Abstract:
A functional architecture is provided for decentralizing the authorization function of an access control system that incorporates user carried access devices, such as smart cards, and door controllers that interact so as to make access decisions. Access to individual rooms is guarded by parameters partially carried by the user carried access devices and partially included in the door controllers.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present application relates to decentralizing the authorization function in the context of physical access control. 
     BACKGROUND OF THE INVENTION 
     Access control is frequently implemented to control the access of users to resources and/or to make decisions about denying or granting access to those resources. In the context of physical access control, these resources are typically rooms or, more generally, restricted areas guarded by entrances or doors. 
     The goal of authorization in access control is usually to specify and evaluate/look-up a set of policies that control the access of users to resources, i.e., making decisions about denying or granting access of users to resources. The goal of secure authorization is usually to communicate this decision in a secure manner. The goal of authentication is usually to verify that a user is who the user says he or she is. The focus herein is primarily on authorization. 
     As shown in  FIGS. 1 and 2 , an access control system  10  traditionally includes card readers  12   1 ,  12   2 , . . . ,  12   n  connected to a centralized controller  14 . The card readers  12   1 ,  12   2 , . . . ,  12   n , for example, are typically stationed at doors or other access points to restricted areas. Each of the card readers  12   1 ,  12   2 , . . . ,  12   n  reads access cards carried by the users, and the card readers  12   1 ,  12   2 , . . . ,  12   n  communicate information read from the access cards to the centralized controller  14 . Locks or other entry control devices  16   1 ,  16   2 , . . . ,  16   n  at the access points to the restricted areas are subsequently instructed by the centralized controller  14  to either permit or deny access. The card readers  12   1 ,  12   2 , . . . ,  12   n  communicate with the centralized controller  14  for every access request. Each of the locks or other entry control devices  16   1 ,  16   2 , . . . ,  16   n  usually correspond to one of the card readers  12   1 ,  12   2 , . . . ,  12   n  and are located at the same access point. 
     In many access control systems, such as the access control system  10  shown in  FIGS. 1 and 2 , neither the card readers  12   1 ,  12   2 , . . . ,  12   n  nor the access cards have any appreciable processing, power, or memory themselves. Hence, such card readers  12   1 ,  12   2 , . . . ,  12   n  and access cards are usually referred to as passive devices. 
     By contrast, the centralized controller  14  of the access control system  10  is usually a well designed and sophisticated device with fail-over capabilities and advanced hardware and algorithms to perform fast decision making. 
     The decision making process of the centralized controller  14  of the access control system  10  is fundamentally based on performing a lookup in a static Access Control List (ACL)  18 . The ACL  18  contains static policy based rules (e.g., one rule in the ACL  18  might provide that user X is not allowed entry into room R), which change only when the policy changes (e.g., the ACL  18  might be changed to provide that user X can henceforth enjoy the privileges of room R). 
     Policies are implemented in a set of rules that governs authorization. The static ACL based policies as mentioned above can be viewed as context-independent policies. In contrast, context-sensitive policies will require a dynamic evaluation of different states of the system including the user&#39;s past history of activities. This evaluation is referred to as dynamic authorization. 
     With the interconnect architecture of  FIGS. 1 and 2 , and with a reasonable number of users of a protected facility, the access control system  10  using static ACL based policies makes decisions quickly, is reliable, and is considered to be reasonably robust. It may be additionally noted that, in current access control systems, context-sensitive policies typically constitute a small fraction of the total policies governing the operation of the system. 
     It is expected that buildings and facilities of the future will require increasingly more intelligent physical access control solutions. For example, access control solutions are being provided with the capability to detect such conditions as intrusion and fire. In general, this increased capability implies that such access control solutions should be provided with the ability to specify conditions that are dynamically evaluated, e.g., disable entry to a particular room in case of a break-in, and/or disable entry to a particular room if its occupancy reaches its capacity limit, and/or allow entry to a normal user only if a supervisor is already present inside the room, etc. This increased capability leads to a significant emphasis on the need for dynamic authorization. That is, if context-sensitive policies form a significant part of the access control policies of a facility, then the facility will appear to adapt its access control enforcement in keeping with the changes in the system. Thus, the facility will appear to be more intelligent as compared to facilities having a lesser number of context dependent, access control policies. 
     Such dynamic authorization can be centrally implemented with the current architecture ( FIG. 1 and 2 ). This centralized implementation will require the context information pertaining to every possible policy to be continuously gathered at the central controller, and upon a request, the controller needs to evaluate this context and needs to arrive at a dynamic authorization decision. 
     While this process can work for small facilities, such a centralized solution will not scale up well with an increase in the number of users, size of the facility, or complexity of the context-sensitive policies, since progressively more and more information will have to be pushed from various sources to the central controller. 
     Due to reasons of flexibility and ease of installation and modification, a general purpose network (e.g., an Internet Protocol (IP) network of a facility) is more attractive for an access control solution in comparison with the special purpose dedicated connections between the various devices and the central controller in  FIGS. 1 and 2 . 
     As shown in  FIG. 3 , an access control system  20  using a more generic interconnect architecture may include card readers  22   1 ,  22   2 , . . . ,  22   n  connected to a network  24  that is either a wired only network, or a wireless only network, or a mixed wired and wireless network. The network  24  includes controllers  26   1 , . . . ,  26   n  and servers  28   1 , . . . ,  28   n . The architecture of  FIG. 3  is not suitable for the centralized access control system  10  shown in  FIGS. 1 and 2 . This unsuitability is due to the fundamental dependency on the central controller for every decision, i.e., a system architecture that necessitates a guaranteed reader-to-controller communication for every access decision will not be a good choice for the more generic and flexible interconnect architecture (such as that shown in  FIG. 3 ). 
     The present application focuses primarily on a decentralized policy evaluation framework for dynamic authorization. Addressed herein are issues of scalability related to dynamic authorization as raised above. The present invention as set out in the claims hereof enables an access control system to leverage a more general purpose network, e.g., the IP network of a facility. 
     Most work in the domain of facility access control is based on a model having a door D that receives an input I (including user id) from an access card (or some other device carried by an user), that sends information i (where i=f (I)) to a central controller E, and that receives a response R from the central controller E. The response R indicates whether or not access is allowed. 
     A purely centralized implementation of access control has only one controller E, whereas a slightly more scalable solution that has multiple controllers with different levels or hierarchies and data caching is shown in European Application EP1320012A2. 
     U.S. Pat. No. 6,570,487 describes an arrangement that is intended to improve the robustness of communications from the doors to the access controllers by providing redundancy of receivers and access controllers (referred to as distributed receivers and distributed access controllers in the literature). 
     One fundamental problem addressed by work related to access control is that of a secure transmission of the response R from the controller E to the door D rather than of determining the response R per se. It may be recalled that determining the privilege grant content of the response R, i.e., computing what should be the access permission, given a certain door D and input I, is the problem of authorization. 
     Core Street has described a technique for making the controller E to door D communication more secure by enabling the door D to figure out if the response R is valid, given the input I. Only the controller E can generate the response R and this response can then be made publicly available. That is, the response R cannot be generated by a non-controller E given the input I and previous responses on similar transactions. 
     Thus, as detailed in U.S. Published Application 20050055567, a barrier to access is provided that includes a controller and at least one administration entity. The controller selectively allows access, and the at least one administration entity generates credentials/proofs. According to the barrier, no valid proofs are determinable given only the credentials and values for expired proofs. The controller receives the credentials and proofs, the controller determines if access is presently authorized, and, if access is presently authorized, the controller allows access. 
     Document WO2003088166A2 shows how the door D can verify the response R by making use of a one way hash function H (N I ) (where N I  is dependant on the input I), and an elapsed time interval of which the door D keeps track. A related document WO2005010685 underlines how this strategy can be useful for disconnected doors—where essentially the response R will be carried by the access card. 
     U.S. Published Application 20030028814 describes a generic microcontroller enabled door reader that can communicate with a smart card. However, its functional architecture uses the card and reader interaction to establish the authenticity of the card and not for authorization. 
     In the last 10-15 years, significant research efforts have been directed towards coming up with an authorization framework, inclusive of a policy specification language and a well defined authorization model that supports dynamic authorization. To a large extent, these frameworks focus on languages that provide flexibility in specifying role based policies and guarantees unambiguous evaluation (decision) with feasible bounds on the run time, and implicitly assume a centralized implementation of the policy evaluation. These approaches concentrate more on access control as modeled on computer systems in general and not on physical access control in buildings. Consequently, while they underline the need and importance of context-dependent or dynamic evaluation of access control policies, the functional architecture remains centralized and focused on languages that provide flexibility in specifying role based policies and guarantees unambiguous evaluation (decision) with feasible bounds on the run time. 
     U.S. Pat. No. 6,647,388 discloses that an access request can be used to extract a policy condition and that the policy condition is evaluated to determine if there is sufficient information available to evaluate, to obtain the necessary information if there is insufficient information to reach a proper decision, and then to grant or deny access on the basis of the evaluated information. However, this processing was designed for access control in computer systems in general and, hence, its functional architecture differs from that of the present invention. 
     Similarly, U.S. Published Application 20050068983 includes a context based access control policy, but is more geared towards software systems where the requesting agent can wait for all the necessary context evaluations to be performed by a separate service module. 
     U.S. Published Application 20050080838 presents a flexible architecture for dynamic policy evaluation in the context of web-services and is significantly different in the functional modules from the present invention. U.S. Pat. No. 6,014,666, U.S. Published Application 20050132048(A1), U.S. Published Application 20030204751(A1), and U.S. Published Application 20050138419(A1) also discuss similar access control mechanisms in the context of general computer systems and software agents. 
     There exist applications and standards that use smart cards where per user information is written back to the cards from specific terminals/controllers that they interact with (e.g., MONEO and CEP). An example is the electronic purse. However, these applications concentrate more on security issues and not so much on the context-dependent run-time policy evaluations. 
     The recent draft of XACML (extensible Access Control Markup Language Version 2.0) under OASIS also addresses access control of general computer systems and focuses on the policy language model. It does include the vision of a distributed access control based on a request response model of many participating entities, and lays down the request/response language protocols for exchanging access control decisions. Thus, it streamlines the terms and their scopes in the context of access control on an internet based network of computing resources, and lays down recommendations of various kinds of data exchanges (and their suggested formats). However, it does not identify any particular functional architecture for decentralized user access control in relation to large facilities. 
     The present invention solves one or more of these or other problems. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a decentralized access control system is provided to make decentralized access authorization decisions. The system comprises the following: at least one access controlling device and at least one user carried device. The access controlling device provides a first parameter that enables a decision relating to access authorization of a user. The at least one user carried device is carried by the user and interacts with the access controlling device, the user carried device stores a second parameter that enables the decision relating to the access authorization of the user at the instance of presenting the user carried device to the access controlling device, and the decision is made as a function of both the first parameter and the second parameter. 
     According to another aspect of the present invention, a smart card, which is useful in a decentralized access control system whereby access authorization decision making is decentralized, comprises a memory and a processor. The memory stores policy rules, the policy rules enable decisions to be made at instances of presenting the smart card to an access controller controlling access to a restricted area, and the decisions relate to access to the restricted area by a user of the smart card. The processor is coupled to the memory and is arranged to enable the decisions based upon the policy rules and a system context transmitted to the smart card. The system context is based on an environment relating to the restricted area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which: 
         FIGS. 1 and 2  show a traditional centralized access control system; 
         FIG. 3  shows a generic interconnect architecture that can be used for access control system; 
         FIG. 4  shows an access control system according to an embodiment of the present invention; 
         FIG. 5  shows a representative one of the smart cards of  FIG. 4 ; 
         FIG. 6  shows a representative one of the readers of  FIG. 4 ; and, 
         FIG. 7  shows a representative one of the door controllers of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     The domain of the control of physical access to a facility involves users (who are free to move) making requests (e.g., swiping a card, pointing a device, etc.) to some physical device (e.g., reader, processor, etc.) for access to some resource. For example, facility access control that guards a user&#39;s physical entry/exit to/from a room or other similar restricted area exemplifies this physical access control space. Facility access control specifies and enforces a set of policies/rules that dictate access of users to spaces such as rooms. Authorization deals with the issues of determining whether to grant or deny access as per the policies/rules that are conditional on dynamically changing aspects of the system. 
     This issue of authorization is addressed herein, as distinct from issues relating to security (i.e., secure communication of authorization decisions) and authentication (identification of an user). Existing access control systems primarily address static policies and typically involve a centralized implementation strategy where all the policies are stored as an access control list (ACL) in a central controller. The readers of existing access control systems are installed at various doors and communicate with the central controller for every access request. These readers receive the allow/deny decisions from the controller, and communicate the decisions back to the user requesting access. This solution cannot be adequately scaled up to meet the needs of future buildings where it is envisioned that (i) the policies/rules are predominantly context-sensitive, (ii) there will be a large number of users, and (iii) connections between readers and controllers will leverage a generic building network. A reader-controller communication for every access request in such a scenario will not be scalable. 
     Therefore, according to one embodiment of the present invention, authorization is decentralized and, consequently, does not rely on communications between the readers and a central controller for access decisions. 
     According to this embodiment of the present invention, users carry devices such as smart cards on which the policies dictating the access of users are stored. These access controlling policies are system context dependent. For example, one policy might provide that a requesting user is allowed access only if the occupancy of the room is less than or equal to a predetermined capacity limit, such as 20 occupants. In such a case, an allow or deny decision is dictated by the system context involving the occupancy of the room. 
     Policies may be specified in a formal language and stored as an executable on the smart cards. System context information is obtained dynamically from the system. Upon an access request from a user, the policies stored on his/her smart card are executed along with the system context information, and an allow/deny decision is made by the smart card and the reader that is installed at the portal to the room to which the card holder desires access. Per-user state information is then written back to the smart card. 
     One embodiment of an access control system  40  for the control of access to a building with interconnects is shown in  FIG. 4 . The access control system  40  implements de-centralized access control (DAC), which is not to be confused with Discretionary Access Control. The de-centralized access control, for example, may be arranged to fall within the domain of non-discretionary access control. 
     The access control system  40  includes user-carried devices  42  (e.g., smart access cards), readers  44  (e.g., device readers), access agents  46  (e.g., portals such as doors), resources  48  (e.g., protected areas such as rooms), an interconnect  50 , policies  52  that are context sensitive and dynamic, and controllers  54 . 
     The user-carried devices  42  have built in computational capabilities and memories, as opposed to passive cards that are commonly used today. Users are required to carry the user-carried devices  42 . The user-carried devices  42  are more simply referred to herein as smart cards. However, it should be understood that the present invention can also relate to user-carried devices other than smart cards. 
     The readers  44  at the doors or other portals are able to read from and write to the user-carried devices  42 . 
     The access agents  46  are access control enabled. The access agents  46  are more simply referred to herein as doors. However, it should be understood that the present invention relates to access agents other than doors. Each of the doors  46 , for example, may be arranged to have one or more readers  44 . For example, each of the doors  46  may be arranged to have two readers  44  with one of the readers  44  on each side of the corresponding door  46 . Also, each of the doors  46 , for example, may be arranged to have a corresponding one of the door controllers  54 . The door controller  54  is connected to the reader  44  and has an actuator for locking and unlocking the corresponding door  46 . The door controller  54  will usually have a wireless/locally wired communication component and some processing capabilities. Each reader can have its own controller too. Also, the functionality of the door controller  54  and the reader  44  can be folded into one integrated unit as well, and a door may have two such units on either side. 
     The resources  48 , for example, may be enclosed spaces or other restricted areas. Access to the resources  48  is permitted by the doors  46  with each of the doors  46  being provided with a corresponding one of the door-controllers  54  to control access through a corresponding one of the doors  46  and into a corresponding one of the resources  48 . 
     The interconnect  50  interconnects the door controllers  54  and is typically a mix of wired and wireless components, and can leverage the facility IP network. It should be understood that the interconnect  50  may instead comprise only wired components or only wireless components, that the wired components may include regular network cables, optical fibers, electrical wires, or any other type of physical structure over which the door controllers  54  can communicate, and that the wireless components may include RF links, optical links, magnetic links, sonic links, or any other type of wireless link over which the door controllers  54  can communicate. 
     The policies  52  include authorization policies that depend on a system context (e.g., refuse entry if the number of people in a room is more than a threshold) and that can be altered dynamically. 
     The smart cards  42  carry information about all the access policies  52  of the corresponding user. Upon an access request, the access decision is made locally by virtue of the interaction between the smart card  42 , which carries the policies  52 , and the door controller  54 , which supplies the context information. In one embodiment, the smart card  42  can use the policy and both the system context and the user&#39;s history in order to make a decision regarding the request for access by the user through the door  46 . 
     The interconnect  50  is used to transfer system-level information to the door-controllers  54  and to program the door-controllers  54 . 
     One example of system level information can be administrative actions, like raising the security level of a facility to high, which need to be communicated to all or to at least some of the door controllers  54  using the interconnect  50 . 
     Another example can be local information as collected from different door controllers  54  of a particular room in order to locally compute the room occupancy using the interconnect  50  to talk amongst themselves. The logs of the different door controllers  54  are also periodically pushed to a central place using the interconnect  50 . 
     The users are expected to re-program, re-flash, or otherwise alter the policies  52  stored on their smart cards  42  on an agreed upon granularity so that they can reflect any change in the policies  52 . In specific instances, all or some door controllers  54  may be instructed to reflash the policies of certain users or a group of users by using the readers  44  attached to the controllers  54  to reflash the user carried devices  42 . 
     Thus, instead of a central controller storing all policies as is done in traditional access control systems, the pertinent portions thereof (i.e., of the policies  52 ) are stored on the user&#39;s smart card  42  in connection with the access control system  40 . The door controller  54  and the smart cards  42  communicate with one another in order to choose the correct policy and hence control access to the room  48 . 
     The policies  52  stored on the smart card  42  may be personal to the user possessing the smart card  42 . For example, the smart card  42  of user A may contain a policy specifying that user A is permitted access to a room only if user B is already in the room. However, the smart card  42  of user C may contain no such policy. 
     To implement and enforce context-sensitive policies, the smart cards  42  carry a policy rule-engine instead of static policies. The door-controllers  54 , by virtue of the interconnect  50 , imposes the system context. The system context, in conjunction with the rule-engine on the smart cards  42 , dynamically makes the access decisions. 
     Thus, the policies  52  are analyzed by a policy analyzer  56  in conjunction with a facility topology  58 , are converted into user-specific rule engines, and are programmed into the smart cards  42 . The door controllers  54  are also programmed/configured by the analyzer  56  in order for them to evaluate the system context in a distributed manner. The door controllers  54  can write user specific history into the smart cards  42  at runtime. The policies  52  are combined with the system context imposed by the door-controllers  54  in order to make access decisions. 
     As an example, one of the rules that is produced by the policy analyzer  56  from the policies  52  might specify that entry into a particular one of the rooms  48  (identified by the facility topology  58 ) is allowed only if occupancy in this particular room is less then twenty (e.g., the capacity limit of this room). The context of this policy is the current occupancy of this room. The door controller  54 , which is charged with imposing the system context, maintains a count of the occupants of the room. When a user with a smart card  42  that has the rule engine corresponding to the above policy requests access to the room, the policy is evaluated by the smart card  42  after applying the system context which it receives from the door controller  54  and makes the access decision to grant or deny access. 
     The policies  52 , for example, may be specified using a formal logical language. The formal logical language may be built on top of certain elementary relations over events and variables using Boolean operations and quantification. The events may be atomic entities relating to the system context and the movement of users inside a facility. The variables may be place holders used to quantify over events. The relationship between an event and a variable determines how a variable represents a particular event and the order of occurrence of events. 
     An administrator can define the policies  52  in a high level English-like specification, which follows a grammar. The grammar in this context refers to a language generation rule. The policy analyzer includes a high level policy parser that parses the policies  52  input by the administrator in accordance with the grammar and translates the policy input into a formal logical language. 
     One formal logical language that can be used for this purpose is the Monadic Second Order (MSO) Logic. This logic is parameterized by a set of events, where events are entities that represent access control requests, decisions, and system context (e.g., a room reaching its maximum occupancy). The events may thus be atomic entities relating to the system context and the movement of users inside a facility. The formal logical language may be built on top of certain elementary relations over events and variables using Boolean operations and quantification. In summary, the syntax of the formal policy language can be MSO logic, tuned to the context of access control, e.g., using application specific knowledge to define the relations over events. 
     The high level parser of the policy analyzer  56  works by first parsing the high level policy to extract pieces of templates for which pre-designated Monadic Second Order formulas can be substituted. The Monadic Second Order formulas of the pieces of templates are then put together, e.g., by means of conjunctions or disjunctions, by the high level parser to obtain a single Monadic Second Order formula corresponding to the policy. 
     The parser uses knowledge of the application domain to effectively perform the translation. Once a grammar for the high-level English-like specification is defined according to the needs of the access control application, parsing can be carried out using well known parsing techniques available from Alfred V. Aho, Ravi Sethi, Jeffrey D. Ullman in “Compilers Principles, Techniques, Tools”, Reading, Mass., Addison-Wesley, 1986, and well known tools disclosed by S. C. Johnson in “YACC—Yet another compiler compiler”, Technical Report, Murray Hill, 1975, and by Charles Donelly and Richard Stallman in “Bison: The YACC-Compatible Parser Generator (Reference Manual)”, Free Software Foundation, Version 1.25 edition, November 1995. 
     In order for the policies specified in Monadic Second Order Logic thus obtained to be operational in terms of enforcing access, they have to be converted into computational/executable machine models. These machine models can then be stored in appropriate locations for execution. Conventional finite state automata may be used as the machine models that execute these policies. A language analyzer of the policy analyzer  56  may be used to constitute the set of algorithms that convert the policies specified in Monadic Second Order Logic into their equivalent finite state automata. A language analyzer algorithm follows well-known theoretical techniques for converting formula into automata. Theorems and techniques from Thomas, W. in “Languages, automata and logic,” in Handbook of Formal Languages, Vol. III, Springer, N.Y., 1997, pp. 389-455 can be implemented as an algorithm for this language analyzer. The automata can then be stored in user carried devices to carry out the decentralized authorization. These automata act as rule engines executing the policies  52 , since, as mentioned above, their construction allows precisely those behaviors that satisfy the policies. All of the policies  52  corresponding to a particular user are collected together and converted into executable automata which are then stored on the user&#39;s smart card  42 . 
     The policy analyzer also use the topology  58  of the facility in which the access control system is to be used. That way, the executable automata are tailored for this topology. The door controllers  54  may also be programmed/configured by the analyzer  56  in order for them to evaluate the system context in a distributed manner. 
     Accordingly, when a user requests access to a room  48 , the corresponding door controller  54  initiates execution of those of the policies  52  stored in the user&#39;s smart card  42 , which results in an access decision (allow/deny) that is unique to that user and to that room. 
     The parser and the language analyzer are together referred to in this disclosure as the high level analyzer or the policy analyzer or simply the analyzer  56 . 
     Examples of dynamic policy types that can be specified using the formal logical language referred above include the following: assisted access, whereby one user can enter the facility only when another designated user is available to provide access; anti-pass back, whereby re-entry is denied if a user is found to have made an unrecorded exit after a valid entry; system state based policies, whereby access is limited, for example, by the number or category of users inside a room; and, temporal policies, whereby a user has access to a facility only during specific interval of time. Different or other policies may be implemented. 
     The policy analyzer  56  analyzes and converts the policies  52  into their equivalent finite state automata. These automata act as rule engines executing the policies  52 . They are constructed to allow precisely those behaviors that satisfy the policies. All of the policies  52  corresponding to a particular user are collected together and converted into executable automata which are then stored on the user&#39;s smart card  42 . When the user requests access to a room  48 , the corresponding door controller  54  initiates execution of those of the policies  52  stored in the user&#39;s smart card  42 , which results in a an access decision (allow/deny) that is unique to that user. 
     The interconnect  50  may be arranged to include a system administrator  59  some of whose functions are discussed below. 
     A representative one of the smart cards  42  is shown in  FIG. 5 . The smart card  42  includes a memory  60 , a processor  62 , a transceiver  64 , and a power source  66 . The memory  60 , for example, may be a flash memory and stores the rule engine that enforces the policies  52  targeted to the user carrying the smart card  42 . 
     The smart card  42  may be arranged to respond to a generic read signal that is transmitted continuously, periodically, or otherwise by the reader  44 , that is short range, and that requests any of the smart cards  42  in its vicinity to transmit its ID, and/or a request for system context, and/or other signal to the reader  44 . In response to the read signal, the smart card  42  transmits the appropriate signal to the reader  44 . 
     Accordingly, when the user presents the user&#39;s smart card  42  to the reader  44 , the transceiver  64  receives from the reader  44  at least the system context provided by the door controller  54 . Based on this system context and the policies  52  stored in the memory  60 , the processor  62  makes the access decision to grant or deny the user access to the room  48  associated with the reader  44  to which the user&#39;s smart card  42  is presented. The processor  62  causes the grant decision to be transmitted by the transceiver  64  to the reader  44 . If desired, the processor  62  may be arranged to also cause the deny decision to be transmitted by the transceiver  64  to the reader  44 . 
     The memory  60  may also be arranged to store a personal ID of the user to which the access card is assigned. When the user presents the smart card  42  to the reader  44 , the processor  62  may be arranged to cause the user&#39;s personal ID to be transmitted by the transceiver  64  to the reader  44 . In this manner, particular users may be barred from specified ones of the rooms  48 , and access by specific users to specific rooms, etc. may be tracked. Also, the door controllers  54  can be arranged to provide back certain system contexts that are targeted to particular users. 
     The memory  60  can also store other information. 
     The processor  62 , for example, may be a microcomputer, a programmable gate array, an application specific integrated circuit (ASIC), a dedicated circuit, or other processing entity capable of performing the functions described herein. 
     The power source  66  may be a battery, or the power source  66  may be arranged to derive its power from transmissions of the readers  44 , or the power source  66  may be any other device suitable for providing power to the memory  60 , the processor  62 , and the transceiver  64 . 
     The transceiver  64  transmits and receives over a link  68 . The link  68  may be a wired link or a wireless link. 
     A representative one of the readers  44  is shown in  FIG. 6 . The reader  44  includes a transceiver  70 , a processor  72 , a transceiver  74 , and a power source  76 . Although not shown, the reader  44  may also include a memory. 
     When the user presents the user&#39;s smart card  42  to the reader  44 , the processor  72  causes the transceiver  74  to send a signal to the door controller  54  that the smart card  42  is being presented to the reader  44 . This signal prompts the door controller  54  to transmit appropriate system context to the reader  44 . The system context supplied by the door controller  54  is received by the transceiver  74  of the reader  44 . The processor  72  causes the system context received from the door controller  54  to be transmitted by the transceiver  70  to the smart card  42 . The access decision made and transmitted by the smart card  42  is received by the transceiver  70 . The processor  72  causes this decision to be transmitted by the transceiver  74  to the door controller  54 . 
     The processor  72 , for example, may be a microcomputer, a programmable gate array, an application specific integrated circuit (ASIC), a dedicated circuit, or other processing entity capable of performing the functions described herein. 
     The power source  76  may be a battery, or the power source  76  may be a plug connectable to a wall or other outlet, or the power source  76  may be any other device suitable for providing power to the transceiver  70 , the processor  72 , and the transceiver  74 . 
     The transceiver  70  transmits and receives over a link  78 . The link  78  may be a wired link or a wireless link. The transceiver  74  transmits and receives over a link  80 . The link  80  may be a wired link or a wireless link. 
     A representative one of the door controllers  54  is shown in  FIG. 7 . The door controller  54  includes a transceiver  90 , a processor  92 , a transceiver  94 , a memory  96 , one or more context detectors  98 , and a power source  100 . 
     When the user presents the user&#39;s smart card  42  to the reader  44  and the reader  44  sends a signal requesting the appropriate system context, the transceiver  90  receives this request signal causing the processor  92  to control the transceiver  90  so as to transmit this system context to the reader  44 . The system context may be stored in the memory  96 . For example, the system context stored in the memory  96  may be user specific and may be stored in the memory  96  by user ID. Thus, when a user&#39;s smart card  42  transmits its user ID to the door controller  54  via the reader  44 , the door controller  54  transmits back system context specific to the user ID that it has received. 
     According to one embodiment of the present invention, at least a portion of the system context results from the context detector  98 . The context detector  98  may simply be a counter that counts the number of users permitted in the room  48  guarded by the door controller  54 . However, the context detector  98  may be arranged to detects additional or other system contexts to be stored in the memory  96  and to be transmitted to the reader  44  and then to the smart card  42 . 
     The transceiver  94  is arranged to exchange communications with the interconnect  50 . 
     The processor  92 , for example, may be a microcomputer, a programmable gate array, an application specific integrated circuit (ASIC), a dedicated circuit, or other processing entity capable of performing the functions described herein. 
     The power source  100  may be a battery, or the power source  100  may be a plug connectable to a wall or other outlet, or the power source  100  may be any other device suitable for providing power to the transceiver  90 , the processor  92 , the transceiver  94 , the memory  96 , and the context detector  98 . 
     The transceiver  90  transmits and receives over a link  102 . The link  102  may be a wired link or a wireless link. The transceiver  94  transmits and receives over a link  104 . The link  104  may be a wired link or a wireless link. 
     Accordingly, context-sensitive policy enforcement is de-centralized. Thus, there is no need for a controller to centrally maintain information about per-user permissions and system context. Instead, access control decisions are made locally, with the door-controllers dynamically maintaining pertinent environmental system context. This de-centralization alleviates the problem of scalability as the number of users and the complexity of the policies grow. 
     Moreover, the access control system  40  is easy to configure and re-configure. At a high level, the readers  44  and/or the door controllers  54  are equipped with the knowledge of what they are protecting, but not how they are protecting and how should they interact and compose the system context, but not with details about an user&#39;s policy or history of activities. The readers  44  and/or door controllers  54  are stateless in this regard, making reconfiguration of the facility easier. 
     Further, effective decentralization and localization of policy decision making also enables meaningful enforcement of at least some access control policies in the event of a disconnected or partially connected reader  44  and/or door controller  54 . For example, policies depending only on a user&#39;s past behavior (and not on other system context) can be enforced even if a door controller  54  is disconnected from the system through the interconnect  50 . 
     While secure authorization is not the primary focus of the present invention, existing mechanisms can be used for a basic secure solution. For example, using symmetric key encryption, where all the access agents and the administrator  59  share a secret key k, with which they will be configured at the time of installation (or on a subsequent facility-wide reset operation, if the key is compromised), the per-user policy engine and states can be encrypted with k on the user-carried devices, and the readers  44  and/or the door controllers  54  can decrypt them using k and further write back encrypted states using k on the user-carried devices. This symmetric key encryption ensures security as long as k is not compromised. The policy on the smart card can be certified by a digital certificate and its validity can be verified by using technologies like those developed by Core street. 
     Certain modifications of the present invention have been discussed above. Other modifications of the present invention will occur to those practicing in the art of the present invention. For example, as described above, the smart cards  42  make the access decision as to whether a user is to be permitted or denied access to a room. The smart card  42  makes this decision based on the policies  52  that it stores and the system context provided by the door controller  54 . Instead, the door controller  54  could make the access decision as to whether a user is to be permitted or denied access to a room based on the policies  52  provided by the smart card  42  and the system context stored in the memory  96  of the door controller  54 . 
     Also, the reader  44  and the door controller  54  are shown as separate devices. Instead, their functions may be combined into a single device. 
     Moreover, the functions of the door controller  54  may be moved to the readers  44  reducing the door controller  54  to a simple lock. 
     In addition, the connections shown in  FIG. 4  may be wired connections, or wireless connections, or a mixture of wired connections and wireless connections. 
     Furthermore, the door controllers  54  may be arranged to log access decisions in a log file so that the decisions logged in the log file can be subsequently collated by a separate process for book-keeping. 
     The system context may be detected by individual door controllers through sensors or context detectors  98  either built into the door controllers  54  or otherwise attached to them. An example of this can be the presence of a certain chemical in a room. The system context may also require the collaboration of different door controllers—e.g., to decide if the occupancy of a room is below a certain threshold. Such contexts, along with each of the individual grants/denials to users are all represented as discrete events happening at the respective controllers  54 . The policy specification language can also define hierarchical events which are formed out of individual events at different controllers. For example, if event e 1  represents the context of “high threshold of a chemical in room A” and event e 2  represents the context of “occupancy in room A&gt;=1”, then the event e 3  defined as “e1 AND e2” represents the system context “personnel hazard in room A”. Such events may be specified as part of the policies  52 . The analyzer  56  can then translate the event definitions to specific actions on the part of the door controllers  54  by which they will detect system context either individually or in collaboration, as required by the policies. 
     Moreover, as discussed above, the interconnect  50  of  FIG. 4  may include the administrator  59 . The system administrator  59  may be used to supply special system contexts that are in addition to any system contexts detected by the context detectors  98 . Such special system contexts, for example, may be used to take care of emergency situations including but not limited to revoking the access rights of a rogue user. 
     Also, the system administrator  59  may be arranged to formally specify policy roles as the policies relate to each user and to assign the users to appropriate ones of these roles. 
     Usually the policies will not differ across every individual, but are likely to be different across groups of individuals. In this sense, a role refers to a certain policy or groups of policies that is applicable to a certain class of user. For example, a “supervisor” is a role that can include the policy of free access to all rooms, whereas a “regular employee” can be a role that includes policies which allow an entry to certain protected rooms only if a “supervisor” is present. 
     However, the access control system  40  may also include user-specific authorization policies. An example of this can be a special user who is not a regular employee at a site but needs better structured access control policies as compared to a visitor. 
     Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.