Abstract:
A telecommunication network comprises an ICS GW configured to receive an access request from a UE; an HSS FE configured to retrieve A3/A8 authentication vectors including an A3/A8 authentication response from an HLR, and further configured to encode the A3/A8 authentication vectors as AKA authentication vectors and send the AKA authentication parameters to an S-CSCF; the ICS GW configured to receive an authentication challenge from the S-CSCF with encoded A3/A8 authentication vectors, and further configured to detect the A3/A8 authentication vectors and issue an A3/A8 authentication challenge to the UE; and the ICS GW configured to receive an A3/A8 authentication response from the UE, and further to encode the A3/A8 authentication response into an AKA authentication response and sending it to the S-CSCF for comparison.

Description:
RELATED APPLICATION 
     This patent applications claims the benefit of U.S. Provisional Patent Application No. 61/980,741, filed on Apr. 17, 2014. 
    
    
     FIELD 
     The present disclosure relates to a system and method for GSM (Global System for Mobile Communications) A3/A8 authentication in an IMS (IP Multimedia Subsystem) network. 
     BACKGROUND 
     “Authentication” is the means by which a cellular network can validate the identity of a subscriber, or more accurately, of a Subscriber Identity Module (SIM) that is inserted into a mobile device. The Authentication Center (AuC) (which is typically part of or co-located with the Home Location Register (HLR) or Home Subscriber Server (HSS)) is configured to authenticate each SIM card that attempts to connect to the GSM core network. Once authentication is successful, the HLR is allowed to manage the SIM and subscriber services. The SIM and AuC have a shared secret that is unique to a given subscriber/SIM. The shared secret is used as an input to an authentication algorithm, either A3 or A8, that are executed at both the mobile device/SIM and the AuC. If the results from the mobile device and AuC match, authentication is deemed successful. 
     Successful authentication results in the establishment of a security association between the SIM and the network. This security association is basically a set of data, such as an integrity key and a ciphering key, which are used to provide security services during the life of that security association. 
     The IMS (IP Multimedia Subsystem) defines a generic architecture for offering multimedia services such as Voice over IP (VoIP). The IMS core network includes the Call Session Control Function (CSCF) and the Home Subscriber Server (HSS). The CSCF facilitates session setup and teardown using SIP (Session Initiation Protocol). The HSS plays the role of a location server in IMS and also serves as a repository for subscriber data. The CSCF is divided into three logical entities: Proxy CSCF (P-CSCF), Interrogating CSCF (I-CSCF), and Serving CSCF (S-CSCF). The P-CSCF is responsible for routing incoming SIP messages to the IMS registrar server and for facilitating policy control. The I-CSCF acts as an inbound SIP proxy server in the IMS. The S-CSCF is the heart of the IMS core network, and facilitates the routing path for mobile originated or terminated session requests and is the most processing intensive node of the IMS core network. 
     Before a user can have access to IMS services, an IMS-capable User Equipment (UE) equipped with a Universal Subscriber Identity Module (USIM) or an IP Multimedia Subscriber Identity Modules (ISIM) must authenticate with the S-CSCF. However, for older SIM-based UEs, access to IMS is not possible as the IMS specifications do not support the SIM-based A3/A8 authentication protocol, but rather define a newer security protocol called AKA or Authentication and Key Agreement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of an exemplary embodiment of GSM A3/A8 authentication in legacy CS networks according to the present disclosure; 
         FIG. 2  is a simplified block diagram of an exemplary embodiment of A3/A8 authentication; 
         FIG. 3  is a simplified block diagram of an exemplary embodiment of UMTS AKA Authentication in Legacy CS Networks according to the present disclosure; 
         FIG. 4  is a simplified block diagram of an exemplary embodiment of AKA authentication; 
         FIG. 5  is a simplified block diagram of an exemplary embodiment of the IMS core; 
         FIG. 6  is a simplified block diagram of an exemplary embodiment of AKA Authentication in IMS Networks; 
         FIG. 7  is a simplified block diagram of an exemplary embodiment of GSM A3/A8 Authentication in an IMS network according to the present disclosure; 
         FIG. 8  is a simplified block diagram of an exemplary embodiment of transporting A3/A8 authentication parameters inside AKA parameters according to the present disclosure; and 
         FIG. 9  is a simplified data flow diagram of an exemplary embodiment of GSM A3/A8 Authentication in an IMS network according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified block diagram of an exemplary embodiment of GSM A3/A8 authentication in legacy CS (circuit switched) networks according to the present disclosure. A UE (User Equipment)  12  with a SIM (Subscriber Identity Module)  14  accesses a legacy CS network  16 . The MSC (Mobile Switching Center)  18  receives A3/A8 authentication vectors from the AuC (Authentication Center)  20  that is typically part of or co-located with the HLR (Home Location Register)  22 . The MSC  18  then issues an A3/A8 authentication challenge to the UE  12 . The UE  12  runs the A3/A8 authentication algorithm and sends its response to the MSC  18 . The MSC  18  then checks to determine whether the UE&#39;s response matches the HLR&#39;s response. If the results from the UE  12  and the HLR  22  match, then authentication is successful and the UE  12  may access services provided by the network. 
       FIG. 2  is a simplified block diagram of an exemplary embodiment of A3/A8 authentication method or function. As described above, older GSM (Global System for Mobile Communications) SIMs (Subscriber Identity Modules) in mobile telephones (User Equipment or UE) use an authentication algorithm called A3 (authentication)  24  and A8 (confidentiality key creation)  26 . The A3/A8 algorithm receives as input a 128-bit subscriber secret, Ki, and a random number, RAND (128-bit). The subscriber secret, Ki, is a secret key that is associated with the IMSI (International Mobile Subscriber Identity) of the subscriber. The AuC is configured to look up the Ki using the IMSI, and the Ki is also stored on the SIM card in the UE. The A3 algorithm generates a Signed Response, SRES (4 bytes), and the A8 algorithm generates a Confidentiality Key or Ciphering Key, Kc (8 bytes). The Kc will be used in an algorithm called A5 to encrypt and decrypt data that will be transmitted on the Um interface with the UE. Therefore, the AuC also performs the same A3/A8 authentication function  24 ′ and  26 ′. If the SRES calculated by the SIM  14  matches the SRES calculated by the AuC  20 , the authentication is successful and the confidentiality key, Kc, can then be used for signaling between the mobile device (UE  12 ) and the GSM network. 
     With the introduction of Universal Mobile Telecommunications System (UMTS), UEs are equipped with USIMs (Universal Subscriber Identity Module) that use a newer and stronger authentication algorithm called AKA (Authentication and Key Agreement). Both A3/A8 and AKA authentication algorithms are supported in legacy circuit switched (CS) networks today. 
     In  FIG. 3 , a UE  30  equipped with a USIM (Universal Subscriber Identity Module)  32  accesses the legacy CS network  16 . The MSC  18  receives AKA authentication vectors from the HLR  22 . The MSC  18  then issues an AKA authentication challenge to the UE  30 . The UE  30  runs the AKA authentication algorithm and sends its response to the MSC  18 . The MSC  18  then checks to determine whether the UE&#39;s response matches the HLR&#39;s response. 
     As shown in  FIG. 4 , the AKA algorithm  34  receives, as input, the subscriber secret K, a sequence number (SQN), and a random number RAND. Its outputs are an Expected Response XRES (8 bytes), an Authentication Token AUTN (16 bytes), a Confidentiality Key CK (16 bytes), and an Integrity Key IK (16 bytes). If the XRES calculated by the USIM  32  matches the XRES calculated by the AuC  20 , the authentication is successful and both the confidentiality key, CK, and integrity key, IK, are then be used in signaling between the mobile device and the network. 
       FIG. 5  is a simplified block diagram of network nodes of an IMS (IP Multimedia Subsystem) core network  40 . The IMS  40  is a standardized Next Generation Network (NGN) architecture defined by the European Telecommunication Standards Institute (ETSI) and the 3rd Generation Partnership Project (3GPP) to provide Internet media services capability. As with the Internet, NGN is built around the Internet Protocol (IP) and its goal is to create a unified system that offers services like video, voice and data by encapsulating them into packets. The NGN architecture can incorporate a variety of wireless and wireline technological alternatives for users to access the global telecommunication network. IMS is a set of standards that defines a generic architecture for offering Voice over IP (VoIP) and multimedia services. The IMS core network  40  includes the Call Session Control Function (CSCF)  42  and the Home Subscriber Server (HSS)  44 . The CSCF  42  node facilitates session setup and teardown using SIP (Session Initiation Protocol). The HSS  44  plays the role of a location server in the IMS and also serves as a single point of service for IMS subscribers and their services. 
     As shown in  FIG. 5 , the CSCF functionality  42  is divided into three logical entities: Proxy CSCF (P-CSCF)  46 , Interrogating CSCF (I-CSCF)  47 , and Serving CSCF (S-CSCF)  48 . The P-CSCF  46  is responsible for routing incoming SIP messages to the IMS registrar server and for facilitating policy control. The I-CSCF  47  acts as an inbound SIP proxy server in the IMS network. The S-CSCF  48  is the heart of the IMS core network  40 , and facilitates the routing path for mobile originated or terminated session requests and is the most processing intensive node of the IMS core network. The S-CSCF  48  acts as the registrar sending an authentication challenge to a UE  50  with authentication vectors supplied by the HSS  44 . With the introduction of IMS, UEs incorporate an ISIMs (IP Multimedia Subscriber Identity Modules)  52 . Once the UE  50  has successfully completed the authentication process, the S-CSCF  48  completes the registration and notifies the HSS  44 . Because the IMS is specified to provide support for AKA authentication, there is no capability to perform the older A3/A8 authentication within the IMS network. 
     In  FIG. 6 , an IMS-capable UE  60  equipped with USIM or ISIM  62  accesses an IMS-capable PS (Packet Switched) network  63 . The S-CSCF  64  of the IMS network receives AKA authentication vectors computed by the AuC  68  from the HSS  66 . The S-CSCF  64  then issues an AKA authentication challenge to the UE  60 . The UE  60  runs the AKA authentication algorithm and sends its response to the S-CSCF  64 . The S-CSCF  64  then checks that the UE&#39;s response matches the response from the HSS  66 . If the responses match, then authentication is successful. 
       FIG. 7  is a simplified block diagram of an exemplary embodiment of GSM A3/A8 Authentication in an IMS network according to the present disclosure. Referring to  FIG. 7  and to the data flow diagram in  FIG. 9 , an IMS Centralized Services Gateway (ICS GW)  70  is a network node that connects legacy GSM/UMTS access networks  72  (and thus legacy GSM/UMTS mobile devices or UEs  74 ) with an IMS core network  76 . A network node, HSS FE (Home Subscriber Server Front End)  78 , appears as an HSS to the IMS core network  76 . A UE  74  equipped with a SIM  80  attempts to access the network. The ICS GW  70  receives the UE&#39;s request and is configured to make this UE  74  appear as an AKA-capable IMS UE to the IMS core network  76 . The ICS GW  70  sends a register message to the S-CSCF  82  with an IMPI (IP Multimedia Private Identity) associated with the IMSI (International Mobile Subscriber Identity) received from the UE  74 . The S-CSCF  82  then requests AKA authentication vectors from the HSS (HSS FE)  78 . The HSS FE  78  retrieves authentication vectors from the HLR  84 /AuC  86 , and detects that it has received A3/A8 vectors (SRES and KC). The HSS FE  78  then encodes or embeds the A3/A8 authentication vector information (SRES and KC) into the parameters defined for AKA (XRES, CK, IK, AUTN) and sends the disguised information to the S-CSCF  82  along with the Random number, RAND. The S-CSCF  82  does not detect the disguised A3/A8 vector information and issues the authentication challenge towards the UE (via the ICS GW  70 ). The ICS GW  70  detects that what is being passed in the AKA parameters is actually A3/A8 vector information and instead issues an A3/A8 authentication challenge containing RAND to the UE  74 . The UE  74  runs the A3/A8 authentication algorithm and sends its response (SRES) to the ICS GW  70 . The ICS GW  70  encodes or embeds the A3/A8 response (SRES) into the parameters defined for AKA. Again, the S-CSCF  82  does not detect the A3/A8 response, and checks that the UE&#39;s response matches the HSS&#39;s response. In this way, the IMS core network is able to perform GSM A3/A8 authentication for a SIM-based UE  74 . 
     In an alternate embodiment, an actual HSS may function in the place of the HSS FE  78 , for example. As a further alternative, a Visiting Subscriber Server (VSS) may implement the functionalities of the HSS FE  78 . 
     Accordingly, the present disclosure provides a way to interface with a UE having an older GSM SIM so that there is support for A3/A8 authentication within an IMS core network that by definition does not support this older authentication algorithm. In the method disclosed herein, the IMS core network thinks that AKA authentication is being used for authentication when in fact GSM A3/A8 is being used. In this way, no change or adaptation in the IMS core network is required to interface with older SIM-based UEs. 
     In the preferred embodiment, A3/A8 parameters are contained within AKA authentication parameters that are passed in the IMS core network. Because the A3/A8 parameters are shorter than the AKA authentication, this can be easily accomplished. For example, the A3/A8 parameters can be accommodated in the manner as illustrated in  FIG. 8 : 
     The GSM SRES parameter  80  is 4 bytes and the UMTS XRES parameter  81  is 8 bytes. Therefore, an XRES parameter  81  can include the SRES parameter  80  and 4 additional bytes of filler bits  82 , which may be set to zeroes, for example. 
     The GSM Kc parameter  84  is 8 bytes and the UMTS CK parameter  85  is 16 bytes. Therefore, a CK parameter  85  can be constructed to contain the Kc parameter  84  (8 bytes) and 8 additional bytes of filler bits  86 , which may be set to zeroes, for example. 
     There is no GSM integrity key and the UMTS IK parameter  88  is 16 bytes. Therefore, an IK parameter  88  can contain 16 bytes of zeroes  89 , for example. 
     There is no GSM authentication token parameter (AUTN)  90  and the UMTS AUTN is 16 bytes, so an AUTN parameter  90  can contain 16 bytes of zeroes  91 , for example. 
     Because the parameter sizes match what is expected with AKA authentication information, the S-CSCF  82  is unaware that A3 and A8 parameters are actually being passed. All of the S-CSCF&#39;s current functionality and checks will still function properly. Only the IMS HSS FE  78  and the ICS GW  70  are aware what authentication parameters are really being used for authentication. 
     The features of the present invention which are believed to be novel are set forth below with particularity in the appended claims. However, modifications, variations, and changes to the exemplary embodiments described above will be apparent to those skilled in the art, and the system and method described herein thus encompasses such modifications, variations, and changes and are not limited to the specific embodiments described herein.