Patent Publication Number: US-7596368-B2

Title: Wireless access point apparatus and method of establishing secure wireless links

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wireless communication network, more particularly to the access point apparatus in a wireless mesh network, and still more particularly to a method of establishing secure wireless links between access points. 
     2. Description of the Related Art 
     The mesh network architecture, which uses wireless links between access points, provides a simple and flexible way to extend the coverage area of a wireless local area network (LAN). Applications are expected to appear in homes, offices, college campuses, and other areas, and standardization of the architecture is under study by the Institute of Electrical and Electronics Engineers (IEEE) within the framework of the IEEE 802.11i group of wireless LAN standards. 
     Wireless LANs in general are at risk from eavesdropping, spoofing, and other well-known forms of tampering, including the setting up of unauthorized access points. In a mesh network these risks are increased, because access points can be set up almost without restriction and communications may be relayed over considerable distances. Authentication and protection of communication is therefore a critical issue, as discussed in, for example, Ji et al, ‘Self-Organizing Security Scheme for Multi-Hop Wireless Access Networks’, IEEE 2004 Aerospace Conference, Big Skye, Mont., March 2004, available as of Feb. 3, 2005 on the Internet at http://www.flacp.fujitsulabs.com/Aerospace04-51.pdf. 
     The above paper proposes an encryption key management system in which all access points in a network share a single group key distributed from a master access point. Consequently, if the group key is compromised at even one access point, the security of the entire network is endangered. 
     When this happens, it is necessary to halt all communication in the network and change the group key. Needless to say, this temporary shutdown of the entire network is a major inconvenience to network users. A similar inconvenience occurs when an access point is temporarily removed from the network for servicing or repair, because that involves a risk of possible key disclosure, and the group key must be changed to forestall the risk. 
     The inconvenience is particularly great when the network is large in scale or is connected to a wired network and forms part of the local infrastructure of the area in which it is used. Since mesh networks are expected to be large in scale and to operate in the infrastructure mode, there is an urgent need for a solution to this problem. 
     That is, from the standpoints of both network security and network operations, there is a need for wireless access point apparatus and connection processing methods that can establish secure wireless links between wireless access points without widespread sharing of encryption keys. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide wireless access point apparatus that can establish secure wireless links between access points in a wireless mesh network by using a separate encryption key for each link. 
     The invented wireless network includes a plurality of access points and at least one authentication server. Each access point has access point apparatus including a supplicant processing unit, an authenticator processing unit, and a function selector. 
     The supplicant processing unit requests authentication processing by a predetermined authentication method and supplies the necessary authentication information to an authenticator device. The authenticator processing unit mediates authentication processing requested by another device, such as another access point, by forwarding the authentication request and authentication information toward the authentication server, and passing information received from the authentication server to the requesting device. The function selector operates when an unconnected access point is detected within communication range, and selects either the supplicant processing unit or the authenticator processing unit. The selected supplicant processing unit or authenticator processing unit then operates in cooperation with the authenticator processing unit or supplicant processing unit at the unconnected access point to establish a secure wireless link between the two access points. 
     Because every access point can operate as either an authenticator or a supplicant, when a new connection between access points is established, the two access points involved can handle the authentication procedure and distribution of encryption keys themselves (with the cooperation of the authentication server), without having to share these encryption keys with other access points. Since the encryption key that secures a wireless link is not stored at any access point other than the access points at the two ends of the link, the security of the link will not be compromised by a problem occurring at another access point. 
     For the same reason, in the event that an encryption key is compromised, the effect is localized and can be isolated without the need to shut down the entire wireless network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the attached drawings: 
         FIG. 1  shows the general structure of a network embodying the present invention; 
         FIG. 2  is a block diagram of wireless access apparatus embodying the invention; 
         FIG. 3  illustrates the process by which a new access point is added to the network in  FIG. 1 ; 
         FIG. 4  illustrates the sequence of steps in the process in  FIG. 3 ; 
         FIG. 5  is a flowchart illustrating the function selection procedure in the present invention; 
         FIG. 6  illustrates the process by which the new access point added in  FIG. 3  is connected to another neighboring access point; and 
         FIG. 7  illustrates the process by which a connection is reestablished after the gateway device in  FIG. 6  recovers from a failure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A wireless mesh network, access point apparatus, and access method embodying the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. 
     It will be assumed that the wireless mesh network uses the access point authentication protocol described in the IEEE 802.1x standards, and that each access point in the network has the authenticator function and the supplicant function described in the IEEE 802.1x standards. 
     Referring to  FIG. 1 , the wireless mesh network  100  includes four access points  101  to  104  and a gateway device  110 . The gateway device  110  is an access point that is also connected to an external network. In this embodiment, the gateway device  110  is connected to an authentication server  120  via a wired network  130 . The authentication server  120  authenticates the access points  101  to  104 , the gateway device  110 , and wireless terminal devices (not shown) by following the IEEE 802.1x authentication protocol. 
     The functions implemented by the authentication server  120  include at least the following: managing registered authentication information relating to the access points  101  to  104 , the gateway device  110 , and wireless terminal devices; generating the seeds of encryption keys for encrypted communication on the wireless links between the access points; and distributing the generated encryption key seeds to the access points  101  to  104  and gateway device  110 . In particular, the authentication server  120  may implement the Remote Authentication Dial-In User Server (RADIUS) functions described in these standards. 
     The authentication protocol followed by the authentication server  120  is not limited to the IEEE 802.1x protocol, provided it allows access points to forward authentication requests from neighboring access points and return authentication results to the neighboring access points, and allows the distribution of encryption key seeds for generating pairwise encryption keys for the wireless links between access points. 
     The access points  101  to  104 , including the gateway device  110 , establish wireless connections or ‘associations’ with wireless terminals (not shown) that are within communication range, and forward information to other access points that are within communication range. Information can thereby reach its destination in a series of hops. An access point mediates communication between its associated wireless terminals, between these wireless terminals and wireless terminals associated with other access points, and between these wireless terminals the wired network  130 , or possibly with other external networks (not shown). 
     In the following description, the term ‘neighboring access point’ will be used to denote an access point (possibly the gateway device  110 ) that is within direct wireless communication range of a given access point. 
     The gateway device  110  implements the functions of an access point, and also connects the wireless mesh network  100  to the wired network  130 , thereby making the functions of devices in the wired network  130  and possibly other external networks available to devices within the wireless mesh network  100 . In particular, the gateway device  110  in this embodiment gives the access points  101  to  104  access to the authentication server  120 . 
     Each access point  101  to  104  and the gateway device  110  has the authenticator function and the supplicant function described in the IEEE 802.1x standards, and can participate in the authentication of neighboring access points, as well submitting authentication requests on its own behalf. 
     Accordingly, when a new access point is added, for example, its authentication processing does not have to be mediated by a master authenticator node as called for in the prior art cited above; any neighboring access point can act as the authenticator. This arrangement avoids the problem of stoppage of authentication because of a failure at the master authenticator node, and helps distribute the load of authentication processing. 
     The access points  101  to  104  and gateway device  110  also generate encryption keys, using encryption key seeds distributed from the authentication server  120  over wireless links between access points that have been successfully authenticated, and manage pairwise encryption keys for each wireless link. Since the encryption keys are generated for individual wireless links and are managed separately at each access point, in the eventuality that an encryption key is compromised, the damage does not spread to the wireless mesh network  100  as a whole. 
     Referring to  FIG. 2 , each access point  101  to  104  and the gateway device  110  has wireless access apparatus  2  comprising at least a mesh network wireless communication unit  21 , a function selector  22 , an authenticator processing unit  23 , a supplicant processing unit  24 , a routing information generator  25 , a terminal wireless communication unit  26 , and a key management unit  27 . The wireless access apparatus  2  also includes an authenticator processing unit for authenticating wireless terminals, but this function has been omitted to avoid obscuring the invention with needless detail. 
     The mesh network wireless communication unit  21  performs wireless communication with neighboring access points by using a predetermined wireless communication method. 
     The function selector  22  selects and activates the authenticator processing unit  23  or supplicant processing unit  24 . The function selector  22  at a given access point operates when an unconnected neighboring access point is detected; that is, when an access point is detected that is within communication range of the given access point but does not yet have a secure wireless link to the given access point. 
     The authenticator processing unit  23  executes the functions of an authenticator as defined in the IEEE 802.1x standards. These functions include, for example, forwarding authentication requests and information from supplicant access points to the authentication server  120 , generating unicast encryption keys (unicast keys) and broadcast encryption keys (broadcast keys), and distributing broadcast keys. Accordingly, the authenticator processing unit  23  has an authentication mediation section  23   a  and an encryption information generator  23   b , the encryption information generator  23   b  including a unicast key generation section  23   c  and a broadcast key generation and distribution section  23   d.    
     The supplicant processing unit  24  executes the functions of a supplicant as defined in the IEEE 802.1x standards. These functions include, for example, submitting an authentication request to an authenticator access point and, like the authenticator processing unit  23 , generating unicast and broadcast keys and distributing broadcast keys. Accordingly, the supplicant processing unit  24  has an authentication request submission section  24   a  and an encryption information generator  24   b , the encryption information generator  24   b  including a unicast key generation section  24   c  and a broadcast key generation and distribution section  24   d.    
     The two encryption information generators  23   b ,  24   b  are shown separately in  FIG. 2 , but they are identical and may be combined into a single shared unicast key generation section and a broadcast key generation and distribution section. 
     A unicast key is used for encrypting communication on a particular wireless link between two access points. The number of unicast keys managed by a wireless access apparatus  2  depends on the number of wireless links that connect the wireless access apparatus  2  to other access points. 
     A broadcast key is used for encrypting communications directed toward a plurality of access points. The number of broadcast keys managed by a wireless access apparatus  2  depends on the number of neighboring access points (and thus depends on the number of wireless links). The wireless access apparatus  2  must store one broadcast key to encrypt outgoing broadcasts, and one broadcast key for each neighboring access point to decrypt broadcasts received from that access point. 
     The routing information generator  25  generates routing information for the wireless mesh network  100  according to a prescribed routing protocol. Known types of routing protocols include reactive protocols that determine routes dynamically when communication takes place, proactive protocols that determine routes in advance, typically when a connection is changed, and hybrid protocols that combine both reactive and proactive techniques. The present embodiment employs a proactive or a hybrid protocol. 
     The terminal wireless communication unit  26  carries out wireless communication with associated terminal devices, also referred to as client terminals (not shown). 
     The key management unit  27  manages the three types of keys described above: unicast keys, broadcast keys for decrypting incoming broadcast communications, and a broadcast key for encrypting outgoing broadcast communications. 
     Each access point also broadcasts a beacon signal to announce its presence in the wireless mesh network  100 . The beacon signal includes information identifying the network  100 , such as a service set identifier (SSID), and information identifying the access point by which it is broadcast, such as a basic service set identifier (BSSID). 
     Next the operations for establishing a secure bidirectional wireless link with a newly added access point, for establishing a new secure bidirectional wireless link to an access point that that is already linked securely to another access point, and for recovering from a gateway failure will be described. 
     Referring to  FIG. 3 , when a new access point  105  is added to the wireless mesh network  100 , it begins by broadcasting its own beacon signal and attempting to receive beacon signals from existing access points in the network. In  FIG. 3 , access point  105  receives the beacon signal broadcast by access point  101 , including the network identifier (SSID) ‘Mesh1’ and access point identifier ‘2’, and the beacon signal broadcast by access point  103 , with network identifier ‘Mesh1’ and access point identifier ‘3’. Access point  105  selects one of the received beacon signals, such as the beacon signal with the greatest received signal strength. In the following description, it will be assumed that access point  105  selects the beacon signal broadcast by access point  101  and proceeds to establish a secure wireless link with access point  101 . 
     The sequence by which this is done is indicated roughly by the steps shown in  FIG. 3 : (1) reception of beacon signals; (2) connection; (3) authentication and distribution of pairwise master key (PMK); and (4) generation of pairwise transient key (PTK). The procedure is illustrated in more detail in  FIGS. 4 and 5 . 
     In step S 101  in  FIG. 4 , access point  101  receives the beacon signal transmitted by access point  105  and access point  105  receives the beacon signal transmitted by access point  101 . At this point access points  101  and  105  can communicate with each other, but do not yet have a secure wireless link. 
     In step S 102 , the function selectors  22  in access points  101  and  105  select the authenticator processing unit  23  or the supplicant processing unit  24  at each access point. The selection procedure is illustrated in  FIG. 5 . Step S 1  in  FIG. 5  is the reception of a beacon signal, corresponding to step S 101  in  FIG. 3 . 
     In step S 2  in  FIG. 5 , the function selector  22  decides whether its access point can connect to the authentication server  120 . Various methods of making this decision are available. In one well-known method, each access point  101  to  105  is pre-equipped with the Internet Protocol (IP) address of the authentication server  120  and the access point transmits a so-called ping packet addressed to the authentication server  120 . If a returning reply or ‘echo’ is received, the function selector  22  decides that the access point can connect to the authentication server  120 . 
     If the authentication server  120  is connectable, the function selector  22  next decides whether the routing information generator  25  has generated routing information for the access point from which the beacon signal was received. This decision can also be made by various methods, such as searching a routing table maintained by the routing information generator  25 . 
     If the routing information generator  25  has generated routing information for the access point from which the beacon signal was received, the function selector  22  begins a role arbitration process by communicating with the function selector  22  at the access point from which the beacon signal was received (step S 4 ). On the basis of this arbitration process, the function selector decides whether its own access point should act as the authenticator or the supplicant (step S 5 ). The function selector then activates the authenticator processing unit  23  (step S 6 ) or the supplicant processing unit  24  (step S 7 ). 
     If the function selector  22  decides that the access point cannot connect to the authentication server  120  (‘No’ in step S 2 ), the access point must operate as the supplicant (step S 7 ). Conversely, if the access point can connect to the authentication server  120  but does not have routing information for the other access point with which it is trying to connect (‘No’ in step S 3 ), it must operate as the authenticator (step S 6 ). 
     In the present case, it will be assumed that the new access point  105  does not have routing information for access point  101 , or cannot connect to the authentication server  120 , and therefore acts as the supplicant. This is reported to access point  101  in the role arbitration process, and access point  101  accordingly acts as the authenticator. 
     Returning to  FIG. 4 , access point  105  executes the supplicant processing unit  24  (step S 103 ) while access point  101  executes the authenticator processing unit  23  (step S 104 ). 
     The authentication protocol specified in the IEEE 802.1x standards is now followed to authenticate access point  105 . In this process, access point  105  submits an authentication request, access point  101  forwards the request to the authentication server  120 , and access point  105  and the authentication server  120  carry out a well-known authentication procedure, with access point  101  acting as an intermediary by relaying communication between the authentication server  120  and access point  105 . If authentication succeeds, the authentication server  120  generates a pairwise master key (PMK), which is a type of seed to be used in generating encryption keys for unicast communication between access points  101  and  105 , and distributes this PMK seed to access points  101  and  105  (step S 106 ). 
     When access points  101  and  105  have received the PMK seed from the authentication server  120 , they confirm that they have identical seed information, and proceed to generate a unicast key such as a pairwise transient key (PTK) from the seed (step S 107 ). This unicast key is stored in the key management unit  27  at each access point  101  and  105  and becomes an individual encryption key for use only on the wireless link between access points  101  and  105 . 
     Next, the authenticator processing unit  23  in access point  101  uses the unicast key to send access point  105  an encrypted message including the broadcast key that access point  101  uses to encrypt outgoing broadcast communications (step S 108 ). Access point  105  receives this message and decrypts it with the unicast key, and the key management unit  27  in access point  105  stores the decrypted broadcast key. 
     Similarly, the supplicant processing unit  24  in access point  105  generates a broadcast key that it will use to encrypt communications broadcast to neighboring access points (step S 109 ) and sends access point  101  a copy of this broadcast key, encrypted with the unicast key (step S 110 ). The key management unit  27  in access point  101  stores a decrypted copy of this broadcast key. 
     In step S 108 , if access point  101  does not already have a broadcast key, it generates one and distributes encrypted copies of the newly generated broadcast key to its other neighboring access points, as well as to access point  105 . Broadcast keys can also be distributed by the broadcast or multicast methods used in the ad hoc mode of communication. 
     After these operations, access point  105  is connected to access point  101  via a secure wireless link, and can also send and receive broadcast communications securely, although so far its broadcast partners are limited to access point  101 . The routing information generator  25  at access point  105  now proceeds to acquire routing information for the other access points  102 ,  103 ,  104  and the gateway device  110 . 
     Referring to  FIG. 6 , after acquiring routing information, access point  105  proceeds to establish a secure wireless link with access point  103 . The procedure is generally similar to the procedure by which it established the secure wireless link with access point  101 : (1) beacon signal reception; (2) authenticator mediation; (2) connection; (4) authentication and pairwise master key distribution; and (5) pairwise transient key generation. This procedure will be described in more detail with reference again to  FIGS. 4 and 5 . 
     In step S 101 , access points  103  and  105  receive each other&#39;s beacon signals. Each accesses point recognizes the other as an unconnected access point that is within communication range. Being within communication range is indicated by, for example, the received signal strength of the beacon signal. 
     In step S 102 , the function selector  22  at each access point  103 ,  105  selects the authenticator processing unit  23  or supplicant processing unit  24 , again following the procedure illustrated in  FIG. 5 . 
     Step S 1  (beacon reception) in  FIG. 5  is the same as step S 101  in  FIG. 4 . 
     Since both access points  103  and  105  are already connected to the wireless mesh network  100 , both can connect to the authentication server  120  and each access point possesses routing information for the other access point. The decisions in steps S 2  and S 3  are accordingly ‘Yes’ at both access points  103  and  105 . 
     This brings both access points to step S 4 , in which they work out the assignment of the authenticator and supplicant roles by predetermined arbitration rules. One known rule compares the media access control (MAC) addresses built into each access point apparatus: the access point with the higher MAC address becomes the authenticator; the access point with the lower MAC address becomes the supplicant. Other possible rules involve a hop-count comparison or a comparison of processing capability. Any of these methods involves an exchange of management information between access points  103  and  105 , which in this case takes place through the access point  101  to which both access points  103  and  105  are already securely linked, and leads to a role decision (step S 5 ) at each access point. In the following description, it will be assumed that access point  103  assumes the role of authenticator (step S 6 ) and access point  105  again assumes the role of supplicant (step S 7 ). 
     Returning to  FIG. 4 , the supplicant processing unit  24  operates at access point  105  (step S 103 ) and the authenticator processing unit  23  operates at access point  103  (step S 104 ). Access point  105  submits another authentication request, which is mediated by access point  103  (step S 105 ), with authentication information passing through access point  101  as shown in  FIG. 6  due to the network topology. 
     The rest of the procedure is substantially the same as before: the authentication server  120  authenticates access point  105  again and distributes a pairwise master key (PMK) to both access points  103 ,  105  (step S 106 ); access points  103  and  105  confirm that they have the same PMK and generate unicast keys or PTKs (step S 107 ); access point  103  sends its broadcast key to access point  105  (step S 108 ); and access point  105  sends its broadcast key to access point  103  (step S 110 ). 
     This procedure establishes a secure wireless link between access points  103  and  105 . In addition, access point  105  can broadcast encrypted information simultaneously to access points  101  and  103 , and can receive and decrypt encrypted broadcasts from either access point  101  or access point  103 . 
     When a new access point is connected to the wireless mesh network  100 , the procedures shown in  FIGS. 3 to 6  are used to establish secure wireless links with all existing access points within communication range of the new access point, one after another. 
       FIG. 7  illustrates the process carried out when a problem occurs in the gateway device  110  and the gateway device  110  has to be taking out of service temporarily and reset to an initial unconnected state. 
     While the gateway device  110  is out of service, the wireless mesh network  100  continues to function normally, except that it cannot provide authentication service or access to other services offered by the wired network  130 . After the gateway device  110  recovers and is connected normally to the wired network  130 , a secure wireless link between the gateway device  110  and access point  101  is reestablished by a procedure that includes (1) reception of a beacon signal, (2) test of connectability to the authentication server  120 , (3) connection, (4) authentication and distribution of a pairwise master key, and (5) generation of a pairwise transient key. This procedure will be described in more detail with reference again to  FIGS. 4 and 5 . 
     In step S 101  in  FIG. 4 , the access point  101  and gateway device  110  receive each other&#39;s beacon signals. 
     In step S 102 , the function selector  22  at each access point  103 ,  110  selects the authenticator processing unit  23  or supplicant processing unit  24 , once again following the procedure illustrated in  FIG. 5 . 
     Step S 1  (beacon reception) in  FIG. 5  is the same as step S 101  in  FIG. 4 . 
     In steps S 2  and S 3 , since access point  101  has lost its connection to the gateway device  110  while the gateway device  110  was out of service, access point  101  cannot access the wired network  130  and cannot connect with the authentication server  120 . Since the gateway device  110  has recovered internally, it can connect to the authentication server  120  via the wired network  130 , but it has lost its routing table and does not have routing information for access point  101 . Accordingly, the gateway device  110  functions as the authenticator (step S 6 ) while access point  101  functions as the supplicant (step S 7 ). 
     Returning to  FIG. 4 , the supplicant processing unit  24  operates at access point  101  (step S 103 ) and the authenticator processing unit  23  operates at the gateway device  110  (step S 104 ). Access point  101  submits an authentication request through the gateway device  110  to the authentication server  120  (step S 105 ). The authentication server  120  authenticates access point  101  and distributes a pairwise master key (PMK) to the gateway device  110  and access point  101  (step S 106 ); the gateway device  110  and access point  101  confirm that they have the same PMK and generate unicast keys or PTKs (step S 107 ); gateway device  110  sends its broadcast key to access point  101  (step S 108 ); and access point  101  sends its broadcast key to the gateway device  110  (step S 109 ). 
     The gateway device  110  and access point  101  now have a secure wireless link for unicast communication, and can also decrypt each other&#39;s broadcast communications. 
     Next, a similar procedure is used to reestablish secure wireless links between the gateway device  110  and any other access points within communication range. 
     If a problem at one of the general access points  101  to  105  forces the access point to be taken out of service, after it recovers, it is reconnected by following the same procedure as when a new access point is added to the access point  101 . 
     In particular, if a security leak occurs and the encryption keys stored at a particular access point are compromised, security can be restored by shutting down the access point at which the leak occurred, then reconnecting it to the network, so that it acquires a new set of encryption keys. To nullify the effect of the leak completely, it may also be necessary for the access point where the leak occurred and the access points to which it was connected to generate new broadcast keys, but this is a relatively simple procedure, not requiring connection to the authentication server  120 . 
     As described above, the invention minimizes the effect of security leaks and provides a wireless network with a high degree of security. 
     Since the invention uses the existing IEEE 802.1x standards, it can be practiced without the need to modify existing authentication servers or change the authentication protocol. A secure wireless mesh network can be constructed with an existing authentication server. 
     As noted above, however, the invention is not limited to use of the IEEE 802.1x standards. 
     Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.