Patent Publication Number: US-11048812-B2

Title: System for reliably accessing a protected resource

Description:
RELATED APPLICATIONS 
     Foreign priority benefits are claimed under 35 U.S.C. § 119 to European application number 18166874.0, filed Apr. 11, 2018, the entire contents of which is hereby incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     This disclosure relates to a system, a method and a computer program for reliably accessing a protected resource at a client system on behalf of a user. 
     BACKGROUND 
     The OAuth (“Open Authorization”) 2.0 authorisation framework enables a third-party application to obtain limited access to an HTTP service, such as access to a protected resource. The third-party application may obtain access to the HTTP service on behalf of a resource owner by orchestrating an approval interaction between the resource owner and the HTTP service. Alternatively, the third-party application can obtain access to the HTTP service on its own behalf. 
       FIG. 1  illustrates an overview of the OAuth 2.0 protocol flow, involving the following four entities: a resource owner  1 , a client  3 , an authorisation server  5  and a resource server  7 . The resource owner  1  is an entity that is capable of granting access to a protected resource. 
     The resource owner  1  may be a person, referred to as a user operating a user device. The client  3  is an entity, or an application, that can make a request for access to the protected resource on behalf of the resource owner  1 , when authorised by the resource owner  1 . The authorisation server  5  is an entity that grants and issues access tokens to the client after successfully authenticating the resource owner and obtaining authorisation. The resource server  7  is an entity that hosts the protected resource and is capable of accepting and responding to requests for the protected resource using access tokens. 
     Access tokens, such as those issued by the authorisation server  5 , are credentials used to access protected resources. An access token is a string representing an authorisation issued to the client. The string is usually opaque to the client. Tokens represent specific scopes and durations of access, granted by the resource owner, and enforced by the resource server and authorisation server. 
     An access token may denote an identifier used to retrieve the authorisation information or may self-contain the authorisation information in a verifiable manner (i.e., a token string consisting of some data and a signature). Access tokens can have different formats, structures, and methods of utilization (e.g., cryptographic properties) based on the resource server security requirements. 
     An access token can be used to identify a client. When an access token is used in this context it may be referred to herein as a grant token, rather than an access token. However, a grant token could equally be described as an access token. 
     The authorisation server  5  can issue refresh tokens, which are credentials used to obtain access tokens. Refresh tokens are issued to the client by the authorisation server and are used to obtain a new access token when the current access token becomes invalid or expires, or to obtain additional access tokens with identical or narrower lifetime and fewer permissions than authorised by the resource owner). Issuing a refresh token is optional at the discretion of the authorisation server. If the authorisation server issues a refresh token, it is included when issuing an access token. 
     A refresh token is a string representing the authorisation granted to the client by the resource owner. The string is usually opaque to the client. The token denotes an identifier used to retrieve the authorisation information. 
     In step  11 , client  3  requests authorisation from the resource owner  1  for access to the protected resource. The authorisation request can be made directly to the resource owner  1 , or indirectly via the authorisation server  5  as an intermediary. 
     In step  12 , the client  3  receives an authorisation grant, which is a credential representing the authorisation provided by the resource owner  1 . This authorisation may be expressed using one of a plurality of “grant types” described in the Authorisation Framework. The authorisation grant type depends on the method(s) used by the client  3  to request authorisation and the type(s) supported by the authorisation server  5 . 
     In step  13 , the client  3  requests an access token by authenticating with the authorisation server  5  and presenting the authorisation grant. The authentication may be expressed using one of a plurality of “authentication types” for authenticating identifying client  3  at the authorisation server  5 . The authentication type depends on the method(s) used by the client  3  to authenticate itself and the type(s) supported by the authorisation server  5 . 
     In step  14 , the authorisation server  5  authenticates the client  3  and validates the authorisation grant, and if valid, issues an access token. 
     In step  15 , the client  3  requests the protected resource from the resource server  7  and authenticates itself at the resource sever  7  by presenting the access token. 
     In step  16 , the resource server  7  validates the access token. If the access token is valid, the resource server  7  serves the request by transmitting the protected resource to the client  3 . 
     As explained above, the authorisation server  5  can support different types of grant method and different types of authentication method. In conventional systems, the client  3  is preconfigured to use one type of grant method and one type of authentication method for communicating with the authorisation server  5  in order to obtain the access token. In these conventional systems, if the type of grant method or the type of authentication method that is supported by the authorisation server  5  changes to a different type of method, the client  3  would use the incorrect grant or authentication method in communicating with the authorisation server  5 . Thus, the client  3  would be incapable of obtaining the access token. 
     There is a need for a flexible and more reliable system that is resilient to changes in the types of grant and authentication methods supported by an authorisation server  5 , so that a client is able to reliably access protected resources. 
     SUMMARY 
     In one aspect of the invention there is provided a computer-implemented method for obtaining an access token for providing access to a protected resource stored at a resource system, the method comprising: storing, at a client system: a plurality of grant method code portions each executable to obtain access to the access token using one of a plurality of types of grant method, wherein each respective type of grant method is different to the other types of grant method; and a plurality of authentication method code portions each executable to authenticate the client system using a different authentication method, wherein each respective type of authentication method is different to the other types of authentication method; storing, at the client system, a configurable database comprising a plurality of authorisation system identifiers each indicative of a respective authorisation system, wherein each of the plurality of authorisation system identifiers is associated with one or more of the plurality of types of grant method which are supported by the respective authorisation system, and each of the plurality of authorisation system identifiers is associated with one or more of the plurality of types of authentication method which are supported by the respective authorisation system; receiving, at the client system, from a user device an access request comprising an instruction for the client system to access a protected resource, the instruction comprising a request identifier indicative of an authorisation system for authorising access to the protected resource; identifying, in the configurable database, a selected grant method type from one or more of the grant method types associated with an authorisation system identifier corresponding to the request identifier; identifying, in the configurable database, a selected authentication method type, from one or more of the authentication method types associated with the authorisation system identifier corresponding to the request identifier; executing, at the client system, the grant method code portion corresponding to the selected grant method type to request the access token for accessing the protected resource; executing, at the client system, the authentication method code portion corresponding to the selected authentication method type to authenticate the client system at the authorisation system; and receiving the access token at the client system from the authorisation sever, in response to executing the grant method code portion and the authentication method code portion. 
     In the method, the client system maintains a configurable database that indicates the authentication and grant method types that are supported by each authorisation system with which the client system can communicate to obtain an access token for accessing the protected resource. Also, the client system stores a plurality of code portions each executable to perform any one of the supported grant or authentication methods. Thus, when the client system requests the access token, the client system is able to determine the correct grant and authentication methods and execute these methods accordingly for receiving the access token. Therefore, the client system is able to adapt to a variety of different authentication and grant methods that may be supported at different times by a variety of authorisation servers, which provides greater flexibility and resilience. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be described, by way of example, with reference to the following drawings, in which: 
         FIG. 1  illustrates a protocol sequence diagram of the OAuth 2.0 framework; 
         FIG. 2  illustrates the general architecture of a system for accessing a protected resource; 
         FIGS. 3 and 4  illustrate a protocol sequence diagram of a computer-implemented invention for accessing a protected resource; 
         FIG. 5  illustrates a flow chart of a computer-implemented method in which a client system is configured to execute a selected authentication method and grant method for obtaining an access token used for accessing a protected resource; 
         FIG. 6  illustrates a flow chart of a computer-implemented method in which a client system refreshes a grant token based on its expiry time; 
         FIG. 7  illustrates a flow chart of computer-implemented method for managing and storing valid and invalid access tokens; 
         FIG. 8  illustrates a schematic diagram of a client system; and 
         FIG. 9  illustrates a schematic diagram of an example device in the system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2 , there is a system  100  for managing access to protected resources. The system  100  comprises one or more user devices  101 , a client system  103  and an external system  104 , which comprises an authorisation system  105  and a resource system  107 . 
     The user device  101  may considered as the resource owner  1 , the client system  103  may be considered as the client  3 , the authorisation system  105  may be considered as the authorisation server  5 , and the resource system  107  may be considered as the resource sever  7  when considering the methods and systems described herein in terms of the OAuth 2.0 protocol. 
     The external system  104  is configured to store data associated with users of devices, such as the user device  101 . The data stored at the external system  104  may comprise protected resources, such as one or more secure data items. 
     Each one of the protected resources may be indicative of private information relating to a user of the external system  104 . In one specific example, each data item stored at the external system  104  comprises financial data relating to the user, such as details that enable the user to make payments or the details of previous financial transactions made by the user. 
     The following systems and methods are described in the context of managing access to financial data in terms of the OAuth 2.0 protocol. However, these systems and methods could be used to manage access to any type of protected resource for which access by unauthorised third parties is to be restricted using another suitable protocol, such as SAML, OpenID and the like. 
     The client system  103  is configured to access protected resources stored at the external system  104 , upon request from a user. 
     The external system  104  may comprise a plurality of sub-systems, for instance, a plurality of servers. In the example described herein, the external system  104  comprises the authorisation system  105  and the resource system  107 . The authorisation server  105  is configured to authorise requests for access to protected resources that are stored at the resource system  107 . Alternatively, the external system  104  may comprise a single server for performing these functions. 
     The client system  103  may comprise a plurality of sub-systems, for instance, a plurality of servers. In the example described herein, the client system  103  comprises a retrieval engine  103   a  and a token management server (TMS)  103   b . The retrieval engine  103   a  is configured to interface with the user device  101 , and the TMS  103   b  is configured to interface with the external system  104 . Alternatively, the client system  103  comprises a single server for performing these functions. 
     In the following examples, secure data and protected resources, are referred to as being accessible by a user, and that the data is associated with the user. For example, the user may have access to an online user account, such as an online banking account, via an account interface provided by the external system  104 . In this scenario, the user may be assigned a unique username and a shared secret (e.g. login information), such as a password, that can be used to access the user account via the account interface. Once the user has accessed the user account, that user is able to access the data via the user account. Therefore, the data is accessible by the user via login information that is unique to the user. 
     The secure data and protected resources that are accessible by the first user may be accessible by the external system  104  itself. The secure data and protected resources may be accessible by the user only, unless otherwise authorised by the user. In other words, the data/protected resource are prevented from being sent to a device or a system that is remote and distinct from the external system  104 , such as the client system  103 , without the corresponding user providing authorisation to the external system  104  for the data to be sent to a remote device or system. 
     Each one of the external system  104 , the client system  103  and the user device  101  are arranged to communicate with one another via a communications network  110 . The communications network  110 , in this example, is the Internet  110 . However, it will be appreciated that any suitable form of communications network  110  could be used. 
     Each one of the external system  104 , the client system  103 , and the user device  101  are web-enabled and may comprise a display, a user interface, a processor and memory. The devices and systems  101 ,  103 ,  104  can be arranged to communicate data between one another via any suitable communications protocol or connection. For instance, the devices and systems  101 ,  103 ,  104  may communicate with one another via a wired and/or a wireless connection. 
     The user device  101  may be any suitable type of personal computing device, such as a laptop computer, a desktop computer, a web-enabled telephone, such as a smartphone, or a tablet device. The client system  103  and the external system  104  may be any suitable type of computing system or collection of computing systems, such as a server or a collection of servers. 
     Referring to  FIGS. 3 and 4 , there is a computer-implemented method in which the client system  103  obtains access to a protected resource associated with the user of the user device  101  from the external system  104 . 
     In step  301 , the user device  101  transmits an access request to the retrieval engine  103   a  at the client system  103 . The access request comprises an instruction for the client system  104  to initiate accessing the protected resource stored at the external system  104 , in other words the access request indicates the user&#39;s intent for the client system  103  to access the protected resource. 
     In response to the access request received from the user device  101 , the client system  103  performs steps  303  to  307  for obtaining a grant token from the external system  104 . The grant token can be used by the client system  103  to identify itself to the external system  104  in order to initiate requests for the protected resource. 
     In step  303 , the retrieval engine  103   a  transmits a request for a grant token to the TMS  103   b.    
     In step  305 , the TMS  103   b  forwards the request for the grant token to the authorisation system  105  at the external system  104 . The authorisation sever  105  validates the request for the grant token. If the request for the grant token is valid, the authorisation system  105  responds by transmitting the grant token to the TMS  103   b.    
     In step  307 , if the grant token has been received, the TMS  103   b  forwards the grant token to the retrieval engine  103   a.    
     Once the client system  103  has received the grant token, steps  309  and  311  can be performed in order to receive an intent identifier from the external system  104 . The intent identifier is an identification label that can be used to identify a specific request from the user for the client system  103  to access a protected resource associated with the user from the external system  104  (i.e. the user&#39;s intent for the client system  103  to access the protected resource). The client system  103  is able to receive the intent identifier, if it has been issued with a valid grant token previously. 
     In step  309 , the retrieval engine  103   a  transmits an intent request to the resource system  107  at the external system  104 . The intent request is sent with the grant token received previously, which identifies the client system  103  to the external system  104 . 
     In step  311 , in response to the intent request, the external system  104  validates the grant token. If the intent request is valid, the resource system  107  responds by transmitting an intent identifier to the TMS  103   a.    
     In step  313 , the retrieval engine  103   a  forwards the intent identifier to the user device  101 . In addition, an instruction to redirect to the authorisation system  105  is sent to the user device  101  along with the intent identifier. The redirect instruction may comprise a URL corresponding with the authorisation system  105 . 
     In step  315 , the user device  101  is redirected to the authorisation system  105  using the URL, and the user device  101  transmits the intent identifier to the authorisation system  105 . The intent identifier is used by the authorisation system  105  to identify the user&#39;s intention for the client system  103  to access the protected resource. 
     In step  317 , the user device  101  and the authorisation system  105  communication with one another in order for the user to provide authorisation for the client system  103  to access the protected resource. This step may involve the user selecting the protected resource, or a portion of the protected resource such as a specific sub-set of data relating a specific user account (or group of accounts) stored at the resource system  107  that are accessible by the user. 
     In step  319 , once the user has provided authorisation for the client system  104  to access the protected resource, the authorisation system  105  transmits an authorisation code to the user device  101 . The authorisation code comprises an indicator that the user has provided authorisation. In this step, the authorisation system  105  transmits an instruction to the user device  101  to redirect to the client system  103 . This redirect instruction may comprise a URL corresponding with the retrieval engine at the client system  103 . 
     In step  321 , the user device  101  is redirected to the retrieval engine  103   a , and the user device  101  transmits the authorisation code to the retrieval engine  103   a.    
     In step  323 , the retrieval engine  103   a  transmits a request for an access token by transmitting the authorisation code to the authorisation system  105 . The authorisation code may be transmitted directly from the retrieval engine  103   a  to the authorisation system  105 , or indirectly via the TMS  103   b.    
     In step  325 , the authorisation system  105  validates the authorisation code. If the authorisation code is valid, the authorisation system  105  responds by transmitting an access token the retrieval engine  103   a . The access token can be used by the retrieval engine  103   a  to access the protected resource stored at the resource system. 
     In step  327 , if the access token has been received successfully, the retrieval engine  103   a  transmits a success message indicating that the access token has been received. Alternatively, if the access token has not been received, the retrieval engine  103   a  transmits a failure message indicating that the access token has not been received. 
     Referring to  FIG. 4 , in step  329  the user device  101  transmits a request to the retrieval engine  103   a  for the client system  103  to retrieve the protected resource. If the access token has been received previously by the client system  103 , the method proceeds to step  337 . Alternatively, if the access token has not been received previously, steps  331  to  335  are performed in order for the client system  103  to obtain the access token in a similar manner to that described above. 
     In step  331 , the retrieval engine  103   a  forwards a request for an access token to the TMS  103   b . Then, in step  333 , the TMS  103   b  forwards the access token request to the authorisation system  105  and, in response, receives the access token. In step  335 , the access token is transmitted from the TMS  103   b  to the retrieval engine  103   a.    
     In step  337 , the retrieval engine  103   a  transmits a request for the protected resource by transmitting the access token to the resource system  107 . In step  339 , the resource system  104  the access token. If the access token is valid, the resource system responds by transmitting the protected resource to the retrieval engine  103   a.    
     In step  341 , once the protected resource has been received by the client system  103 , the retrieval engine transmits the protected resource to the user device  101 . Once received, the protected resource can be displayed at the user device  101  via an app or a browser at the user device  101 . 
     Referring to  FIG. 5 , there is a computer-implemented method  500  performed by the client system  103  that enables the client system  103  to flexibly adapt to the grant and authentication methods supported by the external system  104 . This method can be used in conjunction with the method described above with reference to  FIGS. 3 and 4 . 
     In step  501 , the client system  103  stores a plurality of grant method code portions. Each of the grant method code portions are stored in memory at the client system  103  and are executable by a processor of the client system  103 . Execution of any one of the grant method code portions causes the client system  103  to obtain an access token from the authorisation system  105  of the external system  104  using a specific type of grant method. The grant method corresponding to a particular grant method code portion is different to the grant methods corresponding to other the grant method code portions. 
     As discussed previously, the authorisation system  105  may support one or more of a plurality of different types of grant method, each of which enable the client system  103  to access a protected resource. In the OAuth 2.0 framework there are a number of available types of grant method, (or “grant types” as referred to in the OAuth 2.0 framework). The grant methods may comprise types such at the “authorisation code”, “implicit”, “resource owner password credentials” and “client credentials” grant types. 
     An overview of the “authorisation code” grant type is described above with reference to steps  313  to  327  of  FIG. 3 . In these steps the authorisation system  105  provides an authorisation code in response to the user authorising access the protected resource. The client system  103  is then able to exchange the authorisation token for the access token at the authorisation system  105 . 
     The “client credentials” grant type involves fewer steps than the “authorisation code” grant type. Instead of redirecting the user device  101  to the authorisation system  105  in order to obtain the authorisation code, the client system  101  transmits a request for the access token comprises a client identifier and a client secret, which is some form of shared secret such as a password. The request for the access token comprises an indication of the grant type being used which, in this case is the “client credentials” grant type. The authorisation system  105  validates the client identifier and the client secret. If the client identifier and the client secret are valid, the authorisation system  105  responds with the access token. 
     Although only certain grant types have been described herein, any other suitable grant type(s) could be used in addition to or instead of the grant type(s) described. 
     In step  503 , the client system  103  stores a plurality of authentication method code portions. Each of the authentication method code portions are stored in memory at the client system  103  and are executable by a processor of the client system  103 . Execution of any one of the authentication method code portions causes the client system  103  to authenticate itself to the external system  104  during the process of obtaining the access token. The authentication method corresponding to a particular authentication method code portion is different to the authentication methods corresponding to the other authentication method code portions. 
     As discussed previously, an authorisation system may support one or more of a plurality of different types of authentication method, each of which enabling the client system  103  to authenticate itself to the external system  104 . In the OAuth 2.0 framework there are a number of available types of authentication methods, (or “authentication types”). The authentication methods may comprise types such as the “client secret” authentication method and the “client assertion” authentication method. 
     The “client secret” authentication method defines the way in which the client system  103  authenticates itself at the authorisation system  105  when obtaining the access token. If the “client secret” authentication method is used, the client system  103  will transmit a client secret to the authorisation system  103  when requesting the access token. The client secret is a shared secret, such as a password, assigned to the client system  103 . The authorisation system  105  uses the client secret to authenticate the client system  103  in validating requests for the access token. 
     The “client assertion” authentication method is similar to the “client secret” method. However, in the “client assertion” authentication method the client system  103  transmits an integrity protected version of the client secret. For instance, the client secret may be integrity protected using a digital signature or Message Authentication Code (MAC). In this way, the client secret can be protected from eavesdropping and tampering. 
     Although only certain authentication types have been described herein, any other suitable authentication type(s) could be used in addition to or instead of the authentication type(s) described. 
     In step  505 , the client system  103  stores a configurable database that identifies which authorisation systems support which grant and authentication method types. The configurable database comprises a plurality of authorisation system identifiers that are each indicative of a particular authorisation system. Thus, each authorisation system identifier enables a specific authorisation system to be identified, such as the authorisation system  105  described with reference to  FIG. 2 . 
     Each one of the authorisation system identifiers is stored in association with one or more of the plurality of types of grant method. In other words, each authorisation system identifier is logically linked with one or more of the types of grant method in the database. This enables the database to indicate which grant method type(s) are supported by a specific authorisation system. 
     In addition, each one of the authorisation system identifiers is stored in association with one or more of the plurality of types of authentication method. In other words, each authorisation system identifier is logically linked with one or more of the types of authentication method in the database. This enables the database to indicate which authentication method type(s) are supported by a specific authorisation system. 
     The database is configurable such that it can be updated in order to modify the types of grant method and the authentication method that are associated with each authorisation system identifier. Since the client system  103  is capable of executing a plurality of different types of the authentication and grant methods by virtue of the grant and authentication method code portions, the client system  103  can adapt to changes in the types of method supported by an external system in a quick and simple fashion. 
     In addition, the configurable database can be updated in order to store additional authorisation system identifiers and associated grant and authentication method types. This allows the client system  103  to be configured for communicating with an external system with which the client system  103  has not previously communicated. 
     In steps  505 A-B, the grant and authentication methods associated with one or more of the authorisation systems can be modified. This may occur in response the client system  103  receiving a message that the grant and/or authentication methods supported by an authorisation system have changed. For instance, the client system  103  may receive a message from the external system  104 , or any other system, that a particular grant or authentication method is no longer supported by its authorisation system  105 . The client system  103  may disassociate the unsupported grant or authentication method with the authorisation system identifier in response to this message. 
     In another example, the client system may receive a message from the external system  104 , or any other system, that a particular grant or authentication method that was not previously supported by the authorisation system  105  is now supported. The client system  103  may associate the newly supported grant or authentication method with the authorisation system identifier in response to this message. 
     Instead of receiving a message from another system regarding the supported grant/authentication types and responding accordingly, the database may be configured by an administrator at the client system  103  to reflect which methods are supported by a selected authorisation system. 
     The client system  103  may perform steps  505 C-D in order to maintain an accurate copy of the configurable database. This allows the client system  103  to ensure that the correct grant/authentication methods are used when communicating with the external system  104 . 
     In step  505 C, the client system  103  transmits a database request to a system that hosts the database which indicates the grant/authentication systems that are supported by a selection of authorisation systems. The database hosting system responds to the database request by transmitting at least a portion of the database to the client system  103 . Then, in step  505 D, the client system  103  updates the configurable database using the database received from the database hosting system. In this step, the configurable database is configured to store the one or more of the grant/authentication methods in the received database in association with the corresponding authorisation system identifier. 
     The client system  103  may execute step  505 C in an intermittent fashion. This allows the client system  103  to conserve bandwidth usage by reducing amount of communication between the client system  103  and the system that stores the database. The client system  103  may transmit the database request in accordance with a predetermined schedule. For instance, 30 the predetermined schedule may define a time interval between consecutive database requests. In one example, the time interval between each data request is defined as 24 hours in the predetermined schedule, such that a single database request is sent once a day. This achieves a balance between the aim of maintaining an accurate version of the database at the client system  103  and bandwidth usage. The predetermined schedule may be configurable. For instance, the time interval between adjacent database requests may be configurable by an administrator of the client system  103 . This allows the client system  103  to be tuned in order for the optimum number of database requests to be sent 5 within a given time period. 
     As discussed above, the authorisation system  105  may support different types of grant method and authentication method. However, one of these different types of method may be more or less secure than the other types. For example, the “authorisation code” grant method may be more secure than the “client credentials” grant method. In another example, the “client secret” authentication method may be less secure than the “client assertion” authentication method. 
     In step  507 , the client system  103  ranks the types of grant method and authentication method in the configurable database based on the security strength of each grant type. This step may involve associating a score with each one of the grant method types and each one of the authentication method types, which indicates the security strength of each method. For example, the “authorisation code” grant method may be associated with a score of “10” and the “client credentials” grant method may be associated with a score of “5”. In this way, the respective scores indicate that the “authorisation code” grant method is more secure that the “client credentials” grant method. 
     In another example, the “client secret” authentication method may be associated with a score of “4” and the “client assertion” authentication method may be associated with a score of “9”. In this way, the respective scores indicate that the “client assertion” authentication method is more secure than the “client secret” authentication method. 
     Ranking the grant and authentication methods based on their security strength allows the client system  103  to choose the more secure method, where a choice is available. This enhances the security of the system as a whole, since using less secure methods may expose the system to issues such as eavesdropping. 
     In step  509 , the client system  103  receives an access request from the user device  101  in a similar manner to that described in step  301  of  FIG. 3 . The access request comprises an instruction for the client system  103  to access a protected resource. The access request also comprises a request identifier which indicates the authorisation system with which the client system  103  must communicate with before accessing the protected resource at the corresponding resource system. In this example, the authorisation system is the authorisation system  105  of the external system  104 . 
     In step  511 , the client system  103  compares the authorisation system  105  indicated by the access request against the authorisation systems indicated by the authorisation system identifiers in the configurable database. If a match is found, the grant and authorisation method types associated with the matching authorisation system are identified. 
     The identified authorisation and grant method types are the methods that are supported by the authorisation system  105  with which the client system  103  must communicate in order to service the access request. There may be a plurality grant method types supported by the authorisation system  105 , or there may be only a single grant method type supported by the authorisation system  105 . There may be a plurality of authentication method types supported by the authorisation system  105 , or there may be only a single authentication method type supported by the authorisation system  105 . 
     Steps  511 A-B refer to the scenario in which a plurality of grant and/or authentication types are supported the authorisation system  104 . In step  511 A, the client system  103  selects one of the grant method types and one of the authentication method types identified in step  511 . The method type selected may be based on a variety of different criteria, for instance by choosing the fastest or the most efficient method of each type available. 
     Step  511 B refers to a specific example in which the most secure grant and/authentication method is chosen. In this step, the client system  103  selects the most secure method based on the ranking process performed in step  507 . For instance, the grant method or the authentication method with the highest score is selected for execution. 
     In step  513 , the grant method code portion corresponding with the selected grant method is executed, and the authentication method code portion corresponding with the selected authentication method is executed. 
     In step  514 , if the client system  103  is successfully validated at the authorisation system  104  using the selected grant and authentication methods, the client system  103  receives the access token. Subsequently, the client system  103  can transmit the access token to the resource system  107  in order to obtain the protected resource corresponding with the access request in step  501 . 
     Referring to  FIG. 6 , there is a computer-implemented method  600  that enables the client system  103  to ensure that it has access to a valid grant token, which is required in order to successfully respond to requests from the user to access a protected resource. This method can be used in conjunction with the method described above with reference to  FIGS. 3 and 4 . 
     Each of the grant tokens provided to the client system  103  by the authorisation system  105  may have a corresponding time to expire which is the time at which the identification will not be considered valid at the authorisation system. Thus, an expired grant token cannot be used in the process of obtaining a protected resource from the resource system  107 . If the grant token stored at the client system  103  is not valid (i.e. the token has expired) at a time when an access request (such as the request described with reference to step  301  in  FIG. 3 ) is received, the client system  103  may not be able to service the access request successfully, or at the least there will be a delay in servicing the request. Thus, the method described with reference to  FIG. 6  allows user access requests to be serviced reliably and quickly, by ensuring that a valid grant token is held by the client system  103 . 
     In step  601 , the client system  103  receives an intent message from the user device  101 . The intent message may be the access message described previously. Thus, step  601  may be performed in a similar manner to step  301  described with reference to  FIG. 3 . The access message may be referred to an as intent message because this message indicates the user&#39;s intent for the client system  103  to access the protected resource. 
     In step  603 , the client system  103  transmits a request for an grant token to the authorisation system  105 . As explained above with reference to  FIG. 3 , the grant token is a token that enables the external system  104  to authenticate the identity of the client system  103 . 
     In step  605 , the authorisation system  105  validates the client system&#39;s  103  request for an identification code. If the request is valid, the authorisation system  105  responds by transmitting the grant token to the client system  103 . The authorisation system  105  may transmit an expiry time indicator to the client system  103  that indicates the expiry time of the grant token. In addition, the authorisation system  105  may transmit a refresh token to the client system  103  that can be used to obtain a new (i.e. an unexpired) grant token. 
     Steps  603 - 505  may be performed in a similar manner to that described with reference to steps  303 - 307  in  FIGS. 3 and 4 . Also, steps  603 - 605  may be performed before, after or at the same time as receiving the intent message of step  601 . 
     In step  607 , a time interval is determined which defines the time between receiving the grant token and requesting a new one, and a timer is set using the time interval. The time interval may be a predetermined time interval, such as a 1 minute, 5 minutes or 10 minutes etc. This time interval may be based on the expiry time of the received grant token. For instance, the time interval may be equal to the expiry time of the grant token, such that the client system  103  can initiate the process of requesting a new grant token at the moment the grant token expires. In a specific example, the expiry time of the grant token may be 5 minutes and the time interval may, therefore, be set to 5 minutes. Thus, the client system  103  will request a new grant token 5 minutes after the previous grant token has been received, which is the moment at which the previous token expires. 
     In another example, the time interval may be set as an amount of time less than the expiry time of the grant token. In this way, the client system  103  can initiate the process of requesting a new grant token before the grant token expires. In a specific example, the expiry time of the grant token may be 5 minutes and the time interval may be set to 4 minutes. Thus, the client system  103  can ensure that there is only a small window of time between requesting a new grant token and expiry of the previous token. The time interval may be set such that this window us not greater than a predefined window length (e.g. 1 minute). This window can be configured by configuring the time interval to optimise the number of refresh requests sent, while ensuring that a valid grant token is held at the client system  103 . 
     In another example, the time interval may be set as an amount of time greater than the expiry time of the grant token. In this way, the client system  103  can ensure that the process of requesting a new grant token will occur at a precise moment after the previous grant token has expired. In a specific example, the expiry time of the grant token may be 5 minutes and the time interval may, therefore, by set to 6 minutes. Thus, the client system  103  can ensure that there is only a small window of time between expiry of the grant token and the request for a new one. The time interval may be set such that this window is not greater than a predefined window length (e.g. 1 minute). Again, this window can be configured by configuring the time interval to optimise the number of refresh requests sent, while ensuring that a valid grant token is held at the client system  103 . 
     In step  609 , the client system  103  starts a timer in response to receiving the grant token and using the time interval set in step  607 . The timer is used to determine the moment at which the time interval has elapsed. 
     In step  611 , once the time interval has elapsed the client system  103  transmits a refresh request to the authorisation system  105 . The refresh request may comprise an instruction for a new (i.e. unexpired) grant token to be provided to the client system  103  from the authorisation system  105 . 
     In step  613 , the client system  103  receives the new grant token in response to the refresh request. 
     In step  615 , the client  103  may transmit the most recently received grant token to the resource system  107  in order to initiate a request for the protected resource. Step  615  may be performed in a similar manner that described with reference to step  309  of  FIG. 3 . 
     Steps  601  to  615  may be repeated such that many different grant tokens are received from the authorisation system  107 . In addition, steps  601  to  615  may be repeated for other authorisation system such that many different grant tokens are received from a variety of authorisation systems. The grant tokens received from a particular authorisation system may have similar expiry times. 
     In step  617 , the time interval discussed in connection with steps  607  and  609  is calculated based on the expiry times of the grant tokens received and the specific authorisation systems from which the grant tokens are received. 
     In one example, the client system  103  may calculate a predicted expiry time of the grant tokens received from a particular authorisation system. The predicted expiry time may be calculated by calculating an average of the expiry times of the grant token received from a specific authorisation system, or a plurality of different authorisation systems. 
     The predicted expiry time calculated in step  617  may be used to determine the time interval in step  607 . For instance, the predicted expiry time may equal to, a time less than or a pre-set time greater than the time interval in step  607 . 
     Referring to  FIG. 7 , there is a computer-implemented method  700  that enables the client system  103  reduce the bandwidth and processing resources used by minimising the number of requests for access tokens sent, such as the requests for access tokens described with reference to steps  323  and  325  in  FIG. 3 . Thus, the method described with reference to  FIG. 7  can be used in conjunction with the method described above with reference to  FIGS. 3 and 4 . 
     In step  701 , the client system  103  receives an access request from the user device  101 . This step may be performed in a similar manner to that described with reference to step  301  in  FIG. 3 . 
     In step  703 , the client system  103  transmits a request for an access token to the authorisation server  105  in response to the access request. This step may be performed in a similar manner to step  323 , for instance after steps  303  to  321  have been performed, as described with reference to  FIG. 3 . 
     In step  705 , the client system  103  receives the access token from the authorisation system  105 . This step may be performed in a similar manner to step  325 , as described with reference to  FIG. 3 . In this step, the authorisation system  105  may transmit an expiry time indicator to the client system  103 . The expiry time indicator corresponds to the access token and indicates the expiry time of the access token. After the expiry time indicated by the expiry time the access token cannot be used to access the protected resource. In other words, the expiry time indicator is indicative of the time at which the corresponding access token will not be valid for obtaining the protected resource from the resource system  107 . In step  505 , the authorisation system  105  can transmit a refresh token corresponding with the access token. The refresh token can be used by the client system  3  obtain an unexpired access token from the authorisation system  105 . 
     The expiry time indicator may be indicative of a length of time during which the access token will be valid. For instance, this length of time may be expressed as a number of second or minutes. Alternatively, the expiry time may indicate a point in time at which the access token will no longer be valid. For instance, the expiry time may indicate a specific time during the day. The client system  103  may determine the expiry time of the access token based on the length of time or the point in time indicators by the expiry time indicator. 
     In step  707 , the client system  103  transmits the access token to the resource system  107  in a request to receive the protected resource. This step may be performed in a similar manner to step  337  as described with reference to  FIG. 4 . 
     In step  711 , the access token received from the authorisation system  107  is stored at a token storage unit at the client system  103 . The expiry time indicated by the expiry time indicator may be stored at the token storage unit also. The access token and/or the expiry time indicated by the expiry time indicator may be encrypted and stored at the token storage unit to enhance security. In this step, the access token may be stored independently of the expiry time of the corresponding access token. In other words, the client system  103  stores the access token irrespective of whether the expiry time is short or long. This reduces the processing effort required in analysing the expiry time of each access token. Alternatively, the client system  103  may not store, or may delete, the access token if its expiry time is less than a predetermined threshold. If the access token is stored, the access token may be stored after its expiry time. 
     In step  713 , the client system  103  receives another access request similar to the access request received in step  701 . 
     In step  715 , the client system  103  transmits the stored access token to the authorisation system  107  in another request to receive the protected resource similar to the request sent in step  707 . Thus, the client system  103  uses the stored access token, rather than requesting a new access token to service the user&#39;s access request. This reduces the bandwidth and processing resources used by the client system  103 . 
     In steps  719  to  727 , token storage maintenance requests performed at the client system  103 . These steps assist in conserving storage resources, while making more efficient use of bandwidth and processing resource by minimising the number of requests for new access tokens. 
     In step  719 , the client system  103  compares the time indicated by the expiry time indicator corresponding with one or more of the access tokens stored. 
     In step  721 , if the time indicated by an expiry time indicator is after the current time, the method proceeds to step  723 . Alternatively, if the time indicated by the expiry time indicator is not after the current, the method proceed to step  725 . 
     In step  723 , the access token corresponding with the expiry time that is after the current time is deleted. 
     Steps  719  to  723  may be performed intermittently. For instance, steps  719  may be executed in accordance with a predetermined schedule. In one example, the predetermined schedule defines a time interval between adjacent executions of steps  719  to  723 . This time interval may be configurable at the client system  103  based on a user input received from an administrator. The time interval may be configurable at the client system  103  based on monitored performance of the client system  103 . Configuration of the time interval may occur automatically. 
     Steps  725  and  727  may be executed In order to perform steps  719  to  723  in accordance with the predetermine schedule. In step  725 , the time interval of the predetermined schedule is determined. Then, in step  727  the method proceed to repeat steps  719  to  723  once the time interval has elapsed. 
     Steps  729  and  731  may be performed in the method  700  in order to manage a situation in which an access token has become invalid and, therefore, will not be usable for obtaining the protected resource. When an access token has become invalid it should not be used again, as this will involve unnecessary communications being transmitted in between the client system  103  and the external system  104 . However, deleting access tokens each time they are deemed to be invalid involves a processing burden that it would preferable to avoid. This is particularly relevant when the client system  103  handles a large number of access token for a large number of users with protected resources stored at a variety of external systems. Steps  729  and  731  allow the transmission of invalid access to be prevented, while avoiding the processing burden managing the storing of access tokens an on individual basis. 
     In step  729 , the client system  103  receives a rejection message indicating that the access token is not valid. This message may be received from the external system  104 , for instance via the authorisation system  105  or the resource system  107 . Alternatively, the rejection message may be received from any other system, or may be received via an input at the client system  103 . 
     The rejection message may be received in response to the external system  104  determining that the access token has expired. For example, the client system  103  may transmit the stored access token to the authorisation system  105  in an attempt to access the protected resource. However, the authorisation system  107  may determine that the access token has expired. In response to determining the access token has expired, the authorisation system  107  transmits a rejection message to the client system  103 . 
     In another example, the rejection message may be received in response to the user of the user device  101  revoking their authorisation for the client system  103  to access the protected resource. The user may inform the authorisation system  107  that their authorisation has been revoked. This will invalidate the corresponding access token, and its corresponding refresh token. However, in this scenario the client system  103  will be unaware that access token and the refresh token have been invalidated. Therefore, the client system  103  may continue to transmit requests for access to the protected resource using the invalid access token. The client system  103  will receive a rejection message from the external system  104  in response to each one of these requests because the access token is invalid. However, the client system  103  might still transmit refresh requests using the invalid refresh token, under the assumption that the access token has expired rather than the user&#39;s authorisation having been revoked. All of these processes represent an unnecessary load on the processing resources and bandwidth of the client system  103 . If the access token has a corresponding refresh token, the rejection message comprises an indication that the refresh token is invalid. 
     In step  731 , rather than deleting the access token and the refresh token directly in response to the rejection message, the client system  103  sets an invalidation flag in association with the corresponding access token and refresh token. Setting the invalidation flag involves a lower processing overhead than deleting the tokens. Thus, when the client system  103  is managing many tokens, this will equate to a significant enhancement in efficiency. 
     The invalidation flag may be an addition bit field stored in association with the access token and the refresh token. For instance, if the bit field is set to “1”, this may indicate that the corresponding token is invalid, and, if the bit field is set to “0”, this may indicate that the corresponding token is invalid (or vice versa). 
     In another example, the invalidation flag may be set by modifying the expiry time indicator associated with the access token and/or the refresh token. In this example, the expiry time indicator is set in the past in order to indicate that the token(s) are no longer valid. Thus, when steps  719  to  727  are performed subsequently, the tokens will be deleted as part of the token storage maintenance steps 
     Referring to  FIG. 8 , the client system  103  comprises a communication interface  801  comprising a receiver  803  and a transmitter  805 . The client system  103  comprises a processor  807 , an identification module  813 , a ranking module  815 , a timer module  817 , a time interval calculation module  819 , a token storage maintenance module and a flag setting module  823 . The client system  103  also comprises the retrieval engine  103   a  and the token management server  103   b  described above. 
     There is a storage resource  825  at the client system  103  which comprises a grant method code portion storage resource  827 , an authentication method code storage resource  829 , a configurable database storage resource  831  and a token storage unit  833 . 
     The receiver  803  and the transmitter  805  are configured to receive and transmit message, instructions and tokens to and from the client system  103  as explained above. 
     The storage resource  825  is configured to store grant method code portions, as described above with reference to step  501 , at the grant method code portions storage resource  827 . The storage resource  825  is configured to store authentication method code portions, as described above with reference to step  503 , at the authentication method code portion storage resource  829 . The storage resource  825  is configured to store the configurable database, as described above with reference to step  505 , at the configurable database storage resource  831 . The token storage unit  833  is arranged for storing tokens, such as the access tokens, grant tokens and refresh tokens described above. 
     The ranking module  817  is configured to perform the ranking processes described above, such as those described above with reference to step  507 . The identification module  813  is configured to identify the authentication method and grant methods supported by an authorisation server, as described above with reference to step  511 . The processor  807  is configured to execute instructions, such as the instruction of the selected grant and authentication method code portions, as described with reference to step  513 . The timer module  817  is configured to activate a timer for a time period, for instance as described above with reference to step  607 ,  609 . In addition, the timer module  817  can be used to monitor a current time for comparison with the expiry time of a token, as described above with reference to step  719 . The time interval calculation module  819  is configured to calculate a time interval for setting the timer, as described above with reference to step  725 . The token storage management module  821  is configured to compare the current time indicated by the timer with the expiry time of a token, and to delete a token in response, as described above with reference to steps  721  and  723  above. The flag setting module  823  is configured to set an invalidation flag in association with a token, as described above with reference to step  731 . 
       FIG. 9  shows an exemplary electronic device  901  according to any of the electronic devices or systems of this disclosure (such as the user device  101 , client system  103 , the external system  104 , the authorisation system  105 , the resource system  107 , the retrieval engine  103   a  or the TMS  103   b ). The electronic device  901  comprises processing circuitry  910  (such as a microprocessor) and a memory  912 . Electronic device  901  may also comprise one or more of the following subsystems: a power supply  914 , a display  916 , a transceiver  920 , and an input  926 . 
     Processing circuitry  910  may control the operation of the electronic device  901  and the connected subsystems to which the processing circuitry is communicatively coupled. Memory  912  may comprise one or more of random access memory (RAM), read only memory (ROM), non-volatile random access memory (NVRAM), flash memory, other volatile memory, and other non-volatile memory. 
     Display  916  may be communicatively coupled with the processing circuitry  910 , which may be configured to cause the display  916  to output images representative of the secure data, or protected resources, shared between the entities in the system  100 . 
     The display  916  may comprise a touch sensitive interface, such as a touch screen display. The display  916  may be used to interact with software that runs on the processor  910  of the electronic device  901 . The touch sensitive interface permits a user to provide input to the processing circuitry  910  via a discreet touch, touches, or one or more gestures for controlling the operation of the processing circuitry and the functions described herein. It will be appreciated that other forms of input interface may additionally or alternatively be employed for the same purpose, such as the input  926  which may comprise a keyboard or a mouse at the input device. 
     The transceiver  920  may be one or more long-range RF transceivers that are configured to operate according to communication standard such as LTE, UMTS, 3G, EDGE, GPRS, GSM, and Wi-Fi. For example, electronic device  901  may comprise a first wireless transceiver  921 , such as a cellular transceiver, that is configured to communicate with a cell tower  903  via to a cellular data protocol such as LTE, UMTS, 3G, EDGE, GPRS, or GSM, and a second transceiver  928 , such as a Wi-Fi transceiver, that is configured to communicate with a wireless access point  904  via to a Wi-Fi standard such as 802.11 ac/n/g/b/a. In this regard and for the purposes of all embodiments herein concerning a long-range wireless protocol, a long-range wireless protocol may be a protocol which is capable and designed for communication over 5, 10, 20, 30, 40, 50, or 100 m. This is in contrast to short-range wireless protocol mentioned above. The long-range wireless protocol may communicate utilizing higher power than the short-range wireless protocol. The range (e.g. line of sight distance) between the long-range end nodes (electronic device and router or base station) for the long-range wireless protocol may be greater than the range (e.g. line of sight distance) between the short-range end nodes (e.g. electronic device and wireless beacon). 
     Electronic device  901  may be configured to communicate via the transceiver  920  with a network  940 . Network  940  may be a wide area network, such as the Internet, or a local area network. Electronic device  901  may be further configured to communicate via the transceiver  920  and network  940  with one or more systems or user devices. These servers or user devices may be any one of those described herein. 
     The term “comprising” encompasses “including” as well as “consisting” e.g. a composition “comprising” X may consist exclusively of X or may include something additional e.g. X+Y. 
     The word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y. Where necessary, the word “substantially” may be omitted from the definition of the invention. 
     Unless otherwise indicated each embodiment as described herein may be combined with another embodiment as described herein. 
     The methods described herein may be performed by software in machine readable form on a tangible storage medium e.g. in the form of a computer program comprising computer program code means adapted to perform all the steps of any of the methods described herein when the program is run on a computer and where the computer program may be embodied on a computer readable medium. Examples of tangible (or non-transitory) storage media include disks, thumb drives, memory cards etc. and do not include propagated signals. The software can be suitable for execution on a parallel processor or a serial processor such that the method steps may be carried out in any suitable order, or simultaneously. This acknowledges that firmware and software can be valuable, separately tradable commodities. It is intended to encompass software, which runs on or controls “dumb” or standard hardware, to carry out the desired functions. It is also intended to encompass software which “describes” or defines the configuration of hardware, such as HDL (hardware description language) software, as is used for designing silicon chips, or for configuring universal programmable chips, to carry out desired functions. 
     It will be appreciated that the modules described herein may be implemented in hardware or in software. Furthermore, the modules may be implemented at various locations throughout the system. 
     Those skilled in the art will realise that storage devices utilised to store program instructions can be distributed across a network. For example, a remote computer may store an example of the process described as software. A local or terminal computer may access the remote computer and download a part or all of the software to run the program. Alternatively, the local computer may download pieces of the software as needed, or execute some software instructions at the local terminal and some at the remote computer (or computer network). Those skilled in the art will also realise that by utilizing conventional techniques known to those skilled in the art that all, or a portion of the software instructions may be carried out by a dedicated circuit, such as a DSP, programmable logic array, or the like. 
     Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person. 
     It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. 
     Any reference to ‘an’ item refers to one or more of those items. The term ‘comprising’ is used herein to mean including the method blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements. 
     The steps of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the spirit and scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought. Any of the module described above may be implemented in hardware or software 
     It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this invention. 
     LIST OF NUMBERED EMBODIMENTS 
     1. A computer-implemented method for obtaining an access token for providing access to a protected resource stored at a resource system, the method comprising: 
     storing, at a client system:
         a plurality of grant method code portions each executable to obtain access to the access token using one of a plurality of types of grant method, wherein each respective type of grant method is different to the other types of grant method; and   a plurality of authentication method code portions each executable to authenticate the client system using a different authentication method, wherein each respective type of authentication method is different to the other types of authentication method;       

     storing, at the client system, a configurable database comprising a plurality of authorisation system identifiers each indicative of a respective authorisation system, wherein each of the plurality of authorisation system identifiers is associated with one or more of the plurality of types of grant method which are supported by the respective authorisation system, and each of the plurality of authorisation system identifiers is associated with one or more of the plurality of types of authentication method which are supported by the respective authorisation system; 
     receiving, at the client system, from a user device an access request comprising an instruction for the client system to access a protected resource, the instruction comprising a request identifier indicative of an authorisation system for authorising access to the protected resource; 
     identifying, in the configurable database, a selected grant method type from one or more of the grant method types associated with an authorisation system identifier corresponding to the request identifier; 
     identifying, in the configurable database, a selected authentication method type, from one or more of the authentication method types associated with the authorisation system identifier corresponding to the request identifier; 
     executing, at the client system, the grant method code portion corresponding to the selected grant method type to request the access token for accessing the protected resource; 
     executing, at the client system, the authentication method code portion corresponding to the selected authentication method type to authenticate the client system at the authorisation system; and 
     receiving the access token at the client system from the authorisation sever, in response to executing the grant method code portion and the authentication method code portion. 
     2. The computer-implemented method of embodiment 1 further comprising: 
     modifying, at the client system, the one or more of the plurality of types of grant method in the configurable database associated with at least one of the plurality of authorisation system identifiers stored in the configurable database, thereby forming a modified configurable database; 
     identifying, at the client system, a selected grant method type, in the modified configurable database, from one or more of the grant method types associated with an authorisation system identifier corresponding to the request identifier; and executing, at the client system, the grant method code portion corresponding to the selected grant method type to request the access token for accessing the protected resource. 
     3. The computer-implemented method of embodiment 1 or embodiment 2 further comprising: 
     modifying, at the client system, the one or more of the plurality of types of authentication method in the configurable database associated with at least one of the plurality of authorisation system identifiers stored in the configurable database, thereby forming a modified configurable database at the client system; 
     identifying, at the client system, a selected authentication method type, in the modified configurable database, from one or more of the authentication method types associated with an authorisation system identifier corresponding to the request identifier; and 
     executing, at the client system, the authentication method code portion corresponding to the selected authentication method type to authenticate the client system at the authorisation system. 
     4. The computer-implemented method of any one of the preceding embodiments further comprising: 
     transmitting a database request to a database hosting system from the client system and, in response, receiving at the client system at least a portion of a database; and 
     updating, the one or more of the plurality of types of grant method and/or the one or more of the plurality of types of authentication method associated with at least one of the plurality of authorisation system identifiers stored in the configurable database using the received database. 
     5. The computer-implemented method of embodiment 4 wherein the database request is transmitted intermittently according to a predetermined schedule. 
     6. The computer-implemented method of embodiment 5 wherein the predetermined schedule defines a time interval between consecutive database requests. 
     7. The computer-implemented method of any one of the preceding embodiments wherein at least one of the authorisation system identifiers is associated with a plurality of the one or more types of grant method which are supported by the respective authorisation system, and the method further comprises: 
     identifying in the configurable database, at the client system, the grant method types associated with the authorisation system identifier corresponding to the first identifier; 
     selecting, at the client system, a grant method type from the identified grant method types; and 
     executing, at the client system, the grant method code portion corresponding to the selected grant method type. 
     8. The computer-implemented method of embodiment 7 further comprising: 
     ranking, at the client system, the plurality of types of grant method stored in the configurable database based on the security strength of each grant method type; 
     wherein the step of selecting comprises selecting, from the identified grant method types, the grant method type which is ranked with the highest security strength relative to the other identified grant method types. 
     9. The computer-implemented method of any one of the preceding embodiments wherein at least one of the plurality of authorisation system identifiers is associated with a plurality of the one or more types of authentication method which are supported by the respective authorisation system, and the method further comprises: 
     identifying in the configurable database, at the client system, the authentication method types associated with the authorisation system identifier corresponding to the first identifier; 
     selecting, at the client system, an authorisation method type from the plurality of identified authorisation method types; and 
     executing, at the client system, the authentication method code portion corresponding to the selected authentication method type. 
     10. The computer-implemented method of embodiment 9 further comprising: 
     ranking, at the client system, the plurality of types of authentication method stored in the configurable database based on the security strength of each respective authentication method type; 
     wherein step of selecting comprises selecting, from the identified authentication method types, the authentication method type ranked with the highest security strength relative to the other identified authentication method types. 
     11. The computer-implemented method of any one of the preceding embodiments wherein each respective grant method type is indicative of a grant method supported by the authorisation system indicated by the authorisation system identifier associated with the grant method type.
 
12. The computer-implemented method of any one of the preceding embodiments wherein each respective authentication method type is indicative of an authentication method supported by the authorisation system indicated by the authorisation system identifier associated with the authentication method type.
 
13. The computer-implemented method of any one of the preceding embodiments wherein the protected resource is accessible by the first user via an online account of the first user.
 
14. The computer-implemented method of any one of the preceding embodiments wherein the plurality of grant method code portions comprises an authorisation code grant method code portion which when executed causes the client system to:
 
     redirect the first user device to the authorisation system, wherein redirecting the user device to the authorisation system comprises the user device transmitting a request, to the authorisation system, for an authorisation code; 
     receive an authorisation code from the authorisation system; and 
     transmit the authorisation code to the authorisation system and, in response, receive the access token. 
     15. The computer-implemented method of any one of the preceding embodiments wherein the plurality of grant method code portions comprises a client credentials grant method code portion which when executed causes the client system to: 
     transmit a shared secret to the authorisation system and, in response, receive the access token, without requesting an authorisation code. 
     16. The computer-implemented method of any one of the preceding embodiments wherein the plurality of authentication method code portions comprises a client secret method code portion which when executed causes the client system to: 
     transmit a shared secret to the authorisation system for authenticating the client system at the authorisation system. 
     17. The computer-implemented method of any one of the preceding embodiments wherein the plurality of authentication method code portions comprises a client assertion method code portion which when executed causes the client system to: 
     transmit an integrity protected identifier of the client system to the authorisation system for authenticating the client system at the authorisation system. 
     18. The computer-implemented method of any one of the preceding embodiments further comprising: 
     transmitting the access token from the client system to the resource system and, in response, receiving the protected resource. 
     19. The computer-implemented method of embodiment 18 further comprising: 
     processing, transmitting and/or displaying the protected resource using the client system. 
     20. The computer-implemented method of any one of the preceding embodiments wherein the client system comprises one or more client servers in communication with one another. 
     21. The computer-implemented method of any one of the preceding embodiments wherein the authorisation system comprises one or more authorisation servers in communication with one another. 
     22. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of any one of the preceding embodiments. 
     23. A data carrier signal carrying the computer program of embodiment 22. 
     24. A computer readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method of any one of embodiments 1 to 21. 
     25. A client system for obtaining an access token for accessing a protected resource stored at a resource system, the client system comprising: 
     a storage resource configured to store:
         a plurality of grant method code portions each executable to obtain access to the access token using one of a plurality of types of grant method, wherein each respective type of grant method is different to the other types of grant method;   a plurality of authentication method code portions each executable to authenticate the client system using a different authentication method, wherein each respective type of authentication method is different to the other types of authentication method; and   a configurable database comprising a plurality of authorisation system identifiers each indicative of a respective authorisation system, wherein each of the plurality of authorisation system identifiers is associated with one or more of the plurality of types of grant method which are supported by the respective authorisation system, and each of the plurality of authorisation system identifiers is associated with one or more of the plurality of types of authentication method which are supported by the respective authorisation system;       

     the client system further comprising processing circuitry configured to: 
     receive, from a user device, an access request comprising an instruction for the client system to access a protected resource, the instruction comprising a request identifier indicative of an authorisation system for authorising access to the protected resource; 
     identify a selected grant method type, in the configurable database, from one or more of the grant method types associated with an authorisation system identifier corresponding to the request identifier; 
     identify a selected authentication method type, in the configurable database, from one or more of the authentication method types associated with the authorisation system identifier corresponding to the request identifier; 
     execute the grant method code portion corresponding to the selected grant method type to request the access token for accessing the protected resource; 
     execute the authentication method code portion corresponding to the selected authentication method type to authenticate the client system at the authorisation system; and 
     receive the access token from the authorisation sever, in response to executing the grant method code portion and the authentication method code portion. 
     26. A client system for obtaining an access token for accessing a protected resource stored at a resource system, the client system comprising: 
     a storage resource configured to store:
         a plurality of grant method code portions each executable to obtain access to the access token using one of a plurality of types of grant method, wherein each respective type of grant method is different to the other types of grant method;   a plurality of authentication method code portions each executable to authenticate the client system using a different authentication method, wherein each respective type of authentication method is different to the other types of authentication method; and   a configurable database comprising a plurality of authorisation system identifiers each indicative of a respective authorisation system, wherein each of the plurality of authorisation system identifiers is associated with one or more of the plurality of types of grant method which are supported by the respective authorisation system, and each of the plurality of authorisation system identifiers is associated with one or more of the plurality of types of authentication method which are supported by the respective authorisation system;       

     the client system further comprising: 
     a receiver configured to receive, from a user device, an access request comprising an instruction for the client system to access a protected resource, the instruction comprising a request identifier indicative of an authorisation system for authorising access to the protected resource; 
     an identification module configured to: 
     identify a selected grant method type, in the configurable database, from one or more of the grant method types associated with an authorisation system identifier corresponding to the request identifier;
         identify a selected authentication method type, in the configurable database, from one or more of the authentication method types associated with the authorisation system identifier corresponding to the request identifier;       

     a processor configured to:
         execute the grant method code portion corresponding to the selected grant method type to request the access token for accessing the protected resource;   execute the authentication method code portion corresponding to the selected authentication method type to authenticate the client system at the authorisation system; and       

     wherein the receiver is configured to receive the access token from the authorisation sever, in response to executing the grant method code portion and the authentication method code portion.