Abstract:
This invention provides enhanced safety for operation of a wireless communication terminal authentication system. This invention is an authentication system, which comprises a wireless communication terminal and a base station for establishing wireless communication with the wireless communication terminal. The wireless communication terminal comprises a terminal side receiver transmitter module for exchanging information with the base station, and a terminal side recording module for recording use and thrown type authentication information. The terminal side recording module records plural pieces of use and thrown type authentication information. The terminal side receiver transmitter module transmits one of the plural pieces of use and thrown type authentication information to the base station and then invalidates the transmitted use and thrown type authentication information. The base station performs an authentication process with the received piece of use and thrown type authentication information.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese patent application P2005-172146 filed on Jun. 13, 2005, the content of which is hereby incorporated by reference into this application. 
     FIELD OF THE INVENTION 
     This invention relates to an authentication system for wireless communication terminal authentication by a wireless base station, and more particularly to an authentication system that uses use and thrown type authentication information. 
     BACKGROUND OF THE INVENTION 
     In recent years, a sensor net is widely used. Within the sensor net, many wireless communication terminals are connected by short-distance wireless communication links. However, there are various problems with the sensor net (refer to James Newsome and Elaine Shi and Dawn Song and Adrian Perrig “The sybil attack in sensor networks: analysis &amp; defense”, IPSN&#39;04: Proceedings of the third international symposium on Information processing in sensor networks, Berkeley, Calif., USA, 2004, ISBN 1-58113-846-6, p. 259-268, ACM Press). In a certain situation, corrupt data may flow into the sensor net due to the use of a forged wireless communication terminal. In another situation, communication resources may be occupied to cause a denial of service. 
     To avoid such illegal accesses, it is necessary that the wireless base station perform a stringent authentication process on wireless communication terminals and reject any illegal terminal when it issues a request for connection to the network. 
     Under these circumstances, for instance, JP 2004-282295 A discloses a wireless communication terminal authentication system, which is based on a one-time password authentication method. When the one-time password authentication method is used, the password is changed whenever authentication is performed. 
     Further, for instance, Poupard and J. Stern “Security analysis of a practical ‘on the fly’ authentication and signature generation, Advances in cryptology—Eurocrypt&#39; 98, Lecture Notes in Computer Science 1403, Springer-Verlag” 1998, p. 422-436 proposes a technology for using digital coupons, which cannot be used repeatedly, instead, for instance, of transportation tickets. 
     SUMMARY OF THE INVENTION 
     A challenge-response authentication method is used as a basic version of the one-time password authentication method. The challenge-response type authentication method provides terminal identification and authentication by verifying that a wireless communication terminal possesses a unique encryption key. 
     The challenge-response authentication method can be classified into two types. One method uses a secret key cryptosystem. The other method uses a public key cryptosystem. 
     If the method based on the secret key cryptosystem is used, it is necessary that the wireless base station possess a wireless communication terminal&#39;s secret key. In a wide-area sensor net in which a vehicle-mounted sensor or the like is used, however, the wireless communication terminal moves among plural wireless base stations. When adopting this operation style, it is necessary that the terminal&#39;s secret key be distributed beforehand to many wireless base stations. Therefore, it is highly likely that the information about the secret key will leak. 
     If, on the other hand, the method based on the public key cryptosystem is used, it is not necessary that the wireless communication terminal and wireless base stations share the secret key. Therefore, the wireless communication terminal can communicate with any wireless base station positioned near a move destination. 
     However, when using the method based on the public key cryptosystem, the volume of calculations performed by the terminal increases because the cryptographic process performed for authentication is complicated. In such a situation, processing operations cannot be fully performed by a small-size wireless communication terminal that operates from a small-size battery, solar battery, or power generation device. Thus, the method based on the public key cryptosystem cannot properly be applied to a small-size wireless communication terminal. Under these circumstances, an authentication system is demanded that is applicable to a small-size wireless communication terminal moving over a wide area and permits the terminal to communicate with any wireless base stations. 
     It is an object of this invention to provide a wireless communication terminal authentication system that adopts use and thrown type authentication information to assure enhanced safety of operation. 
     According to an embodiment of this invention, there is provided an authentication system comprising a wireless communication terminal and a base station establishing wireless communication with the wireless communication terminal. The wireless communication terminal comprises a sensor, a terminal side receiver transmitter module exchanging information with the base station, a terminal side authentication module performing an authentication process in relation to the base station, and a terminal side recording module recording use and thrown type authentication information. The base station comprises a base station side receiver transmitter module exchanging information with the wireless communication terminal and a base station side authentication module performing an authentication process in relation to the wireless communication terminal. The terminal side recording module records plural pieces of use and thrown type authentication information. The terminal side receiver transmitter module transmits one of the plural pieces of use and thrown type authentication information to the base station and then invalidates the transmitted use and thrown type authentication information. The base station performs an authentication process with the received piece of use and thrown type authentication information. 
     Since the authentication system according to an embodiment of this invention reduces the volume of calculations performed on the wireless communication terminal side, identification or authentication can be achieved for a small-size wireless communication terminal whose computational resource and power supply capacity are limited. Further, the wireless base station uses only public information to authenticate a wireless communication terminal. Therefore, there is no risk of information leakage. Consequently, highly safe operations can be conducted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: 
         FIG. 1  is a block diagram illustrating an authentication system according to one embodiment of this invention; 
         FIG. 2  is a block diagram illustrating the configurations of a wireless communication terminal, a wireless base station, and an administration server according to one embodiment of this invention; 
         FIG. 3  is a flowchart illustrating an initialization process and authentication process according to one embodiment of this invention; 
         FIG. 4  shows the structure of a list of coupons according to one embodiment of this invention; 
         FIG. 5  shows the structure of a list of terminal authentication keys, which is provided in a wireless base station according to one embodiment of this invention; 
         FIG. 6  shows the structure of a list of terminal authentication keys, which is provided in an administration server according to one embodiment of this invention; 
         FIG. 7  shows the structure of a list of issued coupons according to one embodiment of this invention; and 
         FIG. 8  is a schematic diagram illustrating a wireless communication terminal according to one embodiment of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of this invention will now be described with reference to the accompanying drawings. 
       FIG. 1  is a block diagram illustrating an authentication system according to one embodiment of this invention. 
     The authentication system comprises a communication network  3  and plural wireless communication networks (local networks)  5 . The communication network  3  is connected to the plural local networks  5 . 
     Each local network  5  includes an administration server  4 , a wireless base station (access point)  2 , and plural wireless communication terminals  1 . Each local network  5  includes at least one wireless base station  2 . 
     The administration server  4  is connected to the wireless base station  2  via a network. The administration server  4  communicates with the wireless communication terminals  1  via the wireless base station. Alternatively, the administration server  4  may comprise wireless or wired communication modules, which can directly communicate with the wireless communication terminals  1  without via the wireless base station  2 . 
     The administration server  4  initializes the wireless communication terminals  1  and manages the communication between the wireless base station  2  and wireless communication terminals  1  within the local network  5 . At the time of wireless communication terminal initialization, the administration server  4  writes an encryption key and authentication key in storage media of the wireless communication terminals  1 . Further, the administration server  4  shares the encryption key and authentication key with the wireless base station  2  within the same local network. The wireless communication terminals  1  can safely communicate with the wireless base station  2  in a home network by using the encryption key and authentication key. 
     The wireless base station  2  wirelessly communicates with the wireless communication terminals  1 . Further, the wireless base station  2  is connected to the communication network  3 . The wireless base station  2  collects the information received from the wireless communication terminals  1  and transmits the collected information to the communication network  3 . 
     Each wireless communication terminal  1  includes a power supply, a sensor  11  (refer to  FIG. 2 ), and a wireless communication device. When the wireless communication terminal  1  starts up for the first time, it is initialized by the administration server  4  in a certain local network  5 . The local network to which the wireless communication terminal  1  is connected at the time of initialization is called a home network. 
     The wireless communication terminal  1  can connect to a local network  5  other than the home network and establish connection. The local networks other than the home network are called visiting networks. 
     When the wireless communication terminal  1  physically moves from a local network  5  to a visiting network and issues a request for connection or moves from one visiting network to another and issues a request for connection, each visiting network checks that the wireless communication terminal  1  is a genuine wireless communication terminal that is registered in the authentication system. After verifying that the wireless communication terminal  1  is genuine, each visiting network grants connection permission. The method for authenticating whether the wireless communication terminal  1  is a genuine wireless communication terminal  1  that is registered in the authentication system will be described later with reference to  FIG. 3 . 
       FIG. 2  is a block diagram illustrating the wireless communication terminal  1 , wireless base station  2 , and administration server  4  according to one embodiment of this invention. 
     The wireless communication terminal  1  comprises a sensor  11 , a controller  12 , a non-volatile memory  13 , a radio-frequency module  14 , an antenna  15 , a display unit  102 , and an input unit  103 . The sensor  11 , controller  12 , non-volatile memory  13 , display unit  102 , and input unit  103  are connected to a bus  10 . 
     The sensor  11  detects, for instance, temperature, humidity, illuminance, acceleration, and infrared radiation. The sensor  11  is not an essential component of the wireless communication terminal  1 . 
     The controller  12  includes a CPU for communication control and a RAM that is used as a work area for the CPU operation. Further, the wireless communication terminal  1  includes the non-volatile memory  13 , which is externally attached to the controller  12 . 
     The non-volatile memory  13  stores the information that is used for authentication processing. For example, an EEPROM is used as the non-volatile memory  13 . The non-volatile memory  13  stores authentication processing software  16 , a terminal authentication key identifier  17 , a terminal secret key  18 , and a list of coupons  19 . 
     The authentication processing software  16  calculates a response at the time of authentication processing (refer to  FIG. 3 ). The controller  12  executes the authentication processing software  16  to configure an authentication module on the wireless communication terminal side. 
     The terminal authentication key identifier  17  is generated by the administration server  4  at the time of initialization. The terminal authentication key identifier  17  is used to identify a terminal authentication key that is possessed by the wireless communication terminal  1 . 
     The terminal secret key  18  is generated by the administration server  4  at the time of initialization. The terminal secret key  18  is used to calculate a response at the time of authentication processing. 
     The list of coupons  19  is a list of digital coupons that are issued by the administration server  4 . Once a digital coupon is used for authentication processing, it is invalidated. When a new digital coupon is issued by the administration server  4 , an invalid digital coupon recorded in the list of coupons  19  is updated to the new digital coupon. The list of coupons  19  will be described later with reference to  FIG. 4 . 
     The display unit  102  comprises a display that indicates the status of communication with the wireless base station  2  and the status of remaining digital coupons. The input unit  103  comprises various operating controls (switches, buttons, etc.) for the wireless communication terminal  1 . When the input unit  103  is operated to enter a signal, the signal is transmitted to the controller  12  and a predefined process is performed. The display unit  102  and/or input unit  103  are not essential components of the wireless communication terminal  1 . 
     The antenna  15  transmits signals to and receives signals from the wireless base station  2 . The transmitted/received information is input into/output from the controller  12  via the radio-frequency module  14 . 
     The wireless base station  2  comprises a wireless interface  22 , a controller  21 , a non-volatile memory  24 , and a communication interface  23 . The wireless interface  22 , controller  21 , non-volatile memory  24 , and communication interface  23  are connected via a bus  20 . 
     The wireless interface  22  includes an antenna and exchanges signals with the wireless communication terminal  1 . The controller  21  comprises a CPU for exercising communication control and a RAM that is used as a work area for communication. Further, the wireless base station  2  includes the non-volatile memory  24 , which is externally attached to the controller  21 . 
     The non-volatile memory  24  stores the information that is used for authentication processing. For example, an EEPROM is used as the non-volatile memory  24 . The non-volatile memory  24  stores a list of terminal authentication keys  25  and authentication processing software  26 . 
     The list of terminal authentication keys  25  is a list of terminal authentication keys that are given to wireless communication terminals  1 . The list relates to all wireless communication terminals  1  that may connect to a wireless base station  2  in which the list of terminal authentication keys  25  is stored. The wireless base stations  2  to which a wireless communication terminal  1  may connect are wireless base stations  2  within a home network for the wireless communication terminal  1 , wireless base stations  2  in local networks  5  near the home network, and wireless base stations  2  in local networks within a predefined range to which the wireless communication terminal  1  may move. 
     The authentication processing software  26  generates the random number to be transmitted to the wireless communication terminal  1 , and verifies a received response (refer to  FIG. 3 ). The controller  21  executes the authentication processing software  26  to configure an authentication side of the module on the wireless base station  2 . 
     The communication interface  23  is connected to the communication network  3  via a network (e.g., Ethernet). The communication interface  23  exchanges information with the administration server  4 , which is connected to the communication network  3 . 
     The administration server  4  comprises a controller  45 , a communication interface  46 , and a data storage (e.g. hard disk drive)  41 . The controller  45 , communication interface  46 , and data storage  41  are connected via a bus  40 . 
     The controller  45  includes a CPU, a ROM that stores, a program etc., and a RAM that is used as a work area for the CPU operation. 
     The data storage  41  stores parameters for coupon generation  42 , a list of terminal authentication keys  43 , and a list of issued coupons  44 . 
     The parameters for coupon generation  42  are generated by the administration server  4 . A selected parameter for coupon generation  42  is used to issue a digital coupon at the time of initialization of the wireless communication terminal  1 . The digital coupon can only be used for only one authentication process. This invention is characterized by the fact that an already used digital coupon is invalidated as a used coupon. 
     The list of terminal authentication keys  43  is a list of terminal authentication keys for wireless communication terminals  1  that are initialized by the administration server  4 . When a registration request is issued by the wireless communication terminal  1 , the administration server  4  initializes the wireless communication terminal  1 . Upon completion of initialization, the administration server  4  adds the information about a generated terminal authentication key to the list of terminal authentication keys  43 . 
     The list of issued coupons  44  is a list of digital coupons that are issued by the administration server  4 . No matter whether plural digital coupons are issued to plural wireless communication terminals  1 , the single list of issued coupons  44  stores the information about all the digital coupons. 
       FIG. 3  is a flowchart illustrating an initialization process and an authentication process according to one embodiment of this invention. 
     The administration server  4  registers a wireless communication terminal  1 , issues a digital coupon to the wireless communication terminal  1 , and performs other initialization steps. The coupon issue process may be performed after the registration process or performed simultaneously with the registration process. The wireless communication terminal  1  cannot be properly used if it is not initialized at startup. 
     First of all, the registration process (from  500  to  506 ) for the wireless communication terminal  1  will be described. 
     Upon startup, the wireless communication terminal  1  requests the administration server  4  for registration ( 500 ). If no terminal identifier is includes in a received registration request, the administration server  4  judges that the wireless communication terminal  1 , which transmitted the registration request, was turned on for the first time, and then generates a terminal identifier for identifying the wireless communication terminal  1 . The administration server  4  registers the generated terminal identifier in the list of terminal authentication keys  43 . Further, the administration server  4  transmits the terminal identifier to the wireless communication terminal  1  ( 501 ). 
     Alternatively, a terminal identifier may be set beforehand for each wireless communication terminal  1 . In such an instance, when a wireless communication terminal  1  starts up, it transmits its terminal identifier to the administration server  4  to make a request for registration ( 500 ). The administration server  4  receives a registration request containing the terminal identifier from the wireless communication terminal  1 , and then registers the terminal identifier in the list of terminal authentication keys  43 . Then, as described later, the administration server  4  generates a terminal authentication key and digital coupon in accordance with the terminal identifier. 
     When the terminal identifier is registered in the list of terminal authentication keys  43 , the home network for the wireless communication terminal  1  is determined. 
     Next, the administration server  4  generates a terminal authentication key unique to the wireless communication terminal  1  ( 502 ). At the same time, the administration server  4  generates a terminal authentication key identifier for identifying the terminal authentication key. The generated terminal authentication key and terminal authentication key identifier are associated with the terminal identifier and registered in the list of terminal authentication keys  43  (refer to  FIG. 6 ). 
     The terminal authentication key includes a terminal authentication key secret part  433  and a terminal authentication key public part  434 . The administration server  4  generates a random number s, handles it as the terminal authentication key secret part  433 , and calculates the expression
 
v=g −s  mod p
 
with the selected random number s. The calculated value v is then handled as the terminal authentication key public part  434 . The values g and p are parameters for coupon generation  42 , and their details will be described later.
 
     The terminal authentication key secret part  433  is used for the terminal secret key  18 , whereas the terminal authentication key public part  434  is used for the terminal public key  254 . 
     The algorithm for implementing a use and thrown type digital coupon is not limited to a method that is described in conjunction with the present embodiment. Any identification algorithm in that it is possible to perform in advance major calculations of an authentication process to be performed by the wireless communication terminal  1 . For example, the Okamoto-identification scheme may be used. For the Okamoto-identification scheme, refer to T. Okamoto “Provably secure and practical identification schemes and corresponding signature schemes,” Advances in cryptology—Crypt&#39;92, Lecture Notes in Computer Science 740, Springer-Verlag, 1993, p. 31-53. 
     Next, the administration server  4  transmits the generated terminal authentication key identifier and terminal authentication key secret part (terminal secret key)  18  to the wireless communication terminal  1  ( 503 ). The wireless communication terminal  1  receives the transmitted information and records it in the non-volatile memory  13  ( 504 ). The administration server  4  transmits the generated terminal authentication key identifier and terminal authentication key public part (terminal public key)  254  to a wireless base station  2  to which the wireless communication terminal  1  having the terminal public key  254  may connect ( 505 ). The wireless base station  2  receives the transmitted information and records it in the list of terminal authentication keys  25  ( FIG. 5 ) within the non-volatile memory  24  ( 506 ). 
     Alternatively, the terminal public key  254  may be transmitted to the wireless base station  2  when a connection request is received from the wireless communication terminal  1  as described later. In such an instance, the wireless base station  2  receives the connection request and issues a request to the administration server  4  of the home network for the wireless communication terminal  1  in order to acquire the terminal public key  254 . 
     A coupon issuance process (from  507  to  509 ) will now be described. 
     The administration server  4  generates parameters for coupon generation  42 . The parameters for coupon generation  42  are then used to generate a digital coupon for the wireless communication terminal  1  ( 507 ). The coupon generation parameters  42  possessed by the administration server  4  of the home network determined by the registration process are used in a coupon generation process for the wireless communication terminal  1 . 
     The parameters for coupon generation  42  first generate prime number p, which has a predetermined number of digits, and prime number q, which is smaller than p and a prime factor of p−1. The parameters are then used to generate integer g (0&lt;g&lt;p), which is such that the p-based modular system order of g is q. A pair of integers satisfying these conditions are generated, for instance, by using a technology that is described in Alfred J. Menezes, Paul C. van Oorschot, Scott A. Vanstone “Handbook of Applied Cryptography,” CRC Press, 1996, ISBN: 0-8493-8523-7, chapter 4. 
     As a parameter for coupon generation  42 , one of plural prepared parameters may be used. An alternative is to repeatedly use a single parameter. The parameters for coupon generation  42  may be generated by the administration server  4  as far as secrecy and safety are assured. An alternative is to let the administration server  4  acquire externally generated parameters and use them as the parameters for coupon generation  42 . 
     The digital coupon includes a public part of coupons  192  and a secret part of coupons  191 . The administration server  4  generates a random number x and calculates the public part of coupons (value t) using following expression.
 
t=g x  mod p
 
The value x becomes the secret part of coupons  191 , whereas the value t becomes the public part of coupons  192 . The administration server  4  issues plural use and thrown type digital coupons to the wireless communication terminal  1 . The formula for the use and thrown type digital coupons is not limited. For example, the formula described in Poupard and J. Stern “Security analysis of a practical ‘on the fly’ authentication and signature generation, Advances in cryptology—Eurocrypt&#39; 98, Lecture Notes in Computer Science 1403, Springer-Verlag” 1998, p. 422-436 may be used.
 
     The administration server  4  transmits an issued digital coupon to the wireless communication terminal  1  ( 508 ). The wireless communication terminal  1  receives the digital coupon and registers it in the list of coupons  19  within the non-volatile memory  13  ( 509 ). 
     A wireless communication terminal authentication process (from  621  to  626 ) that is performed by a wireless base station  2  will now be described. 
     The authentication process is performed at a stage that is different from the stage for the initialization process. The initialization process is performed only once when the wireless communication terminal  1  starts up for the first time. On the other hand, the authentication process is performed when the wireless communication terminal  1  moves to a remote network and issues a request for connection to a wireless base station  2 . For example, the authentication process is performed when the wireless communication terminal  1  moves from the local network  5  to a visiting network or from one visiting network to another. 
     When the wireless communication terminal  1  moves to another local network, the wireless communication terminal  1  searches for a wireless base station  2  that the wireless communication terminal  1  can communicate with. 
     When the wireless communication terminal  1  detects the wireless base station  2  that it can communicate with, it selects one of the digital coupons recorded in the list of coupons  19  as the digital coupon to be used. The wireless communication terminal  1  then writes the value “0” as the status  193  of the digital coupon in order to record that the digital coupon is already used and invalid ( 620 ). 
     Next, a connection request is transmitted to the detected wireless base station  2 . The connection request contains one terminal authentication key identifier  17  and one public part of coupons  442  ( 621 ). 
     Upon receipt of the connection request, the wireless base station  2  refers the status  255  ( FIG. 5 ) of the list of terminal authentication keys  25  to judge whether the terminal public key  254  corresponding to the received terminal authentication key identifier  17  is valid. 
     If the received terminal authentication key identifier  17  is not found or the corresponding terminal public key  254  is invalid, the wireless base station  2  notifies the wireless communication terminal  1  of access denial. A case where the terminal public key  254  is invalid will be described with reference to  FIG. 5 . 
     If the terminal public key  254  is valid, the wireless base station  2  generates a random number c ( 622 ), and transmits the generated random number c to the wireless communication terminal  1  ( 623 ). 
     In accordance with the received random number c, the secret part of coupons  191  (x), and the terminal secret key  18  (s), the wireless communication terminal  1  calculates a response (y) using following expression ( 624 ),
 
 y=x+sc 
 
and transmits the calculated response y to the wireless base station  2  ( 625 ).
 
     The wireless base station  2  uses the received response y, random number c, and terminal public key  254  (v) to calculate
 
g y v c  mod p,
 
and judges whether the obtained calculation result agrees with the public part of coupons  442  (t) received in step  621  ( 626 ). If the two values agree with each other, the wireless base station  2  concludes that the received response y is obtained from a wireless communication terminal  1  registered in the authentication system, and then gives access permission to the wireless communication terminal  1 . If, on the other hand, the two values do not agree with each other, the wireless base station  2  concludes that the wireless communication terminal  1  is not registered in the authentication system, and then notifies the terminal of access denial ( 627 ).
 
     The above process is performed to authenticate the wireless communication terminal  1 . 
     When the use and thrown type digital coupons are used for authentication processing as described above, the volume of calculations performed on the wireless communication terminal  1  side is 1/1000 the volume of calculations performed with the public key cryptosystem. Therefore, identification or authentication can be achieved for a small-size wireless communication terminal  1  whose computational resource and power supply capacity are limited. Further, the wireless base station  2  uses only public information to authenticate the wireless communication terminal  1 . Therefore, there is no risk of information leakage. Consequently, highly safe operations can be conducted. 
     After connection permission is granted subsequently to successful authentication of the wireless communication terminal  1 , the wireless base station  2  may establish encrypted communication or use a message authentication code to establish communication for the purpose of maintaining the secrecy of wirelessly communicated information and preventing wirelessly communicated information from being altered. For this purpose, the wireless base station  2  and wireless communication terminal  1  share an encryption key or message authentication code key. The message authentication code key is used for message authentication code generation and authentication. 
     The above-mentioned keys are shared in compliance with the Diffie-Hellman key exchange protocol. Alternatively, the administration server  4  may generate the encryption key or message authentication code key at the time of initialization of the wireless communication terminal  1 . When the wireless communication terminal  1  connects to the wireless base station  2 , the administration server  4  may transmit the encryption key or message authentication code key to the wireless base station  2 . 
     The above keys may also be generated by the method described below. 
     First of all, the administration server  4  generates a secret parameter u and random number z, and calculates the expressions
 
t=g z  mod p
 
and
 
k=g zu  mod p.
 
     The administration server  4  transmits the calculated values (t and k) to the wireless communication terminal  1 . The wireless communication terminal  1  receives the values (t and k) and records them in the non-volatile memory  13 . 
     Further, the administration server  4  transmits the value u to the wireless base station  2 . The wireless base station  2  receives the value u and stores it in the non-volatile memory  24 . 
     Next, the wireless communication terminal  1  transmits the value t, which is recorded in the non-volatile memory  13 , to the wireless base station  2 . The wireless base station receives the value t, and calculates the value t u  in accordance with the received value t. The value t u  serves as the encryption key or message authentication code key. The key is now shared by the wireless communication terminal  1  and wireless base station  2 . 
       FIG. 4  shows the structure of the list of coupons  19  according to one embodiment of this invention. This list is stored in the non-volatile memory  13  of the wireless communication terminal  1 . 
     The list of coupons  19  includes a secret part of coupons  191 , a public part of coupons  192 , and a status  193 . 
     A field of the secret part of coupons  191  is used to record a random number x that is selected by the administration server  4  at the time of coupon issuance processing. The random number x is a positive integer (e.g., a 10-digit hexadecimal integer). 
     A field of the public part of coupons  192  is used to record a value t that is calculated from the random number x. The value t is a positive integer (e.g., a 10-digit hexadecimal integer). 
     A field of the status  193  is used to record information that indicates whether the associated digital coupon is valid. If the associated digital coupon is already used, it is invalid so that the value “0” is recorded in the field for the status  193 . If, on the other hand, the associated digital coupon is not used yet, it is valid so that the value “1” is recorded in the field for the status  193 . 
     The data recorded in the list of coupons  19  are updated to the data of digital coupons added by an additional coupon issuance process that is described later. 
       FIG. 5  shows the structure of the list of terminal authentication keys  25 , which is provided in the wireless base station  2  according to one embodiment of this invention. This list is stored in the non-volatile memory  24  of the wireless base station  2 . 
     The list of terminal authentication keys  25  includes a terminal authentication key identifier  253 , a terminal public key  254 , and a status  255 . 
     The terminal authentication key identifier  253  is a unique identification number that is assigned to each terminal authentication key. A field of the terminal authentication key identifier  253  is used to record terminal authentication key identifiers  253  assigned to all wireless communication terminals  1  that may connect to the wireless base station  2 . 
     A field of the terminal public key  254 , is used to record a calculated value v. The value v is a positive integer (e.g., a 20-digit hexadecimal integer). 
     A field of the status  255  is used to record information that indicates whether a wireless communication terminal  1  having the associated terminal authentication key is validly operating. When the associated wireless communication terminal  1  is validly operating, the value “1” is recorded as the status  255 . If, on the other hand, the associated wireless communication terminal  1  is invalid, the value “0” is recorded as the status  255 . 
     A wireless communication terminal  1  is invalid when, for instance, it is lost. When the user of a wireless communication terminal  1  reports that the wireless communication terminal  1  is stolen or otherwise lost, the wireless communication terminal  1  is invalidated. The wireless communication terminal  1  is also invalidated when it is rendered inoperative due to damage or defect. 
     Upon receipt of a connection request from a wireless communication terminal  1 , the wireless base station  2  judges whether the terminal authentication key identifier  253  includes in the connection request is recorded in the list of terminal authentication keys  25 . If, for instance, the wireless communication terminal  1  has not gone through the initialization process, no terminal authentication key identifier  253  is assigned to it. It is therefore not recorded in the list of terminal authentication keys  25 . In such an instance, the wireless base station  2  notifies the wireless communication terminal  1  of access denial. 
     If the received terminal authentication key identifier  253  is recorded in the list of terminal authentication keys  25 , the associated status  255  is referred to judge whether the wireless communication terminal  1 , which transmitted the connection request, is validly operating. If, for instance, the wireless communication terminal  1  is stolen, the value “0” is recorded as the status  255  of the wireless communication terminal  1 . It is therefore judged that the wireless communication terminal  1  is invalid. In this instance, the wireless base station  2  notifies the wireless communication terminal  1  of access denial. 
     If, on the other hand, the wireless communication terminal  1  is validly operating, the associated terminal public key  254  is used for authentication purposes. If the result of authentication indicates that the wireless communication terminal  1  is registered in the authentication system, the communication with the wireless communication terminal  1  begins. 
       FIG. 6  shows the structure of the list of terminal authentication keys  43 , which is provided in the administration server  4  according to one embodiment of this invention. This list is stored in the data storage  41  of the administration server  4 . 
     The list of terminal authentication keys  43  includes a terminal identifier  431 , a terminal authentication key identifier  432 , a terminal authentication key secret part  433 , a terminal authentication key public part  434 , and a status  435 . 
     A field of the terminal identifier  431  is used to record the terminal identifiers  431  of wireless communication terminals  1  whose home network is a local network to which an administration server  4  having the list of terminal authentication keys  43  belongs. The terminal identifier  431  is a unique identification number that is assigned to a wireless communication terminal  1 . 
     A field of the terminal authentication key identifier  432  is used to record the terminal authentication key identifier of a terminal authentication key that is assigned to a wireless communication terminal  1  having the associated terminal identifier  431 . The terminal authentication key identifier  432  is the same as the terminal authentication key identifier  253  in the list of terminal authentication keys  25  that is stored in each wireless base station  2 . However, the list of terminal authentication keys  25  records the terminal authentication key identifiers of all wireless communication terminals  1  that may connect to the wireless base station  2 . On the other hand, the list of terminal authentication keys  43  records the terminal authentication key identifiers of only wireless communication terminals  1  whose home network is a local network  5  containing the administration server  4 . 
     A field of the terminal authentication key secret part  433  is used to record a random number s that is selected by the administration server  4  at the time of registration processing. The random number s is a positive integer (e.g., a 20-digit hexadecimal integer). 
     A field of the terminal authentication key public part  434  is used to record a value v that is calculated from the random number s. The value v is a positive integer (e.g., a 20-digit hexadecimal integer). 
     The terminal authentication key secret part  433  is the same as the terminal secret part  253  of the list of terminal authentication keys  25 , which is stored in each wireless base station  2 . The terminal authentication key public part  434  is the same as the terminal public part  254  of the list of terminal authentication keys  25 , which is stored in each wireless base station  2 . However, the list of terminal authentication keys  25  records the terminal secret parts and terminal public parts of all wireless communication terminals  1  that may connect to a wireless base station  2 . On the other hand, the list of terminal authentication keys  43  records the terminal authentication key secret parts  434  and terminal authentication key public parts  433  of only wireless communication terminals  1  whose home network is a local network  5  containing the administration server  4 . 
     A field of the status  435  is used to record information that indicates whether the associated wireless communication terminal  1  is validly operating. When the associated wireless communication terminal  1  is validly operating, the value “1” is recorded as the status  435 . If, on the other hand, the associated wireless communication terminal  1  is invalid, the value “0” is recorded as the status  435 . 
     A wireless communication terminal  1  is invalid when, for instance, it is lost or rendered inoperative due to damage or defect. 
       FIG. 7  shows the structure of the list of issued coupons  44  according to one embodiment of this invention. This list is stored in the data storage  41  of the administration server  4 . 
     The list of issued coupons  44  includes a secret part of coupons  441 , a public part of coupons  442 , a terminal identifier  443 , and a status  444 . The information recorded in this list indicates what digital coupons are issued to what wireless communication terminals  1 . 
     A field of the secret part of coupons  441  is used to record a random number x that is selected at the time of coupon issuance processing. The random number x is a positive integer (e.g., a 10-digit hexadecimal integer). The secret part of coupons  441  is the same as the secret part of coupons  191  that is included in the list of coupons  19 , which is stored in each wireless communication terminal  1 . 
     A field of the public part of coupons  442  is used to record a value t that is calculated from the random number x. The value t is a positive integer (e.g., a 10-digit hexadecimal integer). The public part of coupons  442  is the same as the public part of coupons  192  that is included in the list of coupons  19 , which is stored in each wireless communication terminal  1 . 
     A field of the terminal identifier  443  is used to record the terminal identifier  443  of a wireless communication terminal  1  to which the associated digital coupon is issued. The terminal identifier  443  is the same as the terminal identifier  431  that is included in the list of coupons  19 , which is stored in each wireless communication terminal  1 . 
     For the sake of convenience, the list shown in  FIG. 7  records the information about two digital coupons for each wireless communication terminal  1 . More specifically, two digital coupons are issued to each wireless communication terminal  1  by performing a coupon issuance process once. However, the number of digital coupons issued by performing a coupon issuance process once is not limited to two. Any number of digital coupons may be issued by a single coupon issuance process. 
     A field of the status  444  is used to record information that indicates whether the associated issued digital coupon is valid. The information indicating whether a digital coupon is valid is received at predetermined timing from a wireless communication terminal  1  and recorded. If the associated digital coupon is already used, it is invalid so that the value “0” is recorded as the status  444 . If, on the other hand, the associated digital coupon is not used yet, it is valid so that the value “1” is recorded as the status  444 . 
     The field for the status  444  is not an essential element. However, when the field for the status  444  is provided, the administration server  4  can manage the information indicating the use of digital coupons possessed by individual wireless communication terminals  1 . 
     Each time a digital coupon is issued to a wireless communication terminal  1 , the information about the issued digital coupon is added to the list of issued coupons  44 . 
       FIG. 8  is a schematic diagram illustrating a wireless communication terminal  1  according to one embodiment of this invention. 
     The wireless communication terminal  1  includes a display screen  101  and an operating control button (operating control unit)  103 . 
     The display screen  101  includes an LCD (liquid-crystal display). The display screen  101  indicates the status of communication with a wireless base station  2  and shows an indicator  102 . 
     The indicator  102  shows the status of remaining digital coupons (the number of valid digital coupons currently possessed by a wireless communication terminal  1 ). This makes it possible to notify the user of the wireless communication terminal  1  of the number of remaining digital coupons. A bar (hatched portion in the figure) in the indicator  102  shows the number of remaining digital coupons. The interior of the indicator  102  may alternatively be graduated to indicate the number of remaining digital coupons. Another alternative is to numerically indicate the number of remaining digital coupons. 
     Still another alternative is to let the indicator  102  display the number of digital coupons as the remaining battery power. When the wireless communication terminal  1  moves over a wide area to use many digital coupons in a situation where this alternative is adopted, it looks as if the remaining battery power of the wireless communication terminal  1  were decreased. This also makes it possible to urge the user of the wireless communication terminal  1  to recharge the wireless communication terminal  1  (additional digital coupon issuance). 
     The operating control button  103  issues a request for additional digital coupon issuance to the administration server  4 . When the indicator  102  indicates that the number of remaining digital coupons is small, the user operates the operating control button  103  to issue a request for additional digital coupon issuance to the administration server  4 . 
     The display screen  101 , indicator  102 , and/or operating control button  103  are not essential components of the wireless communication terminal  1 . 
     An additional digital coupon issuance process will now be described. 
     When the number of valid digital coupons recorded in the list of coupons  19  is smaller than a predetermined one, the wireless communication terminal  1  requests the administration server  4  to issue new additional digital coupons. The additional issuance of new digital coupons may be automatically requested. Alternatively, the user may operate the operating control button  103  to issue an additional issuance request. 
     Upon receipt of the request from the wireless communication terminal  1 , the administration server  4  generates digital coupons with the parameters for coupon generation  42 , and transmits the generated digital coupons to the wireless communication terminal  1  via the communication network  3  and wireless base station  2 . 
     To avoid an illegal use of digital coupons, additional digital coupons are issued only when the wireless communication network  1  is positioned within the home network. In other words, additional digital coupons are issued only when the wireless communication network  1  is connected to a wireless base station  2  within the home network. Upon receipt of a request for additional coupon issuance, in this instance, the administration server  4  confirms the address of the wireless base station  2  to which the wireless communication terminal  1 , which has issued the request, is connected. The administration server  4  fulfills the request for additional coupon issuance only when the administration server  4  verifies that the wireless base station  2  is within the home network of the wireless communication terminal  1 . 
     To assure the safety of the contents of the digital coupons to be issued, the administration server  4  may generate a message authentication code key at the time of initialization. This key is shared by the wireless communication terminal  1  and administration server  4 . When a coupon issuance process is performed, the administration server  4  adds a message authentication code to the issued digital coupons. Upon receipt of the digital coupons, the wireless communication terminal  1  uses a previously owned message authentication code key to generate a message authentication code for the received digital coupons. The generated message authentication code is then compared against the message authentication code attached to the received digital coupons. The received digital coupons are recorded in the list of coupons  19  only when the message authentication codes match. This makes it possible to prevent the wireless communication terminal  1  from receiving illegal digital coupons. 
     A wireless communication terminal authentication method for providing user privacy protection will now be described. 
     The wireless communication terminal  1  may be attached to a part of a human body, for instance, for health management purposes. In this instance, the behavior record and other personal information about the user are collected through the network and conveyed to the network administrator. If the network administrator differs from the administrator of the authentication system, it is necessary to provide privacy protection for the user of the wireless communication terminal  1 . 
     An anonymous authentication technology based on the use of a provisional name is known as a conventional authentication method. When this method is used, however, the scale of the authentication process performed on the wireless communication terminal  1  side is greater than when the aforementioned public key cryptosystem is used. Therefore, it is difficult to apply the anonymous authentication technology to a small-size wireless communication terminal  1  that operates from a small-size battery or solar battery. 
     Under such circumstances, the same terminal authentication key is assigned to plural wireless communication terminals  1  or plural terminal authentication keys are commonly assigned to plural wireless communication terminals  1 . This makes it difficult to identify individual wireless communication terminals  1  by a terminal authentication key. In this manner, it is possible to provide privacy protection for the users of the wireless communication terminals  1 . 
     Each wireless communication terminal  1  may possess plural terminal authentication keys. In such an instance, terminal authentication keys other than those invalidated can be used no matter whether leaked terminal public keys are invalidated. Even if one terminal authentication key is invalidated, the communication can be maintained unless all the terminal authentication keys possessed by the wireless communication terminal are invalidated. Thus, the availability of the wireless communication terminal  1  increases with the maintenance cost reduced. 
     For example, plural wireless communication terminals  1  share plural terminal authentication keys. The wireless base station  2  selects one of the plural terminal secret keys  18  to perform authentication. 
     When the above method is used, the wireless base station  2  can verify that the authenticated wireless communication terminal  1  is one of the wireless communication terminals  1  having terminal authentication keys issued by the administration server  4 . However, it is not possible to identify the wireless communication terminal  1 . The administration server  4  for the home network, on the other hand, can identify the wireless communication terminal  1  by comparing the digital coupon used for authentication with the list of issued coupons  44 . 
     While the present invention has been described in detail and pictorially in the accompanying drawings, the present invention is not limited to such detail but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.