Abstract:
A user permission check system with less CPU throughput while ensuring non-repudiation is provided. In order to solve the above-described problem, in the present invention firstly, a MAC function that does not require a CPU to have high processing power is utilized. Additionally, a message is encrypted with a plurality of secret keys and the plurality of keys are distributed to a plurality of servers to make them have the keys in order to ensure validity of the message as a proof of non-repudiation. Subsequently, each server proves the validity of the message within its own range and the validity of the message is ensured by aggregating these individual results, thereby implementing the non-repudiation.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a technique regarding checking a user permission and prevention of repudiation of the user permission. 
       BACKGROUND ART 
       [0002]    It is occasionally necessary to check a user permission in order to execute processing by, for example, an information system composed of client equipment and a server, or an automotive system composed of equipment mounted on an automobile and a server for communicating with such equipment. In these cases, the information system or the automotive system transmits a message to a user to ask for a permission and gets confirmation when the user returns a message of permission. Under this circumstance, the information system or the automotive system can keep a log of these exchanged messages in order to retrospectively explain that the permission has been obtained. However, if validity of the log is questioned, the information system or the automotive system needs to prove the validity of the log. In order to prove the validity, it is only necessary to show that the message of the user permission which is kept in the log was created certainly by the relevant user and has not been falsified. A property in which a creator of such message and its content cannot be denied retrospectively is called “non-repudiation.” An electronic signature technique of public key cryptography technology disclosed in U.S. Pat. No. 5,231,668 is known as a technique that enables the non-repudiation. The technique in U.S. Pat. No. 5,231,668 can retrospectively prove that the relevant message was created certainly by the user, as the user assigns their signature by using their own signature key. 
       SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0003]    Regarding the method described in U.S. Pat. No. 5,231,668, it is necessary to process algorithms of the public key cryptography technology when generating the signature and when verifying the signature. However, the algorithm processing of the public key cryptography technology consumes more throughput and longer CPU time than those of algorithm processing of common key encryption technology. Particularly, in a case of a CPU with low specs which embedded devices such as on-board equipment, the CPU time becomes longer. Furthermore, if the CPU is occupied with for a long time, other processing cannot be executed. Thus, in a case of timely controlling particularly such as that on on-board equipments, the control can be influenced even on the order of several msec, so that the public key encryption system cannot sometimes be used due to safety issues. 
         [0004]    Therefore, it is an object of the present invention to provide a user permission check system that operates with less CPU throughput and has the property of non-repudiation. 
       Means to Solve the Problems 
       [0005]    In order to solve the above-described problem, in the present invention, firstly, a MAC function, in which high processing power is not requested regarding a CPU, is used. Additionally, in order to ensure validity of the message as a proof of the non-repudiation, a message is encrypted with a plurality of secret keys, and the plurality of keys are separately stored in a plurality of servers. Each server proves the validity of the message within its own range, and the validity of the message is ensured by aggregating these individual results, thereby the non-repudiation is realized. 
       Advantageous Effects of the Invention 
       [0006]    As mentioned above, according to the present invention, it is possible to provide a user permission check system with less CPU throughput while ensuring non-repudiation. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1  is a diagram illustrating a system example of a first embodiment according to the present invention. 
           [0008]      FIG. 2  is a schematic diagram illustrating a user terminal. 
           [0009]      FIG. 3  is a schematic diagram illustrating a first server. 
           [0010]      FIG. 4  is a schematic diagram illustrating a second server. 
           [0011]      FIG. 5  is a schematic diagram illustrating the format of a message telegram. 
           [0012]      FIG. 6  is a flowchart illustrating a procedure for checking a user permission. 
           [0013]      FIG. 7  is a flowchart illustrating a procedure for denying retrospective repudiation of the user permission. 
           [0014]      FIG. 8  is a diagram illustrating a system example of the second embodiment according to the present invention. 
           [0015]      FIG. 9  is a schematic diagram illustrating a user terminal. 
           [0016]      FIG. 10  is a schematic diagram illustrating the format of a message telegram. 
           [0017]      FIG. 11  is a flowchart illustrating a procedure for checking the user permission. 
           [0018]      FIG. 12  is a flowchart illustrating a procedure for denying the retrospective repudiation of the user permission. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    When a user permitted operations with a user terminal and then retrospectively repudiates the permission, an embodiment will be described regarding a system capable of denying this repudiation. 
         [0020]      FIG. 1  is an overall configuration diagram of the system according to a first embodiment. In the first embodiment, a user terminal  200  and a first server  300  are connected each other via a first network  100 . The first server and a second server  400  are connected each other via a second network  110 . Note that it is not necessarily limited that the first network  100  and the second network  110  are separated, thus they may be unified together. 
         [0021]      FIG. 2  is a schematic diagram of the user terminal  200  according to the present embodiment. As illustrated, the user terminal  200  comprises a control unit  210 , a data transmitter/receiver  220 , a memory unit  230 , a display device  240 , an input device  250 , and a MAC generator  260 . 
         [0022]    The user terminal  200  is controlled by the control unit  210 , and is linked to the first network  100  via the data transmitter/receiver  220 . 
         [0023]    The memory unit  230  can store data, and stores a first common key  231  and a second common key  232 . 
         [0024]    The MAC generator  260  comprises a MAC generation function mechanism  261 . 
         [0025]    The MAC generation function mechanism  261  is assigned with a message and a common key, and generates a corresponding message authentication code (MAC) by calculating a MAC generating function Genmac. Describing with a mathematical expression, the MAC generating function Genmac is represented by the following Mathematical Expression 1. 
         [0000]      mac=Genmac( k, m ) 
         [0026]    However, k denotes the above-mentioned common key, m represents the above-mentioned message, and mac denotes the above-mentioned corresponding MAC, in the above expression. 
         [0027]    A cryptographic MAC function is available as the above-mentioned MAC generating function Genmac. There are examples such as a MAC function configured in a CMAC usage mode of a block cipher AES, a MAC function configured in HMAC using a hash function SHA-256, and a dedicated MAC function Chaskey. 
         [0028]    The MAC generator  260  generates a message telegram  500  with a MAC added thereto by receiving and processing a message, which is an object to add the MAC, a header, a first common key, and a second common key from the control unit  210 . 
         [0029]      FIG. 5  is a diagram illustrating a format  500  of a message telegram. As described in the explanation regarding the MAC generator, the message telegram is composed of a header  510 , a message  520 , a first MAC  530 , and a second MAC  540 . The header  510  is described with attribute information of the message telegram, for example, information such as the size of the message telegram and an ID of a common key used to generate the first MAC and the second MAC. 
         [0030]    A processing sequence executed by the MAC generator  260  for generating the message telegram  500  will be described below. Note that in the following explanation, h denotes the above-mentioned header, m denotes the above-mentioned message, k_1 denotes the above-mentioned first common key, k_2 denotes the above-mentioned second common key, m_out denotes the above-mentioned message telegram, mac_1 denotes the first MAC, and mac_2 represents the second MAC. 
         [0031]    Firstly, the MAC generator  260  delivers data obtained by coupling the above-mentioned header and the above-mentioned message together, and the first common key to the MAC generation function mechanism, thereby obtaining the first MAC. As a mathematical expression, the above-described procedure is represented by the following Mathematical Expression 2. 
         [0000]      mac_1=Genmac( k _1,  h∥m ) 
         [0032]    Next, the MAC generator  260  delivers the data obtained by coupling the above-mentioned header and the above-mentioned message together, and the second common key to the MAC generation function mechanism, thereby obtaining the second MAC. As a mathematical expression, the above-described procedure can be represented by the following Mathematical Expression 3. 
         [0000]      mac_2=Genmac( k _2,  h∥m ) 
         [0033]    The MAC generator  260  couples the above-mentioned header, the above-mentioned message, the above-mentioned first MAC, and the above-mentioned second MAC each other, and generates a message telegram (the following Mathematical Expression 4). 
         [0000]        m _out= h∥m∥ mac_1∥mac_2
 
         [0034]    Note that this format  500  is not only retained by the terminal  200 , but also shared by the first server  300  and the second server  400 , and is used upon verification. 
         [0035]    Note that the MAC generator and the control unit are realized specifically by having a processing unit such as a CPU compute programs which are read in the memory. They may be realized with one CPU, or with separate CPUs. 
         [0036]      FIG. 3  is a schematic diagram of the first server  300  according to the present embodiment. The first server  300  includes a control unit  310 , a data transmitter/receiver  320 , a memory unit  330 , and a MAC verification function  340 . The first server  300  is controlled by the control unit  310 , and is linked via the data transmitter/receiver  320  to the first network  100  and the second network  110 . The memory unit  330  can store data, and stores the first common key  231  and a message telegram log  331 . 
         [0037]    The MAC verification unit  340  includes a MAC verification function mechanism  341 . 
         [0038]    The MAC verification function mechanism  341  is assigned with a verification object message, a common key, and a verification object MAC, and has a function that verifies whether a combination of the verification object message and the verification object MAC is a valid combination or not. The MAC verification function mechanism  341  performs the next operation. Firstly, the MAC verification function mechanism  341  calculates a right MAC from the input verification object message and the input common key by means of a Genmac function. Under this circumstance, the Genmac function is the same function as that used for the MAC function mechanism  261 . Next, the MAC verification function mechanism  341  compares the calculated right MAC with the input verification object MAC; and when the calculated right MAC matches the input verification object MAC, the MAC verification function mechanism  341  determines that the verification has been performed successfully, and outputs “1” as a function value; and when the calculated right MAC does not match the input verification object MAC, the MAC verification function mechanism  341  determines that the verification has failed, and outputs “0” as the function value. The MAC verification function is expressed with a pseudo code as below. Note that in this expression, Vermac represents the MAC verification function, m denotes the input verification object message, k denotes the input common key, and mac denotes the input verification object MAC.
       function of Vermac(m, k, mac):   mac_tmp←Genmac(k, m)   if (mac_tmp==mac) then   return 1   end if   return 0       
 
         [0045]    Operation of the MAC verification unit  340  will be described. The MAC verification unit  340  receives the message telegram and the first common key from the control unit  310 , and verifies the validity of the message telegram. Firstly, the MAC verification unit  340  extracts the header, the message, and the first MAC from the message telegram in accordance with the format  500  of the message telegram. Next, the MAC verification unit delivers the data obtained by coupling the header and the message together as a verification object message, the first common key as the common key, and the first MAC as the verification object MAC to the MAC verification function mechanism  341 , thereby obtaining the verification result. The MAC verification unit returns the obtained verification result to the control unit  310 . 
         [0046]    Note that the MAC verification unit and the control unit are realized specifically by having a processing unit such as a CPU compute program which is read in the memory. They may be realized by one CPU, or separate CPUs. 
         [0047]    Under this circumstance, which part of the message telegram  500  is the MAC that relates to the first server is determined in advance; and according to that, the MAC verification unit can take out the first MAC. 
         [0048]      FIG. 4  is a schematic diagram of the second server  400  according to the present embodiment. The second server  400  includes a control unit  410 , a data transmitter/receiver  420 , a memory unit  430 , and an MAC verification function  440 . 
         [0049]    The memory unit  430  can store data, and stores the second common key  232 . 
         [0050]    The MAC verification unit  440  is equipped with a MAC verification function mechanism  441 . 
         [0051]    The MAC verification function mechanism  441  has the same function as that of the above-mentioned MAC verification function mechanism  341 . 
         [0052]    Operation of the MAC verification unit  440  will be described. The MAC verification unit  440  receives the message telegram and the second common key from the control unit  410 , and verifies the validity of the message telegram. Firstly, the MAC verification unit  440  takes out the header, the message, and the second MAC from the message telegram in accordance with the format  500  of the message telegram. Next, the MAC verification unit delivers the data obtained by coupling the header and the message together as a verification object message, the second common key as the common key, and the second MAC as the verification object MAC to the MAC verification function mechanism  441 , thereby obtaining the verification result. The MAC verification unit returns the obtained verification result to the control unit  410 . 
         [0053]    Note that as described in  FIG. 3 , the MAC verification unit and the control unit are realized specifically by having a processing unit such as a CPU compute program which is read in the memory. They may be realized by one CPU, or separate CPUs. 
         [0054]    Which part of the message telegram  500  is the MAC that relates to the first server is determined in advance; and in accordance with this part, the MAC verification unit can extracts the first MAC. 
         [0055]      FIG. 6  is a flowchart illustrating a processing sequence for checking the user permission according to the present embodiment. 
         [0056]    Firstly, the first server  300  transmits a message to the user terminal  200  to request for a permission to execute processing (Step  601 ). The control unit  210  of the user terminal  200  receives a permission request telegram via the data transmitter/receiver  220 , and outputs a screen with the permission request message written thereon to the display device  240  (Step  602 ). The user inputs either permission or rejection in response to the permission request by using the input device  250  of the user terminal  200  (Step  603 ). 
         [0057]    When the permission is input in Step  603 , the control unit  210  of the user terminal  200  acquires the first common key  231  and the second common key  232  from the memory unit  230 , and transmits a message of permission, a header, and the first common key to the MAC generator  260 , and the MAC generator generates a message telegram  500  from the message of the permission, the header, the first common key, and the second common key, and returns the message telegram  500  to the control unit (Step  604 ). The control unit  210  transmits the above-mentioned message telegram via the data transmitter/receiver  220  to the first server  300  (Step  605 ) 
         [0058]    The control unit  310  of the first server receives the above-mentioned message telegram via the data transmitter/receiver  320 , acquires the first common key  231  from the memory unit  330 , and delivers the message telegram and the first common key to the MAC verification unit  340 . The MAC verification unit verifies the validity of the message telegram, and returns the verification result to the control unit (Step  606 ) 
         [0059]    When the verification has successfully been performed in Step  606 , the control unit  310  transmits the message telegram via the data transmitter/receiver  320  to the second server  400  (Step  607 ). The control unit  410  of the second server receives the above-mentioned message telegram via the data transmitter/receiver  420 , receives the second common key  232  from the memory unit  430 , and delivers the message telegram and the second common key to the MAC verification unit  440 . The MAC verification unit verifies the validity of the message telegram and returns the verification result to the control unit  410  (Step  608 ). 
         [0060]    When the verification has successfully been performed in Step  608 , the control unit  420  transmits a message of the verification success via the data transmitter/receiver  420  to the first server  300  (Step  609 ). The control unit  310  of the first server  300  receives the message of the verification success via the data transmitter/receiver  320 , adds the above-mentioned message telegram to the message telegram log  331 , and transmits a message indicating that the processing can be executed, to the user or the server terminal that will execute the processing for which the user was requested for their permission in Step  601  (Step  610 ). 
         [0061]    The user terminal or the server, which has received the message indicating that the processing can be executed, executes the processing (Step  611 ). 
         [0062]    When the user inputs the rejection in Step  603 , the user terminal  200  transmits a message of rejection to the first server (Step  620 ) and the processing proceeds to Step  640 . 
         [0063]    When the verification of the validity has failed in Step  606 , the processing proceeds to Step  640 . 
         [0064]    When the verification of the validity has failed in Step  608 , the second server  400  transmits a message of the verification failure to the first server (Step  630 ) and the processing proceeds to Step  640 . 
         [0065]    In Step  640 , the first server  300  transmits a message indicating that the processing is cannot be executed, to the server or the user terminal scheduled to execute the processing for which the user was requested for their permission in Step  601  (Step  640 ). The user terminal and the server, which has received the message indicating the processing cannot be executed, cancels the processing without executing it (Step  641 ). 
         [0066]    Note that an example of the operation of the system in Step  601  where the above-mentioned first server  300  requests for the user permission can include the operation where the first server  300  transmits an update program of the user terminal  200  to the user terminal  200 , and the user terminal  200  updates the program by executing the update program. Another example of the operation of the system can include the operation where the user terminal  200  updates the program by executing an update program received from the first server in advance. 
         [0067]    Note that when the user repudiates their permission regarding the operation of the system for which the user gave any permission in the past, the first server, the second server, or the third party can deny the user&#39;s repudiation. 
         [0068]      FIG. 7  is a flowchart illustrating a procedure for denying the repudiation. 
         [0069]    Firstly, a third party or the server which intends to deny the user&#39;s repudiation of permission demands that the first server  300  confirms the user permission (Step  801 ). The control unit  310  of the first server acquires a message telegram of the relevant user permission from the message telegram log  331  of the storage unit  330  (Step  802 ). The control unit of the first server delivers the above-mentioned message telegram, the first common key  331 , which has been acquired from the storage unit, to the MAC verification unit  340 , and the MAC verification unit verifies the validity of the message telegram on the basis of the above-mentioned message telegram and the above-mentioned first common key (Step  803 ). 
         [0070]    When the verification has been performed successfully in Step  803 , the control unit transmits the message telegram to the second server (Step  804 ). The control unit of the second server delivers the above-mentioned message telegram and the second common key, which has been acquired from the storage unit, to the MAC verification unit  340 . The MAC verification unit  340  verifies the validity of the message telegram on the basis of the above-mentioned message telegram and the second common key (Step  805 ). 
         [0071]    When the verification has been performed successfully in Step  805 , the second server transmits a message of the verification success to the first server (Step  806 ). The first server transmits a message indicating that the user permission has been confirmed, and the above-mentioned message telegram to the server or the third party in Step  801  (Step  807 ). 
         [0072]    When the verification has failed in Step  803 , the processing proceeds to Step  830 . 
         [0073]    When the verification has failed in Step  805 , the second server transmits a message of the verification failure to the first server (Step  820 ) and the processing proceeds to Step  830 . 
         [0074]    In Step  830 , the first server transmits a message indicating that the repudiation of the user permission cannot be denied, to the server or the third party in Step  801  (Step  830 ). 
         [0075]    Note that when the third party can access the message telegram log  331  of the first server  300 , the MAC verification unit  340  of the first server  300 , and the MAC verification unit  440  of the second server, the third party can execute all the processing steps executed by the first server and the second server with respect to the respective steps of the flowchart in  FIG. 8 . 
         [0076]    Note that the first MAC and the second MAC of the message telegram generated by the user terminal are not limited to those represented by Expressions 2 and 3, and may be an MAC generated by nesting MAC generation by the first common key and MAC generation by the second common key. Under this circumstance, nesting means to make the MAC generated with the second common key a part of the MAC generation input by the first common key or to make the MAC generated with the first common key a part of the MAC generation input by the second common key. For example, mac_12 and mac_21 expressed in the following Mathematical Expressions 6: 
         [0000]      mac_12=mac_2∥Genmac( k _1,  h∥m ∥mac_2)
 
         [0000]    and; the following Mathematical Expression 7: 
         [0000]      mac_21=mac_1∥Genmac( k _2,  h∥m ∥mac_1)
 
         [0000]    may respectively be the first MAC and the second MAC. 
         [0077]    When mac_12 and mac_21 are respectively the first MAC and the second MAC, the MAC generator of the user terminal and the MAC verification unit of the server generate and verify the MAC in accordance with Mathematical Expressions 6 and 7, respectively. 
       Second Embodiment 
       [0078]    A second embodiment is an embodiment which has expanded the first embodiment and in which the number of servers is expanded to N including the first server to the N-th server while there are two servers in the first embodiment. 
         [0079]      FIG. 8  is an overall configuration diagram of a system according to the second embodiment. A user terminal  1200  and a first server  1300  are linked to each other via a first network  1100 , and the N servers such as the first to N-th servers are linked to each other via a second network  1110 . 
         [0080]      FIG. 9  is a schematic diagram of the user terminal  1200 . The user terminal has the same configuration as that of the first embodiment, and is composed of a control unit, a data transmitter/receiver, a memory unit, a MAC generator, a display device, and an input device. The memory unit  1230  stores N common keys including first to N-th common keys. 
         [0081]    The MAC generator is equipped with the same MAC generation function mechanism  1261  as that of the first embodiment. 
         [0082]    The MAC generator receives a message which is an object to add a MAC, a header, N common keys including a first common key to an N-th common key from the control unit and processes them, thereby generates a message telegram  1500  with the MAC added thereto. 
         [0083]    Note that an explanation regarding the configuration in common with the explanation regarding  FIG. 2  is omitted. 
         [0084]      FIG. 10  is a diagram illustrating the format of a message telegram  1500 . 
         [0085]    A processing sequence of the MAC generator  1260  for generating the message telegram  1500  will be described below. Note that h denotes the above-mentioned header, m denotes the above-mentioned message, k_i denotes an i-th common key, m_out denotes the above-mentioned message telegram, and mac_i denotes an i-th MAC, in the following explanation. 
         [0086]    Firstly, the MAC generator generates N pieces of MACs. For example, regarding the i-th MAC, the MAC generator delivers data obtained by coupling the above-mentioned header and the above-mentioned message together, and the i-th common key to the MAC generation function mechanism, thereby obtaining the i-th MAC. Describing with a mathematical expression, the above-described procedure can be represented by the following expression. 
         [0000]      mac_ i =Genmac( k _ i, h∥m ) 
         [0087]    The MAC generator generates a message telegram by coupling the above-mentioned header, the above-mentioned message, and the N pieces of MACs including a first MAC to an N-th MAC. 
         [0000]        m out= h∥m ∥mac_1∥mac_2∥ . . . ∥mac_ N  
 
         [0088]    The first server has the same configuration as that of the first server  300  according to the first embodiment, and is composed of a control unit, a data transmitter/receiver, a memory unit, and a MAC verification unit. The memory unit stores a first common key. 
         [0089]    Each server from second to N-th servers has the same configuration as that of the second server  400  according to the first embodiment, and is composed of a control unit, a data transmitter/receiver, a memory unit, and an MAC verification unit. A common key stored in the memory unit varies depending on each server and a k-th common key is stored in a k-th server. 
         [0090]    The MAC verification unit of each server from the first server to the N-th server is equipped with a MAC verification function mechanism  341  which is similar to that of the first embodiment. 
         [0091]    The MAC verification unit of an i-th server receives a message telegram and an i-th common key from the control unit and verifies the validity of the message telegram. Firstly, the MAC verification unit takes out the header, the message, and an i-th MAC from the message telegram in accordance with the format  1500  of the message telegram. Next, the MAC verification unit delivers the data obtained by coupling the header and the message together as a verification object message, the i-th common key as a common key, and the i-th MAC as a verification object MAC to the MAC verification function mechanism, thereby obtaining the verification result. Then, the MAC verification unit returns the obtained verification result to the control unit. 
         [0092]      FIG. 11  is a flowchart illustrating a processing execution checking procedure according to the second embodiment. Incidentally, m in Steps represents an integer larger than N/2 and equal to or less than N. 
         [0093]    Firstly, the first server  130  transmits a message to the user terminal  1200  to request for permission to execute processing (Step  1001 ). The data transmitter/receiver  1220  of the user terminal  1200  receives a permission request telegram, and outputs a screen with the permission request message written thereon to the display device  1240  (Step  1002 ). The user inputs either permission or rejection in response to the permission request by using the input device  1250  of the user terminal  1200  (Step  1003 ). 
         [0094]    When the permission is input in Step  1003 , the control unit  1210  of the user terminal  1200  acquires N common keys, including first to N-th common keys, from the memory unit  1230 , and delivers a message of permission, a header, and the N common keys to the MAC generator  1260 , and the MAC generator generates a message telegram  1500 , and returns the message telegram to the control unit (Step  1004 ). The control unit  1210  transmits the message telegram via the data transmitter/receiver  1220  to the first server  1300  (Step  1005 ). 
         [0095]    The data transmitter/receiver of the first server receives the above-mentioned message telegram, and transmits the message telegram via the data transmitter/receiver to N−1 servers including the second server to the N-th server (Step  1006 ). 
         [0096]    Regarding each i-th server which has acquired the message telegram, the control unit of the server acquires the i-th common key from the memory unit, and delivers the message telegram and the i-th common key to the MAC verification unit of the server, and the MAC verification unit verifies the validity of the message telegram, and returns the verification result to the control unit (Step  1007 ). The control unit of each i-th server transmits the above-mentioned verification result to the first server (Step  1008 ). The first server counts the number of verification successes from among the gathered N pieces of verification results, and checks if the number of successes is m or more (Step  1009 ). 
         [0097]    When the number of verification successes is m or more in Step  1009 , the control unit of the first server  1300  adds the above-mentioned message telegram to the message telegram log of the storage unit, and transmits a message indicating that the processing can be executed, via the data transmitter/receiver to the user terminal or the server that will execute the processing for which the user was requested for their permission in Step  1001  (Step  1010 ). The user terminal or the server, which has received the message of the processing execution, executes the processing (Step  1011 ). 
         [0098]    When the user inputs the rejection in Step  1003 , the user terminal  1200  transmits a message of rejection to the first server (Step  1020 ), and the processing proceeds to Step  1030 . 
         [0099]    When the number of verification successes is less than m in Step  1008 , the processing proceeds to Step  1030 . 
         [0100]    In Step  1030 , the first server  1300  transmits a message indicating that the processing cannot be executed, to the server or the user terminal scheduled to execute the processing for which the user was requested for their permission in Step  1001  (Step  1030 ). The user terminal or the server, which has received the message indicating the processing cannot be executed, cancels the processing without executing it (Step  1031 ). 
         [0101]    Note that when the user repudiates their permission regarding the operation of the system for which the user gave the permission in the past, the server or the third party can deny the repudiation.  FIG. 12  is a flowchart illustrating a procedure for denying the repudiation. 
         [0102]    Firstly, the third party or the server which intends to deny the user&#39;s repudiation of permission demands that the first server confirm the user permission (Step  1401 ). The control unit of the first server acquires a message telegram of the relevant user permission from the message telegram log of the storage unit (Step  1402 ). The control unit of the first server transmits the above-mentioned message telegram to N−1 servers including the second server to the N-th server (Step  1403 ). The control unit of the first server transmits the above-mentioned message telegram to the N−1 servers including the second server to the N-th server (Step  1403 ). The control unit of each server delivers the above-mentioned message telegram and the common key, which it has acquired from the storage unit and retains, to the MAC verification unit and the MAC verification unit verifies the validity of the message telegram on the basis of the above-mentioned message telegram and the common key (Step  1404 ). Each server transmits the verification result to the first server (Step  1405 ). The first server counts the number of verification successes from among the gathered N pieces of verification results and checks if the number of successes is m or more (Step  1406 ). 
         [0103]    When the number of verification successes is m or more in Step  1406 , the first server  1300  transmits a message indicating that the user permission has been confirmed, and the above-mentioned message telegram to the server or the third party in Step  1401  (Step  1407 ). 
         [0104]    When the number of verification successes is less than m in Step  1406 , the first server transmits a message indicating that the repudiation of the user permission cannot be denied, to the server or the third party in Step  1401  (Step  1420 ). 
         [0105]    Note that when the third party can access the message telegram log of the first server  1300  and all the MAC verification units of the first server to the N-th server, the third party can execute all the processing Steps executed by the first server to the N-th server with respect to the respective steps of the flowchart in  FIG. 12 . 
         [0106]    Note that the i-th MAC of the message telegram generated by the MAC generator  1260  of the user terminal  1200  in the second embodiment is not limited to that expressed by Expression  6 . For example, the i-th MAC may be mac_i expressed in the following expression. 
         [0000]      mac_ i ′=othermac_ i ∥Genmac( k _ i, h∥m ∥othermac_ i )
 
         [0000]    In the above expression, othermac_i denotes an arbitrary value, other than k_i, which can be calculated by using the N−1 common keys, the header h, the message m, and the Genmac function. 
       REFERENCE SIGNS LIST 
       [0107]      100 : first network 
         [0108]      110 : second network 
         [0109]      200 : user terminal 
         [0110]      210 : control unit 
         [0111]      220 : data transmitter/receiver 
         [0112]      230 : memory unit 
         [0113]      231 : first common key 
         [0114]      232 : second common key 
         [0115]      240 : display device 
         [0116]      250 : input device 
         [0117]      260 : MAC generator 
         [0118]      261 : MAC generation function mechanism 
         [0119]      300 : first server 
         [0120]      310 : control unit 
         [0121]      320 : data transmitter/receiver 
         [0122]      330 : memory unit 
         [0123]      331 : message telegram log 
         [0124]      340 : MAC verification unit 
         [0125]      341 : MAC verification function mechanism 
         [0126]      400 : second server 
         [0127]      410 : control unit 
         [0128]      420 : data transmitter/receiver 
         [0129]      430 : memory unit 
         [0130]      440 : MAC verification unit 
         [0131]      441 : MAC verification function mechanism 
         [0132]      500 : message telegram 
         [0133]      510 : header 
         [0134]      520 : message 
         [0135]      530 : first MAC 
         [0136]      540 : second MAC 
         [0137]      1100 : first network 
         [0138]      1110 : second network 
         [0139]      1200 : user terminal 
         [0140]      1210 : control unit 
         [0141]      1220 : data transmitter/receiver 
         [0142]      1230 : memory unit 
         [0143]      1240 : display device 
         [0144]      1250 : input device 
         [0145]      1260 : MAC generator 
         [0146]      1261 : MAC generation function mechanism 
         [0147]      1300 : first server 
         [0148]      1500 : message telegram