Abstract:
A Proximity Request is handled within a cellular network, by communicating a Map Request from the Proximity Service Function to an Application Server, in response to a first Proximity Request on behalf of a first User Equipment. A Map Response is then communicated from the Application Server to the Proximity Service Function, in response to the Map Request. One or both of: the Map Request; and the Map Response comprises a cryptographic signature to allow authentication of the first Proximity Request.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention provides any individual or combination of: an Application Server; a Proximity Service Function for a cellular network; and a User Equipment. A method of handling a Proximity Request within a cellular network is also provided. 
       BACKGROUND TO THE INVENTION 
       [0002]    The Third Generation Partnership Project (3GPP) have been developing enhancements to cellular systems to allow their operation for public safety communications. A significant part of these developments concerns proximity-based services, known as ProSe. A number of documents have been agreed within this process, including Technical Report (TR) 23.703 and Technical Specification (TS) 23.303. 
         [0003]    TS 23.303 v.12.0.0 discusses the functionality to enable a proximity request to be made and fulfilled. This is when a first User Equipment (UE) requests information about the proximity of a second UE to it. A number of different scenarios are described, including the case where the two UEs each belong to a different Public Land Mobile Network (PLMN). Referring first to  FIG. 1 , there is shown a schematic diagram of an existing reference architecture with the network entities and interfaces used to provide this functionality. 
         [0004]    The first UE (UE A)  10  is in a first PLMN (PLMN A). A second UE (UE B)  60  is in a second PLMN (PLMN B). Each UE operates a ProSe application, UE A  10  operating a Prose application  11  and UE B  60  operating a Prose application  61 . The interface between the UE A  10  and UE B  60  is termed PC5 (each interface being defined by a respective reference point). The PLMN A has a first ProSe function  20  and the PLMN B has a second ProSe function  40 , with an interface between the first ProSe function  20  and the second ProSe function  40  being termed PC6. Where the UE A  10  is roaming, there also exists a PC7 interface (not shown) between a Prose function in the PLMN in which the UE A  10  is roaming (PLMN C) and the first ProSe function  20  in PLMN A. 
         [0005]    The first ProSe function  20  has an interface PC4a with a first Home Subscriber Server (HSS) and an interface PC4b with a first Secure User Plane Location (SUPL) Location Platform (SLP)  25 . Similarly, the second ProSe function  40  has an interface PC4a with a second HSS and an interface PC4b with a second SLP  45 . The first ProSe function  20  has an interface PC2 with a first ProSe Application Server (AS)  31  and the second ProSe function  40  has an interface PC2 with a second ProSe AS  32 . The PC2 interface is used to define the interaction between the ProSe Application Server  32  and ProSe functionality  40  provided by the 3GPP EPS via ProSe Function (for example, name translation) for EPC-level ProSe discovery. Both the first ProSe function  20  and the second ProSe function  40  have interfaces PC3 with the UE A  10 . 
         [0006]    Each UE has an Application Layer User ID (ALUID), which will be the identifier used at each UE for itself. The UE will also use the ALUID of any other UE for which it wishes to make a proximity request. Separately, each UE has an associated Evolved Packet Core (EPC) ProSe User ID (EPUID), which uniquely identifies the UE registered for ProSe and is allocated by the ProSe function. An Application server (such as the first AS  31  and second AS  32 ) is the entity that discloses the EPUID of a particular target UE to a ProSe Function, when provided with the ALUID for that UE. This allows the ProSe Function to send a request to another Prose Function, managing that target UE, for the target UE&#39;s location information for a particular period of time. 
         [0007]    Referring to  FIG. 2 , there is shown a schematic diagram of the messages communicated for a proximity request to be effected in a known way. This is based on FIG. 5.5.5-1 of 3GPP TS 23.303 v.12.0.0. Where the same features as shown in  FIG. 1  have been used, identical reference numerals have been employed. Whilst  FIG. 1  shows two Application Servers, this drawing shows a single Application Server  30 , which will be responsible for managing the proximity request on behalf of UE A  10 . 
         [0008]    A full description of the messages sent can be found in section 5.5.5 of 3GPP TS 23.303 v.12.0.0. In brief, the following summary is provided though. The proximity request message  1  is sent from the UE A  10  to the ProSe Function A  20 , identifying the ALUID for UE A  10  and the ALUID for UE B  60 . It also identifies the Application Server  30  (using the Application ID field) to be used. The Map Request message  2  is then sent from the ProSe Function A  20  to the Application Server  30 , identifying the ALUID for UE B  60  and the Application Server  30  sends a Map Response message  3 , with the EPUID for UE B  60 . This can then be provided by ProSe Function A  20  to ProSe Function B  40  in a second Proximity Request message  4 , which can be used to request the location for UE B  60 . 
         [0009]    Typically, the Prose Function A  20  will only act in response to a genuine Proximity Request message  1  from UE A  10 . However, no mechanism currently exists to ensure that the ProSe Function A  20  of the initiating PLMN A, which sends a proximity request for the target UE B  60  to the ProSe Function B  40  (over the PC6 or PC7 interface) has been requested by the UE A  10  of the initiating PLMN A. 
         [0010]    Proximity requests can provide location information about a UE that is outside the PLMN making the request. As a result, they provide powerful functionality and it is desirable to prevent their abuse. Achieving this without causing significant disruption or adding major complexity to the system is a challenge though. 
       SUMMARY OF THE INVENTION 
       [0011]    Against this background and in a first aspect, there is provided an Application Server (AS) for interfacing with a cellular network, comprising: a proximity service function interface, configured to receive a Map Request from a first Proximity Service Function on behalf of a first User Equipment, UE; and security logic, configured to authenticate the Map Request and, if the Map Request is authenticated, to communicate a Map Response to the first Proximity Service Function. 
         [0012]    The AS therefore acts as an authentication system for checking that the Map Request is genuine. In particular, the AS may authenticate that the Map Request originates from a Proximity Request by the first UE. It is advantageous for the AS to do the authentication, since it communicates directly with the ProSe function making the proximity request and therefore is well placed to check that the ProSe function is not abusing the functionality without causing significant difficulties to the system. The Map Request is typically used to provide a first identifier (such as an ALUID) for a second UE (which is to be the target of the proximity request). The Map Response is normally used to provide a second identifier (such as an EPUID) for the second UE, the second identifier generally being used to allow the Proximity Request to proceed. The Map response may also identify a second Proximity Service Function associated with the second UE (typically the ProSe Function in the PLMN of the second UE), which may be termed a PFID. Typically, each PLMN will have at least one respective ProSe Function. 
         [0013]    A risk in the existing technique for a Proximity Request is that just one Proximity Request from the UE will allow a ProSe Function to keep a record of [ALUID_B, EPUID_B, PFID_B] mapping (that is, for the target, second UE) and have the capability to send a Proximity Request later (as described in step 4 of the Proximity Request procedure of section 5.5.5 of 3GPP TS 23.303 v.12.0.0). This may be possible even though the UE did not actually send a proximity request. This could lead to significant tracking of users by other PLMNs that may be very difficult to detect by the PLMN which serves particular ProSe UEs. Authentication of the Map Request at the AS (or one of the other authentications described herein or a combination thereof) may help to address this issue. 
         [0014]    The approach taken therefore makes a number of assumptions. Firstly, it is noted that no trust is given that a ProSe Function located in a different PLMN will not issue a proximity request for the purpose of tracking a particular ALUID. Even with Service Level Agreements, such trust may be difficult to establish. Moreover, we need not assume that the application server will always be located in the same PLMN of the requesting ProSe Function. In fact, the application server may be under control of a public safety organisation or a consumer third party service provider. Hence, it need not necessarily be a Mobile Network Operator-hosted or owned service. Finally, if the Application server is located outside a PLMN, it should be established whether the ProSe Function in charge of a particular EPUID, should check that an incoming proximity request is being genuinely requested by a particular UE, not the ProSe Function ALUID. Although the AS carries out this checking, the ProSe Function can carry out some further checking, as will be discussed herein. 
         [0015]    There are various ways that the AS can authenticate the Map Request. One approach works as follows. The AS may further comprise a User Equipment interface, configured to establish a cryptographic key (which may comprise one or both of: a signature key; and a verification key) with the UE, prior to receiving the Map Request. The signature key may be stored at the UE and the verification key stored at the AS. Then, the Map Request may further comprise a cryptographic signature from the UE. The security logic may be configured to authenticate the Map Request by validating the cryptographic signature using the established cryptographic key (usually the verification key). If the established cryptographic key can be used to validate the cryptographic signature sent with (as part of) the Map Request, the Map Request may be deemed genuine. The security logic may be further configured to communicate at least part of the established cryptographic key (such as a certification key) with the Map Response. 
         [0016]    In a second approach (which is optionally combined with the first approach), the security logic is configured to check for suspicious characteristics of the Map Request. For example, the security logic may be configured to authenticate (that is, check for suspicious characteristics of) the Map Request on the basis of one or more of: previous Map Requests received from the first Proximity Service Function (for example, in respect of one or more of their number, frequency or content); and a presence detection for the UE at the Application Server. The latter may use an interface between the AS and the first ProSe function, such as the PC2 interface. 
         [0017]    Particularly in this second approach, the security logic may be further configured to determine that at least one operation should be authorised for the Proximity Service Function. Moreover, the security logic may be configured to send an indication of this together with the Map Response. Then, the security logic may be further configured to determine at least one authorised operation for the Proximity Service Function. In this case, the security logic may be configured to send an indication of the at least one authorised operation together with the Map Response. 
         [0018]    In the second approach, the security logic is advantageously further configured to generate an Application Server cryptographic signature using an established cryptographic key (which may comprise one or both of: a signature key; and a verification key) and to communicate the Application Server cryptographic signature together with the Map Response. The established cryptographic key may be established at the AS. Then, the security logic may be further configured to communicate at least part of the established cryptographic key (such as a certification key) with the Map Response. 
         [0019]    Thus, the first approach uses a cryptographic (encryption) key established between the first UE and the AS and the second approach uses a cryptographic (encryption) key established at the AS. Both approaches employ a cryptographic signature using the established cryptographic key as a token for subsequent security purposes. The approaches can also be combined, as noted above. Whichever approach (or combination) is used, the proximity service function interface may be further configured to receive a Key Fetching Request from a second Proximity Service Function. The proximity service function interface may then send at least part of the established cryptographic key (such as the verification key) to the second Proximity Service Function in response to the received Key Fetching Request. 
         [0020]    In a second aspect, there is provided a Proximity Service Function for a cellular network, comprising: a User Equipment interface, configured to receive a first Proximity Request from a first User Equipment, UE; an Application Server interface, configured to send a Map Request to an Application Server, in response to the received first Proximity Request and to receive a Map Response from the Application Server; and a Proximity Service Function interface, configured to send a second Proximity Request to another Proximity Service Function based on the first Proximity Request and the Map Response, the second Proximity Request comprising (or including) a cryptographic signature that is based on the first Proximity Request to allow the other Proximity Service Function to authenticate the second Proximity Request. 
         [0021]    The Proximity Service Function may therefore be configured to communicate with: a first UE (in order to receive the Proximity Request comprising a first identifier for a second, target UE); an AS (in order to convert the first identifier for the second UE into a second identifier for the second UE); and another Proximity Service Function (in order to pass on the Proximity Request and thereby receive location information for the second UE). Importantly, the communication with the other Proximity Service Function includes a cryptographic signature that is based on the Proximity Request. This may allow the other Proximity Service Function to authenticate the Proximity Request in an efficient way. 
         [0022]    As with the first aspect, there are a number of approaches for obtaining the cryptographic signature. In a first approach, the User Equipment interface is configured to receive a UE cryptographic signature from the first UE. Then, the Application Server interface may be further configured to send the UE cryptographic signature to the Application Server. The second Proximity Request sent by the Proximity Service Function interface may comprise the UE cryptographic signature. In this approach, the User Equipment interface may be further configured to receive a cryptographic key from the User Equipment. Then, the Proximity Service Function interface may be further configured to send the cryptographic key to the other Proximity Service Function. 
         [0023]    In a second approach (which may be combined with the first approach), the Application Server interface may be further configured to receive an Application Server cryptographic signature from the Application Server. Then, the second Proximity Request sent by the Proximity Service Function interface may comprise the Application Server cryptographic signature. In this approach, the Application Server interface may be further configured to receive a cryptographic key from the Application Server. Then, the Proximity Service Function interface may be further configured to send the cryptographic key to the other Proximity Service Function. 
         [0024]    In a third aspect (which may be combined with the second aspect), there is provided a Proximity Service Function for a cellular network, comprising: a Proximity Service Function interface, configured to receive a Proximity Request from another Proximity Service Function on behalf of a first User Equipment, UE, the Proximity Request comprising a cryptographic signature; and security logic, configured to validate the cryptographic signature and authenticate the received Proximity Request thereby. Thus, the Proximity Service Function may be configured to authenticate a received Proximity Request from another Proximity Service Function (which will typically come from another PLMN) using the received cryptographic signature. This is an efficient way to authenticate the Proximity Request without causing disruption to the Proximity Request procedure. 
         [0025]    Optionally, the Proximity Request identifies an Application Server associated with the Proximity Request. Then, the Proximity Service Function may further comprise: an Application Server interface, configured to send a Key Fetching Request to the identified Application Server, in response to the received Proximity Request and, preferably, to receive a cryptographic key from the Application Server in response. Then, the security logic may be configured to validate the cryptographic signature using the received cryptographic key. 
         [0026]    Preferably, the Proximity Service Function interface is further configured to send a Proximity Request Acknowledgement to the other Proximity Service Function in response to the security logic authenticating the received Proximity Request. The Proximity Request Acknowledgement may comprise a location for a second UE, the second UE being identified in the Proximity Request. 
         [0027]    In a fourth aspect, there is provided a User Equipment (UE) for a cellular network, comprising: a Proximity Service Function interface, configured to send a Proximity Request to a Proximity Service Function of the cellular network, the Proximity Request comprising a cryptographic signature for the UE. In this way, the Proximity Request can be authenticated within the cellular network. This corresponds with the first approach taken in authentication within the first and second aspects. 
         [0028]    Beneficially, the Proximity Request identifies an Application Server associated with the Proximity Request. Then, the UE may further comprise: an Application Server interface, configured to establish a cryptographic key with the identified Application Server prior to sending the Proximity Request. The cryptographic key is advantageously used to compute the cryptographic signature. 
         [0029]    Any combination of each of the first, second, third and fourth aspects is also provided, particularly as a system. Each of the first, second, third and fourth aspects may comprise hardware (such as communication equipment, digital logic, a processor system), firmware, software of a combination thereof. Each of the first, second, third and fourth aspects may be termed a network entity or a part of a network entity in a general way. 
         [0030]    In a fifth aspect, there is provided a method of handling a Proximity Request within a cellular network, comprising: communicating a Map Request from the Proximity Service Function to an Application Server, in response to a first Proximity Request on behalf of a first User Equipment, UE; and communicating a Map Response from the Application Server to the Proximity Service Function, in response to the Map Request. One or both of: the Map Request; and the Response comprises a cryptographic signature to allow authentication of the first Proximity Request. Thus, this may describe the process that takes place at the network entity of the first or second aspects. 
         [0031]    In the preferred embodiment, the method further comprises: communicating the first Proximity Request from the first User Equipment to the first Proximity Service Function of the cellular network. The step of receiving the Map Response from the Application Server is preferably in response to the Application Server authenticating the Map Request. 
         [0032]    Optionally, the Map Request comprises a cryptographic signature from the first UE. Then, the step of communicating the Map Request may comprise communicating the cryptographic signature from the Proximity Service Function to the Application Server. 
         [0033]    Additionally or alternatively, the Map Response may comprise a cryptographic signature from the Application Server. Then, the step of communicating the Map Response may comprise communicating the cryptographic signature from the Application Server to the Proximity Service Function. In embodiments, the method may further comprise: communicating a second Proximity Request from the first Proximity Service Function to a second Proximity Service Function based on the first Proximity Request and the Map Response. 
         [0034]    The method optionally further comprises: establishing a cryptographic key with the Application Server prior to communication of the first Proximity Request, the cryptographic key being used to compute the cryptographic signature. The first Proximity Request may identify the Application Server. 
         [0035]    The second Proximity Request may comprise: the cryptographic signature from the first UE; the cryptographic signature from the Application Server; or both. 
         [0036]    This method may optionally comprise process features corresponding with functionality discussed herein for any one or more of the first, second, third and fourth aspects. In a yet further aspect, a computer program is provided, configured to carry out any method disclosed herein when operated by a processor. 
         [0037]    The combination of any structural, apparatus, system or method features and aspects is also provided even if not explicitly disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    The invention may be put into practice in various ways, a number of which will now be described by way of example only and with reference to the accompanying drawings in which: 
           [0039]      FIG. 1  shows a schematic diagram of an existing reference architecture with the network entities and interfaces used to provide Proximity Request functionality; 
           [0040]      FIG. 2  shows a schematic diagram of the messages communicated for a proximity request to be effected in an known way using the architecture shown in  FIG. 1 ; 
           [0041]      FIG. 3  schematically illustrates the messages communicated for a proximity request to be effected in accordance with a first embodiment of the invention; and 
           [0042]      FIG. 4  schematically illustrates the messages communicated for a proximity request to be effected in accordance with a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0043]    Before discussing specific embodiments in detail, the following general concepts are outlined. Firstly, the AS  30  can be used to check that the ProSe Function is not abusing the Proximity Request procedure by means of a Map Request. Secondly, a cryptographic signature can be used in the exchange between the ProSe Function of one PLMN and the ProSe Function of another PLMN to confirm that the Proximity Request procedure is not being abused. This can be achieved using a cryptographic signature that originates at the UE (for instance, with a key that is shared with the AS), using a cryptographic signature that originates at the AS or both. The cryptographic signature (or cryptographic signatures) can be checked at the ProSe Function of the other PLMN to confirm that it has originated with the UE or AS. Specific embodiments are now described. 
         [0044]    Referring now to  FIG. 3 , there is shown a schematic illustration of the messages communicated for a proximity request to be effected in accordance with a first embodiment of the invention. Where the same features are shown as in previous drawings, identical reference numerals have been used. 
         [0045]    This approach uses a UE-signed proximity request. UE A  10  signs all the proximity requests sent to its ProSe Function A over the PC3 interface. The signature key for UE A  10  can be provided by the Application Server  30  over the PC1 interface. Alternatively, the UE A  10  could generate a signature and verification key pair, securely stores the signature key in its memory and export the verification key to the Application Server  30 . 
         [0046]    The UE&#39;s verification key is provided to ProSe Function B  40  by the Application Server  30  over the PC2 interface. ProSe Function B  40  is assured of the authenticity of the proximity request received from ProSe Function A  20  over the PC6 interface by verifying a cryptographic signature with a verification key from the UE A  10 . 
         [0047]    It should be noted that the ProSe Application Server  30  may be outside of one or both PMLN A and PLMN B. Thus, with reference to  FIG. 1 , it is easier to consider the first AS  31  and second AS  32  as a single AS  30 . Then, the PC1 interface is maintained at each respective UE and each PC2 interface is maintained to each respective ProSe function from the same entity. Effectively, first AS  31  and second AS  32  can be combined. 
         [0048]    The procedure shown in  FIG. 3  will now be described, with reference to the Proximity Request procedure in clause 5.5.5 of 3GPP TS 23.303 v.12.0.0 to support authenticity of the request. 
         [0049]    In step  100 , UE A  10  and the Application Server  30  perform a key establishment “ceremony”, which results in a signature key stored in UE A  10  and a verification key in the Application Server  30 . The communication between UE A  10  and the Application Server  30  is end-to-end secured over the PC1 interface. 
         [0050]    In step  101 , the UE A  10  sends as part of the Proximity Request the following additional data: a signature (UEA_signature) of the cryptographic hash of the concatenation of the EPUID_A, the ALUID_A, the ALUID_B and a timestamp value to ProSe Function A  20 . These should therefore be transmitted to the Application Server  30  and the ProSe function B  40  so that both may verify the cryptographic signature with the verification key for UE A  10 . The timestamp value corresponds to the system time of UE A  10 . 
         [0051]    Next follows step  102 . The ProSe Function A  20  sends as part of the Map Request message the following additional data: the UEA_signature and the timestamp provided by UE A to the Application Server  30 . 
         [0052]    Step  103  provides that the Application Server  30  verifies the UEA_signature with the verification key associated with ALUID_A. An error message is returned if the verification fails. If the verification is successful, a Map Response is sent (as in the conventional technique) and the Application Server  30  may optionally add in the Map Response the associated certificate of ALUID_A&#39;s verification key. 
         [0053]    Then in step  104 , the ProSe Function A  20  sends as part of the Proximity Request to ProSe Function B  40 , the following additional data: the UEA_signature, ALUID_A, ALUID_B and the timestamp, the Application ID and optionally the certificate associated with ALUID_A. 
         [0054]    Optionally in step  105 , if the certificate associated with ALUID_A or verification key was not part of the Proximity request, the ProSe Function B  40  sends a Verification Key fetching request to the Application Server (identifiable from its Application ID) for the verification key associated with ALUID_A. Subsequently, the Application Server  30  returns the verification key associated with ALUID_A in step  106 . 
         [0055]    Whether or not steps  105  and  106  take place, if the verification of UEA_signature is successful, then the procedure continues the procedure from step 5 in clause 5.5.5 of 3GPP TS 23.303 v.12.0.0 (as discussed above). 
         [0056]    Thus, this approach provides a means of assuring the authenticity of the proximity request received at an initiating ProSe function from another ProSe Function by verifying a cryptographic signature with a verification key from UE within the initiating PMLN, that is the UE signs the proximity request. 
         [0057]    Referring now to  FIG. 4 , there is shown a schematic illustration of the messages communicated for a proximity request to be effected in accordance with a second embodiment of the invention. Again, where the same features are shown as in previous drawings, identical reference numerals have been used. 
         [0058]    This approach uses an Application Server-signed proximity request. In this approach, UE A  10  does not sign the proximity requests sent to its ProSe Function A  20 , but trusts the Application Server  30  to control the authorization of the proximity request sent on its behalf. 
         [0059]    The AS  30  therefore determines whether and the extent to which the ProSe Function A  20  can make Proximity Requests. The authorization criteria can be based, for example, on detection mechanisms of very high volume of incoming proximity requests from a ProSe Function that does not match with the frequency usage of the ProSe Application by the users, it can be based on a presence detection mechanism over the PC1 interface, or a combination of these possibly with other factors. 
         [0060]    The ProSe Function A  20  requests an authorization from the Application Server  30  for each proximity request it should transmit over the PC2 interface. The Application Server  30  returns a parameter which specifies the operations that are authorized (for example, authorized to send only one request, authorized to send a specific number of requests until a particular date or similar). A signature is also provided by the AS  30  in the Map Response. The ProSe Function B  40  is assured of the authenticity of the proximity request received from the ProSe Function A  20  by verifying the signature with a verification key from the Application server  30 . The token verification key is fetched over the PC2 interface between the ProSe Function B  40  and the Application Server  30 . 
         [0061]    The procedure shown in  FIG. 4  will now be described, with reference to the Proximity Request procedure in clause 5.5.5 of 3GPP TS 23.303 v.12.0.0 to support authenticity of the request. 
         [0062]    Step  201  is the same as step 1 of the procedure in clause 5.5.5 of 3GPP TS 23.303 v.12.0.0 and step  202  is the same as step 2 of the procedure in clause 5.5.5 of 3GPP TS 23.303 v.12.0.0. 
         [0063]    In step  203 , the Application Server  30  returns as part of the Map Response the following additional data: the authorized operations (for example, authorized to send only one request, authorized to send X requests until particular date, etc.); a timestamp; the signature (AS_signature) of the cryptographic hash of the concatenation of the ALUID_A, the ALUID_B, the authorized operations and the timestamp value; and optionally the associated certificate CertKey_AS of Application Server&#39;s verification key. 
         [0064]    Next in step  204 , the ProSe Function A  20  sends as part of the Proximity Request to the ProSe Function B  40  the following additional data: the AS_signature; ALUID_A; ALUID_B; the timestamp; the authorized operations; the Application ID; and optionally the certificate CertKey_AS. 
         [0065]    This is optionally followed by step  205  if the certificate CertKey_AS was not part of the Proximity request or if the certificate CertKey_AS or verification key were not stored in internal memory. Then, the ProSe Function B  40  sends a Verification Key fetching request to the Application Server  30  (identifiable from its Application ID) for a verification key associated with the Application ID. The Application Server  30  returns the verification key associated with Application ID in step  206 . 
         [0066]    Whether or not steps  205  and  206  take place, if the verification of AS_signature is successful, then the procedure continues from step 5 in clause 5.5.5 of 3GPP TS 23.303 v.12.0.0. 
         [0067]    Hence, the ProSe Application Server itself is used to control the authorisation of the proximity request sent on its behalf. A second ProSe Function is assured of the authenticity of the proximity request received from a first ProSe Function by verifying a signature with a verification key from the ProSe Application server. 
         [0068]    Although specific embodiments have now been described, the skilled person will understand that various modifications and variations are possible. Indeed, a combination of the two embodiments is possible. Moreover, only a part of a functionality described in each embodiment may need to be implemented. For example, the functionality associated with the AS  30  may be implemented, even though the functionality associated with the UE, ProSe function or both is not available. 
         [0069]    It will be understood that the specific messages and their contents may vary. For example, the messages and cryptographic signatures need not comprise a combination or a hash of all of the information discussed above. Rather a subset of that information can be used. Additionally or alternatively, other information can be used. 
         [0070]    It will also be understood that, wherever an entity (such as a Proximity Service Function) is described as validating a cryptographic signature, this validation computation could be offloaded to an external entity, which may be part of the cellular network or external to it. 
         [0071]    The implementations described herein deal with the scenario when the UE A  10  (making the Proximity Request) and the UE B  60  (the target or subject of the Proximity Request) are both in their Home PLMN. However, it will be recognised that if either are roaming (therefore in a Visited PLMN), the approach may be adapted, especially with reference to the implementations discussed in 3GPP TS 23.303 v.12.0.0.