Abstract:
An encryption key distribution method capable of performing fast roaming by differentially distributing encryption keys in advance, according to access authorization classes, in a wireless access network and a roaming method using distributed encryption keys are provided. The encryption key distribution method comprises: receiving a command to communicate with an access point not available for communication using an encryption key currently selected in the encryption key set; determining access authorization of the access point not available for communications; selecting an encryption key from the encryption key set obtained in advance corresponding to the determined access authorization; and using the selected encryption key to encrypt a transmission message and communicate with the access point not available for communication. The method reduces the delay time, which is caused by encryption key distribution while a wireless terminal is traveling, such that roaming and a hand-off are carried out quickly and safely, and user convenience and safe transmission of data are achieved.

Description:
This application claims priority from Korean Patent Application No. 2002-38882, filed on Jul. 5, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for distributing encryption keys in a wireless network, and more particularly, to an encryption key distribution method capable of performing fast roaming by differentially distributing encryption keys in advance, according to access authorization classes in a wireless access network and a roaming method using distributed encryption keys. 
   2. Description of the Related Art 
   In a wireless network, to guarantee confidentiality of data and user authentication, data is encrypted prior to transmission. For encryption, an encryption key is needed. At present, an encryption key is shared between a wireless station (STA) and an access point (AP) in advance of access. 
   However, the number of wireless local area network (LAN) users is continuously increasing. But the time required for exchanging encryption keys during roaming or a hand-off is too long. This is due primarily to the method of sharing encryption keys at the roaming time. Accordingly, when the mobile characteristic of the wireless LAN is considered, the existing encryption key exchange method causes inconvenience to users. 
   Therefore, it is desirable that the delay time caused by exchanging encryption keys during roaming or hand-off is minimized. To this end, it is desirable that encryption keys are differentiated according to access authorization classes based on the attributes of users. 
   In a wireless access network, in order to perform encryption in a wireless link, STAs share encryption keys with APs. When a STA is roaming between APs or a hand-off occurs, the STA initiates a process for obtaining from the APs a shared key used for encryption. At this time, the biggest problem against fast secure roaming of the STA is the delay time caused by exchanging encryption keys. 
   In the prior art, in a wireless network formed of a wide area network (WAN), LANs, APs, and STAs, all STAs connected to one AP use an identical encryption key. Accordingly, when a STA is trying to access another AP included in the same LAN or another LAN the STA should use another encryption key. Also, whenever there is roaming or a hand-off, the STA should receive another corresponding encryption key. 
   Accordingly, in existing methods, whenever an STA is roaming or there is a hand-off between APs, the STA must initiate the process of obtaining from the AP an encryption key. Therefore, it is difficult to perform fast secure roaming when an STA is in motion. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method by which various encryption keys are used for different access authorization types when the access authorization of an access point (AP) is set in advance. The various keys are obtained in advance by a wireless station (STA) so that fast secure roaming is performed and delay caused by exchanging encryption keys is minimized. 
   According to an aspect of the present invention, there is provided a method for allocating encryption keys according to access authorization classes, wherein an access authorization to an access point is set in advance, encryption keys are differentiated according to access authorization types, and the wireless station obtains the differentiated encryption keys in advance. 
   According to another aspect of the present invention, there is provided a method for allocating encryption keys according to access authorization classes, comprising:
     (a) a wireless station sending an authentication request to an access point and the access point, which is requested to perform authentication, determining the access authorization to the access point;   (b) according to the determination result, obtaining an encryption key and generating a shared key set including the obtained encryption keys;   (c) the wireless station requesting a LAN authentication server to perform authentication, and the LAN authentication server, which is requested to perform authentication, determining the access authorization to an access point belonging to the LAN;   (d) according to the determination result, obtaining an encryption key and updating the shared key set by adding the encryption key to the shared key set;   (e) the wireless station requesting a WAN authentication server to perform authentication and the WAN authentication server, which is requested to perform authentication, determining the access authorization to an access point belonging to the WAN; and   (f) according to the determination result, obtaining an encryption key and updating the shared key set by adding the encryption key to the shared key set.   

   According to still another aspect of the present invention, there is provided a roaming method for a wireless station using encryption keys allocated according to access authorization classes, comprising:
     (a) setting an access authorization to an access point in advance, differentiating encryption keys according to the access authorization type, and a wireless station obtaining in advance an encryption key set, including the differentiated encryption keys for respective access points;   (b) receiving a command to communicate with an access point not available for communication using the encryption key currently selected in the encryption key set;   (c) determining the access authorization of the access point which is not available for communication;   (d) selecting an encryption key from the encryption key set corresponding to the determined access authorization; and   (e) by using the selected encryption key, encrypting a transmission message and communicating with the access point which was not available for communication.   

   According to yet still another aspect of the present invention, there is provided a computer readable medium having embodied thereon a computer program for the methods. 
   According to a further aspect of the present invention, there is provided an apparatus for allocating encryption keys according to access authorization classes, comprising: 
   an access authorization determining unit which determines the access authorization class for communication between a wireless station and an access point; 
   an encryption key storing unit which stores encryption keys according to classes in advance; and 
   an encryption key allocation unit which reads from the encryption key storing unit an encryption key corresponding to the determination result of the access authorization determining unit, and transfers the value to the wireless station. 
   According to an additional aspect of the present invention, there is provided a computer readable medium having embodied thereon the structure of a wireless data packet, which is used in a wireless network comprising a wireless station and an access point. The wireless data packet structure for allocating encryption keys is set according to access authorization classes, comprising: 
   a data packet header which is transmitted through the wireless network; an access authorization information storing field which indicates the access authorization for communication between the wireless station and the access point; 
   an encrypted data field in which data contents to be transmitted are encrypted and stored; and 
   an error correction field, which is used to correct data error. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail, preferred embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a diagram showing a prior art method for using an ordinary encryption key; 
       FIG. 2  is a diagram showing a method for using an encryption key according to the present invention; 
       FIG. 3  is a flowchart showing method steps for allocating encryption keys during initial authentication of the present invention; 
       FIG. 4  is a diagram showing the structure of a packet header for encryption according to access authorization classes; and 
       FIG. 5  is a block diagram of an apparatus for allocating encryption keys according to access authorization classes. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , a wireless network is formed of a WAN, LANs, access points (APs), and wireless stations (STAs). The WAN is a geographically dispersed telecommunications network used to indicate a telecommunications structure covering a wider area than that of a LAN. In general, while the coverage of a LAN may be limited to one building, a part of a school, a laboratory, or a production plant, a WAN is a communications network connecting distant areas that may be separated geographically, such as a nation with another nation, or a continent with another continent. 
   As shown in  FIG. 1 , STAs  131 ,  132 , and  133  connected to an AP  102  use an identical encryption key. Other APs  101 ,  103 , and  104  require different encryption keys, therefore STAs  131 ,  132 , and  133  connected to another AP  101 ,  103 , and  104  must use another identical encryption key. For example, STAs  131 ,  132 , and  133  connected to AP  101  must use encryption key # 1 - 1   111 , and an STAs  131 ,  132 , and  133  connected to AP  102  must use encryption key # 1 - 2   112 . Accordingly, STA 1   131 , STA 2   132 , and STA 3   133  all use encryption key # 1 - 2   112  to access AP  102 . 
   An STA must use a unique encryption key when that STA is connected to a unique AP on the same LAN or to an AP on another LAN. Therefore, whenever roaming or a hand-off is performed, the STA should receive an encryption key corresponding to the AP to which the STA is connected. For example, in order to communicate with AP  141  on LAN 2   140 , the STA 1   131  requires a new encryption key # 2 - 3   142  instead of its current encryption key # 1 - 2   112 . 
     FIG. 2  illustrates a method for using an encryption key of the present invention. To use a classified encryption key suggested in the present invention, there should be access to authorization classes. Encryption keys are classified into four classes as follows:
     class  1  indicates access authorization to an AP to which an STA belongs,   class  2  indicates access authorization to predetermined APs of a LAN to which an STA belongs,   class  3  indicates access authorization to all APs of a LAN to which an STA belongs, and   class  4  indicates access authorization to multiple APs of the WAN.   
   When initial authentication is performed, an STA obtains all allowed encryption keys. For example, STA 1   210  requires
     class  1  encryption key # 1   221  in order to communicate with AP 1 - 2   220 ,   class  2  encryption key # 2   231  in order to communicate with AP 1 - 3   230 ,   class  3  encryption key # 3   241  in order to communicate with AP 1 - 4   240 , and   class  4  encryption key # 4   252  in order to communicate with AP 2 - 2   251  from another LAN, LAN 2   250 .
 
Accordingly, the STA 1   210  obtains a set of encryption keys # 1  through # 4   260 .
   

   Preferably, classes have priorities. For informational access authorization, priorities are set in the order of class  1 &gt;class  2 &gt;class  3 &gt;class  4 , while for the authorization to use a network, priorities are set in the order of class  1 &lt;class  2 &lt;class  3 &lt;class  4 . The informational access authorization determines which class STA can use the requested AP first, to prioritize a plurality of STAs attempting to access the AP at the same time. The authorization to use a network is interpreted as an authorization to use a network more than others. That is, class  4  indicates that an STA can communicate with all APs and has a higher authorization to use the network. Encryption keys are differentiated according to classes, and encryption keys corresponding to the classes of an STA are allocated to the STA in advance when initial authentication of the STA is performed. 
   When an STA is switching from one AP to another AP or from an AP on one LAN to an AP on another LAN, the STA selects one of the encryption keys in the shared key set, which is allocated when the STA begins first roaming, and performs encryption. In this manner, the STA can reduce delay time caused by encryption key exchange. 
     FIG. 3  is a flowchart illustrating a method for allocating encryption keys during initial authentication including a method for differentiating encryption keys according to access authorization classes in a wireless access network of the present invention. To allocate encryption keys according to the classes in a wireless network formed of a WAN, LANs and APs, a STA  310  communicates directly with an AP  320  by requesting authentication in step  350 . An encryption key set shared by the STA  310  and AP  320  is referred to as a shared key set (SK). 
   The AP  320  follows the following authentication procedure. When authentication begins, the AP  320  determines whether or not the STA  310  corresponds to class  1  in step  351 . If the STA  310  corresponds to class  1 , the AP  320  generates a shared key SK={SK 1 } in step  352 , or sets SK={null} in step  353  if no correspondence to class  1  is found. Then, the AP 320  sends an authentication request to a LAN authentication server  330  in step  354 . 
   The LAN authentication server  330  determines whether or not the STA  310  corresponds to class  2  in step  355 . If the STA  310  corresponds to class  2 , the LAN authentication server  330  generates a new shared key SK 2  and by combining it with the existing shared key SK, forms a new shared key set SK=SKU{SK 2 } in step  356 . If STA  310  does not correspond to class  2 , step  356  is skipped and processing continues in the decision step  357 . Then, in step  357  LAN authentication server  330  determines whether or not the STA  310  corresponds to class  3 . If the STA  310  corresponds to class  3 , the LAN authentication server  330  generates a new shared key SK 3  and forms a new shared key set SK=SKU{SK 3 } in step  358 . If the existing shared key SK={null} in step  355 , the LAN authentication server  330  first generates SK={SK 2 } and then forms SK={SK 2 }U{SK 3 }. If STA  310  does not correspond to class  3 , step  358  is skipped. 
   Next, the STA  310  sends an authentication request to a WAN authentication server  340  in step  359 . In step the WAN authentication server  340  determines whether or not the STA  310  corresponds to class  4 . If the STA  310  corresponds to class  4 , the WAN authentication server  340  adds a shared key SK 4  to the shared key set SK transmitted by the LAN authentication server  330 . By doing so, the WAN authentication server  340  generates a new shared key set SK=SKU{SK 4 } in step  361 , finishes the authentication, and transmits the shared key set SK to the STA  310  in step  362 . If STA  310  does not correspond to class  4 , step  361  is skipped and processing restarts in step  362 . If the WAN authentication server  340  does not authenticate the STA  310 , it transmits the shared key set SK, provided by the LAN authentication server  330 , to the STA  310  and finishes the authentication. If the shared key set SK is SK={null} at that time, the authentication is refused. 
   A method for secure fast roaming using encryption keys according to access authorization classes is shown in  FIG. 2 . After obtaining a shared key set SK in initial authentication, the STA encrypts data by using SK 1  and communicates that data to an AP to which the STA is assigned. Turning now to  FIG. 4 ,  FIG. 4  illustrates a message format used in a method for expressing the access authorization in a header of transmission message packet communicated between STA and AP. When the STA switches to an AP of class  2 , the STA should use SK 2  for encrypting data. Likewise, when the STA communicates with an AP of class  3 , the STA uses SK 3 , and when the STA communicates with an AP of class  4 , the STA encrypts data by using SK 4 . The method for expressing access authorizations of a header is shown in  FIG. 4 . 
     FIG. 4  illustrates the structure of a packet header for encryption according to access authorization classes, that packet is transmitted and received through a wireless transmission network, it comprises a header  410 , an access authorization information field  420 , an encrypted data field  430 , and an error correction field  440 . Accordingly, by allocating two bits in the packet access authorization information field  420  for storing access authorization information, four classes can be expressed by the possible combinations of the two bits. For example, “00”, “01”, “10”, and “11” may indicate class  1 ,  2 ,  3 , and  4 , respectively. 
     FIG. 5  illustrates an apparatus for allocating encryption keys according to access authorization classes comprises an access authorization determining unit  510 , an encryption key storing unit  520 , and an encryption key allocation unit  530 . The access authorization determining unit  510  determines the access authorization class attributed to communication between an STA and an AP. The types of classes include
     class  1  that indicates access authorization to an AP to which the STA is assigned,   class  2  that indicates access authorization to predetermined APs included in a LAN to which the STA is assigned,   class  3  that indicates access authorization to all APs included in the LAN to which the STA is assigned, and   class  4  that indicates access authorization to multiple APs included in the WAN.   
   If an STA requests authentication to an AP, the access authorization determining unit  510  determines the access authorization class of the STA. The encryption key storing unit  520  stores encryption keys according to the classes in advance. Based on the determination result of the access authorization determining unit  510 , the encryption key allocation unit  530  reads the encryption key from the encryption key storing unit  520 , and transfers the value to the STA. 
   The present invention may be embodied in machine code, which can be read by a computer, on a computer readable recording medium. The computer readable recording medium includes all kinds of recording apparatuses on which computer readable data is stored. The computer readable recording media includes but is not limited to storage media such as magnetic storage media, e.g., ROMs, floppy disks, hard disks, etc., optically readable media, e.g., CD-ROMs, DVDs, etc., memory, e.g., random access (RAM) and Flash, and carrier waves, e.g., transmissions over the Internet. 
   Optimum embodiments have been explained above and are shown. However, the present invention is not limited to the preferred embodiment described above, and it is apparent that variations and modifications by those skilled in the art can be effected within the spirit and scope of the present invention defined in the appended claims. Therefore, the scope of the present invention is not determined by the above description but by the accompanying claims. 
   As described above, the present invention describes encryption key differentiation according to access authorization classes and a fast secure roaming method using the keys. By doing so, the present invention reduces the delay time, which is caused by encryption key distribution while a wireless terminal is traveling, such that roaming and hand-off are carried out quickly and safely, and user convenience and safe transmission of data are achieved.