Patent Publication Number: US-11039346-B2

Title: Handover of a device which uses another device as relay

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a national stage entry of, and claims priority to, PCT/EP2017/078380, filed on Nov. 7, 2017, which claims priority to European Patent Application EP 16197578.4, filed in the European Patent Office on Nov. 7, 2016, both of which are hereby incorporated in their entirety herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a system for receiving encrypted information in a mobile communication network. 
     The invention further relates to a method of receiving encrypted information in a mobile communication network. 
     The invention also relates to a computer program product enabling a computer system to perform such a method. 
     BACKGROUND OF THE INVENTION 
     The amount of devices that is able to connect to the Internet is expected to grow enormously, especially as a result of everyday objects having network connectivity. Not only do users communicate via Internet-connected devices, but devices also send data, e.g. sensor data or maps, to other devices (machine-to-machine communication) over the Internet. It is estimated that by the year 2020 50 billion devices will be connected. Therefore, it is no surprise that industry efforts and investments are directed towards this field. 
     The category of mobile devices, e.g. mobile phones, tablets, wearable devices and devices embedded in vehicles, is an important category of devices that is able to connect to the Internet. In order to e.g. reduce power consumption, it is advantageous to tether certain devices, e.g. wearable devices, to other devices, e.g. smart phones. When these certain devices do not trust these other devices, but do trust the mobile communication network to which these other devices are connected, protection of data between these certain devices and the mobile communication becomes important. 
     WO 2016/003750 discloses techniques for securely receiving critical communication content associated with a critical communication service. A secure connection from the critical communication service to remote user equipment (UE) may be established through a relay UE in order for the remote UE to securely receive critical communication content from the critical communication service. The remote UE and the critical communication service agree on common key material that can be used to securely relay a master session key, which may be for use by only the remote UE to decrypt encrypted critical communication content sent from the critical communication service and routed through the relay UE. 
     A drawback of these techniques is that each service requires the setup of an own end-to-end secure tunnel. A solution in which a secure tunnel between a remote UE and the mobile communication network may be used for multiple services is preferable. 3GPP TS 33.401 (v13.4.0; System Architecture Evolution (SAE); Security architecture) specifies how to setup a secure tunnel between a UE and an eNodeB in an LTE mobile communication network. This secure tunnel can be used for multiple services. However, 3GPP TS 33.401 does not specify how the secure tunnel can be maintained or re-established between a remote UE and an eNodeB in case of a handover, e.g. when a remote UE switches to another relay UE or when a relay UE used by a remote UE connects to another eNodeB. 
     SUMMARY OF THE INVENTION 
     It is a first object of the invention to provide a system for receiving encrypted information in a mobile communication network, which helps a device which uses another device as relay maintain or re-establish a secure tunnel after a handover without requiring the device to setup a secure tunnel with each service separately. 
     It is a second object of the invention to provide a method of receiving encrypted information in a mobile communication network, which helps a device which uses another device as relay maintain or re-establish a secure tunnel after a handover without requiring the device to setup a secure tunnel with each service separately. 
     According to the invention, the first object is realized in that the system for receiving encrypted information in a mobile communication network comprises a communication interface and at least one processor configured to maintain a list of one or more devices associated with said system in a memory, wherein for each first device of said list which uses a second device of said list as a relay to said mobile communication network, said relation between said first device and said second device is recorded in said list and a security endpoint is recorded for said first device in said list, further configured to use said communication interface to receive encrypted information from a device, further configured to determine from said list whether said device is used by a further device as a relay to said mobile communication network, further configured to use said communication interface to forward said encrypted information to a security endpoint associated with said further device if said device is used by said further device as a relay to said mobile communication network and said security endpoint is not said system, and further configured to decrypt said encrypted information if said device is not used by a further device as a relay to said mobile communication network. The system may comprise, for example, a base station, e.g. an LTE eNodeB. 
     The inventors have recognized that by using a single security endpoint (in the mobile communication network) for devices which use another device as relay, a secure tunnel does not need to be setup for each service separately, while the secure tunnel can be maintained after a handover, because the security endpoint does not need to be the system (e.g. the eNodeB) that the other device (e.g. the relay UE) is connected to. The inventors have recognized that for devices which do not use another device as relay, it is beneficial to setup a secure tunnel between the device (e.g. UE) and the system (e.g. eNodeB) directly, e.g. in accordance with 3GPP TS 33.401. As an additional advantage, since the device that uses another device as relay does not need to be involved in maintaining or re-establishing a secure tunnel, the overhead that would be caused by such an involvement is avoided, which is especially beneficial in case of small data transfer. 
     Said at least one processor may be configured to decrypt said encrypted information if said device is used by a further device as a relay to said mobile communication network and said system is associated as a security endpoint with said further device. This allows the system e.g. a base station, to be the security endpoint. As long as the device does not switch to a further device that is connected to a different base station and the further device does not connect to a different base station, no forwarding may be needed. 
     Said at least one processor may be configured to use said communication interface to connect (i.e. to connect directly, e.g. by establishing a wireless link) to a second device which is used by a first device as a relay, said second device previously connecting to a further system, and to record in said list said further system as said security endpoint for said first device. Said at least one processor may be configured to use said communication interface to receive a notification that a first device is using or wants to use a second device connected to said system as a relay and if said first device previously used a third device as a relay and said third device was connected to a further system, to record in said list said further system as said security endpoint for said first device. 
     When the system and the further system are base stations, base stations can be used as security endpoints and if no security endpoint has been associated with the first device yet, the base station with which the first device is already associated may be used as security endpoint so that that encrypted communication is forwarded to this base station. The first device may be associated with another base station than the base station that the second device that it uses as a relay is connected to and recorded in the list of this other base station when the first device uses a second device as relay that previously connected to this other base station or when the first device is using or previously used a third device as relay which was connected to this other base station, for example. 
     Said at least one processor may be configured to use said communication interface to receive information identifying a security endpoint, e.g. a serving gateway, and to record said security endpoint for one or more devices associated with said system in said list. For example, said at least one processor may be configured to use said communication interface to receive information identifying a certain serving gateway for further devices that use a device as a relay to said mobile communication network from a mobility management function. The same serving gateway may be used for all devices that use another device as relay, for example. If the base station, e.g. eNodeB, is not used as security endpoint for devices that use another device as relay, then it does not have to perform decryption as often and may be able to serve more devices. 
     According to the invention, the second object is realized in that the method of receiving encrypted information in a mobile communication network comprises maintaining a list of one or more devices associated with a system of a mobile communication network, wherein for each first device of said list which uses a second device of said list as a relay to said mobile communication network, said relation between said first device and said second device is recorded in said list and a security endpoint is recorded for said first device in said list, receiving at said system encrypted information from a device, determining from said list whether said device is used by a further device as a relay to said mobile communication network, forwarding said encrypted information to a security endpoint associated with said further device if said device is used by said further device as a relay to said mobile communication network and said security endpoint is not said system, and decrypting said encrypted information if said device is not used by a further device as a relay to said mobile communication network. 
     Said method may further comprise decrypting said encrypted information if said device is used by a further device as a relay to said mobile communication network and said system is associated as a security endpoint with said further device. 
     Said method may further comprise connecting to a second device which is used by a first device as a relay, said second device previously connecting to a further system, and recording in said list said further system as said security endpoint for said first device. 
     Said method may further comprise receiving a notification that a first device is using or wants to use a second device connected to said system as a relay and if said first device previously used a third device as a relay and said third device was connected to a further system, recording in said list said further system as said security endpoint for said first device. 
     Said method may further comprise receiving information identifying a security endpoint, e.g. a serving gateway, and recording said security endpoint for one or more devices associated with said system in said list. 
     Said method may further comprise receiving information identifying a certain serving gateway for further devices that use a device as a relay to said mobile communication network from a mobility management function. 
     Moreover, a computer program for carrying out the methods described herein, as well as a non-transitory computer readable storage-medium storing the computer program are provided. A computer program may, for example, be downloaded by or uploaded to an existing device or be stored upon manufacturing of these systems. 
     A non-transitory computer-readable storage medium stores at least one software code portion, the software code portion, when executed or processed by a computer, being configured to perform executable operations comprising: maintaining a list of one or more devices associated with a system of a mobile communication network, wherein for each first device of said list which uses a second device of said list as a relay to said mobile communication network, said relation between said first device and said second device is recorded in said list and a security endpoint is recorded for said first device in said list, receiving at said system encrypted information from a device, determining from said list whether said device is used by a further device as a relay to said mobile communication network, forwarding said encrypted information to a security endpoint associated with said further device if said device is used by said further device as a relay to said mobile communication network and said security endpoint is not said system, and decrypting said encrypted information if said device is not used by a further device as a relay to said mobile communication network. 
     As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a device, a method or a computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit”, “module” or “system.” Functions described in this disclosure may be implemented as an algorithm executed by a processor/microprocessor of a computer. Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied, e.g., stored, thereon. 
     Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of a computer readable storage medium may include, but are not limited to, the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of the present invention, a computer readable storage medium may be any tangible medium that can contain, or store, a program for use by or in connection with an instruction execution system, apparatus, or device. 
     A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device. 
     Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber, cable, RF, etc., or any suitable combination of the foregoing. Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java(™), Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer, or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
     Aspects of the present invention are described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the present invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor, in particular a microprocessor or a central processing unit (CPU), of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer, other programmable data processing apparatus, or other devices create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of devices, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the blocks may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustrations, and combinations of blocks in the block diagrams and/or flowchart illustrations, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention are apparent from and will be further elucidated, by way of example, with reference to the drawings, in which: 
         FIG. 1  is a block diagram of an embodiment of the system of the invention; 
         FIG. 2  is a flow diagram of a first embodiment of the method of receiving encrypted information in a mobile communication network of the invention; 
         FIG. 3  is a flow diagram of a second embodiment of the method of receiving encrypted information in a mobile communication network; 
         FIG. 4  is a flow diagram of a third embodiment of the method of receiving encrypted information in a mobile communication network; 
         FIG. 5  shows keys used in an LTE mobile communication network; 
         FIG. 6  is a flow diagram showing a first example of a device starting to use a further device as a relay in a first embodiment of the method; 
         FIG. 7  is a flow diagram showing a second example of a device starting to use a further device as a relay in the first embodiment of the method; 
         FIG. 8  is a flow diagram showing an example of the further device of  FIG. 6  being handed over to another base station; 
         FIG. 9  is a flow diagram showing an example of the device of  FIG. 6  starting to use another device as a relay instead of the further device; 
         FIG. 10  is a flow diagram showing an example of a device starting to use a further device as a relay in a second embodiment of the method; 
         FIG. 11  is a flow diagram showing an example of the further device of  FIG. 10  being handed over to another base station; 
         FIG. 12  is a block diagram of an exemplary cellular telecommunication system used in an embodiment of the device and the system of the invention; and 
         FIG. 13  is a block diagram of an exemplary data processing system for performing the method of the invention. 
     
    
    
     Corresponding elements in the drawings are denoted by the same reference numeral. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a mobile device  1  (also referred to as remote UE in this description), devices  7  and  9  (also referred to as relay UEs in this description), which may relay the encrypted information for mobile device  1  to the mobile communication network, a first access point  17  and a second access point  11  (also referred to as eNBs in this description). Second access point  11  is an embodiment of a system for receiving encrypted information in a mobile communication network. First access point  17  may be an embodiment of the system for receiving encrypted information in a mobile communication network as well. First access point  17  and second access point  11  are nodes of the mobile communication network and are connected to the Core Network  21 . The Core Network  21  comprises a mobility management function (e.g. an LTE MME)  25  and a serving gateway (e.g. an LTE S-GW)  23 , amongst others. 
     The second access point  11  comprises a communication interface  13  and a processor  15 . The processor  15  is configured to maintain a list of one or more devices associated with the system in a memory  14 , wherein for each first device of the list which uses a second device of the list as a relay to the mobile communication network, the relation between the first device and the second device is recorded in the list and a security endpoint is recorded for the first device in the list. The processor  15  is further configured to use the communication interface  13  to receive encrypted information from the device  9 , determine from the list whether the device  9  is used by a further device, e.g. the mobile device  1 , as a relay to the mobile communication network, to use the communication interface  13  to forward the encrypted information to a security endpoint associated with the mobile device  1  upon determining that the device  9  is used by the mobile device  1  as a relay to the mobile communication network and the security endpoint is not the second access point  11 , and to decrypt the encrypted information upon determining that the device  9  is not used by a further device as a relay to the mobile communication network. 
     The mobile device  1  may be, for example, a wearable device like virtual reality glasses, a smart watch, augmented reality glasses, earphones, a hearing aid, a glucose sensor, a body temperature sensor, a blood pressure sensor, an insulin pump, a heart rate sensor, a GPS sensor or an accelerometer. Alternatively, the mobile device  1  may be, for example, a car that connects to the mobile device of the driver. The mobile device  1  may also be a device that the user may carry, use or interact with only occasionally and is connected to his/her mobile device only occasionally, e.g. a game console, a wireless toy, a wireless keyboard, a tablet, a screen, a beamer, or a musical instrument. The devices  7  and  9  may each be a smart phone, a laptop or a tablet, for example. The devices  7  and  9  may each have a slot for a UICC (also called a SIM card) or be provisioned with an embedded or enhanced version thereof for storage of credentials, for example. 
     In the embodiment shown in  FIG. 1 , the second access point  11  comprises one processor  15 . In an alternative embodiment, the second access point  11  comprises multiple processors. In the embodiment shown in  FIG. 1 , the memory  14  is part of the second access point  11 . In an alternative embodiment, the memory  14  is part of another device. 
     The communication interface  13  of the second access point  11  may use WiFi, Ethernet or one or more cellular communication technologies such as GPRS, CDMA, UMTS and/or LTE to communicate with the mobile device  1  and the further devices  7  and  9 , for example. The processor  15  is preferably a general-purpose processor, e.g. an Intel or an AMD processor. The processor  15  may comprise multiple cores, for example. The processor  15  may run a Unix-based or Windows operating system, for example. The second access point  11  may comprise other components typical for a component in a mobile communication network, e.g. a power supply and a random access memory. The memory  14  may comprise solid state memory, e.g. one or more Solid State Disks (SSDs) made out of Flash memory, and/or one or more hard disks, for example. 
     In a first embodiment of the second access point  11 , the processor  15  is configured to decrypt the encrypted information upon determining that the device  9  is used by the mobile device  1  as a relay to the mobile communication network and the second access point  11  is associated as a security endpoint with the mobile device  1 . The processor  15  may be configured to use the communication interface  13  to connect to the device  9 , which is used by the mobile device  1  as a relay, the device  9  previously connecting to a first access point  17 , and to record in the list the first access point  17  as the security endpoint for the mobile device  1 . The processor  15  may be configured to use the communication interface  13  to receive a notification that the mobile device  1  is using or wants to use the device  9  connected to the second access point  11  as a relay and if the mobile device  1  previously used a device  7  as a relay and the device  7  was connected to a first access point  17 , to record in the list the first access point system  17  as the security endpoint for the mobile device  1 . 
     In a second embodiment of the second access point  11 , the processor  15  is configured to use the communication interface  13  to receive information identifying a security endpoint and to record the security endpoint for one or more devices associated with the second access point  11  in the list. The security endpoint may comprise a serving gateway  23 . The processor  15  may be configured to use the communication interface  13  to receive information from a mobility management function  25  identifying the serving gateway  23  for further devices, including mobile device  1 , that use device  9  as a relay to the mobile communication network (and possibly for all further devices that use any device as a relay). 
     A flow diagram of a first embodiment of the method of receiving encrypted information in a mobile communication network of the invention is shown in  FIG. 2 . A step  61  comprising maintaining a list of one or more devices associated with a system of a mobile communication network, wherein for each first device of the list which uses a second device of the list as a relay to the mobile communication network, the relation between the first device and the second device is recorded in the list and a security endpoint is recorded for the first device in the list. In parallel, a step  65  comprises receiving at the system encrypted information from a device. 
     A step  67  comprises determining from the list whether the device is used by a further device as a relay to the mobile communication network. A step  69  comprises forwarding the encrypted information to a security endpoint associated with the further device upon determining that the device is used by the further device as a relay to the mobile communication network and the security endpoint is not the system. Alternatively, a step  71  may be performed. Step  71  comprises decrypting the encrypted information upon determining that the device is not used by a further device as a relay to the mobile communication network. After step  69  or step  71  has been performed, the method proceeds to step  65  again. 
     A flow diagram of a second embodiment of the method of receiving encrypted information in a mobile communication network is shown in  FIG. 3 . In the second embodiment, an additional step  73  comprises decrypting the encrypted information upon determining that the device is used by a further device as a relay to the mobile communication network and the system is associated as a security endpoint with the further device. Step  73  is an alternative to steps  69  and  71 . After step  69 , step  71  or step  73  has been performed, the method proceeds to step  65  again. 
     In the second embodiment, step  61  comprises a step  81  of connecting to a second device which is used by a first device as a relay, the second device previously connecting to a further system, and a step  83  of recording in the list the further system as the security endpoint for the first device. Step  61  further comprises a step  85  of receiving a notification that a first device is using or wants to use a second device connected to the system as a relay. Upon determining in decision point  86  that the first device previously used a third device as a relay and the third device was connected to a further system, a step  87  of recording in the list the further system as the security endpoint for the first device is performed. 
     A flow diagram of a third embodiment of the method of receiving encrypted information in a mobile communication network is shown in  FIG. 4 . In the third embodiment, step  61  comprises a step  91  of receiving information identifying a security endpoint and a step  93  of recording the security endpoint for one or more devices associated with the system in the list. The security endpoint may comprise a serving gateway, for example. In particular, step  91  may comprise receiving information identifying a certain serving gateway for further devices that use a device as a relay to the mobile communication network from a mobility management function. 
     The invention is explained in more detail with the help of  FIGS. 5 to 11 . Although the provided examples are based on the LTE standard and refer to LTE components, the invention may also be used in other cellular communication networks and with similar components of those networks. 
       FIG. 5  shows keys used for encryption between UE and eNB in LTE. RRC and NAS signaling may be encrypted using RRC and NAS keys, respectively. For user plane encryption, a KUP key may be used. 
     All key derivations described hereinafter are performed using the following formula (specified in 3GPP TS 33.220): derived key=HMAC-SHA-256 (Key, S). 
     For brevity, the formula is also denoted as “KDF (Key, S)”. The string S is built up from several components, one being an FC-value to separate the key derivations. Also, the String S often contains lengths (denoted Ln) in addition to the value itself. So, a string is constructed, for example, from the following values:
     FC=1   P0=KPN   P1=TNO-NL   L0=3   L1=6   

     The final string S is given by: S=FC∥P0∥L0∥P1∥L1=1KPN3TNO-NL6. Hereinafter, Len will refer to any Ln value and concatenation is denoted by ‘∥’ or ‘/’. 
     The key derivations in LTE work as follows for initial attach (specified in 3GPP TS 33.401):
         USIM and AuC (Authentication Center) derive CK, IK  113  from K  111  (using for example the Milenage algorithm) in block  101 .   UE and HSS derive K ASME    117  from CK, IK  113  and the Serving Network ID (PLMNID)  115  in block  103 . K ASME =KDF(CK∥IK, FC∥SN_id∥Len∥SQON XOR AK∥Len).   UE and MME derive the NAS Keys (K NASenc    119  and K NASInt    121 ), an K eNodeB  key  123  and an NH key (not shown) from K ASME    117  in block  105 . K eNodeB =KDF (K ASME , FC∥NAS Count∥Len). NH=KDF (K eNodeB , FC∥KeNodeB/NH∥Len).       

     UE and eNodeB derive the user plane key K UPenc    125  and the RRC keys (K RRCint    127  and K RRCenc    129 ) from K eNodeB  in block  107 . 
     Key derivations for K eNodeB  are specified in 3GPP TS 33.401. 3GPP TR 33.899 v0.4.1 specifies options for key hierarchies. 3GPP TR 33.899 v0.5.0 specifies security requirements for relay security, in particular to being able to protect sessions and uniquely identify the remote UE, which are enabled by the present invention by deriving appropriate keys and using these keys to protect the session. 
     Ciphering on the link between the eNB and the UE may be used to prevent UE tracking based on cell level measurement reports, handover message mapping, or cell level identity chaining. Another reason to provide ciphering for the link between the eNB and the UE is to (optionally) protect the user plane. 
     The following situations are considered as hand overs:
     a) Switching between direct and relayed mode for the remote UE   b) Handing over the relay UE and the remote UE together   c) Hand over between relay UEs by the remote UE   

     Situation a) is depicted in  FIG. 6  as an example of the first embodiment. In step  131 , UE 1  attaches to the Core Network (CN) via eNB 1  in the normal way. In step  133 , UE 1  discovers a new relay (UE 2 ) based on ProSe (Proximity Services) discovery. ProSe specifies that one of the UEs may broadcast and one of the UEs is listening and so either UE could be the broadcasting UE. Step  133  may involve UE 2  transmitting such a broadcast message or a series of messages to UE 1 . The broadcast message could be directed to UE 1  only, meaning that it could be addressed to UE 1  and be visible for everyone to see or that it could be protected by some encryption or obfuscation technique that allows only UE 1  to learn its contents. Ultimately, in case UE 1  and UE 2  are already on the same network, for example a local area network, the broadcast could be exchanged over the alternative network. Also, the broadcast message could be exchanged on an alternative radio network, such as short range networks like WiFi and Bluetooth. The method of message exchange does not matter as long as the message(s) exchanged between UE 1  and UE 2  contain(s) information that allows UE 1  to learn that UE 2  is a possible relay, by either exposing its capabilities, group memberships or by broadcasting a known identifier. In case UE 1  would broadcast, it would ask for a relay and UE 2  could simply answer by saying ‘yes’. Also note that the discovering UE may send a match report to the ProSe Function in order to determine whether the broadcast received is a relevant discovery. Alternatively, if UE 1  is broadcasting, UE 2  could reply to the broadcast and so the ‘broadcast’ (step  133 ) and ‘connect’ (step  135 ) arrows change direction. Proximity Services is specified in 3GPP TS 23.303 and TS 33.303. 
     In step  135 , UE 1  connects to UE 2  (it is assumed that UE 2  asks for relay resources from the network and obtains these relay resources from the network). In step  137 , UE 2  confirms to UE 1  that the connection has been established. If the direction of the arrow (i.e. of the broadcast) is reversed in step  133 , the direction of the arrows may need to be reversed in steps  135  and  137  as well. 
     In step  139 , UE 1  asks eNB 1  for a handover to UE 2 . Normally, the network is in charge of handovers, so UE may not be able to literally ask for a handover. Instead, it could send a ‘relay discovered’ message or it could send a measurement report which includes the signal from UE 2  as measurement. Also, a message could be used in which UE 1  literally sends a ‘please handover to discovered relay UE 2 ’ message. Lastly, the handover could be initiated by the network after a ProSe match report was received from UE 1  or UE 2 . In that case, step  139  could be performed by the network instead. 
     In step  141 , eNB 1  starts a search to find out who UE 2  is. (1) One solution is that UE 2  has told UE 1  its radio identifier when they connected and UE 1  may have included the radio identifier. (2) Another option is that UE 1  forwards the ProSe identifier of UE 2  and that the eNB does a lookup of the identifier via the Core Network. (3) The eNB may also try to page UE 2  based on the identifier it got from UE 1 . (4) Yet another alternative is that UE 2  forwards the handover request to eNB 1  including its GUTI/TMSI or C-RNTI or even a bearer identifier of the bearer to be switched. In case step  139  is performed by the network, the eNB may still will have to make a mapping between the identifiers known in the Core Network and the identifiers known to the Core Network, or alternatively, the Core Network will have to map between the different identifiers of ProSe and EPS bearer identities in order to identify the respective eNBs that the UEs are connected to and inform the respective eNB (eNB 1  in this case) of the findings. 
     eNB 1  may also find out that UE 2  is connected to another eNB. This is described in relation to  FIG. 7 . In the flow diagram shown in  FIG. 6 , UE 2  is also connected to eNB 1 . Once eNB 1  has found out who UE 2  is, it will assign new radio resources to UE 2  and assign a DRB ID (Data Radio Bearer Identifier) to the bearer for those radio resources. It will also tell UE 2  that these radio resources are for relay of data from UE 1  in an RRC message in step  143 . The new radio bearer(s) for UE 1  traffic that are set up between eNB 1  and UE 2  may use ‘NULL’ encryption or may be unencrypted bearers. Also, eNB 1  may specify that no integrity protection is necessary. Instead, integrity protection for the RRC or UP (if used) may be provided by UE 1 . 
     UE 1  then switches to UE 2  relay in step  148  upon receiving a command thereto from the eNB 1  in step  145 . New keys are derived by UE 1  and eNB in steps  146  and  147  (in this example before switching to UE 2 ) as follows: K eNB *=KDF(K eNB , FC∥Cell ID∥Len∥EARFCN_DL∥Len). 
     UE 1  establishes a PC-5 bearer between UE 1  and UE 2  in step  149 . UE 2  forwards traffic between the PC-5 bearer and the radio bearer for UE 1 . If there is no one-to-one relation between PC-5 bearers and radio bearers, UE 1  will have to indicate to UE 2  for which bearer the traffic is meant. UE 2  will then have to inspect each packet that it receives over the PC-5 bearer (possibly reassemble packets) and forward them to the correct radio bearer. In return, UE 2  will have to keep UE 1  informed about possible packet counters and retransmissions which may affect the counters (and therefore, the encryption). 
       FIG. 7  shows what would change in the example of  FIG. 6  when eNB 1  finds out in step  141  that UE 2  is connected to another eNB, eNB 2  in this case. eNB 1  now initiates a handover of UE 1  to eNB 2  in step  151  and optionally provides eNB 2  with the information that it had just obtained about the switched UE 1 . Next, UE 1  is informed in step  153  by eNB 1  that it should connect to eNB 2  to request the handover to UE 2 . In step  155 , UE 1  connects to eNB 2  and requests the handover to UE 2 . If eNB 2  did not receive information about who UE 2  is from eNB 1  in step  151 , it finds out in step  157 . Steps  163  to  169  correspond to steps  143  to  149  of  FIG. 6 , but the role of eNB 1  is now assumed by eNB 2 . 
     Situation b) is depicted in  FIG. 8  for an example of the first embodiment. UE 2  is connected to eNB 1 , as shown in  FIG. 6 . 
     In step  173 , UE 2  signals eNB 1  the need for a handover. UE 2  may do this using measurement reports and eNB 1  may finally issue the command to handover. In step  181 , UE 2  and eNB 2  arrange the handover of UE 2  from eNB 1  to eNB 2 . In step  183 , eNB 1  and eNB 2  arrange the handover of UE 1  from eNB 1  to eNB 2 . UE 1  is not informed of the handover. In step  185 , eNB 1  informs eNB 2  that eNB 2  should forward encrypted information originating from UE 1  to eNB 1 . This allows eNB 1  to decrypt the encrypted information, route the decrypted information to a further network node in the Access, Core Network or further packet network in case the termination point of the information is not eNB 1  itself, and encrypt information received from a further node in the network that is meant to be received by UE 1  (now shown in  FIG. 8 ). An example of such a further node is a Serving Gateway (S-GW), for example for user plane traffic, a mobility management function or entity for signaling and small data transfer or even another eNB, for example eNB 2 , for radio resource control signaling. 
     In step  187 , UE 1  transmits encrypted information to UE 2 . In step  188 , UE 2  relays the received encrypted information to eNB 2 , to which UE 2  is currently connected. In Step  189 , eNB 2  forwards the encrypted information to eNB 1  (the security endpoint), to which UE 2  was previously connected. No new keys need to be derived. 
     Situation c) is depicted in  FIG. 9  as an example of the first embodiment. Steps  193  to  201  are similar to steps  133  to  141  of  FIG. 6 , but now it is UE 3  instead of UE 2  that UE 1  requests to use as a relay and UE 3  is connected to eNB 2  instead of to eNB 1 . UE 1  previously used UE 2  (which was connected to eNB 1 ) as a relay, but UE 2  may now be out of range of UE 1  or UE 2  may no longer be available, e.g. switched off or in flight mode. 
     When eNB 2  finds out in step  201  that UE 1  is associated with eNB 1 , because UE 1  previously used UE 2  as relay and UE 2  was connected to eNB 1 , eNB 2  and eNB 1  arrange the handover of UE 1  to eNB 2  in step  183 . However, eNB 1  remains the security endpoint for UE 1 . In step  185 , eNB 1  informs eNB 2  that eNB 2  should forward encrypted information originating from UE 1  to eNB 1 . This allows eNB 1  to decrypt the encrypted information, route the decrypted information to a further network node in the Access, Core Network or further packet network in case the termination point of the information is not eNB 1  itself, and encrypt information received from a further node in the network that is meant to be received by UE 1  (not shown in  FIG. 9 ). An example of such a further node is a Serving Gateway (S-GW), for example for user plane traffic, a mobility management function or entity for signaling and small data transfer or even another eNB, for example eNB 2 , for radio resource control signaling. 
     In step  203 , UE 2  is informed that UE 1  is gone so that it may clean up its resources. In step  205 , UE 1  transmits encrypted information to UE 3 . In step  207 , UE 3  relays the received encrypted information to eNB 2 , to which UE 3  is connected. In Step  209 , eNB 2  forwards the encrypted information to eNB 1 , to which UE 2  was connected when UE 1  used UE 2  as relay. No new keys need to be derived. 
     Situation a) is depicted in  FIG. 10  as an example of the second embodiment. In this second embodiment, a specific S-GW is used for the remote UEs. In this way, no K eNB  is necessary, but a newly introduced K SGW.  In this second embodiment, the user plane is encrypted and not the radio channel. 
     In step  131 , UE 1  attaches to the network. During the attach procedure, the MME recognizes UE 1  as a potential remote UE (for example, the subscription profile indicates that it is a low-power device) and so, the MME selects a specific S-GW for remote UEs. This specific S-GW should be security enhanced so that it can handle security contexts. 
     In step  133 , UE 1  discovers a relay, UE 2 , e.g. using Proximity Services. In Step  135 , UE 1  connects to UE 2 . In Step  137 , UE 2  confirms to UE 1  that the connection has been established. In step  139 , UE 1  asks eNB 1  for a handover to UE 2 . For the radio resource allocation, the eNB 1  finds out who UE 2  is in step  141 , so that it knows how to allocate the radio resources. At the same time, eNB 1  informs the MME in step  211  of the fact that UE 1  has requested to be relayed through UE 2 . In step  213 , the MME informs the S-GW of the pending hand over. 
     Once eNB 1  has found out who UE 2  is, it will assign new radio resources to UE 2  and assign a DRB ID (Data Radio Bearer Identifier) to the bearer for those radio resources. It will also tell UE 2  that these radio resources are for relay of data from UE 1  in an RRC message in step  143 . The new radio bearer(s) for UE 1  traffic that are set up between eNB 1  and UE 2  may use ‘NULL’ encryption or may be unencrypted bearers. Also, eNB 1  may specify that no integrity protection is necessary. Instead, integrity protection for the RRC or UP (if used) may be provided by UE 1 . UE 1  then switches to UE 2  in step  148  upon receiving a command thereto from the eNB 1  in step  145 . 
     The S-GW and UE 1  now start a security negotiation in step  215 , during which they agree on:
         Which algorithm(s) is/are used.   Key derivation input parameters, which could be for example two randoms (nonces) agreed by the UE and the S-GW. If no negotiation is preferred, it could be the NAS Counter.       

     Alternatively, the role of the S-GW in step  215  could also be assumed by the MME, which may, after having derived the key, forward the key to the S-GW. Alternatively, eNB 1  forwards the key it has to the S-GW, no negotiation is necessary. The key may then be derived using an incrementing counter or something alike. 
     At the end of the security negotiation of step  215 , the S-GW sends back a ‘secure mode command’ to UE 1  to start encrypting and/or integrity protecting the user plane with the newly agreed key and algorithms. The S-GW also informs the eNB 1  in step  216  that the security context between the UE 1  and the S-GW has been established. 
     In steps  217  and  219 , the UE 1  and S-GW derive their respective keys. As described in relation to step  215 , the following key derivations may be used, for example: K SGW *=KDF(K ASME , FC∥NONCE_SGW∥Len∥NONCE_UE∥Len, K SGW *=KDF(K ASME , FC∥Counter∥Len) or K SGW *=KDF(K eNB , FC∥“S_GW”∥Len∥NONCE_SGW∥Len). In step  221 , a secure tunnel is established between the S-GW and UE 1 . The signaling between eNB 1  and UE 1  is done by signaling to UE 2 . 
     Situation b) is depicted in  FIG. 11  as an example of the second embodiment. UE 2  is connected to eNB 1 , as shown in  FIG. 10 . 
     In step  231 , UE 1  transmits encrypted information to UE 2 . In step  233 , UE 2  relays the received encrypted information to eNB 1 , to which UE 2  is currently connected. In Step  235 , eNB 1  forwards the encrypted information to the S-GW (the security endpoint). 
     In step  173 , UE 2  signals eNB 1  the need for a handover. UE 2  may do this using measurement reports and eNB 1  may finally issue the command to handover. In step  181 , UE 2  and eNB 2  arrange the handover of UE 2  from eNB 1  to eNB 2 . In step  183 , eNB 1  and eNB 2  arrange the handover of UE 1  from eNB 1  to eNB 2 . UE 1  is not informed of the handover. In step  185 , eNB 1  informs eNB 2  that eNB 2  should forward encrypted information originating from UE 1  to the S-GW. This allows the S-GW to decrypt the encrypted information and route the decrypted information to a further network node in the Access, Core Network or further packet network in case the termination point of the information is not S-GW itself, and encrypt information received from a further node in the network that is meant to be received by UE 1  (not shown in  FIG. 11 ). An example of such a further node is a Packet Gateway, for example for user plane traffic. Another example of such a node is a MME or a node in the Access Network, such as an eNB. Even though the S-GW sits in the user plane, the embodiment includes the possibility to exchange signaling messages between nodes in the Core or Access Network and the UE 1  via the encrypted user plane between the UE 1  and the S-GW, for example by setting a flag in each message that indicates whether the message is user or control plane. The advantage of such an implementation would be that no further derivation of air interface keys may be required and no further signaling keys are required; another advantage is that the relaying UE (UE 2 ) has only one endpoint to direct the communication of UE 1  to, namely the specified S-GW. Such a design would therefore be simple to implement. In the examples mentioned above, some signaling traffic goes via the user plane; similarly, it would be possible to send some user data via the control plane in case the role of the S-GW in this embodiment is assumed by a signaling entity, for example a Mobility Management Function in the Core Network or an MME in LTE. 
     In step  237 , UE 1  transmits encrypted information to UE 2 . In step  239 , UE 2  relays the received encrypted information to eNB 2 , to which UE 2  is currently connected. In Step  241 , eNB 2  forwards the encrypted information to the S-GW (the security endpoint). No new keys need to be derived. 
     Advantageously, this embodiment enables a situation where the UE 1  may temporarily be unavailable because it goes into some sort of battery saving mode. For example, in case that UE 1  is a smartwatch and UE 2  is a smartphone, the two UEs may assume that for most of the day they will remain in close proximity. As such, they may set a reasonable time interval of for example 5 minutes where they exchange a message to check that the other UE is still there. In the case that UE 1  would like to send data, it is only required to check that the UE 2  (or another associated UE) is proximity and available for data transfer. The UE 1  does not need to exchange further signaling with the network, for example because no mobility management is necessary, and can use the stored security context (the K SGW  and other keys) to protect the data straight away. 
     In the telecommunications system  500  of  FIG. 12 , three generations of networks are schematically depicted together for purposes of brevity. A more detailed description of the architecture and overview can be found in 3GPP Technical Specification TS 23.002 ‘Network Architecture’ which is included in the present application by reference in its entirety. Other types of cellular telecommunication system can alternatively or additionally be used, e.g. a 5G cellular telecommunication system. 
     The lower branch of  FIG. 12  represents a GSM/GPRS or UMTS network. 
     For a GSM/GPRS network, a radio access network (RAN) system  520  comprises a plurality of nodes, including base stations (combination of a BSC and a BTS), not shown individually in  FIG. 12 . The core network system comprises a Gateway GPRS Support Node  522  (GGSN), a Serving GPRS Support Node  521  (SGSN, for GPRS) or Mobile Switching Centre (MSC, for GSM, not shown in  FIG. 12 ) and a Home Location Register  523  (HLR). The HLR  523  contains subscription information for user devices  501 , e.g. mobile stations MS. 
     For a UMTS radio access network (UTRAN), the radio access network system  520  also comprises a Radio Network Controller (RNC) connected to a plurality of base stations (NodeBs), also not shown individually in  FIG. 12 . In the core network system, the GGSN  522  and the SGSN  521 /MSC are connected to the HLR  523  that contains subscription information of the user devices  501 , e.g. user equipment UE. 
     The upper branch of the telecommunications system in  FIG. 12  represents a next generation network, commonly indicated as Long Term Evolution (LTE) system or Evolved Packet System (EPS). 
     The radio access network system  510  (E-UTRAN), comprises base stations (evolved NodeBs, eNodeBs or eNBs), not shown individually in  FIG. 12 , providing cellular wireless access for a user device  501 , e.g. user equipment UE. The core network system comprises a PDN Gateway (P-GW)  514  and a Serving Gateway  512  (S-GW). The E-UTRAN  510  of the EPS is connected to the S-GW  512  via a packet network. The S-GW  512  is connected to a Home Subscriber Server HSS  513  and a Mobility Management Entity MME  511  for signalling purposes. The HSS  513  includes a subscription profile repository SPR for user devices  501 . 
     For GPRS, UMTS and LTE systems, the core network system is generally connected to a further packet network  502 , e.g. the Internet. 
     Further information of the general architecture of an EPS network can be found in 3GPP Technical Specification TS 23.401 ‘GPRS enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access’. 
       FIG. 13  depicts a block diagram illustrating an exemplary data processing system that may perform the methods as described with reference to  FIGS. 2 to 4  and  FIGS. 6 to 11 . 
     As shown in  FIG. 13 , the data processing system  600  may include at least one processor  602  coupled to memory elements  604  through a system bus  606 . As such, the data processing system may store program code within memory elements  604 . Further, the processor  602  may execute the program code accessed from the memory elements  604  via a system bus  606 . In one aspect, the data processing system may be implemented as a computer that is suitable for storing and/or executing program code. It should be appreciated, however, that the data processing system  600  may be implemented in the form of any system including a processor and a memory that is capable of performing the functions described within this specification. 
     The memory elements  604  may include one or more physical memory devices such as, for example, local memory  608  and one or more bulk storage devices  610 . The local memory may refer to random access memory or other non-persistent memory device(s) generally used during actual execution of the program code. A bulk storage device may be implemented as a hard drive or other persistent data storage device. The processing system  600  may also include one or more cache memories (not shown) that provide temporary storage of at least some program code in order to reduce the number of times program code must be retrieved from the bulk storage device  610  during execution. 
     Input/output (I/O) devices depicted as an input device  612  and an output device  614  optionally can be coupled to the data processing system. Examples of input devices may include, but are not limited to, a keyboard, a pointing device such as a mouse, or the like. Examples of output devices may include, but are not limited to, a monitor or a display, speakers, or the like. Input and/or output devices may be coupled to the data processing system either directly or through intervening I/O controllers. 
     In an embodiment, the input and the output devices may be implemented as a combined input/output device (illustrated in  FIG. 13  with a dashed line surrounding the input device  612  and the output device  614 ). An example of such a combined device is a touch sensitive display, also sometimes referred to as a “touch screen display” or simply “touch screen”. In such an embodiment, input to the device may be provided by a movement of a physical object, such as e.g. a stylus or a finger of a user, on or near the touch screen display. 
     A network adapter  616  may also be coupled to the data processing system to enable it to become coupled to other systems, computer systems, remote network devices, and/or remote storage devices through intervening private or public networks. The network adapter may comprise a data receiver for receiving data that is transmitted by said systems, devices and/or networks to the data processing system  600 , and a data transmitter for transmitting data from the data processing system  600  to said systems, devices and/or networks. Modems, cable modems, and Ethernet cards are examples of different types of network adapter that may be used with the data processing system  600 . 
     As pictured in  FIG. 13 , the memory elements  604  may store an application  618 . In various embodiments, the application  618  may be stored in the local memory  608 , he one or more bulk storage devices  610 , or separate from the local memory and the bulk storage devices. It should be appreciated that the data processing system  600  may further execute an operating system (not shown in  FIG. 13 ) that can facilitate execution of the application  618 . The application  618 , being implemented in the form of executable program code, can be executed by the data processing system  600 , e.g., by the processor  602 . Responsive to executing the application, the data processing system  600  may be configured to perform one or more operations or method steps described herein. 
     Various embodiments of the invention may be implemented as a program product for use with a computer system, where the program(s) of the program product define functions of the embodiments (including the methods described herein). In one embodiment, the program(s) can be contained on a variety of non-transitory computer-readable storage media, where, as used herein, the expression “non-transitory computer readable storage media” comprises all computer-readable media, with the sole exception being a transitory, propagating signal. In another embodiment, the program(s) can be contained on a variety of transitory computer-readable storage media. Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, ROM chips or any type of solid-state non-volatile semiconductor memory) on which information is permanently stored; and (ii) writable storage media (e.g., flash memory, floppy disks within a diskette drive or hard-disk drive or any type of solid-state random-access semiconductor memory) on which alterable information is stored. The computer program may be run on the processor  602  described herein. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of embodiments of the present invention has been presented for purposes of illustration, but is not intended to be exhaustive or limited to the implementations in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the present invention. The embodiments were chosen and described in order to best explain the principles and some practical applications of the present invention, and to enable others of ordinary skill in the art to understand the present invention for various embodiments with various modifications as are suited to the particular use contemplated.