Abstract:
A Terminal Identity Token is created for identifying a User Equipment (UE) connected to a radio base station in a radio system. The UE communicates with the radio base station via a secure communication associated with an existing cryptographic key. The Terminal Identity Token is created based on a physical cell identity of a target cell known to both the UE and the radio base station, the terminal identity, and the existing key. By using the Terminal Identity Token, a secure communication can be established and enhanced without having to provide for additional security network components or additional signaling.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/746,879, filed 8 Jun. 2010, which was the National Stage of International Application No. PCT/EP2008/058450, filed 1 Jul. 2008, which claims the benefit of U.S. Provisional Application No. 61/012,814, filed 11 Dec. 2007, the disclosures of each of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method and a device for providing secure communication in a cellular radio system. 
     BACKGROUND 
     The evolved packed system (EPS) is a standardized cellular telecommunication standard, standardized within the Third Generation Partnership Project (3GPP). EPS is a part the long-term evolution (LTE) of third-generation cellular systems designed to meet demands for higher user bit rates. Within the EPS, Access Stratum (AS) traffic is protected by cryptographic means. In particular the user plane is confidentiality protected and the Radio Resource Control (RRC) signaling is both confidentiality and integrity protected. The keys used for providing encryption are derived from a cryptographic key called K_eNB. 
     At handovers of mobile station, also referred to as User Equipment (UE), from one base station the K_eNB of an originating source base station is transformed in the source evolved Node B (eNB), i.e. the base station into a transformed key called K_eNB* before it is delivered to the target eNB. Presently, the target eNB transforms the K_eNB* together with a user target eNB Cell Radio Network Temporary Identifier (C-RNTI). Hereby it is possible to provide continued encryption between the UE and the target base station using the transformed cryptographic key. 
     Furthermore it has been decided that not only the intended target cell should be prepared to accept a particular mobile station but also other base stations shall be able to do so. The underlying reason is to help recover from radio link failure, and in particular failed handovers. To facilitate acceptance by other base stations besides the target base station, the source base station eNB sends key information and a Terminal Identity Token (TeIT) to the set of “to-be-prepared” base stations. Typically the source base station eNB sends key information and a Terminal Identity Token (TeIT) to base stations located close to the target base station and/or close to the source base station. However, if the same security token is shared by all eNBs in the to-be-prepared set, any one of those could masquerade as the mobile station, at least until AS protection is enabled. 
     A problem within the existing proposed standard is that the same transformed key K_eNB* should not be used by all base stations as this would allow all base stations in the to-be-prepared set to generate the K_eNB finally used by the base station after hand over, see the contribution to SA3, Td S3a070975. One proposed solution is that the system generates initial data which is used in transformation of K_eNB for a given base station eNB in the to-be-prepared set of base stations. This initial data is then forwarded together with the corresponding base station key K_eNB* to the base station eNB. Also, the document “Key refresh in SAE/LTE, S3-070234”, XP-002445697 describes a method where data is sent over the air-interface to generate input when generating a new base station key. 
     However, there exists a constant demand to reduce complexity and enhance security in existing telecommunication systems. Hence there exists a need for an improved method for providing secure communication in a cellular radio system. 
     SUMMARY 
     It is an object of the present invention to provide an improved method for providing secure communication in a cellular radio system. 
     This object and others are obtained by the method, radio system node and User Equipment and as set out in the appended claims. Thus, by creating a radio base station key and/or a Terminal Identity Token using data known to both the mobile station and the radio base station a secure communication can be established and enhanced without having to provide for additional security network components or additional signaling. 
     In accordance with one embodiment a derived radio base station key in a cellular radio system is generated. The derived radio base station key is created in response to a determined set of public data bits and an existing cryptographic key used for secure communication between a radio base station and a User Equipment. The public data can for example be data bits associated with the Radio Access Technology such as data bits identifying the physical cell identity. Hereby a cryptographic base station key specific for each radio base station is derived for each radio base station thereby increasing the security in the system. Moreover the specific cryptographic key(s) can be derived without additional signaling and/or need for generating specific input data when deriving a cryptographic key being specific for each radio base station, which reduces the complexity and provides a high level of security. 
     In accordance with one embodiment a Terminal Identity Token for identifying a User Equipment, UE, connected to a radio base station in a radio system is created. The UE is adapted to communicate with the radio system via a secure communication associated with an existing encryption key. When creating the Terminal Identity Token a set of data bits known to both the UE and the source radio base station is determined. The Terminal Identity Token is then generated in response to the determined set of data bits, the terminal identity and the existing key. Hereby a Terminal Identity Token is derived that is specific for each radio base station thereby increasing the security in the system. 
     In accordance with one embodiment a method of identifying a User Equipment, UE, in a radio system is provided. The UE communicates with the radio system via a secure communication associated with an existing encryption key. A first Terminal Identity Token is generated in a radio base station to which the User Equipment is currently connected. The first Terminal Identity Token is then distributed to a number of other radio base stations of the radio system. A second Terminal Identity Token is also generated in the User Equipment. The second token is transmitted to one of the other radio base stations. When the second token is received by a radio station the UE is identified by comparing the first and second Terminal Identity Token. Both the first and second Terminal Identity Tokens are created in response to the terminal identity and the existing key. Hereby a terminal dropping a connection can reconnect to the system via a secure identification procedure. 
     The present invention also extends to nodes and User Equipments adapted to implement the methods in as set out above. 
     Using the methods, nodes and User Equipments in accordance with the invention will provide a more efficient and secure procedure for providing secure communication in a radio system. This is obtained by using data available for the radio base station and the User Equipment when deriving a cryptographic key or a Terminal Identity Token. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which: 
         FIG. 1  is a view illustrating a cellular radio system, 
         FIG. 2  is a flow chart illustrating steps performed in a procedure for creating a radio base station key, 
         FIG. 3  is a flow chart illustrating steps performed in a source radio base station when preparing for verifying the authenticity of a moving mobile station, 
         FIG. 4  is a flowchart illustrating steps performed in a target radio base station when verifying the authenticity of a mobile station, 
         FIG. 5  is a flow chart illustrating steps performed in a mobile station when verifying the authenticity of the mobile station to a cellular radio system, 
         FIG. 6  is a view of a radio base station, and 
         FIG. 7  is a view of a User Equipment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following reference in the exemplary embodiments described will be made to an LTE system. However, the invention is not limited to an LTE system but is applicable to any radio system using radio base station keys for protecting data transmitted to and from a mobile station associated with the radio base station. 
     In  FIG. 1  a schematic view of a cellular radio system  100  providing encrypted communication for a mobile station also termed User Equipment (UE)  101  is shown. The UE  101  transmits and receives data to and from a radio base station  103 . In the case when the cellular radio system is an LTE system the radio base station  103  is commonly termed evolved NodeB (eNB). When the UE  101  moves around in the geographical area covered by the cellular radio system  100  it will sometimes be necessary to hand over the connection from one radio base station to another radio base station. Also sometimes the UE may drop the connection to the cellular radio system  100  and may then need to reconnect to the cellular radio system. In both these scenarios it is desired to maintain a secure connection between the cellular radio system  100  and the UE  101 . 
     In case the UE moves from an area covered by the radio base station  103  towards an area covered by a radio base station  105  the cellular radio system prepares for a handover from the source radio base station  103  to the target radio base station  105 . Also because it can sometimes be difficult to predict which radio base station that a mobile station  101  will be handed over to a number of other radio base stations may also be prepared for handover. The “be prepared” radio base stations are  FIG. 1  represented by a single radio base station  107 . 
     During handover a new radio base station key needs to be derived that can be used for continued secure communication between the mobile station  101  and the radio base station  105 ,  107  to which the connection is transferred after handover. The new base station key can be termed a transformed or derived base station key. In the case the cellular radio system is an LTE system the transformed key can be labeled K_eNB*. 
     In accordance with one aspect of the present invention, the information for creating unique transformed base station key K_eNB* in the to-be-prepared set can be based on the least significant bits of an identity which is known by (or is made known to) both the mobile station UE and the radio base station eNB. For example the nine-bit E-UTRAN physical cell identity, here denoted PhyCell_ID can be used or some other cell specific data determined by the Radio Access Technology (RAT) context. The transformation can in accordance with one embodiment make use of a Pseudo-Random Function (PRF) or a hash function with source base station key K_eNB and the cell data such as the PhyCell_ID bits as input. Also other input parameters can be included. Examples of other parameters can be C-RNTI or any other user specific information, data identifying when the key can be used, etc. 
     The mobile station will know the PhyCell_ID bits from its Radio Access Technology (RAT) context. In the exemplary embodiment above the derivation of a target base station key K_eNB* for a given base station eNB with PhyCell_ID can in one exemplary embodiment be written as:
         K_eNB*=PRF(K_eNB_Source), PhyCell_ID bits, Other_parameters)       

     In accordance with another aspect of the present invention, a Terminal Identity Token TeIT can be formed in a corresponding way and be made unique per base station eNB, i.e. it can also be derived by applying a PRF on the mobile station identity, the base station key K_eNB of the source and the PhyCell_ID bits of the receiving base station eNB. Also other input parameters can be included. Examples of other parameters can be C-RNTI or any other user specific information, data identifying when the key can be used, etc. 
     Furthermore, when a mobile station is required to prove its identity it can be adapted to generate the corresponding identity token. This can typically be the case during handover when the User Equipment connects to a new radio base station and the system needs to verify the identity of the User equipment or if the connection to a User Equipment is dropped and the User Equipment needs to re-connect to the system. 
     In accordance with one embodiment a Terminal Identity Token 1 (TeIT1) can be defined as:
         TeIT1=PRF(K_eNB_Source, Terminal_ID, PhyCell_ID bits, Other_parameters)       

     In  FIG. 2  a flow chart illustrating steps performed when generating a base station encryption key for a secure connection between a mobile station  101  and a cellular radio system  100  when the secure connection is handed over from one source radio base station  103  to a target radio base station  105 ,  107  is shown. First in a step  201 , the system detects that there may be a handover. For example the system may determine that the mobile station  101  is close to the cell border of the source radio base station based on radio measurements. Next, in a step  203  the source radio base station generates and transmits a transformed Base station Key to the target base station  105 . In step  203  the source base station  103  can also send a transformed Base station Key to a set of “be prepared” radio base stations  107 . The transformed base station Key can be derived in accordance with the above. In accordance with one embodiment the source base station also transmits a Terminal Identity Token in a step  205 . The Terminal Identity Token can for example be a token generated as the token TeIT1 as described above. Thereupon handover can be performed in a conventional manner as indicated by step  207 . 
     In accordance with another embodiment of the present invention the source base station eNB can be adapted to distribute a common token, TeIT3, to all base stations in the to-be-prepared set. This token can be the output from a PRF applied to the output from a second PRF, which takes at least the terminal identity and K_eNB as input. Also other input parameters can be provided. Examples of other parameters can be C-RNTI or any other user specific information, data identifying when the key can be used, etc. 
     In accordance with one exemplary embodiment, when a mobile station transmits its identity token information, TeIT2, it transmits the PRF of the terminal identity and the base station key K_eNB. The receiving base station can apply the outer PRF on the TeIT2 received from the terminal and compare the outcome against the identity token, i.e. the TeIT3 received from the source base station. If the two entities correspond, the terminal identity is determined to have been established. Expressed differently the Terminal Identity Tokens 2 an 3 can be written:
         TeIT2=PRF(K_eNB_Source, Terminal_ID, Other_parameters)   TeIT3=PRF(TeIT2, Other_parameters2)       

     A comparison in a base station of TeIT2 received from mobile station and TeIT3 received from source eNB can be performed as follows:
         TeIT3=?=PRF(TeIT2, Other parameters2),   where=?=denotes a comparison operation.       

     In the above descriptions the Terminal_ID can for example be the C-RNTI assigned to the terminal in the source base station eNB or any other user specific information, data identifying when the key can be used, etc. 
     Thus if for example a connection for a UE  101  is dropped and the UE  101  needs to reconnect to the cellular radio system  100 , all the radio base stations having access to the TeIT3 identity token information can verify the authenticity of a mobile station transmitting the identity token TeIT2. 
     In accordance with one embodiment the transformed base station key K_eNB* for the target base station eNB can be derived in the same way as for the radio base station eNBs in the to-be prepared set. The target base station eNB can then receive the same type of information as all other prepared base stations eNBs as the hand over may fail and the mobile station will then try to reconnect to the intended target base station eNB. 
     In  FIG. 3  a flowchart illustrating steps performed in a source radio base station when preparing for verifying the authenticity of a moving mobile station is shown. First in a step  301  a source radio base station determines to transmit a terminal identity token to a number of other radio base stations. The reason for transmitting the terminal identity token may for example be that there is a handover procedure in progress. The terminal identity token can for example be generated as the TeIT3 token described above in a step  303 . Next, the token is transmitted to the other radio base stations in a step  305 . The other radio base stations can typically be adjacent radio base stations, which the mobile station is likely to connect to in the near future. 
     In  FIG. 4  a flowchart illustrating steps performed in a target radio base station when verifying the authenticity of a mobile station. First in a Step  401  the target radio base station receives a token identity TeIT3 from a source radio base station. Next, in a step  403 , the target radio base station receives a terminal identity token TeIT2 from a mobile station. Thereupon, in a step  405  the target base station compares the token identity TeIT3 with the terminal token identity TeTI2. Finally, in a step  407  the target radio base station verifies the authenticity of the mobile station based on the comparison in step  405 . 
     In  FIG. 5  a flow chart illustrating steps performed in a mobile station when verifying the authenticity of the mobile station to a cellular radio system. First, in a step  501 , the mobile station is triggered to send an authentication message. For example during handover or when a connection is dropped the mobile station may need to (re-) authenticate itself to a radio base station of the cellular radio system. Next in a step  503 , the mobile station generates a terminal identity token. The terminal identity token can be generated as the terminal identity token TeIT2 described above. Finally, the mobile station transmits an authentication message to a radio base station of the cellular radio network in a step  505  based on which the cellular radio system can authenticate the mobile station. 
     In  FIG. 6  an exemplary radio base station  103  adapted to generate a transformed cryptographic radio base station key in accordance with the above is depicted. The radio base station comprises a module  601  for selecting data to be used when creating a cryptographic key or a Terminal identity Token in accordance with the above. The module  601  is connected to a module  603  adapted to generate a cryptographic key or a Terminal identity Token in accordance with the above. The exemplary radio base station  103  may also comprise an identifier module  605  adapted to identify a UE connecting to the radio base station using a Terminal Identity Token as described above. 
     In  FIG. 7  an exemplary User Equipment (UE)  101  adapted to generate a Terminal Identity Token in accordance with the above is depicted. The UE comprises a module  701  for selecting data to be used when creating a Terminal identity Token in accordance with the above. The module  701  is connected to a module  703  adapted to generate a Terminal identity Token in accordance with the above. A Terminal Identity Token generated in the module  703  can be transmitted by a token transmitter  705  connected to the module  703 . 
     Using the method and system as described herein will provide a more efficient procedure for providing secure communication in a cellular radio system both in a handover situation and in situations calling for authentication of a mobile station.