UE and a first key index for the transmitting UE from the transmitting UE by a receiving UE, if the transmitting UE generates a first security key for a connection to the receiving UE, generating a second security key for the connection to the transmitting UE and transmitting the key request message including the ID of the transmitting UE and the first key index to an MME by the receiving UE, receiving a key response message including the first security key from the MME by the receiving UE, transmitting a key response message including an ID of the receiving UE and a second key index for the receiving UE to the transmitting UE by the receiving UE, and communicating with the transmitting UE using the first and second security keys by receiving UE, while the connection between the transmitting UE and the receiving UE is maintained. 
         [0017]    In another aspect of the present disclosure, an originating UE for managing a security key in a D2D communication system includes a transmitter for transmitting a key request message including an ID of the originating UE to a server through an MME, a receiver for receiving a key response message including security parameters from among the security parameters and a security key transmitted by the server, the security key being derived using a secret key of the originating UE, and a controller for deriving the security key using the security parameters and the secret key of the originating UE. The transmitter and the receiver communicate with a terminating UE using the security key by the originating UE, while a connection between the originating UE and the terminating UE is maintained. 
         [0018]    In another aspect of the present disclosure, a transmitting UE for managing a security key in a D2D communication system includes a transmitter for transmitting a key request message including an ID of the transmitting UE to a server through an MME and transmitting a first D2D key request message including an ID of a receiving UE through the MME, a receiver for receiving a key response message including security parameters from among the security parameters and a first security key transmitted by the server, the first security key being derived using a secret key of the transmitting UE and receiving a first D2D key response message including the security parameters and a second security key transmitted by the server, the second security key being derived using a secret key of the receiving key, and a controller for deriving the first security key using the security parameters and the secret key of the transmitting UE. The transmitter and the receiver communicate with the receiving UE using the first and second security keys, while a connection between the transmitting UE and the receiving UE is maintained. 
         [0019]    In another aspect of the present disclosure, a transmitting UE for managing a security key in a D2D communication system includes a controller for generating a first security key for a connection to a receiving UE, a transmitter for transmitting a key information message including an ID of a transmitting UE, the first security key, and a first key index for the transmitting UE to a server through an MME, and transmitting a first key request message including the ID of the transmitting UE and the first key index to the receiving UE, if the MME authenticates the key information message, and a receiver for receiving a first key response message including an ID of the receiving UE and a second key index for the receiving UE from the receiving UE and transmitting a second key request message including the ID of the receiving UE and the second key index to the MME and receiving a second key response message including a second security key derived using a secret key of the receiving UE from the MME. The transmitter and the receiver communicate with the receiving UE using the first and second security keys, while the connection between the transmitting UE and the receiving UE is maintained. 
         [0020]    In another aspect of the present disclosure, a terminating UE for managing a security key in a D2D communication system includes a transmitter for transmitting a D2D key request message including an ID of an originating UE to a server through an MME and transmitting a key response message including the security parameters to the originating UE, and a receiver for receiving a D2D key response message including security parameters and a security key transmitted by the server, the security key being derived using a secret key of the originating UE. The transmitter and the receiver communicate with the originating UE using the security key, while a connection between the originating UE and the terminating UE is maintained. 
         [0021]    In another aspect of the present disclosure, a receiving UE for managing a security key in a D2D communication system includes a transmitter for transmitting a D2D key request message including an ID of a transmitting UE to a server through an MME and transmitting a key response message including the security parameters to the transmitting UE, a receiver for receiving a D2D key response message including security parameters and a first security key derived using a secret key of the transmitting UE, and a controller for deriving a second security key using the security parameters included in a key response ACK message received from the transmitting UE and a secret key of the receiving UE. The transmitter and the receiver communicate with the transmitting UE using the first and second security keys by the receiving UE, while a connection between the transmitting UE and the receiving UE is maintained. 
         [0022]    In another aspect of the present disclosure, a receiving UE for managing a security key in a D2D communication system includes a receiver for receiving a key request message including an ID of a transmitting UE and a first key index for the transmitting UE from the transmitting UE, if the transmitting UE generates a first security key for a connection to the receiving UE, and receiving a key response message including the first security key from an MME, and a transmitter for generating a second security key for the connection to the transmitting UE, transmitting the key request message including the ID of the transmitting UE and the first key index to the MME, and transmitting a key response message including an ID of the receiving UE and a second key index for the receiving UE to the transmitting UE. The transmitter and the receiver communicate with the transmitting UE using the first and second security keys, while the connection between the transmitting UE and the receiving UE is maintained. 
         [0023]    Thus, the present invention provides a method for managing a security key in a D2D communication system. The method can increase the reliability of a security key and maximize security. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a diagram illustrating a signal flow for an example of generating security keys and exchanging the security keys between User Equipments (UEs) in a Device-to-Device (D2D) communication system according to a first method of the present disclosure; 
           [0025]      FIGS. 2 a  and 2 b    are diagrams illustrating a signal flow for another example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to the first method of the present disclosure; 
           [0026]      FIGS. 3 a  and 3 b    are diagrams illustrating a signal flow for an example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to a second method of the present disclosure; 
           [0027]      FIGS. 4 a  and 4 b    are diagrams illustrating a signal flow for another example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to the second method of the present disclosure; 
           [0028]      FIGS. 5 a  and 5 b    are diagrams illustrating a signal flow for an example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to a third method of the present disclosure; 
           [0029]      FIG. 6  is a diagram illustrating a signal flow for another example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to the third method of the present disclosure; 
           [0030]      FIG. 7  is a block diagram of a UE that manages a security key in a D2D communication system according to an embodiment of the present disclosure; 
           [0031]      FIG. 8  is a block diagram of an MME that manages a security key in a D2D communication system according to an embodiment of the present disclosure; and 
           [0032]      FIG. 9  is a block diagram of a server that manages a security key in a D2D communication system according to an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Reference will be made to preferred embodiments of the present disclosure with reference to the attached drawings. A detailed description of a generally known function and structure of the present disclosure will be avoided lest it should obscure the subject matter of the present disclosure. In addition, although the terms used in the present disclosure are defined in consideration of functions of the present disclosure, the terms may be changed according to the intention of a user or an operator, or customs. Therefore, the present disclosure must be understood, not simply by the actual terms used but by the meanings of each term lying within. 
         [0034]    In a legacy communication system wherein a UE communicate with the base station, a User Equipment (UE) derives its key implicitly and a Base Station (BS) acquires a key specific to the UE from an authenticator. 
         [0035]    On the other hand, although UEs may have their own master keys in a Device-to-Device (D2D) communication system, the following issues may be faced in terms of security key management. 
         [0036]    Which key between a master key and a derived key should the UEs use? 
         [0037]    How a security key is to be derived from a master key? 
         [0038]    Which master key between the master keys of the UEs is to be used to derive security keys? 
         [0039]    Is a single key required for each direction or both directions? 
         [0040]    How are different keys to be exchanged, with security ensured? 
         [0041]    Embodiments of the present disclosure provide first, second, and third methods to solve the problems involved in security management, which will be described below. 
         [0042]    Method 1 
         [0043]    One security key is used for both transmission of an originating UE and transmission of a terminating UE. 
         [0044]    The security key for communication between the UEs is derived from a secret key of a UE. A security key is specific to one connection. That is, if a first UE (UE 1 ) is connected to a second UE (UE 2 ) and a third UE (UE 3 ), different security keys are used for the connection between UE 1  and UE 2  and the connection between UE 1  and UE 3 . 
         [0045]    The lifetime of a security key is equal to the duration of a connection. In the case where UE 1  is connected to UE 2 , upon release of the connection, a security key for the connection is also released. Upon re-setup of the connection, a new security key is generated, as follows. 
         [0000]      Security Keyconnection=KDF(RAND,connection count of originating UE,secret key of originating UE). 
         [0046]    RAND is generated for all new connections between the originating UE and other UEs and the connection count is increased for each new connection. 
         [0047]    The originating UE internally derives a security key, while the terminating UE receives the security key from a network. Security parameters are generated and maintained in the network. 
         [0048]    The first method will be described in greater detail in relation to signaling procedures illustrated in  FIGS. 1, 2   a , and  2   b.    
         [0049]    Method 2 
         [0050]    Separate security keys are used for transmission of a transmitting UE and transmission of a receiving UE. 
         [0051]    A security key for communication between UEs is derived from a secret key of a UE and is specific to a connection and/or a transmission direction. 
         [0052]    The security key lasts as long as the connection is maintained. That is, if UE 1  is connected to UE 2 , a security key is released upon release of the connection. Upon re-setup of the connection, a new security key is generated. 
         [0053]    A connection-specific security key is generated using a secret key of a transmitting UE. That is, if UE 1  is connected to UE 2 , a secret key of UE 1  is used for transmission of UE 1  and a secret key of UE 2  is used for transmission of UE 2 . A security key is generated by the following equation. 
         [0000]      Security Keyconnection=KDF(RAND,connection count of transmitting UE,secret key of transmitting UE) 
         [0054]    where RAND is generated for every new connection between the transmitting UE and other UEs and the connection count increases for each new connection. 
         [0055]    The transmitting UE internally derives a security key, whereas the receiving UE receives a security key from a network. Security parameters are generated and maintained by the network. 
         [0056]    Method 2 will be described later in greater detail in relation to signaling procedures illustrated in  FIGS. 3 a    to  4   b.    
         [0057]    Method 3 
         [0058]    Separate security keys are used for transmission of a transmitting UE and transmission of a receiving UE. 
         [0059]    A security key for communication between UEs is derived from a secret key of a UE and is specific to a connection and/or a transmission direction. 
         [0060]    The security key lasts as long as the connection is maintained. That is, if UE 1  is connected to UE 2 , a security key is released upon release of the connection. Upon re-setup of the connection, a new security key is generated. 
         [0061]    A connection-specific security key is generated using a secret key of a transmitting UE. That is, if UE 1  is connected to UE 2 , a secret key of UE 1  is used for transmission of UE 1  and a secret key of UE 2  is used for transmission of UE 2 . The security key is generated by the following equation. 
         [0000]      Security Keyconnection=KDF(RAND,connection count of transmitting UE,secret key of transmitting UE) where RAND is generated for every new connection between the transmitting UE and other UEs and the connection count increases for each new connection. 
         [0062]    The transmitting UE internally derives a security key, whereas the receiving UE receives a security key from a network. Security parameters are generated and maintained by a UE. 
         [0063]    Method 3 will be described later in greater detail in relation to signaling procedures illustrated in  FIGS. 5 a , 5 b   , and  6 . 
         [0064]      FIG. 1  is a diagram illustrating a signal flow for an example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to a first method of the present disclosure. 
         [0065]    Referring to  FIG. 1 , the D2D communication system includes UE 1   100 , UE 2   110 , a Mobility Management Entity (MME)  120 , and a server such as a Home Subscriber Server (HSS)/Proximity Service (ProSe) server  130 . 
         [0066]    UE 1   100  transmits a page or key request message to UE 2   110  (operation  101 ). The page or key request message includes an Identifier (ID) of UE 1   100 , UE 1  ID. 
         [0067]    UE 2   110  transmits a D2D key request message to the MME  120  (operation  103 ). The D2D key request message includes the ID of UE 1   100  as a primary UE, that is, UE 1  ID and an ID of UE 2   110  as a secondary UE, that is, UE 2  ID. Herein, UE 2  ID is selectively included in the D2D key request message. That is, only when it is necessary to determine whether UE 2   110  has been authorized and/or when UE 2  ID is used in generating a security key, UE 2  ID is included in the D2D key request message. Non-Access Stratum (NAS) security is applied to the D2D key request message. 
         [0068]    The MME  120  authenticates/validates whether the D2D key request message received in operation  103  has been transmitted by an authorized UE (operation  105 ) and transmits a D2D key request message to the HSS/ProSe server  130  (operation  106 ) if authentication/validation is successful. The D2D key request message includes the ID of UE 1   100  as a primary UE, that is, UE 1  ID and the ID of UE 2   110  as a secondary UE, that is, UE 2  ID. Herein, UE 2  ID is selectively included in the D2D key request message. That is, only when it is necessary to determine whether UE 2   110  has been authorized and/or when UE 2  ID is used in generating a security key, UE 2  ID is included in the D2D key request message. 
         [0069]    The HSS/ProSe server  130  derives a connection-specific security key using a secret key of primary UE i.e. UE 1  (operation  107 ) and transmits a D2D key response message for the D2D key request message received in operation  106  to the MME  120  (operation  109 ). The HSS/ProSe server  130  maintains connection counter for each UE. The D2D key response message includes the connection-specific security key and security parameters. The connection-specific security key may be, for example, K′-D2D-UE 1  and the security parameters may include, for example, a RAND function, a connection count, and a Message Authentication Code (MAC). 
         [0070]    The MME  120  transmits a D2D key response message for the D2D key request message received in operation  103  to UE 2   110  (operation  111 ). The D2D key response message includes the connection-specific security key and the security parameters. NAS security is applied to the D2D key response message. 
         [0071]    UE 2   110  stores the connection-specific security key received in operation  111  (operation  113 ). Then UE 2   110  transmits a page or key response message for the page or key request message received in operation  101  to UE 1   100  (operation  115 ). The page or key response message includes the security parameters and UE 2  ID. UE 2  ID is selectively included in the page or key response message. That is, when UE 2  ID is used in generating a security key and is not available to UE 1   100 , UE 2  ID is included in the page or key response message. UE 1   100  may indicate whether UE 2  ID is known to UE 1   100  by a key request message at operation  101 . 
         [0072]    UE 1   100  authenticates the page or key response message received in operation  115  (operation  117 ) and derives the connection-specific security key from the security parameters (i.e. connection counter, RAND) included in the authenticated page or key response message and the secret key of UE 1   100  (operation  119 ). Authentication is done using the MAC. UE 1   100  generates MAC and compares with the received MAC. UE 1   100  transmits a page or key response Acknowledgement (ACK) message to UE 2   110  (operation  121 ). Subsequently, UE 1   100  communicates with UE 2   110  using the connection-specific security key (operation  123 ). The connection-specific security key lasts for the duration of the connection. 
         [0073]    Particularly, the connection-specific security key and the MAC are generated using the secret key of UE 1   100 , the RAND function, and the connection count of UE 1   100 . In an alternate embodiment, the ID of UE 2 , UE 2  ID may also be used additionally in generation of connection-specific security key. In an another alternate embodiment, the ID of UE 2 , UE 2  ID and the ID of UE 1 , UE  1  ID both may also be used additionally in generation of connection-specific security key. 
         [0074]    The secret key of UE 1   100  is known only to UE 1   100  and the HSS/ProSe server  130 . The secret key of a specific UE is not provided to another UE and a key derived from the secret key is not transmitted wirelessly. 
         [0075]    With reference to  FIG. 1 , a method for generating security keys and exchanging the security keys between UEs when a primary UE is UE 1  and a secondary UE is UE 2  has been described, by way of example. If both UE 1  ID and UE 2  ID are used in generating a security key, the primary UE and the secondary UE may be exchanged in the signaling procedure illustrated in  FIG. 1 . 
         [0076]      FIGS. 2 a  and 2 b    are diagrams illustrating a signal flow for another example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to the first method of the present disclosure. 
         [0077]    Compared to the signaling procedure of  FIG. 1 , a key index (key-idx or key index) is used in the signaling procedure of  FIGS. 2 a  and 2 b   . The use of a key index may reduce the overhead of a page response message. 
         [0078]    Referring to  FIGS. 2 a  and 2 b   , the D2D communication system includes UE 1   200 , UE 2   210 , an MME  220 , and a server, for example, an HSS/ProSe server  230 . 
         [0079]    UE 1   200  transmits a page or key request message to UE 2   110  (operation  201 ). The page or key request message includes an ID of UE 1   200 , UE 1  ID. 
         [0080]    UE 2   210  transmits a D2D key request message to the MME  220  (operation  203 ). The D2D key request message includes the ID of UE 1   200  as a primary UE, that is, UE 1  ID and an ID of UE 2   210  as a secondary UE, that is, UE 2  ID. Herein, UE 2  ID is selectively included in the D2D key request message. That is, only when it is necessary to determine whether UE 2   210  has been authorized and/or when UE 2  ID is used in generating a security key, UE 2  ID is included in the D2D key request message. NAS security is applied to the D2D key request message. 
         [0081]    The MME  220  authenticates/validates whether the D2D key request message has been transmitted by an authorized UE (operation  205 ) and transmits a D2D key request message to the HSS/ProSe server  230  (operation  207 ) if the authentication/validation is successful. The D2D key request message includes the ID of UE 1   200  as a primary UE, that is, UE 1  ID, the ID of UE 2   210  as a secondary UE, that is, UE 2  ID, and key-idx. Herein, UE 2  ID is selectively included in the D2D key request message. That is, only when it is necessary to determine whether UE 2   110  has been authorized and/or when UE 2  ID is used in generating a security key, UE 2  ID is included in the D2D key request message. Key-idx is set to a null value. 
         [0082]    The HSS/ProSe server  230  derives a connection-specific security key using a secret key of UE 1  (operation  209 ) and transmits a D2D key response message for the D2D key request message received in operation  207  to the MME  120  (operation  211 ). The D2D key response message includes the connection-specific security key and key-idx. 
         [0083]    The MME  220  transmits a D2D key response message for the D2D key request message received in operation  203  to UE 2   210  (operation  213 ). The D2D key response message includes the connection-specific security key and key-idx. NAS security is applied to the D2D key response message. 
         [0084]    UE 2   210  stores the connection-specific security key received in operation  213  (operation  215 ). Then UE 2   210  transmits a page or key response message for the page or key request message received in operation  201  to UE 1   100  (operation  217 ). The page or key response message includes key-idx and UE 2  ID. UE 2  ID is selectively included in the page or key response message. That is, when UE 2  ID is used in generating a security key and is not available to UE 1   200 , UE 2  ID is included in the page or key response message. UE 1   200  may indicate whether UE 2  ID is known to UE 1   200  by a key request message at operation  201 . 
         [0085]    UE 1   200  transmits a D2D key request message to the MME  220  (operation  219 ). The D2D key request message includes the ID of UE 1   100  as a primary UE, that is, UE 1  ID, the ID of UE 2   110  as a secondary UE, that is, UE 2  ID. Herein, UE 2  ID is selectively included in the D2D key request message. That is, only when it is necessary to determine whether UE 2   210  has been authorized and/or when UE 2  ID is used in generating a security key, UE 2  ID is included in the D2D key request message. NAS security is applied to the D2D key request message. 
         [0086]    The MME  220  authenticates whether the D2D key request message received in operation  219  has been transmitted by an authorized UE (operation  221 ) and transmits a D2D key request message to the HSS/ProSe server  230  (operation  223 ). The D2D key request message includes the ID of UE 1   100  as a primary UE, that is, UE 1  ID, the ID of UE 2   110  as a secondary UE, that is, UE 2  ID, and key-idx. Herein, UE 2  ID is selectively included in the D2D key request message. That is, only when it is necessary to determine whether UE 2   210  has been authorized and/or when UE 2  ID is used in generating a security key, UE 2  ID is selectively included in the D2D key request message. 
         [0087]    The HSS/ProSe server  230  transmits a D2D key response message for the D2D key request message received in operation  223  to the MME  220  (operation  225 ). The D2D key response message includes a connection-specific security key. 
         [0088]    The MME  220  transmits a D2D key response message for the D2D key request message received in operation  219  to UE 1   200  (operation  227 ). The D2D key response message includes the connection-specific security key. NAS security is applied to the D2D key response message. 
         [0089]    UE 1   200  stores the connection-specific security key received in operation  227  (operation  229 ) and transmits a page or key response ACK message in response to the page or key response message received in operation  217  to UE 2   210  (operation  231 ). 
         [0090]    The connection-specific security key is generated using the secret key of UE 1   200 , the RAND function, and the connection count of UE 1   200 . In an alternate embodiment, the ID of UE 2 , UE 2  ID may also be used additionally in generation of connection-specific security key. In an another alternate embodiment, the ID of UE 2 , UE 2  ID and the ID of UE 1 , UE  1  ID both may also be used additionally in generation of connection-specific security key. 
         [0091]    The secret key of UE 1   200  is known only to UE 1   200  and the HSS/ProSe server  230 . The secret key of a specific UE is not provided to another UE and a key derived from the secret key is not transmitted wirelessly. 
         [0092]      FIGS. 3 a  and 3 b    are diagrams illustrating a signal flow for an example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to a second method of the present disclosure. 
         [0093]    Referring to  FIGS. 3 a  and 3 b   , the D2D communication system includes UE 1   300 , UE 2   310 , an MME  320 , and a server, for example, an HSS/ProSe server  330 . 
         [0094]    UE 1   300  transmits a page or key request message to UE 2   310  (operation  301 ). The page or key request message includes an ID of UE 1   300 , UE 1  ID. 
         [0095]    UE 2   310  transmits a D2D key request message to the MME  320  (operation  303 ). The D2D key request message includes the ID of UE 1   300  as a primary UE, that is, UE 1  ID and an ID of UE 2   310  as a secondary UE, that is, UE 2  ID. Herein, UE 2  ID is selectively included in the D2D key request message. That is, only when it is necessary to determine whether UE 2   310  has been authorized and/or when UE 2  ID is used in generating a security key, UE 2  ID is included in the D2D key request message. NAS security is applied to the D2D key request message. 
         [0096]    The MME  320  authenticates/validates whether the D2D key request message received in operation  303  has been transmitted by an authorized UE (operation  305 ) and transmits a D2D key request message to the HSS/ProSe server  330  (operation  307 ) if the authentication/validation is successful. The D2D key request message includes the ID of UE 1   300  as a primary UE, that is, UE 1  ID and the ID of UE 2   310  as a secondary UE, that is, UE 2  ID. Herein, UE 2  ID is selectively included in the D2D key request message. That is, only when it is necessary to determine whether UE 2   310  has been authorized and/or when UE 2  ID is used in generating a security key, UE 2  ID is included in the D2D key request message. 
         [0097]    The HSS/ProSe server  330  derives a connection-specific security key using a secret key of UE 1   300  (operation  309 ) and transmits a D2D key response message for the D2D key request message received in operation  307  to the MME  320  (operation  311 ). In  FIGS. 3 a  and 3 b   , the connection-specific security key derived from the secret key of UE 1  is referred to as a first connection-specific security key (connection-specific security key  1 ). The D2D key response message includes connection-specific security key  1  and security parameters (i.e. RAND, Connection counter and MAC). 
         [0098]    The MME  320  transmits a D2D key response message for the D2D key request message received in operation  303  to UE 2   310  (operation  313 ). The D2D key response message includes connection-specific security key  1  and the security parameters. NAS security is applied to the D2D key response message. 
         [0099]    UE 2   310  stores connection-specific security key  1  received in operation  313  (operation  315 ). Then UE 2   310  transmits a page or key response message for the page or key request message received in operation  301  to UE 1   300  (operation  317 ). The page or key response message includes the security parameters and UE 2  ID. UE 2  ID is selectively included in the page or key response message. That is, when UE 2  ID is used in generating a security key and is not available to UE 1   300 , UE 2  ID is included in the page or key response message. UE 1   300  may indicate whether UE 2  ID is known to UE 1   300  by a key request message. 
         [0100]    UE 1   300  authenticates the page or key response message received in operation  317  (operation  319 ) and derives connection-specific security key  1  (operation  321 ) using the security parameters (i.e. connection counter, RAND) included in the authenticated page or key response message and the secret key of UE 1   300 . 
         [0101]    The connection-specific security key and the MAC are generated using the secret key of UE 1   300 , the RAND function, and the connection count of UE 1   300 . In an alternate embodiment, the ID of UE 2 , UE 2  ID may also be used additionally in generation of connection-specific security key. In an another alternate embodiment, the ID of UE 2 , UE 2  ID and the ID of UE 1 , UE  1  ID both may also be used additionally in generation of connection-specific security key. 
         [0102]    UE 1   300  transmits a D2D key request message to the MME  320  (operation  323 ). The D2D key request message includes the ID of UE 2  as a primary UE, UE 2  ID and the ID of UE 1  as a secondary UE, UE 1  ID. Herein, UE 1  ID is selectively included in the D2D key request message. That is, only when it is necessary to determine whether UE 1  has been authorized and/or when UE 1  ID is used in generating a security key, UE 1  ID is included in the D2D key request message. NAS security is applied to the D2D key request message. 
         [0103]    The MME  320  authenticates/validates whether the D2D key request message has been transmitted by an authorized UE (operation  325 ) and transmits a D2D key request message to the HSS/ProSe server  330  (operation  327 ). The D2D key request message includes the ID of UE 2   310  as a primary UE, UE 2  ID and the ID 
         [0104]    of UE 1   300  as a secondary UE, UE 1  ID. Herein, UE 1  ID is selectively included in the D2D key request message. That is, only when it is necessary to determine whether UE 1  has been authorized and/or when UE 1  ID is used in generating a security key, UE 1  ID is included in the D2D key request message. 
         [0105]    The HSS/ProSe server  330  derives a connection-specific security key using a secret key of UE 2  (operation  329 ) and transmits a D2D key response message for the D2D key request message received in operation  327  to the MME  320  (operation  331 ). In  FIGS. 3 a  and 3 b   , the connection-specific security key derived from the secret key of UE 2  is referred to as a second connection-specific security key (connection-specific security key  2 ). The D2D key response message includes connection-specific security key  2  and security parameters (i.e. RAND and Connection counter). 
         [0106]    The MME  320  transmits a D2D key response message for the D2D key request message received in operation  323  to UE 1   300  (operation  333 ). The D2D key response message includes connection-specific security key  2  and the security parameters. NAS security is applied to the D2D key response message. 
         [0107]    UE 1   300  authenticates the D2D key response message received in operation  333  (operation  335 ) and stores connection-specific security key  2  included in the D2D key response message (operation  337 ). Then UE 1   300  transmits a key response ACK message for the key response message received in operation  317  to UE 2   310  (operation  339 ). The key response ACK message includes the security parameters. 
         [0108]    UE 2   310  derives connection-specific security key  2  and communicates with UE 1   300  using connection-specific security key  2  (operation  343 ). That is, connection-specific security key  1  is used for communication directed from UE 1   300  to UE 2   310  and connection-specific security key  2  is used for communication directed from UE 2   310  to UE 1   300 . 
         [0109]    Particularly, connection-specific security key  1  and a MAC at operation  309  are generated using the secret key of UE 1   300 , the RAND function, and the connection count of UE 1   300 . Connection-specific security key  1  is generated, selectively taking into account the ID of UE 2 , UE 2  ID. 
         [0110]    Connection-specific security key  2  is generated using the secret key of UE 2   310 , the RAND function, and the connection count of UE 2   310  at operation  329 . In an alternate embodiment, the ID of UE 2 , UE 2  ID may also be used additionally in generation of connection-specific security key. In an another alternate embodiment, the ID of UE 2 , UE 2  ID and the ID of UE 1 , UE  1  ID both may also be used additionally in generation of connection-specific security key. 
         [0111]    The secret keys of UE 1   300  and UE 2   310  are known only to UE 1   300  and the HSS/ProSe server  330 . The secret key of a specific UE is not provided to another UE and a key derived from the secret key is not transmitted wirelessly. 
         [0112]      FIGS. 4 a  and 4 b    are diagrams illustrating a signal flow for another example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to the second method of the present disclosure. 
         [0113]    Compared to the third method, the proposed fourth method may reduce the overhead of a page response message. 
         [0114]    Referring to  FIGS. 4 a  and 4 b   , the D2D communication system includes UE 1   400 , UE 2   410 , an MME  420 , and a server, for example, an HSS/ProSe server  430 . 
         [0115]    UE 1   400  transmits a page or key request message to UE 2   410  (operation  401 ). The page or key request message includes an ID of UE 1   400 , UE 1  ID. 
         [0116]    UE 2   410  transmits a D2D key request message to the MME  420  (operation  403 ). The D2D key request message includes the ID of UE 1   400 , UE 1  ID and an ID of UE 2   410 , UE 2  ID. NAS security is applied to the D2D key request message. 
         [0117]    The MME  420  authenticates whether the D2D key request message received in operation  403  has been transmitted by an authorized UE (operation  405 ) and transmits a D2D key request message to the HSS/ProSe server  430  (operation  407 ). The D2D key request message includes UE 1  ID and UE 2  ID. 
         [0118]    The HSS/ProSe server  430  derives security key  1  using a secret key of UE 1  (operation  409 ) and derives security key  2  using a secret key of UE 2  (operation  411 ). 
         [0119]    The HSS/ProSe server  430  transmits a D2D key response message for the D2D key request message received in operation  407  to the MME  420  (operation  413 ). The D2D key response message includes security key  1 , key-idx-UE 1 , security key  2 , and key-idx-UE 2 . 
         [0120]    The MME  420  transmits a D2D key response message for the D2D key request message received in operation  403  to UE 2   410  (operation  415 ). The D2D key response message includes security key  1 , key-idx-UE 1 , security key  2 , and key-idx-UE 2 . Herein, key-idx-UE 1  is a key index for UE 1   400  and key-idx-UE 2  is a key index for UE 2   410 . NAS security is applied to the D2D key response message. 
         [0121]    UE 2   410  stores security key  1  and security key  2  received in operation  415  (operation  417 ). Then UE 2   410  transmits a page or key response message for the page or key request message received in operation  401  to UE 1   400  (operation  419 ). The page or key response message includes key-idx-UE 1 , key-idx-UE 2 , and UE 2  ID. UE 2  ID is selectively included in the page or key response message. That is, when UE 2  ID is used in generating a security key and is not available to UE 1   400 , UE 2  ID is included in the page or key response message. UE 1   400  may indicate whether UE 2  ID is known to UE 1   400  by a key request message at operation  401 . 
         [0122]    UE 1   400  transmits a D2D key request message to the MME  420  (operation  421 ). The D2D key request message includes UE 1  ID, key-idx-UE 1 , UE 2  ID, and key-idx-UE 2 . NAS security is applied to the D2D key request message. 
         [0123]    The MME  420  authenticates whether the D2D key request message received in operation  421  has been transmitted by an authorized UE (operation  423 ) and transmits a D2D key request message to the HSS/ProSe server  430  (operation  425 ). The D2D key request message includes UE 1  ID, key-idx-UE 1 , UE 2  ID, and key-idx-UE 2 . 
         [0124]    The HSS/ProSe server  430  transmits a D2D key response message for the D2D key request message received in operation  425  to the MME  420  (operation  427 ). The D2D key response message includes security key  1  and security key  2 . 
         [0125]    The MME  420  transmits a D2D key response message for the D2D key request message received in operation  421  to UE 1   400  (operation  429 ). The D2D key response message includes security key  1  and security key  2 . NAS security is applied to the D2D key response message. 
         [0126]    UE 1   400  stores security key  1  and security key  2  received in operation  429  (operation  431 ). Then UE 1   400  and UE 2   410  communicate using security key  1  and security key  2  (operation  433 ). That is, security key  1  is used for communication directed from UE 1   400  to UE 2   410  and security key  2  is used for communication directed from UE 2   410  to UE 1   400 . 
         [0127]      FIGS. 5 a  and 5 b    are diagrams illustrating a signal flow for an example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to a third method of the present disclosure. 
         [0128]    Referring to  FIGS. 5 a  and 5 b   , the D2D communication system includes UE 1   500 , UE 2   510 , and an MME  520 . 
         [0129]    UE 1   500  generates security key  1  for a connection to UE 2   510  (operation  501 ) and transmits a D2D key information message to the MME  520  (operation  503 ). 
         [0130]    The D2D key information message includes UE 1  ID, security key  1 , and key-idx-UE 1 . NAS security is applied to the D2D key information message. 
         [0131]    The MME  520  authenticates the D2D key information message and stores related information, that is, the information included in the D2D key information message (operation  505 ). The MME  520  transmits a D2D key information ACK message for the D2D key information message received in operation  503  to UE 1   500  (operation  507 ). NAS security is applied to the D2D key information ACK message. 
         [0132]    UE 1   500  transmits a D2D key request message to UE 2   510  (operation  509 ). The D2D key request message includes UE 1  ID and key-idx-UE 1 . 
         [0133]    UE 2   510  generates security key  2  for a connection to UE 1   500  (operation  511 ) and transmits a D2D key information message to the MME  520  (operation  513 ). The D2D key information message includes UE 2  ID, security key  2 , and key-idx-UE 2 . NAS security is applied to the D2D key information message. 
         [0134]    The MME  520  transmits a D2D key information ACK message for the D2D key information message received in operation  513  to UE 2   510  (operation  515 ). The MME  520  authenticates the D2D key information message and stores related information, that is, the information included the D2D key information message (operation  517 ). NAS security is applied to the D2D key information ACK message. 
         [0135]    UE 2   510  transmits a D2D key request message to the MME  520  (operation  519 ). The D2D key request message includes UE 1  ID and key-idx-UE 1 . 
         [0136]    In network multiple MMEs may be there. UE 1  may be associated with MME 1  and UE 2  may be associated with MME 2 . 
         [0137]    In this case, MME of UE 2  sends the key request message with UE 1  ID and key-idx-UE 1  to MME of UE 1  and gets the security KEY  1 . MME of UE  2  can determine whether the MME of UE 1  is different based on UE 1  ID or alternately the MME ID of MME of UE 1  can be received by UE 2  at operation  509  and received by MME of UE 2  at operation  519 . If the MME of UE 1   500  is different, the MME  520  sends a request to the MME of UE 1   500  (operation  521 ). 
         [0138]    The MME  520  transmits a D2D key response message for the D2D key request message received in operation  519  to UE 2   510  (operation  523 ). The D2D key response message includes security key  1  and NAS security is applied to the D2D key response message. 
         [0139]    UE 2   510  stores security key  1  included in the D2D key response message (operation  525 ) and transmits a D2D key response message to UE 1   500  (operation  527 ). The D2D key response message includes UE 2  ID and key-idx-UE 2 . 
         [0140]    UE 1   500  transmits a D2D key request message to the MME  520  (operation  529 ). The D2D key request message includes UE 2  ID and key-idx-UE 2  received from UE 2   510  in operation  527 . NAS security is applied to the D2D key request message. 
         [0141]    If the MME of UE 2   510  is different, the MME  520  requests to the MME of UE 2   510  (operation  531 ). 
         [0142]    The MME  520  transmits a D2D key response message for the D2D key request message received in operation  529  to UE 1   500  (operation  523 ). The D2D key response message includes Security Key  2 . NAS security is applied to the D2D key response message. 
         [0143]    UE 1   500  stores security key  2  (operation  535 ). 
         [0144]      FIG. 6  is a diagram illustrating a signal flow for another example of generating security keys and exchanging the security keys between UEs in a D2D communication system according to the third method of the present disclosure. 
         [0145]    Referring to  FIG. 6 , the D2D communication system includes UE 1   600 , UE 2   610 , and an MME  620 . 
         [0146]    UE 1   600  generates security key  1  for a connection to UE 2   610  (operation  601 ) and transmits a service request message to the MME  620  (operation  603 ). The service request message includes UE 1  ID and security key  1 , and additionally, UE 2  ID or a discovery code.  FIG. 6  will be described on the assumption that the service request message includes a discovery code. NAS security is applied to the service request message. 
         [0147]    The MME  620  authenticates the service request message and stores related information, that is, the information included in the service request message (operation  605 ). The MME  620  determines UE 2  ID from the discovery code, determining that the service request message does not include UE 2  ID (operation  607 ). If the service request message includes UE 2  ID, step  607  may be omitted. Then, the MME  620  transmits a page message to UE 2   610  (operation  609 ). The page message includes information indicating direct connection and UE 1  ID. 
         [0148]    UE 2   610  generates security key  2  for a connection to UE 1   600  (operation  611 ) and transmits a service request message to the MME  620  (operation  613 ). The service request message includes UE 2  ID and security key  2 , and additionally, UE 1  ID or a discovery code. NAS security is applied to the service request message. 
         [0149]    The MME  620  transmits a service response message for the service request message received in operation  613  to UE 2   610  (operation  615 ). The service response message includes security key  1 . NAS security is applied to the service response message. 
         [0150]    UE 2   610  stores security key  1  included in the service response message received in operation  615  (operation  617 ). 
         [0151]    Then, the MME  610  transmits a service response message for the service request message received in operation  603  to UE 1   600  (operation  619 ). The service response message includes security key  2 . 
         [0152]    UE 1   600  stores security key  2  included in the service response message received in operation  619  (operation  621 ). 
         [0153]      FIG. 7  is a block diagram of a UE that manages a security key in a D2D communication system according to an embodiment of the present disclosure. 
         [0154]    Referring to  FIG. 7 , a UE  700  includes a receiver  702 , a controller  704 , and a transmitter  706 . 
         [0155]    The controller  704  provides overall control to the UE  700 . Particularly, the controller  704  controls overall operations related to management of a security key according to an embodiment of the present disclosure. The overall operations related to management of a security key have been described before with reference to  FIGS. 1 to 6  and thus their description is not provided herein. 
         [0156]    The receiver  702  and the transmitter  706  receive and transmit messages under the control of the controller  704 . The messages have been described before with reference to  FIGS. 1 to 6  and their detailed description is not provided herein. 
         [0157]      FIG. 8  is a block diagram of an MME that manages a security key in a D2D communication system according to an embodiment of the present disclosure. 
         [0158]    Referring to  FIG. 8 , an MME  800  includes a receiver  802 , a controller  804 , and a transmitter  806 . 
         [0159]    The controller  804  provides overall control to the MME  800 . Particularly, the controller  804  controls overall operations related to management of a security key according to an embodiment of the present disclosure. The overall operations related to management of a security key have been described before with reference to  FIGS. 1 to 6  and thus their description is not provided herein. 
         [0160]    The receiver  802  and the transmitter  806  receive and transmit messages under the control of the controller  804 . The messages have been described before with reference to  FIGS. 1 to 6  and their detailed description is not provided herein. 
         [0161]      FIG. 9  is a block diagram of a server that manages a security key in a D2D communication system according to an embodiment of the present disclosure. 
         [0162]    Referring to  FIG. 9 , a server  900  includes a receiver  902 , a controller  904 , and a transmitter  906 . 
         [0163]    The controller  904  provides overall control to the server  900 . Particularly, the controller  904  controls overall operations related to management of a security key according to an embodiment of the present disclosure. The overall operations related to management of a security key have been described before with reference to  FIGS. 1 to 4B  and thus their description is not provided herein. 
         [0164]    The receiver  902  and the transmitter  906  receive and transmit messages under the control of the controller  804 . The messages have been described before with reference to  FIGS. 1 to 4B  and their detailed description is not provided herein. 
         [0165]    Although the disclosure has been described with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and variations can be made in the present disclosure without departing from the spirit or scope of the disclosure described in the appended claims. Accordingly, the scope of the disclosure should be determined by the appended claims and their legal equivalents, not by the specific embodiments described herein. 
         [0166]    It will be appreciated that the method and apparatus for managing a security key according to the embodiments of the present disclosure may be implemented in the form of hardware, software, or a combination of hardware and software. Any such software may be stored in a volatile or non-volatile storage device such as a Read-Only Memory (ROM), or in a memory such as a Random Access Memory (RAM), a memory chip, a memory device or a memory integrated circuit, or in a storage medium, such as a Compact Disc (CD), a Digital Versatile Disc (DVD), a magnetic disk or a magnetic tape, which is optically or magnetically recordable and simultaneously, is readable by a machine, regardless of whether the software can be deleted or rewritten. It will be appreciated that a memory is an example of a storage medium readable by a machine (for example, a computer) suitable for storing a program or programs including instructions for implementing the embodiments of the present disclosure. 
         [0167]    Accordingly, the present disclosure includes a program including a code for implementing a method claimed in any claim of this specification, and a machine-readable storage medium for storing this program. Also, this program may be electronically conveyed via any medium such as a communication signal transmitted through a wired or wireless connection, and the present disclosure suitably includes equivalents of this program. 
         [0168]    In the embodiments of the present disclosure, the program may be received from the apparatus for managing a security key and stored. The program providing apparatus may include a memory for storing a program including instructions that make a graphic processing device to perform a predetermined content protection method and information required for the content protection method, a communication unit for conducting wired or wireless communicating with the graphic processing device, and a controller for transmitting the program to the transmission and reception device automatically or upon request of the graphic processing device.