Patent Publication Number: US-2023153411-A1

Title: Authentication server, authentication system, control method of authenticationserver, and storage medium

Description:
TECHNICAL FIELD 
     The present invention relates to an authentication server, an authentication system, a control method of an authentication server, and a storage medium. 
     BACKGROUND ART 
     In recent years, various kinds of services using biological information have begun to spread. For example, face authentication is used for various kinds of procedures (check-in procedures, procedures at baggage drop-off counters, etc.) performed in airports and for check-in procedures at hotels, etc. 
     In the case of a service using face authentication, processing is performed as follows. First, a terminal (a terminal installed at an airport or a hotel) acquires a face image of a user and generates feature values (a feature vector) that characterize the face image. The generated feature values are transmitted to a server on a network. 
     The server includes a database that stores biological information and personal information (a name, an address, etc.) of the user who wishes to receive the service using face authentication. When the server acquires a matching request from the terminal, the server searches the above database (performs matching) and determines the biological information and the personal information corresponding to the matching request from the terminal. The server transmits the determined personal information to the terminal, and the terminal installed at the airport or the like performs an operation based on the acquired personal information. 
     For example, PTL 1 discloses a private lodging management server in a private lodging service system. This management server performs identification of a lodger by using an image of the guest captured by a mobile terminal and unlocks a room door. 
       1 CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Unexamined Patent Application Publication No. JP201 8-101235 
     SUMMARY OF INVENTION 
     Technical Problem 
       FIG.  2    in PTL 1 illustrates a configuration in which the server that performs authentication based on biological information manages personal information in association with the biological information. This configuration could cause a serious problem if information is leaked from the database in the server. In particular, biological information of a person, such as a face image, a fingerprint image, and a vein pattern, does not change for his or her entire life. That is, biological information cannot be changed even if information leakage occurs. Leakage of a combination of such biological information and personal information to a third party could cause an irreversible situation. 
     As measures against the above information leakage, personal information or the like can be encrypted. However, if encrypted text and a key for decrypting the encrypted text are stored in the same server, the key and the encrypted text could be leaked simultaneously. Thus, encrypting personal information or the like is not an ultimate solution. 
     It is a main object of the present invention to provide an authentication server, an authentication system, a control method of an authentication server, and a storage medium that enable more secure authentication using biological information. 
     Solution to Problem 
     According to a first aspect of the present invention, there is provided an authentication server including: a user registration unit that acquires a first ID that uniquely determines a user in a system and first biological information that is used for authentication of the user; a service registration unit that processes a service registration request that is transmitted from a service provider of a service that the user wishes to use and that includes the first ID and a second ID that identifies the service provider; and a storage unit, wherein the service registration unit generates a third ID that is uniquely determined by a combination of the user and the service provider, generates an encryption key, and transmits the third ID and the encryption key to the service provider, and wherein the storage unit stores the first biological information, the first ID, the second ID, the third ID, and personal information of the user encrypted with the encryption key in association with each other. 
     According to a second aspect of the present invention, there is provided an authentication system including: an authentication server; and a management server, wherein the authentication server includes a user registration unit that acquires a first ID that uniquely determines a user in the system and first biological information that is used for authentication of the user, a service registration unit that processes a service registration request that is transmitted from a service provider of a service that the user wishes to use and that includes the first ID and a second ID that identifies the service provider, and a first storage unit, wherein the service registration unit generates a third ID that is uniquely determined by a combination of the user and the service provider, generates an encryption key, and transmits the third ID and the encryption key to the service provider, wherein the first storage unit stores the first biological information, the first ID, the second ID, the third ID and personal information of the user encrypted with the encryption key in association with each other, and wherein the management server includes a personal information acquisition unit that acquires the first ID and personal information of the user from the user, a service registration request unit that acquires the third ID and the encryption key by transmitting the service registration request to the authentication server and transmits the encrypted personal information of the user to the authentication server, and a second storage unit that stores the encryption key and the third ID in association with each other. 
     According to a third aspect of the present invention, there is provided a control method of an authentication server, the control method including: acquiring a first ID that uniquely determines a user in a system and first biological information that is used for authentication of the user; receiving a service registration request that is transmitted from a service provider of a service that the user wishes to use and that includes the first ID and a second ID that identifies the service provider; generating a third ID that is uniquely determined by a combination of the user and the service provider, and an encryption key; transmitting the third ID and the encryption key to the service provider; and storing the first biological information, the first ID, the second ID, the third ID, and personal information of the user encrypted with the encryption key in association with each other. 
     According to a fourth aspect of the present invention, there is provided a computer-readable storage medium storing a program causing a computer mounted on an authentication server to perform processing for: acquiring a first ID that uniquely determines a user in a system and first biological information that is used for authentication of the user; receiving a service registration request that is transmitted from a service provider of a service that the user wishes to use and that includes the first ID and a second ID that identifies the service provider; generating a third ID that is uniquely determined by a combination of the user and the service provider, and an encryption key; transmitting the third ID and the encryption key to the service provider; and storing the first biological information, the first ID, the second ID, the third ID and personal information of the user encrypted with the encryption key in association with each other. 
     Advantageous Effects of Invention 
     The individual aspects of the present invention provide an authentication server, an authentication system, a control method of an authentication server, and a storage medium that enable more secure authentication using biological information. The advantageous effects of the present invention are not limited to the above advantageous effect. The present invention may provide other advantageous effects, instead of or in addition to the above advantageous effect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating an outline of an example embodiment. 
         FIG.  2    is a diagram illustrating an example of a schematic configuration of an authentication system according to a first example embodiment. 
         FIG.  3    is a diagram illustrating an operation in a user registration phase in the authentication system according to the first example embodiment. 
         FIG.  4    is a diagram illustrating an operation in a service registration phase in the authentication system according to the first example embodiment. 
         FIG.  5    is a diagram illustrating an operation in a service provision phase in the authentication system according to the first example embodiment. 
         FIG.  6    is a diagram illustrating an example of a processing configuration of an authentication server according to the first example embodiment. 
         FIG.  7    is a diagram illustrating an operation of a user registration unit in the authentication server according to the first example embodiment. 
         FIG.  8    is a diagram illustrating an operation of the user registration unit in the authentication server according to the first example embodiment. 
         FIG.  9    is a diagram illustrating an example of an authentication information database. 
         FIG.  10    is a diagram illustrating an example of the authentication information database. 
         FIG.  11    is a diagram illustrating an example of the authentication information database. 
         FIG.  12    is a diagram illustrating an example of a processing configuration of a management server according to the first example embodiment. 
         FIG.  13    is a diagram illustrating an operation of a personal information acquisition unit in the management server according to the first example embodiment. 
         FIG.  14    is a diagram illustrating an example of a user information database. 
         FIG.  15    is a diagram illustrating an example of a processing configuration of an authentication terminal according to the first example embodiment. 
         FIG.  16    is a sequence diagram illustrating an example of an operation relating to the service registration phase in the authentication system according to the first example embodiment. 
         FIG.  17    is a sequence diagram illustrating an example of an operation relating to the service provision phase in the authentication system according to the first example embodiment. 
         FIG.  18    is a diagram illustrating an operation in a service registration phase in an authentication system according to a second example embodiment. 
         FIG.  19    is a diagram illustrating an operation in a service provision phase in the authentication system according to the second example embodiment. 
         FIG.  20    is a diagram illustrating an example of an authentication information database. 
         FIG.  21    is a diagram illustrating an operation in a service registration phase in an authentication system according to a third example embodiment. 
         FIG.  22    is a diagram illustrating an operation in a service provision phase in the authentication system according to the third example embodiment. 
         FIG.  23    is a diagram illustrating an example of a user information database. 
         FIG.  24    is a diagram illustrating an example of an authentication information database. 
         FIG.  25    is a diagram illustrating an example of a hardware configuration of an authentication server. 
     
    
    
     EXAMPLE EMBODIMENT 
     First, an outline of an example embodiment will be described. In the following outline, various components are denoted by reference characters for the sake of convenience. That is, the following reference characters are used as examples to facilitate the understanding of the present invention. Thus, the description of the outline is not intended to impose any limitations. In addition, unless otherwise specified, an individual block illustrated in the drawings represents a configuration of a functional unit, not a hardware unit. An individual connection line between blocks in the drawings signifies both one-way and two-way directions. An arrow schematically illustrates a principal signal (data) flow and does not exclude bidirectionality. In the present description and drawings, elements that can be described in a like way will be denoted by a like reference character, and redundant description thereof will be omitted as needed. 
     An authentication server  100  according to an example embodiment includes a user registration unit  101 , a service registration unit  102 , and a storage unit  103  (see  FIG.  1   ). The user registration unit  101  acquires a first ID that uniquely determines a user in a system and first biological information that is used for authentication of the user. The service registration unit  102  processes a service registration request that is transmitted from a service provider of a service that the user wishes to use and that includes the first ID and a second ID that identifies the service provider. The service registration unit  102  generates a third ID uniquely determined by a combination of the user and the service provider, generates an encryption key, and transmits the third ID and the encryption key to the service provider. The storage unit  103  stores the first biological information, the first ID, the second ID, the third ID, and personal information of the user encrypted with the encryption key in association with each other. 
     The storage unit  103  of the authentication server  100  stores biological information of users, various IDs, and encrypted personal information. The encryption key (the encryption key generated by the service registration unit  102 ) for decrypting the encrypted personal information is stored by the service provider, and the encryption key does not exist in the authentication server  100 . Thus, even if information is leaked from the authentication server  100 , because the contents of personal information will not be disclosed to third parties, the participants (the system users and service providers) in the authentication system can use the authentication based on the authentication server  100  more securely. 
     Hereinafter, specific example embodiments will be described in more detail with reference to drawings. 
     First Example Embodiment 
     A first example embodiment will be described in more detail with reference to drawings. 
     [System Configuration] 
       FIG.  2    is a diagram illustrating an example of a schematic configuration of an authentication system according to a first example embodiment. As illustrated in  FIG.  2   , the authentication system includes an authentication center and a plurality of service providers. 
     Each service provider that participates in the authentication system provides a service using biological authentication. Examples of the services provided by the service providers include payment services at retail stores, etc. and accommodation services at hotels, etc. Examples of the services provided by the service providers also include emigration and immigration examination at airports and ports. The service providers disclosed in the present application may provide any services that can be provided by using biological authentication. 
     An authentication server  10  is installed in the authentication center. The authentication server  10  operates as an authentication station that performs authentication using biological information. The authentication server  10  may be a server installed on the premises of the authentication center or may be a server installed on a cloud. 
     Examples of the biological information of a user include data (feature values) calculated from physical features unique to this individual, such as a face, a fingerprint, a voiceprint, a vein, a retina, or an iris pattern of an eye of the user. Alternatively, the biological information of a user may be image data of a face image or a fingerprint image of the user, for example. The biological information of a user may be any information including physical features of the user. 
     The authentication server  10  is a server apparatus for realizing an individual service based on biological authentication. The authentication server  10  processes an “authentication request” transmitted from an individual service provider and transmits a corresponding result of the authentication processing to the service provider. 
     The individual service provider has a management server and an authentication terminal. 
     For example, a management server  20  and a plurality of authentication terminals  30  are installed at a service provider S1. A management server  20  and a plurality of authentication terminals  31  are installed at a service provider S2. 
     In the following description, when a certain component needs to be distinguished from the other components, a reference numeral on the right side of the corresponding hyphen will be used. As the apparatuses included in the service provider S1 operate in the same way as those included in the service provider S2, the following description will be made with a focus on the service provider S1. 
     The apparatuses illustrated in  FIG.  2    are connected to each other. For example, the authentication server  10  and the individual management server  20  are each connected to wired or wireless communication means so that the authentication server  10  and the individual management server  20  can communicate with each other. 
     The individual management server  20  is a server that controls and manages overall operations of the corresponding service provider. For example, if a service provider is a retail store, the corresponding management server  20  performs merchandise inventory control, etc. If the service provider is a hotel operator, the corresponding management server  20  manages information about guest reservations, for example. 
     In addition to the above functions relating to the provision of a service, the individual management server  20  has control and management functions relating to biological authentication of the users. 
     The individual authentication terminal  30  is an apparatus that functions as an interface for the users who visit at the corresponding service provider. The users receive various services via the authentication terminal  30 . For example, if the service provider is a retail store, a user performs payment by using the authentication terminal  30 . If the service provider is a hotel operator, a user performs a check-in procedure by using the authentication terminal  30 . 
     The configuration illustrated in  FIG.  2    is an example and does not limit the configuration or the like of the authentication system disclosed in the present application. For example, two or more authentication servers  10  may be installed in the authentication center. At least one authentication terminal  30  is installed by the individual service provider. Alternatively, the functions of the management server  20  and the functions of the individual authentication terminal  30  may be integrated, and in this case, a single apparatus having the integrated functions may be installed to provide a service using biological authentication. At each service provider, a plurality of authentication terminals  30  may be connected to a single management server  20  as illustrated in  FIG.  2    or a single authentication terminal  30  may alternatively be connected to a single management server  20 . 
     [Schematic System Operation] 
     Next, a schematic operation in the authentication system according to the first example embodiment will be described. 
     The operation in the authentication system includes three phases. 
     The first phase is a phase (a user registration phase) in which a user performs system registration. 
     The second phase is a phase (a service registration phase) in which the user performs service registration. 
     The third phase is a phase (a service provision phase) in which a service using biological authentication is provided to the user. 
     [User Registration Phase] 
       FIG.  3    is a diagram illustrating an operation in the user registration phase in the authentication system according to the first example embodiment. 
     A user who wishes to receive a service using biological authentication performs user registration in advance. The user determines information (a user ID (Identifier) and a PW (password)) for determining himself or herself in the authentication system and registers the information in the system. In the drawings including  FIG.  3   , the user ID will be denoted as “uID”. 
     In addition, the user registers his or her biological information (for example, a face image) in the system. The user registers the above three items of information (the user ID, the password, and the biological information) in the system by using any means. For example, the user may mail a document in which the above three items of information are written to the authentication center, and an employee at the authentication center may enter the above three items of information to the authentication server  10 . Alternatively, the user may mail an external storage device such as a USB (Universal Serial Bus) memory in which the above three items of information are stored to the authentication center. 
     Alternatively, the user may enter his or her face image captured by operating his or her terminal  40 , a user ID, and a password to the authentication server  10 . Examples of the terminal  40  include a portable terminal device such as a smartphone, a portable phone, a game console, or a tablet and a computer (a personal computer or a laptop computer). 
     The authentication server  10  generates feature values (a feature vector formed by a plurality of feature values) from the acquired face image and stores the feature values, the user ID, and the password in association with each other. Specifically, the authentication server  10  adds a new entry in its authentication information database and stores the above three items of information in association with each other. 
     In this way, in the user registration phase, a first ID (for example, a user ID) that uniquely determines a user in the system and first biological information used for authentication of the user are registered in the system. In the first example embodiment, a user ID and a password are used as the identifier (the first ID) that uniquely determines a user in the system. However, if each user uses a unique user ID, the user ID may be used as the above identifier (the first ID). 
     [Service Registration Phase] 
       FIG.  4    is a diagram illustrating an operation in the service registration phase in the authentication system according to the first example embodiment. 
     Upon completing the user registration, the user selects a service provider from which the user wishes to receive a service using biological authentication and registers the selected service provider in the system. For example, in  FIG.  2   , if the user wishes to receive a service from the service provider S1, the user registers the service provider S1 in the system. 
     The user registers personal information (for example, his or her name, etc.) necessary for receiving a service from the selected service provider in the system. Examples of the personal information include his or her name, age, and gender. In addition to the above personal information, the user registers the user ID and the password determined in the user registration phase in the system. 
     In the disclosure of the present application, the personal information is defined as information that does not include biological information of the user (the authentication target user). That is, biological information and feature values generated from the biological information are excluded from “personal information” in the disclosure of the present application. 
     The user enters the above three items of information (the personal information, the user ID, and the password) to the selected service provider by using any means. For example, the user mails a medium (a paper medium, an electronic medium) in which the above three items of information are written to the selected service provider. An employee of the service provider enters the above three items of information to the corresponding management server  20 . The user may enter the above three items of information to the corresponding management server  20  by operating an authentication terminal  30  installed at the service provider. 
     Alternatively, as illustrated in  FIG.  4   , the user enters the above three items of information to the management server  20  by operating his or her terminal  40 . In this case, the user enters the above three items of information on a web page managed and operated by the service provider. 
     After the management server  20  acquires the above three items of information (the personal information, the user ID, and the password), the management server  20  transmits a “service registration request” to the authentication server  10 . Specifically, the management server  20  transmits a service registration request including a service provider ID, the user ID, and the password to the authentication server  10 . 
     The service provider ID is identification information for uniquely identifying the service provider (for example, a retail store participating in the authentication infrastructure using biological authentication) included in the authentication system. In the example in  FIG.  2   , a different service provider ID is assigned to each of the service providers S1 and S2. 
     An individual service provider ID is an ID assigned to each service provider, not to each service. For example, in  FIG.  2   , even if the service providers S1 and S2 provide the same kind of service (for example, an accommodation service), if the service providers S1 and S2 are different business operators, different IDs are assigned to these service providers. 
     The authentication server  10  and the management server  20  share the service provider ID by using any method. For example, when a service provider participates in the authentication infrastructure, the authentication server  10  can generate a service provider ID and distribute (transmit) the generated service provider ID to the service provider. In the drawings including  FIG.  4   , the service provider ID will be denoted as “spID”. 
     Upon receiving the service registration request, the authentication server  10  searches its authentication information database by using the user ID and the password included in the request as a key and determines the corresponding user. Thereafter, the authentication server  10  generates a “service user ID”. 
     The service user ID is identification information that uniquely identifies the correspondence relationship between (a combination of) the user and the service provider. For example, in the example in  FIG.  2   , a value that is set as a service user ID defined by a combination of a user U1 and the service provider S1 and a value that is set as a service user ID defined by a combination of the user U1 and the service provider S2 are different values. 
     The authentication server  10  stores the user ID, the password, the feature values, the service provider ID, and the above-generated service user ID in association with each other. In the drawings including  FIG.  4   , the service user ID will be denoted as “suID”. 
     The authentication server  10  transmits the above-generated service user ID to the management server  20  that has transmitted the service registration request. The authentication server  10  gives the service user ID to the management server  20  by transmitting a response including the service user ID to the management server  20 . 
     The management server  20  stores the service user ID acquired from the authentication server  10  and the personal information of the user in association with each other. The management server  20  adds a new entry in a user information database and stores the above information (the personal information and the service user ID). 
     The user repeats the registration operation as described above for each service provider from which the user wishes to receive a service using biological authentication. In other words, the user does not perform the registration operation for service providers from which the user does not wish to receive services. 
     As described above, in the service registration phase, the service provider of a service that the user wishes to use transmits a service registration request including a first ID (for example, the user ID) and a second ID (for example, the service provider ID) to the authentication server  10 . When processing the service registration request, the authentication server  10  generates a third ID (for example, the service user ID) uniquely determined by a combination of the user and the service provider. The authentication server  10  transmits the third ID to the service provider. The service provider (the management server  20 ) stores the personal information of the user and the third ID in association with each other. 
     [Service Provision Phase] 
       FIG.  5    is a diagram illustrating an operation in the service provision phase in the authentication system according to the first example embodiment. 
     Upon completion of the registration of a service (the service registration phase), the user visits the corresponding service provider. The user stands in front of an authentication terminal  30 . 
     The authentication terminal  30  acquires biological information from the user standing in front of the authentication terminal  30 . Specifically, the authentication terminal  30  captures an image of the user and acquires a face image. The authentication terminal  30  transmits the acquired face image to the management server  20 . 
     The management server  20  generates feature values from the acquired face image. The management server  20  transmits an authentication request including the generated feature values and its service provider ID to the authentication server  10 . 
     The authentication server  10  extracts the feature values from the authentication request and performs matching processing (1-to-N processing; N will hereinafter denote a positive integer) by using the extracted feature values and the feature values registered in the authentication information database. 
     The authentication server  10  determines the user by performing the matching processing and determines the service user ID corresponding to the service provider ID included in the authentication request among the plurality of service user IDs associated with the determined user. 
     The authentication server  10  transmits the determined service user ID to the management server  20  that has transmitted the authentication request. The authentication server  10  transmits a response including the determined service user ID (a response to the authentication request) to the management server  20 . 
     The management server  20  searches its user information database by using the acquired service user ID as a key and determines the personal information corresponding to the service user ID. The service provider (the management server  20 , the authentication terminal  30 ) provides the user with a service based on the determined personal information (for example, a payment procedure, a check-in procedure, etc.). 
     As described above, in the service provision phase, the authentication server  10  receives an authentication request including second biological information of a user and a second ID (a service provider ID) from a service provider. The authentication server  10  determines a third ID (a service user ID) by using the first and second biological information and the second ID. The authentication server  10  transmits the determined third ID to the service provider. When providing a service to the user, the management server  20  determines the personal information of the user by using the third ID acquired by transmitting the authentication request to the authentication server  10 . 
     The service provider provides a service to the user by using the determined personal information. 
     Next, details of the individual apparatuses included in the authentication system according to the first example embodiment will be described. 
     [Authentication Server] 
       FIG.  6    is a diagram illustrating an example of a processing configuration (processing modules) of the authentication server  10  according to the first example embodiment. As illustrated in  FIG.  6   , the authentication server  10  includes a communication control unit  201 , a user registration unit  202 , a database management unit  203 , a service registration unit  204 , an authentication unit  205 , and a storage unit  206 . 
     The communication control unit  201  is means for controlling communication with other apparatuses. Specifically, the communication control unit  201  receives data (packets) from the management servers  20 . In addition, the communication control unit  201  transmits data to the management servers  20 . The communication control unit  201  gives data received from other apparatuses to other processing modules. The communication control unit  201  transmits data acquired from other processing modules to other apparatuses. In this way, other processing modules transmit and receive data to and from other apparatuses via the communication control unit  201 . 
     The user registration unit  202  is means for realizing the above user registration. The user registration unit  202  acquires the user ID, the password, and the biological information (a face image) of a user (a user who wishes to receive a service using biological authentication; a system user). 
     The user registration unit  202  acquires the above three items of information (the user ID, the password, and the biological information) by using any means. For example, the user registration unit  202  displays a GUI (Graphical User Interface) or an input form on which the user ID and the password are determined on a terminal  40 . For example, the user registration unit  202  displays a GUI as illustrated in  FIG.  7    on the terminal  40 . 
     The user registration unit  202  determines whether the user ID and password acquired on the GUI or the like have already been registered. If the user ID and password have not been registered yet, the user registration unit  202  displays a GUI for acquiring biological information of the user on the terminal  40 . 
     For example, the user registration unit  202  displays a GUI as illustrated in  FIG.  8    on the terminal  40 . For example, the user presses a “Select File” button illustrated in  FIG.  8    and specifies the image data of a face image to be registered in the system. The specified face image is displayed on a preview area (displayed as “Selected Face Image” in  FIG.  8   ). To register a previewed face image, the user presses the “OK” button. 
     For example, the user registration unit  202  acquires the user ID, the password, and the biological information (a face image) on the GUIs as illustrated in  FIGS.  7    and  FIG.  8    and generates feature values (a feature vector formed by a plurality of feature values) from the face image. 
     Specifically, the user registration unit  202  extracts feature points from the acquired face image. An existing technique can be used to extract the feature points, and therefore, detailed description thereof will be omitted. For example, the user registration unit  202  extracts the eyes, nose, mouth, etc. from the face image as the feature points. Next, the user registration unit  202  calculates the location of an individual feature point and the distance between feature points as feature values and generates a feature vector formed by the plurality of feature values (vector information that characterizes the face image). 
     The user registration unit  202  gives the user ID, the password, and the feature values generated as described above to the database management unit  203 . 
     The database management unit  203  is means for managing the authentication information database. The authentication information database stores information (a user ID and a password) that determines a system user, biological information (feature values) of the user, a service provider ID that determines a service provider, and a service user ID that determines the user in a corresponding service in association with each other. 
     When the database management unit  203  acquires the above three items of information (the user ID, the password, and the feature values) from the user registration unit  202 , the database management unit  203  adds a new entry in the authentication information database. For example, when the database management unit  203  acquires the above three items of information about the user U1, the database management unit  203  adds an entry illustrated in the lowest row in  FIG.  9   . At the time of the user registration, since the service provider ID and the service user ID have not been generated yet, no information is set in these fields. 
     The service registration unit  204  is means for allowing a system user to register an individual service. The service registration unit  204  processes a service registration request acquired from the management server  20  of a service provider. 
     The service registration unit  204  searches the authentication information database by using the user ID and the password included in an acquired service registration request as a key. The service registration unit  204  checks the service provider ID field corresponding to the determined user (the user determined from a combination of the user ID and the password). 
     The service registration unit  204  determines whether the service provider ID included in the service registration request acquired from the management server  20  is set in the service provider ID field. If the service provider ID acquired from the management server  20  is already registered in the database, the service registration unit  204  notifies the management server  20  to that effect. In this case, since the service (the service provider) that the user wishes to register is already registered in the authentication information database, the service registration unit  204  transmits a “negative response” as a response to the service registration request. 
     In contrast, if the service provider ID included in the service registration request is not set in the service provider ID field corresponding to the determined user, the service registration unit  204  generates a service user ID corresponding to the user and the service provider. 
     As described above, a service user ID is identification information uniquely determined from a combination of a user and a service provider. For example, the service registration unit  204  calculates a hash value by using a user ID, a password, and a service provider ID and uses the calculated hash value as a service user ID. Specifically, the service registration unit  204  calculates a combined value of a user ID, a password, and a service provider ID, calculates a hash value of the combined value calculated, and uses the hash value as a service user ID. 
     The above generation of a service user ID by using a hash value is only an example. The method for generating a service user ID is not limited to the above generation method. A service user ID may be any information that can uniquely identify a combination of a system user and a service provider. For example, each time the service registration unit  204  processes a service registration request, the service registration unit  204  may generate a unique number and use this unique number as a service user ID. 
     After generating a service user ID, the service registration unit  204  gives the user ID, the password, the service provider ID, and the service user ID to the database management unit  203 . The database management unit  203  registers the two IDs (the service provider ID and the service user ID) in the authentication information database. For example, if the user U1 registers a service of the service provider S1, the above two IDs are added in an entry illustrated in the lowest row in  FIG.  10   . 
     Since the service registration is performed per service provider, there are cases in which a plurality of service providers and a plurality of service user IDs are set for a single user. For example, if the user U1 registers a service of the service provider S1 and a service of the service provider S2, the entries in the second row and the third row in  FIG.  11    are generated. If the user U2 registers a service of the service provider S1, an entry in the lowest row in  FIG.  11    is generated. 
     The authentication information database illustrated in  FIG.  11   , etc. is an example. The information stored in the authentication information database is not limited to the information illustrated in  FIG.  11   , etc. For example, in place of feature values for authentication, a face image may be registered in the authentication information database. That is, at each authentication, feature values may be generated from a face image registered in the authentication information database. 
     When a service provider ID and a service user ID are registered in the authentication information database, the service registration unit  204  notifies the management server  20  that the service registration request has been processed successfully. The service registration unit  204  transmits a “positive response” as a response to the service registration request. Specifically, the service registration unit  204  transmits a response including the service user ID to the management server  20 . 
     The authentication unit  205  is means for performing authentication processing for system users. The authentication unit  205  processes the authentication requests received from the management servers  20  of the service providers. 
     The authentication unit  205  extracts the feature values and the service provider ID from an authentication request. The authentication unit  205  searches the authentication information database by using the extracted feature values and service provider ID as a key and determines a corresponding service user ID. 
     The authentication unit  205  performs 1-to-N matching by setting the feature values extracted from an authentication request as the matching target feature values and by setting the feature values stored in the database as the registered feature values. Specifically, the authentication unit  205  calculates the similarity between a set of matching target feature values and each of a plurality of sets of registered feature values. For the individual similarity, the chi-squared distance, the Euclidean distance, or the like may be used. A longer distance represents a lower similarity, and a shorter distance represents a higher similarity. 
     The authentication unit  205  determines whether, among the plurality of sets of feature values registered in the database, there is at least one set of feature values whose similarity to the set of matching target feature values is a predetermined value or more. Next, if the authentication unit  205  finds a plurality of sets of feature values whose similarity to the set of matching target feature values is a predetermined value or more, the authentication unit  205  determines a set of feature values whose similarity is the highest among the plurality of sets of feature values. If the authentication unit  205  finds such set of feature values, the authentication unit  205  determines whether there is an entry matching the service provider ID included in the authentication request among the at least one service provider ID associated with the user, the at least one service provider ID having been determined by the above 1-to-N matching. 
     If there is an entry as described above (if the authentication unit  205  succeeds in the above two determination operations), the authentication unit  205  determines that the authentication of the user has succeeded. Specifically, the authentication unit  205  transmits a “positive response” to the management server  20  that has transmitted the authentication request. In this case, the authentication unit  205  generates a response including the service user ID in the determined entry (a response to the authentication request) and transmits the response to the management server  20 . 
     If at least one of the above two determination operations fails, the authentication unit  205  determines that the authentication of the user has failed. In this case, the authentication unit  205  transmits a “negative response” to the management server  20  that has transmitted the authentication request. 
     For example, in the example in  FIG.  11   , if feature values “FV1” and a service provider ID “S1” are included in an authentication request, the entries (a user) in the second and third rows are first determined by the feature values FV1, and the entry in the second row is next determined by the service provider ID “S1”. As a result, the above authentication request is processed successfully, and a positive response including a service user ID “U1S1” is transmitted to the management server  20 . 
     In contrast, if feature values “FV2” and a service provider ID “S2” are included in an authentication request, the entry in the lowest row is determined by the feature values. However, since the service provider ID in the entry indicates “S1”, not “S2”, the above authentication request is not processed successfully. As a result, a negative response is transmitted to the management server  20 . 
     The storage unit  206  stores information necessary for operations of the authentication server  10 . The authentication information database is established in the storage unit  206 . 
     [Management Server] 
       FIG.  12    is a diagram illustrating an example of a processing configuration (processing modules) of a management server  20  according to the first example embodiment. As illustrated in  FIG.  12   , the management server  20  includes a communication control unit  301 , a personal information acquisition unit  302 , a service registration request unit  303 , a database management unit  304 , an authentication request unit  305 , and a storage unit  306 . 
     The communication control unit  301  is means for controlling communication with other apparatuses. Specifically, the communication control unit  301  receives data (packets) from the authentication server  10  and the corresponding authentication terminals  30 . In addition, the communication control unit  301  transmits data to the authentication server  10  and the corresponding authentication terminals  30 . The communication control unit  301  gives data received from other apparatuses to other processing modules. The communication control unit  301  transmits data acquired from other processing modules to other apparatuses. In this way, other processing modules transmit and receive data to and from other apparatuses via the communication control unit  301 . 
     The personal information acquisition unit  302  is means for acquiring personal information needed when the corresponding service provider provides a service. For example, when the service provider is a “retail store”, the personal information acquisition unit  302  acquires, in addition to the name of a user, etc., information about payment (for example, credit card information or bank account information). When the service provider is a “hotel operator”, the personal information acquisition unit  302  acquires, in addition to the name, etc., reservation information about accommodation (for example, accommodation date, etc.). 
     The personal information acquisition unit  302  acquires, in addition to the above personal information such as the name, the user ID and the password that the user has determined at the time of the system registration. 
     The personal information acquisition unit  302  acquires the personal information, the user ID, and the password by using any means. For example, the personal information acquisition unit  302  displays a GUI or a form for entering the above information on a terminal  40  (see  FIG.  13   ). Alternatively, information as illustrated in  FIG.  13    may be displayed on a web page managed and operated by the corresponding service provider. Alternatively, a terminal  40  may download an application provided by the service provider, and the application may display a GUI or a form as illustrated in  FIG.  13   . In particular, the web page may be a web page for managing information about the members of the service provider. That is, the members of an individual service provider may perform service registration on a web page for managing information about themselves. 
     The personal information acquisition unit  302  gives the personal information, the user ID, and the password acquired by using a GUI or the like to the service registration request unit  303 . 
     The service registration request unit  303  is means for requesting (asking) the authentication server  10  to register a service to be used by a user. 
     The service registration request unit  303  selects the user ID and the password from the above three items of information (the personal information, the user ID, and the password) acquired from the personal information acquisition unit  302 . The service registration request unit  303  transmits a service registration request including the selected user ID and password and the corresponding service provider ID to the authentication server  10 . 
     The service registration request unit  303  acquires a response to the service registration request from the authentication server  10 . If the acquired response is a “negative response”, the service registration request unit  303  notifies the user to that effect. For example, the service registration request unit  303  notifies the user that the service registration has already been performed. 
     If the acquired response is a “positive response”, the service registration request unit  303  notifies the user that the service registration has succeeded. In addition, the service registration request unit  303  gives the service user ID included in the response and the personal information acquired from the personal information acquisition unit  302  to the database management unit  304 . 
     The database management unit  304  is means for managing the user information database. The user information database is a database for managing information about the service provision target users (the system users). The user information database stores personal information (for example, the name, etc.) of an individual user and an individual service user ID acquired from the authentication server  10  in association with each other. 
     Upon acquiring the above information (the personal information and the service user ID) from the service registration request unit  303 , the database management unit  304  adds a new entry in the user information database. For example, when the management server  20  of the service provider S1 acquires the above information about the user U1, an entry illustrated in the lowest row in  FIG.  14    is added. 
     The authentication request unit  305  is means for requesting the authentication server  10  to perform the authentication of the user. 
     Upon acquiring biological information (a face image) from a corresponding authentication terminal  30 , the authentication request unit  305  generates feature values from the face image. The authentication request unit  305  transmits an authentication request including the generated feature values and the corresponding service provider ID to the authentication server  10 . 
     If the response from the authentication server  10  is a “negative response” (if the authentication fails), the authentication request unit  305  notifies the authentication terminal  30  to that effect. 
     If the response from the authentication server  10  is a “positive response” (if the authentication succeeds), the authentication request unit  305  extracts the service user ID included in the response transmitted from the authentication server  10 . The authentication request unit  305  searches the user information database by using the service user ID as a key and determines a corresponding entry. 
     The authentication request unit  305  reads out the personal information set in the personal information field of the determined entry and transmits the personal information to the authentication terminal  30 . For example, in the example in  FIG.  14   , if the service user ID is “U1S1”, the personal information in the lowest row is transmitted to the authentication terminal  30 . 
     The storage unit  306  stores information necessary for operations of the management server  20 . The user information database is established in the storage unit  306 . 
     [Authentication Terminal] 
     An authentication terminal  30  acquires personal information of a user from the corresponding management server  20  by transmitting biological information acquired from the user to the management server  20 . The authentication terminal  30  provides a service to the user by using the acquired personal information. 
       FIG.  15    is a diagram illustrating an example of a processing configuration (processing modules) of an authentication terminal  30  according to the first example embodiment. As illustrated in  FIG.  15   , the authentication terminal  30  includes a communication control unit  401 , a biological information acquisition unit  402 , a service provision unit  403 , a message output unit  404 , and a storage unit  405 . 
     The communication control unit  401  is means for controlling communication with other apparatuses. Specifically, the communication control unit  401  receives data (packets) from the corresponding management server  20 . In addition, the communication control unit  401  transmits data to the corresponding management server  20 . The communication control unit  401  gives data received from other apparatuses to other processing modules. The communication control unit  401  transmits data acquired from other processing modules to other apparatuses. In this way, other processing modules transmit and receive data to and from other apparatuses via the communication control unit  401 . 
     The biological information acquisition unit  402  is means for acquiring biological information (a face image) of a user by controlling a camera. The biological information acquisition unit  402  captures an image of a person in front of the authentication terminal  30  regularly or at predetermined timing. The biological information acquisition unit  402  determines whether a face image of the person is included in the acquired image. If a face image is included in the acquired image, the face image is extracted from the acquired image data. 
     An existing technique can be used for the face image detection and extraction processing performed by the biological information acquisition unit  402 , and therefore, detailed description thereof will be omitted. For example, the biological information acquisition unit  402  may extract a face image (a face area) from the image data by using a learning model learned by a CNN (Convolutional Neural Network). Alternatively, the biological information acquisition unit  402  may extract a face image by using a technique such as template matching. 
     The biological information acquisition unit  402  gives the extracted face image to the service provision unit  403 . 
     The service provision unit  403  is means for providing a predetermined service to the user. The service provision unit  403  transmits a face image acquired from the biological information acquisition unit  402  to the management server  20 . The management server  20  transmits, as a reply, personal information (for example, the name, etc.) corresponding to the face image. The service provision unit  403  provides a service to the user by using the personal information transmitted as a reply. 
     The message output unit  404  is means for outputting various messages to the user. For example, the message output unit  404  outputs a message relating to a user authentication result and a message relating to a service to be provided. The message output unit  404  may display a message by using a display device such as a liquid crystal monitor or plays an audio message by using an audio device such as a speaker. 
     The storage unit  405  stores information necessary for operations of the authentication terminal  30 . 
     [System Operation] 
     Next, operations in the authentication system according to the first example embodiment will be described. Hereinafter, while operations in the service registration phase and the service provision phase will be described, an operation in the user registration phase will be omitted. 
       FIG.  16    is a sequence diagram illustrating an example of an operation relating to the service registration phase in the authentication system according to the first example embodiment. 
     A management server  20  acquires personal information (information necessary for providing a service), a user ID, and a password from a user (step S 01 ). 
     The management server  20  transmits a service registration request including the acquired user ID and password and its service provider ID to the authentication server  10  (step S 02 ). 
     The authentication server  10  generates a service user ID by using the acquired user ID, password, and service provider ID (step S 03 ). 
     The authentication server  10  stores the service provider ID and the service user ID in its authentication information database (step S 04 ). 
     The authentication server  10  transmits a response including the service user ID (a response to the service registration request) to the management server  20  (step S 05 ). 
     The management server  20  stores the personal information acquired in step S 01  and the service user ID acquired from the authentication server  10  in association with each other in its user information database (step S 06 ). 
       FIG.  17    is a sequence diagram illustrating an example of an operation relating to the service provision phase in the authentication system according to the first example embodiment. Since the service registration phase has already been completed at the time of the service provision phase, the authentication server  10  already holds, as the first biological information, the first feature values generated from a face image of the user. 
     An authentication terminal  30  acquires a face image (biological information) of the user and transmits the acquired face image to the management server  20  (step S 11 ). 
     The management server  20  generates feature values (second feature values) from the acquired face image (step S 12 ). The management server  20  handles the generated feature values as the second biological information. 
     The management server  20  transmits an authentication request including the generated feature values and its service provider ID to the authentication server  10  (step S 13 ). 
     The authentication server  10  performs authentication processing by using the feature values and the service provider ID included in the authentication request and determines the corresponding service user ID (step S 14 ). 
     The authentication server  10  transmits a response including the determined service user ID (a response to the authentication request) to the management server  20  (step S 15 ). 
     The management server  20  searches its user information database by using the acquired service user ID and determines the corresponding personal information (step S 16 ). 
     The management server  20  transmits the determined personal information to the authentication terminal  30  (step S 17 ). 
     The authentication terminal  30  provides a service by using the acquired personal information (step S 18 ). 
     As described above, in the authentication system according to the first example embodiment, while the biological information about the user is stored in the authentication server  10 , the biological information is not stored in the service provider. In addition, while the personal information about the user is stored in the management server  20  managed and operated by the service provider, the personal information is not stored in the authentication server  10 . In the authentication system according to the first example embodiment, the above items of information are stored at separate locations. Thus, the authentication system provides an authentication infrastructure that is robust against information leakage. That is, biological information (in particular, feature values) not associated with personal information is only a series of numerical values and is not valuable to criminals, etc. Thus, even if information is leaked from the authentication server  10 , the impact is limited. With the configuration as described above, the participants in the authentication system (the users receiving services and the service providers providing these services) can securely use the authentication system. 
     Second Example Embodiment 
     Next, a second example embodiment will be described in detail with reference to drawings. 
     In the first example embodiment, biological information used for the authentication of a user is stored in the authentication center, and personal information needed when the user uses a service is stored in a service provider. With this configuration, an authentication system highly resistant to information leakage (robust against information leakage) is established. 
     However, with the configuration according to the first example embodiment, there is still a possibility that the personal information is leaked from the service provider. In the second example embodiment, an authentication system highly resistant to information leakage from the service provider will be described. 
     As the authentication system according to the second example embodiment can have the same configuration as that according to the first example embodiment, the description corresponding to  FIG.  2    will be omitted. In addition, as the authentication server  10 , the individual management server  20 , and the individual authentication terminal  30  according to the second example embodiment can have the same processing configurations as those according to first example embodiment, the description thereof will be omitted. 
     The following description will be made with a focus on the difference between the first and second example embodiments. 
     The authentication system according to the second example embodiment differs from the authentication system according to the first example embodiment in the operations in the service registration phase and the service provision phase. 
     A schematic operation in the service registration phase according to the second example embodiment will be described with reference to  FIG.  18   . Upon receiving a service registration request from a management server  20 , the authentication server  10  generates an encryption key. The generated encryption key is associated with the corresponding user ID, password, service provider ID, and service user ID, and the associated data is stored in the authentication information database. The authentication server  10  transmits the generated encryption key and the service user ID to the management server  20 . The management server  20  generates encrypted text Enc of the corresponding personal information acquired from the user by using the acquired encryption key and stores the encrypted personal information and the service user ID in association with each other. The management server  20  deletes the personal information in plain text. 
     A schematic operation in the service provision phase according to the second example embodiment will be described with reference to  FIG.  19   . Upon receiving an authentication request from a management server  20 , the authentication server  10  searches its authentication information database and determines the corresponding service user ID and encryption key. The authentication server  10  transmits a response including the determined service user ID and encryption key (a response to the authentication request) to the management server  20 . The management server  20  searches its user information database by using the acquired service user ID as a key and determines the corresponding personal information (encrypted personal information). The management server  20  decrypts the encrypted personal information by using the encryption key acquired from the authentication server  10 . The management server  20  transmits the corresponding personal information in plain text to a corresponding authentication terminal  30 . The authentication terminal  30  provides a service by using the personal information. 
     Next, detailed operations of the authentication server  10  and the individual management server  20  according to the second example embodiment will be described. 
     When the service registration unit  204  in the authentication server  10  receives a service registration request (a request including a user ID, a password, and a service provider ID) from a management server  20 , the service registration unit  204  generates an encryption key. The encryption key is a key for encrypting the personal information entered by the user to the corresponding service provider and for decrypting the encrypted personal information. The service registration unit  204  generates an encryption key for an individual pair of a user and a service provider. 
     For example, the service registration unit  204  generates a common key, which is a key that enables both encryption and decryption. For example, the service registration unit  204  calculates a hash value, which is a value obtained by combining a user ID, a password, and a service provider ID. The service registration unit  204  generates a common key by using the calculated hash value as a “seed”. 
     The user ID and the password are each a value unique to each user, and the service provider ID is a value unique to each service provider. Thus, a hash value (a seed for generating a key) calculated from a combined value of these values is a value unique to a combination of a user and a service provider. An encryption key calculated from a “seed”, which is generated for each combination of a user and a service provider, is also a value unique to each combination of a user and a service provider. 
     The service registration unit  204  gives the generated encryption key, etc. to the database management unit  203 . 
     The database management unit  203  updates the authentication information database by using the acquired encryption key. For example, the authentication information database according to the second example embodiment includes an encryption key field as illustrated in  FIG.  20   . 
     As illustrated in  FIG.  20   , an encryption key K(U1S1) is generated for a combination of the user U1 and the service provider S1, and an encryption key K(U1S2) is generated for a combination of the user U1 and the service provider S2. 
     The service registration unit  204  transmits a response including the service user ID and the encryption key generated in response to the service registration request to the management server  20  that has transmitted the service registration request. 
     The service registration request unit  303  in the management server  20  receives the response to the service registration request. The service registration request unit  303  extracts the encryption key from the response and encrypts the personal information (for example, the name, etc.) of the user. The service registration request unit  303  gives the encrypted personal information and the service user ID to the database management unit  304 . Next, the service registration request unit  303  deletes the original personal information (the personal information in plain text) and the encryption key. 
     The database management unit  304  adds an entry including the encrypted personal information and the service user ID in the user information database. The user information database stores the encrypted personal information and the service user ID (a third ID) in association with each other. 
     When the authentication unit  205  of the authentication server  10  receives an authentication request (an authentication request including the feature values of a user and a service provider ID) from a management server  20 , the authentication unit  205  performs authentication processing by using these items of information. If the authentication succeeds, the service user ID and an encryption key that correspond to the combination of the user and the service provider are determined. 
     The authentication unit  205  transmits a response including the determined service user ID and encryption key (a response to the authentication request) to the management server  20 . 
     The authentication request unit  305  of the management server  20  searches the user information database by using the acquired service user ID as a key and acquires the corresponding personal information (encrypted personal information). 
     The authentication request unit  305  decrypts the encrypted personal information by using the encryption key (common key) acquired from the authentication server  10 . The authentication request unit  305  transmits the personal information in plain text to the authentication terminal  30 . If the authentication request unit  305  succeeds in decrypting the personal information, the authentication request unit  305  deletes (discards) the encryption key acquired from the authentication server  10 . 
     It is desirable that the authentication terminal  30  delete the personal information in plain text after providing a service to the user. 
     As described above, the authentication server  10  according to the second example embodiment generates an encryption key in response to a service registration request and transmits the generated encryption key and service user ID (third ID) to the corresponding management server  20 . The management server  20  encrypts the personal information of the user by using the acquired encryption key and stores the encrypted personal information and the service user ID in association with each other. In addition, the authentication server  10  receives an authentication request including biological information of a user and a service provider ID (second ID) from a service provider. The authentication server  10  determines a service user ID (third ID) and an encryption key by using the two items of biological information and a service provider ID. The authentication server  10  transmits the determined service user ID and encryption key to the corresponding service provider. When the corresponding management server  20  provides a service to the user, the management server  20  determines the encrypted personal information by using the service user ID acquired by transmitting an authentication request to the authentication server  10 . In addition, the management server  20  decrypts the encrypted personal information by using the encryption key acquired from the authentication server  10 . The service is provided by using the decrypted personal information. 
     As described above, according to the second example embodiment, the personal information held by the individual service provider is encrypted. Even if the encrypted personal information is leaked, not a serious problem is caused. This is because security and privacy problems occur when the encrypted personal information is decrypted and the content thereof is acquired by a third party. The encryption keys to decrypt the encrypted personal information are stored in the authentication server  10 , and each time a service is provided to a user, an encryption key is transmitted from the authentication server  10  to the corresponding service provider. As the authentication system according to the second example embodiment is configured in this way, it is hard to assume a situation in which information leakage from a management server  20  and information leakage from the authentication server  10  occur simultaneously (during the same period of time). Thus, even if encrypted personal information is leaked, if the encryption key is protected by the authentication center, this information leakage will not be a serious problem. In addition, since the management server  20  deletes the encryption key after the personal information is decrypted, the service provider itself cannot access the personal information. Therefore, the employees, etc. of the service provider are prevented from improperly acquiring the personal information, for example. 
     Third Example Embodiment 
     Next, a third example embodiment will be described in detail with reference to drawings. 
     According to the second example embodiment, a service provider encrypts personal information, and the authentication server  10  holds an encryption key for decrypting the encrypted text. According to the second example embodiment, since the encrypted text and the encryption key are separately stored, an authentication system highly resistant to leakage of the personal information can be provided. However, for example, improvement in information processing technology could break the encryption of encrypted text leaked from a management server  20 . That is, it is not desirable for even encrypted personal information to be leaked from a service provider. 
     According to a third example embodiment, the authentication server  10  holds encrypted personal information, and the management servers  20  hold the corresponding encryption keys. In this way, an authentication system highly resistant to leakage of personal information can be provided. 
     As the authentication system according to the third example embodiment can have the same configuration as that according to the first example embodiment, the description corresponding to  FIG.  2    will be omitted. In addition, as the authentication server  10 , the management servers  20 , and the authentication terminals  30  according to the third example embodiment can have the same processing configurations as those according to the first example embodiment, description thereof will be omitted. 
     Hereinafter, the third example embodiment will be described with a focus on the difference from the first and second example embodiments. 
     The authentication system according to the third example embodiment differs from those according to the first and second example embodiments in the operations in the service registration phase and the service provision phase. 
     A schematic operation in the service registration phase according to the third example embodiment will be described with reference to  FIG.  21   . The authentication server  10  generates an encryption key for encrypting personal information of a user. The generated encryption key is transmitted to a management server  20 , along with a service user ID. 
     The management server  20  generates encrypted text of the personal information by using the acquired encryption key and transmits a personal information registration request including the generated encrypted text (encrypted text Enc illustrated in  FIG.  21   ) and the service user ID to the authentication server  10 . The authentication server  10  searches its authentication information database by using the service user ID as a key and determines a corresponding entry. The authentication server  10  stores the encrypted personal information in the determined entry. After transmitting the personal information registration request, the management server  20  deletes the personal information in plain text. In addition, the management server  20  stores the encryption key and the service user ID acquired from the authentication server  10  in association with each other in its user database. 
     A schematic operation in the service provision phase according to the third example embodiment will be described with reference to  FIG.  22   . Upon receiving an authentication request, the authentication server  10  determines a corresponding service user ID and encrypted personal information by using its authentication information database. The authentication server  10  transmits a response (a response to the authentication request) including the determined service user ID and personal information to the management server  20 . The management server  20  searches its user information database by using the acquired service user ID as a key and determines a corresponding encryption key. The management server  20  decrypts the personal information acquired from the authentication server  10  by using the determined encryption key. The management server  20  transmits the personal information in plain text to an authentication terminal  30 . The authentication terminal  30  provides a service by using the personal information. 
     Next, detailed operations of the authentication server  10  and the individual management server  20  according to the third example embodiment will be described. 
     When the service registration unit  204  of the authentication server  10  receives a service registration request (a request including a user ID, a password, and a service provider ID) from a management server  20 , the service registration unit  204  generates an encryption key (a common key) according to the second example embodiment. 
     The service registration unit  204  transmits a response including a service user ID and an encryption key generated in response to the service registration request to the management server  20  that has transmitted the service registration request. 
     The service registration request unit  303  of the management server  20  receives a response to the service registration request. The service registration request unit  303  extracts the encryption key from the response and encrypts personal information (for example, the name, etc.) of the user. After encrypting the personal information, the service registration request unit  303  deletes the personal information in plain text. 
     The service registration request unit  303  generates a “personal information registration request” including the encrypted personal information and the service user ID and transmits the “personal information registration request” to the authentication server  10 . 
     In addition, the service registration request unit  303  gives the service user ID and the encryption key acquired from the authentication server  10  to the database management unit  304 . The database management unit  304  adds a new entry including the service user ID and the encryption key in the user information database (see  FIG.  23   ). As illustrated in  FIG.  23   , the user information database stores the service user ID and the encryption key in association with each other. 
     The service registration unit  204  of the authentication server  10  receives the personal information registration request. The service registration unit  204  gives the personal information registration request to the database management unit  203 . 
     The database management unit  203  extracts the service user ID from the personal information registration request and searches the authentication information database by using the ID as a key. The database management unit  203  stores the encrypted personal information in the determined entry. As a result, for example, an authentication information database having a personal information field as illustrated in  FIG.  24    is obtained. The authentication information database stores the encrypted personal information in association with the biological information of the user, the user ID, etc. 
     As illustrated in  FIG.  24   , encrypted text Enc(U11) of personal information entered by the user U1 to the service provider S1 and encrypted text Enc(U12) of personal information entered by the user U1 to the service provider S2 are registered in the database. 
     When the authentication unit  205  of the authentication server  10  receives an authentication request (an authentication request including feature values of the user and the service provider ID) from the management server  20 , the authentication unit  205  performs authentication processing by using these items of information. If the authentication unit  205  succeeds in the authentication, the service user ID and the encrypted personal information corresponding to the combination of the user and the service provider are determined. 
     The authentication unit  205  transmits a response (a response to the authentication request) including the determined service user ID and encrypted personal information to the management server  20 . 
     The authentication request unit  305  of the management server  20  searches the user information database by using the acquired service user ID as a key and determines a corresponding encryption key. 
     The authentication request unit  305  decrypts the encrypted personal information acquired from the authentication server  10  by using the encryption key. The authentication request unit  305  transmits the personal information in plain text to a corresponding authentication terminal  30 . If the authentication request unit  305  succeeds in decrypting the personal information, the authentication request unit  305  deletes (discards) the encrypted personal information acquired from the authentication server  10 . 
     As described above, in the authentication system according to the third example embodiment, the encrypted personal information is stored in the authentication server  10 , and the encryption key for decrypting the personal information is stored in the management server  20 . 
     Normally, robust measures against unauthorized access and the like by a third party are taken for the authentication server  10  installed at an authentication center. In contrast, there are cases in which measures against the above unauthorized access and the like are insufficient for a management server  20  installed at a service provider. Thus, there is a difference in security strength between the authentication server  10  and the management server  20 , and personal information is stored in the authentication server  10  having a higher security strength (it is hard to assume that information will leak from the authentication server  10 ). 
     Thus, compared with the second example embodiment in which the management server  20  stores encrypted personal information, the risk of leakage of encrypted personal information is lower. In addition, while the management server  20  stores the encryption key and the service user ID, such information does not give any useful information to a third party. Thus, even if information is leaked from the management server  20  having a lower security strength, not a serious problem will be caused. 
     Next, a hardware configuration of an individual apparatus that constitutes the authentication system will be described.  FIG.  25    is a diagram illustrating an example of a hardware configuration of the authentication server  10 . 
     The authentication server  10  can be configured by an information processing apparatus (a so-called computer) and has a configuration illustrated as an example in  FIG.  25   . For example, the authentication server  10  includes a processor  311 , a memory  312 , an input-output interface  313 , a communication interface  314 , etc. The components such as the processor  211  are connected to an internal bus, etc. so that these components can communicate with each other. 
     The hardware configuration of the authentication server  10  is not limited to the configuration illustrated in  FIG.  25   . The authentication server  10  may include hardware not illustrated or may be configured without the input-output interface  313  if desired. In addition, the number of components, such as the number of processors  311 , included in the authentication server  10  is not limited to the example illustrated in  FIG.  25   . For example, a plurality of processors  311  may be included in the authentication server  10 . 
     For example, the processor  311  is a programmable device such as a CPU (Central Processing Unit), an MPU (Micro Processing Unit), or a DSP (Digital Signal Processor). Alternatively, the processor  311  may be a device such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit). The processor  311  executes various kinds of programs including an operating system (OS). 
     The memory  312  is a RAM (Random Access Memory), a ROM (Read-Only Memory), an HDD (Hard Disk Drive), an SSD (Solid State Drive), or the like. The memory  312  stores an OS program, an application program, and various kinds of data. 
     The input-output interface  313  is an interface for a display device and an input device not illustrated. For example, the display device is a liquid crystal display or the like. For example, the input device is a device, such as a keyboard or a mouse, which receives user operations. 
     The communication interface  314  is a circuit, a module, or the like for performing communication with other apparatuses. For example, the communication interface  314  includes a NIC (Network Interface Card) or the like. 
     The functions of the authentication server  10  are realized by various kinds of processing modules. The processing modules are realized, for example, by causing the processor  311  to execute a program stored in the memory  312 . In addition, this program can be recorded in a computer-readable storage medium. The storage medium may be a non-transient (non-transitory) storage medium, such as a semiconductor memory, a hard disk, a magnetic recording medium, or an optical recording medium. That is, the present invention can be embodied as a computer program product. In addition, the above program may be updated by downloading a program via a network or by using a storage medium in which a program is stored. In addition, the above processing modules may be realized by semiconductor chips. 
     The management servers  20 , the authentication terminals  30 , etc. can each be configured by an information processing apparatus, as in the case of the authentication server  10 . As each of these apparatuses can also have the same basic hardware configuration as that of the authentication server  10 , description thereof will be omitted. For example, the individual authentication terminal  30  includes a camera for capturing an image of an individual user. 
     The authentication server  10  includes a computer and can realize its functions by causing the computer to execute a program. In addition, the authentication server  10  executes an authentication server control method by using this program. 
     [Variations] 
     The configurations, operations, etc. of the authentication systems according to the above example embodiments are examples and do not limit the present system configuration, etc. 
     In the above example embodiments, a user (a system user) determines a user ID and a password, and this user registered in the system is determined by the user ID and the password. However, the authentication system may determine an ID (identifier) that uniquely determines the system user. For example, in the user registration phase, the authentication server  10  acquires biological information (a face image or feature values) of a user. The authentication server  10  may generate an ID based on the biological information. For example, the authentication server  10  may calculate a hash value from feature values of a face image and may use the calculated hash value in place of the user ID and the password. Feature values of a face image differ depending on the user, and a hash value generated from these feature values also differs depending on the user. Thus, the hash value can be used as the ID of the system user. 
     In the above example embodiments, the user registration phase and the service registration phase are performed at different timings. However, these phases may be performed at substantially the same timing. For example, an authentication terminal  30  installed at a service provider from which a user wishes to receive a service may be used for the above registration phases. Specifically, a user may use an authentication terminal  30  to perform user registration (entering biological information, a user ID, and a password) and to continuously perform service registration (entering personal information, the user ID, and the password). In this case, the authentication terminal  30  includes the user registration function (the user registration unit  202 ) of the authentication server  10  and the personal information acquisition function (the personal information acquisition unit  302 ) of the management server  20 . 
     A plurality of authentication terminals  30  of a service provider may not need to be installed on the same premises, in the same building, etc. If a plurality of authentication terminals  30  are managed and operated by the same service provider, these authentication terminals  30  may be installed at locations spatially away from each other. 
     In the above example embodiments, a single service provider ID is assigned to a single service provider. However, a single service provider ID may be assigned to a plurality of service providers. That is, a plurality of service providers may be managed as a group, and a service provider ID may be issued per group. For example, in a case in which the service providers S1 and S2 coordinate with each other and provide the same service, a shared service provider ID may be issued to these service providers S1 and S2. 
     In the above example embodiments, biological information relating to “feature values generated from a face image” is transmitted from a management server  20  to the authentication server  10 . However, biological information relating to “a face image” may be transmitted from a management server  20  to the authentication server  10 . In this case, the authentication server  10  generates feature values from the acquired face image and performs authentication processing (matching processing). 
     In the above example embodiments, an authentication terminal  30  acquires a face image, and the corresponding management server  20  generates feature values from the face image. However, the authentication terminal  30  may generate feature values from a face image and may transmit the generated feature values to the corresponding management server  20 . That is, the management server  20  does not need to generate the feature values. 
     In the above example embodiments, in the service registration phase, when a user registers his or her personal information, the user enters a user ID and a password to a service provider (see  FIG.  13   ). However, in place of the user ID and the password, biological information (a face image) of the user may be entered to the service provider. In this case, the corresponding management server  20  transmits a service registration request including feature values generated from the face image and its service provider ID to the authentication server  10 . The authentication server  10  performs matching processing by using the feature values included in the request and the feature values registered in its authentication information database and determines the corresponding user. If the authentication server  10  succeeds in the user determination (authentication), the authentication server  10  issues a service user ID. In this way, even if the user forgets his or her user ID or password, the user can easily perform the service registration. Alternatively, the service provider may acquire biological information (a face image) of a user, in addition to the user ID and the password. In this case, if the user ID, password, and biological information match, the authentication server  10  may issue a service user ID (two-factor authentication using the biological information and password may be performed). 
     A service provider may cache (temporarily store) information acquired from the authentication server  10  or information acquired from an authentication terminal  30 . For example, a management server  20  caches biological information acquired from an authentication terminal  30  and an authentication result (a service user ID) based on the biological information for a predetermined period. Upon acquiring biological information from an authentication terminal  30 , first the corresponding management server  20  checks the data cached therein. If there is cached data that matches the acquired biological information, the management server  20  does not transmit an authentication request to the authentication server  10 . The management server  20  determines the corresponding personal information by using the service user ID included in the cached data. Alternatively, the management server  20  may cache a combination of biological information and personal information. Alternatively, depending on the kind of the service, a different condition for deleting cached data may be used. For example, if an accommodation service is provided by a hotel company, the management server  20  may delete corresponding cached data when the duration of the stay of the guest ends. 
     In the above example embodiments, while the authentication server  10  generates an encryption key, the individual management server  20  may generate an encryption key. In this case, in the case of the second example embodiment, a management server  20  generates an encryption key and encrypts personal information. The management server  20  stores the encrypted personal information and the corresponding service user ID in association with each other. Next, the management server  20  transmits the encryption key to the authentication server  10 . The authentication server  10  stores the acquired encryption key in association with the corresponding user ID, etc. In the case of the third example embodiment, a management server  20  transmits the encrypted personal information to the authentication server  10 . The management server  20  stores the generated encryption key and the corresponding service user ID in association with each other. The authentication server  10  stores the acquired personal information (the encrypted text of the personal information), the corresponding user ID, etc. in association with each other. The management server  20  deletes the original personal information and the encryption key at the timing described in the second or third example embodiment. Since an individual management server  20  generates an encryption key, the number of communications between the authentication server  10  and the management server  20  can be reduced in the case of the third example embodiment. 
     In the above example embodiments, a common key is used as an encryption key for encrypting personal information. However, “private key” and “public key” may be used as the encryption key for encrypting personal information. In this case, the authentication server  10  stores a private key in association with a user ID, etc. In addition, the authentication server  10  gives a public key to an individual management server  20 . The management server  20  uses the public key to encrypt personal information. Upon receiving an authentication request, the authentication server  10  transmits the corresponding service user ID and private key to the management server  20 . The management server  20  decrypts the personal information by using the acquired private key. 
     Data transmission and reception modes among the individual apparatuses (the authentication server  10 , the individual management server  20 , and the individual authentication terminal  30 ) are not limited to any particular modes. For example, data transmitted or received among these apparatuses may be encrypted. Among these apparatuses, biological information is transmitted and received. Thus, to appropriately protect the biological information, it is desirable that encrypted data be transmitted and received. 
     In the flowcharts and sequence diagrams used in the above description, a plurality of steps (processes) are sequentially described. However, the order of the execution of the steps performed in the individual example embodiment is not limited to the described order. In the individual example embodiment, the order of the illustrated steps may be changed to the extent that a problem is not caused on the content of the individual example embodiment. For example, individual processes may be executed in parallel. 
     The above example embodiments have been described in detail to facilitate the understanding of the present application disclosed and not to mean that all the configurations described above are needed. In addition, if a plurality of example embodiments have been described, each of the example embodiments may be used individually or a plurality of example embodiments may be used in combination. For example, part of a configuration according to one example embodiment may be replaced by a configuration according to another example embodiment. For example, a configuration according to one example embodiment may be added to a configuration according to another example embodiment. In addition, addition, deletion, or replacement is possible between part of a configuration according to one example embodiment and another configuration. 
     The industrial applicability of the present invention has been made apparent by the above description. That is, the present invention is suitably applicable, for example, to authentication systems for authentication of customers of retail stores, hotel operators, etc. 
     A part or the entirety of the example embodiments described above may be described as in the following supplementary notes, but is not limited to the followings. 
     [Supplementary Note 1] 
     1. An authentication server including: 
     a user registration unit that acquires a first ID that uniquely determines a user in a system and first biological information that is used for authentication of the user; 
     a service registration unit that processes a service registration request that is transmitted from a service provider of a service that the user wishes to use and that includes the first ID and a second ID that identifies the service provider; and 
     a storage unit, 
     wherein the service registration unit generates a third ID that is uniquely determined by a combination of the user and the service provider, generates an encryption key, and transmits the third ID and the encryption key to the service provider, and 
     wherein the storage unit stores the first biological information, the first ID, the second ID, the third ID, and personal information of the user encrypted with the encryption key in association with each other. 
     [Supplementary Note 2] 
     The authentication server according to supplementary note 1, further including an authentication unit that receives an authentication request including second biological information of the user and the second ID from the service provider, determines the third ID and the encrypted personal information of the user by using the first and second biological information and the second ID, and transmits the determined third ID and the encrypted personal information of the user to the service provider. 
     [Supplementary Note 3] 
     The authentication server according to supplementary note 1 or 2, wherein the user registration unit acquires a password of the user, and the storage unit stores the first biological information, the first ID, the password, the second ID, the third ID and the encrypted personal information of the user in association with each other. 
     [Supplementary Note 4] 
     The authentication server according to supplementary note 3, wherein the service registration unit calculates a hash value by using the first ID, the password, and the second ID and uses the calculated hash value as the third ID. 
     [Supplementary Note 5] 
     The authentication server according to any one of supplementary notes 1 to 4, wherein the service registration unit generates the encryption key for a pair of the user and the service provider. 
     [Supplementary Note 6] 
     The authentication server according to any one of supplementary notes 1 to 5, wherein the service registration unit generates a common key as the encryption key. 
     [Supplementary Note 7] 
     The authentication server according to any one of supplementary notes 1 to 6, wherein the first biological information includes a feature value generated from a face image of the user. 
     [Supplementary Note 8] 
     An authentication system including: 
     an authentication server; and 
     a management server, 
     wherein the authentication server includes 
     a user registration unit that acquires a first ID that uniquely determines a user in the system and first biological information that is used for authentication of the user, 
     a service registration unit that processes a service registration request that is transmitted from a service provider of a service that the user wishes to use and that includes the first ID and a second ID that identifies the service provider, and 
     a first storage unit, 
     wherein the service registration unit generates a third ID that is uniquely determined by a combination of the user and the service provider, generates an encryption key, and transmits the third ID and the encryption key to the service provider, 
     wherein the first storage unit stores the first biological information, the first ID, the second ID, the third ID and personal information of the user encrypted with the encryption key in association with each other, and 
     wherein the management server includes 
     a personal information acquisition unit that acquires the first ID and personal information of the user from the user, 
     a service registration request unit that acquires the third ID and the encryption key by transmitting the service registration request to the authentication server and transmits the encrypted personal information of the user to the authentication server, and 
     a second storage unit that stores the encryption key and the third ID in association with each other. 
     [Supplementary Note 9] 
     The authentication system according to supplementary note 8, 
     wherein the authentication server further includes an authentication unit that receives an authentication request including second biological information of the user and the second ID from the service provider, determines the third ID and the encrypted personal information of the user by using the first and second biological information and the second ID, and transmits the determined third ID and the encrypted personal information of the user to the service provider, and 
     wherein the management server further includes an authentication request unit that determines the encryption key by using the third ID acquired by transmitting the authentication request to the authentication server when a service is provided to the user, and 
     wherein the encrypted personal information of the user acquired from the authentication server is decrypted by determined encryption key. 
     [Supplementary Note 10] 
     The authentication system according to supplementary note 8 or 9, 
     wherein the personal information of the user acquired by the management server is deleted after the personal information of the user being encrypted with the encryption key acquired from the authentication server. 
     [Supplementary Note 11] 
     The authentication system according to any one of supplementary notes 8 to 10, further including an authentication terminal that acquires personal information of the user from the management server by transmitting biological information acquired from the user to the management server and that provides a service to the user by using the acquired personal information. 
     [Supplementary Note 12] 
     The authentication system according to supplementary note 11 citing supplementary note 9, 
     wherein the authentication server stores a first feature value generated from a face image of the user as the first biological information, 
     wherein the authentication terminal transmits the face image of the user to the management server, and 
     wherein the management server uses a second feature value generated from the face image as the second biological information. 
     [Supplementary Note 13] 
     The authentication system according to any one of supplementary notes 8 to 12, wherein the personal information does not include biological information of the user. 
     [Supplementary Note 14] 
     A control method of an authentication server, the control method including: 
     acquiring a first ID that uniquely determines a user in a system and first biological information that is used for authentication of the user; 
     receiving a service registration request that is transmitted from a service provider of a service that the user wishes to use and that includes the first ID and a second ID that identifies the service provider; 
     generating and the service provider, and an encryption key; transmitting the third ID and the encryption key to the service provider; and 
     storing the first biological information, the first ID, the second ID, the third ID, and personal information of the user encrypted with the encryption key in association with each other. 
     [Supplementary Note 15] 
     A computer-readable storage medium storing a program causing a computer mounted on an authentication server to perform processing for: 
     acquiring a first ID that uniquely determines a user in a system and first biological information that is used for authentication of the user; 
     receiving a service registration request that is transmitted from a service provider of a service that the user wishes to use and that includes the first ID and a second ID that identifies the service provider; 
     generating a third ID that is uniquely determined by a combination of the user and the service provider, and an encryption key; 
     transmitting the third ID and the encryption key to the service provider; and 
     storing the first biological information, the first ID, the second ID, the third ID and personal information of the user encrypted with the encryption key in association with each other. 
     The entire disclosure of the above patent literature is incorporated herein by reference thereto. While the example embodiments of the present invention have thus been described, the present invention is not limited to these example embodiments. It is to be understood to those skilled in the art that these example embodiments are only examples and that various variations are possible without departing from the scope and sprit of the present invention. That is, the present invention of course includes various variations and modifications that could be made by those skilled in the art in accordance with the overall disclosure including the claims and the technical concept. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  100  authentication server 
           20  management server 
           30  authentication terminal 
           40  terminal 
           101 ,  202  user registration unit 
           102 ,  204  service registration unit 
           103 ,  206 ,  306 ,  405  storage unit 
           201 ,  301 ,  401  communication control unit 
           203 ,  304  database (DB) management unit 
           205  authentication unit 
           302  personal information acquisition unit 
           303  service registration request unit 
           305  authentication request unit 
           311  processor 
           312  memory 
           313  input-output interface 
           314  communication interface 
           402  biological information acquisition unit 
           403  service provision unit 
           404  message output unit