Estimation system, estimation method, and estimation program

Abstract

In a server (10) of an estimation system (1), a generation unit (15a) generates a new public key and a new secret key of an applicant who operates a domain when renewing a server certificate (14a). An application unit (15b) submits an application for renewal of the server certificate (14a) to a CA (20) by using the signature of the generated new public key based on an old secret key before the renewal. In the CA (20), a verification unit (25a) decrypts the signature of said application by using an old public key to verify the applicant. When the verification is successful, an issuing unit (25b) issues the server certificate (14a) including information for deriving an elapsed period from the initial issuance of the server certificate (14a) to the applicant.

Description

Estimation system, estimation method, and estimation program

[0001] The present invention relates to an estimation system, an estimation method, and an estimation program.

[0002] A phenomenon called drop catch is known in which a domain is revoked and then immediately reacquired by a third party. When a drop catch occurs by a malicious third party, there is a risk of causing damage such as the placement of unauthorized content (see Non-Patent Document 1).

[0003] "[Warning] To avoid security risks, we ask corporate customers who used the old Visionalist to delete the 'tracer.jp' tag," [online], May 18, 2022, NTT Com Online, [Retrieved May 16, 2023], Internet<URL:https: / / www.nttcoms.com / news / 2022051801 / >

[0004] However, it has traditionally been difficult to identify drop catch events. For example, if the zone file of the TLD (Top Level Domain) to which a domain belongs is continuously monitored and a domain is deleted from the zone file and then reappears very quickly, it is possible that a drop catch event has occurred in that domain. To do this, it is necessary to compare the zone information at the time the domain is accessed with past zone information, but it is not practical to compare it every time a browser is accessed or a script is executed on a daily basis.

[0005] Alternatively, the WHOIS information for the domain in question is monitored, and if the owner information changes, it is determined to be a drop-catch event. However, due to the promotion of General Data Protection Regulation (GDPR) and the increase in hosting domains, there are limitations on the information that can be obtained from the WHOIS database, and it can be difficult to identify when a site operator has changed.

[0006] Alternatively, server certificate information from previous accesses can be stored and compared each time the site is accessed. If the stored certificate information differs from the stored information, the site is deemed to have been dropped. However, this method is difficult because the transfer of stored information must be carried out over a path that fully guarantees mutual reliability. Furthermore, when a certificate is updated, the server must notify the client, and the client must re-store the updated certificate, which is also difficult.

[0007] The present invention has been made in view of the above, and has an object to make it possible to easily estimate a drop-catch event.

[0008] In order to solve the above-mentioned problems and achieve the object, the estimation system of the present invention is characterized by having: a generation unit that generates a new public key and a new private key for an applicant who operates a domain when renewing a certificate; an application unit that applies to a certification authority for certificate renewal using a signature of the new public key generated using the old private key before renewal; a verification unit that decrypts the requested signature using the old public key to verify the applicant; and an issuance unit that issues a certificate including information for deriving the elapsed time since the certificate was first issued to the applicant if the verification is successful.

[0009] According to the present invention, it is possible to easily estimate a drop catch event.

[0010] FIG. 1 is a diagram for explaining an overview of the estimation system. FIG. 2 is a diagram for explaining an overview of the estimation system. FIG. 3 is a schematic diagram illustrating an example of a general configuration of the estimation system. FIG. 4 is a diagram for explaining processing by the issuing unit. FIG. 5 is a diagram for explaining processing by the issuing unit. FIG. 6 is a sequence diagram illustrating an estimation processing procedure. FIG. 7 is a flowchart illustrating the estimation processing procedure. FIG. 8 is a diagram illustrating an example of a computer that executes an estimation program.

[0011] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to this embodiment. In addition, in the description of the drawings, the same parts are designated by the same reference numerals.

[0012] 1 and 2 are diagrams for explaining an overview of the estimation system. When a server certificate is renewed, the estimation system of this embodiment verifies that the applicant of the current server certificate and the applicant of the newly issued server certificate are the same. Here, "server certificate renewal" refers to renewal that is performed while the current server certificate is valid.

[0013] 1, in the estimation system 1, when applying for renewal of a server certificate, an applicant on the server 10 side generates a key pair of a new public key and a new private key (see (1)), and creates a Certificate Signing Request (CSR) based on this key pair (see (2)). The applicant also sends the created CSR and a signature of the new public key encrypted with the old private key corresponding to the server certificate that has been in use (see (3)) to a certification authority (hereinafter referred to as CA, Certificate Authorities) 20 (see (4)).

[0014] When issuing a server certificate (see (5) to (8)), CA20 verifies that the current server certificate has not expired and decrypts this signature using the corresponding old public key (see (7)). In this way, CA20 verifies whether the person applying for the server certificate renewal is the same person as the person applying for the current server certificate.

[0015] Furthermore, if the verification is successful, CA 20 returns the server certificate to server 10, including information from which the period of time elapsed since the initial issuance of the server certificate to the applicant can be derived, as will be described later (see (9)). Here, since it is guaranteed that the applicant is the same before and after the past renewal, the derived period of time elapsed since the initial issuance of the server certificate can be said to be the period of continuous operation by the same operator, as shown in the example of Figure 2.

[0016] As a result, when the client 30 requests a connection to the server 10 (see (10)), it can refer to the presented server certificate (see (11) to (12)) and find out how long the domain has been continuously operated by the same operator. Note that it is assumed that the client 30 has a function such as an add-on that reads information from the server certificate that can derive the elapsed time since the initial issuance of the server certificate.

[0017] Then, for example, if the time elapsed between the previous access and the current access is longer than the period of continuous operation by the same operator, the client 30 considers whether to allow access, taking into account the possibility that the operator may have changed.

[0018] Alternatively, for example, if the period of continuous operation by the same operator is short, less than the specified period during which drop catch could be abused and damage could occur, the client 30 determines that there may be a change in operator or that drop catch has occurred, and then considers whether to allow access.

[0019] The information incorporated into the server certificate issued by CA20 that can be used to derive the number of days that have elapsed since the initial issuance of the server certificate to the applicant is, for example, the same applicant period, which indicates the number of days of continuous operation by the applicant of the current server certificate in the past, or information indicating the date and time of the initial issuance of the applicant's server certificate, as will be described later.

[0020] In addition, the functions of this embodiment are implemented in the intermediate CA and do not apply to the root CA, but the CSR that the intermediate CA applies to the root CA is within the scope of the RFC specifications, as described below, and signing by the root CA can be implemented without any problems.

[0021] [Configuration of Estimation System] Fig. 3 is a schematic diagram illustrating the overall configuration of an estimation system. As illustrated in Fig. 3, the estimation system 1 of this embodiment includes a server 10, a CA 20, and a client 30. The server 10 transmits a CSR requesting an update of the server guarantee to the CA 20. The CA verifies the CSR and returns the updated server guarantee.

[0022] The client 30 accesses the site operated by the server 10 via a browser by a user or mechanically using a script. At that time, the client 30 checks the server certificate 14a of the server 10 to confirm the period of operation of the server 10 by the same operator, and determines whether the operator has changed or whether there is a possibility of a drop catch.

[0023] [Configuration of Server] The server 10 is realized by a general-purpose computer such as a workstation, and includes an input unit 11 , an output unit 12 , a communication control unit 13 , a storage unit 14 , and a control unit 15 .

[0024] The input unit 11 is realized by input devices such as a keyboard and a mouse, and inputs various instruction information such as a command to start processing to the control unit 15 in response to input operations by an operator. The output unit 12 is realized by a display device such as a liquid crystal display, a printing device such as a printer, or the like.

[0025] The communication control unit 13 is realized by a NIC (Network Interface Card) or the like, and controls communication between an external device via a network and the control unit 15. For example, the communication control unit 13 controls communication between the control unit 15 and the CA 20, the client 30, a management device that manages various data used in the estimation process described below, and the like.

[0026] The storage unit 14 is realized by a semiconductor memory element such as a RAM (Random Access Memory) or a flash memory, or a storage device such as a hard disk or an optical disk. The storage unit 14 stores in advance the processing program that operates the server 10, data used during execution of the processing program, and the like, or temporarily stores the data each time processing is performed. For example, the storage unit 14 stores a server certificate 14a and the like that is generated and used in the estimation process described below. The storage unit 14 may be configured to communicate with the control unit 15 via the communication control unit 13.

[0027] The control unit 15 is realized using a CPU (Central Processing Unit), an NP (Network Processor), an FPGA (Field Programmable Gate Array), or the like, and executes a processing program stored in memory. As a result, the control unit 15 functions as a generation unit 15a and an application unit 15b, as exemplified in FIG. 3, and executes the estimation process described below. Note that these functional units may be implemented in different hardware. The control unit 15 may also include other functional units.

[0028] When updating a server certificate, the generation unit 15a generates a new public key and a new private key for the applicant who operates the domain. Specifically, the generation unit 15a generates a pair of a new public key for the server certificate for which an update is being requested and a new private key corresponding to the new public key.

[0029] The application unit 15b applies to the CA 20 for renewal of the server certificate 14a using the signature of the new public key generated using the old private key before renewal. Specifically, the application unit 15b creates a CSR requesting renewal of the server certificate, and transmits this CSR and the signature of the new public key encrypted with the old private key corresponding to the server certificate 14a that has been in use to the CA 20. At that time, the application unit 15b includes in the CSR the value of information for deriving the period of time that has elapsed since the initial issuance of the server certificate 14a to the applicant, as will be described later.

[0030] Furthermore, the application unit 15b receives an updated server certificate issued by an issuing unit 25b of the CA 20 (described later) and updates the server certificate 14a in the storage unit 14.

[0031] [Configuration of the Certification Authority (CA)] The CA 20 includes a control unit 25 implemented by a CPU, NP, FPGA, or the like, and executing a processing program stored in memory. The CA 20 also includes a storage unit 24 implemented by a semiconductor memory element such as RAM or flash memory. The CA 20 also includes a communication control unit (not shown), and communicates with the server 10, other network devices, and the like via the communication control unit. The CA 20 also includes an output unit (not shown) implemented by a display device such as a liquid crystal display, a speaker, or the like.

[0032] As shown in FIG. 3 , the control unit 25 functions as a verification unit 25a and an issuance unit 25b. First, the verification unit 25a decrypts the requested signature using the old public key to verify the applicant. Specifically, the verification unit 25a references a CT (Certificate Transparency) Log, which is a log of the previous server certificate, and acquires the old public key corresponding to the applicant's current server certificate 14a. The verification unit 25a also confirms that the current server certificate has not expired. The verification unit 25a then verifies whether the CSR signature can be decrypted using the old public key. In this way, the verification unit 25a verifies whether the applicant for server certificate renewal is the same person as the applicant for the current server certificate 14a.

[0033] If the verification fails, the verification unit 25a notifies the server 10 and urges the server 10 to resubmit a correct signature.

[0034] On the other hand, if the verification is successful, the issuing unit 25b issues a server certificate including information for deriving the elapsed time since the initial issuance of the certificate to the applicant. Specifically, for example, the verifying unit 25a verifies whether the information for deriving the elapsed time since the initial issuance of the certificate to the applicant, which is included in the CSR, is correct, and if the verification is successful, the issuing unit 25b writes the information in the server certificate and issues an updated server certificate.

[0035] The issuing unit 25b also returns the issued updated server certificate to the server 10. In this case, the server 10 updates the current server certificate 14a using the returned updated server certificate, as described above.

[0036] Here, since it is guaranteed that the applicant is the same before and after the previous renewal, the period that has elapsed since the initial issuance of the certificate to the applicant can be said to be the period of continuous operation by the same person, as shown in the example in Figure 2.

[0037] Therefore, the issuing unit 25b includes information for deriving the period of time that has elapsed since the initial issuance of the server certificate to the applicant in a predetermined extension field of the server certificate. For example, the issuing unit 25b incorporates information for deriving the period of time that has elapsed since the initial issuance of the server certificate to the applicant into the server certificate by using the public key certificate period extension field defined in X.509 of RFC5280.

[0038] Information for deriving the period of time that has elapsed since the first issuance of a server certificate to an applicant is, for example, the period of time that the applicant for the server certificate has been the same, or the date and time of the first issuance of a certificate to the applicant.

[0039] 4 and 5 are diagrams for explaining the processing of the issuing unit. For example, the issuing unit 25b sets a period during which the applicant for the server certificate remains the same, as shown in FIG. 4. In FIG. 4(a) and FIG. 5(a), the type-id is expressed as a character string for the sake of explanation, but in reality it is an OID based on ASN.1.

[0040] 4A, the issuing unit 25b defines "ApplicantPeriod" in the "OtherName" extension field in the Subject Alternative Name of the public key certificate period defined in X.509 of RFC 5280. This ApplicantPeriod (same applicant period) refers to the period during which the site has been continuously operated by the same applicant, i.e., the same operator, from the time the currently referenced server certificate first became valid.

[0041] For example, as shown by the underlined area β in FIG. 4B , an ApplicantPeriod is set in the server certificate. In this case, the derivation unit 35b of the client 30 (described later) determines the number of days (10 days) that have elapsed since the server certificate was issued by calculating the difference between the "Not Before" date and time (February 22, 2023) in the Validity field in bold in area α and the current date and time (e.g., March 5, 2023). Furthermore, the ApplicantPeriod in area β determines the number of days (360 days) that the site has been continuously operated by the same operator since the server certificate was validated. Adding the number of days that have elapsed since the server certificate was issued and the number of days that the site has been continuously operated by the same operator since the server certificate was validated derives the period that has elapsed since the initial issuance of the server certificate to the same applicant, i.e., the number of days (370 days) that the site has been continuously operated by the same operator.

[0042] The first CSR from the applicant does not contain a previous private key and does not include a signature. In this case, ApplicantPeriod = 0.

[0043] Alternatively, the issuing unit 25b sets the issuance date and time of the initial server certificate for the same applicant, as shown in Fig. 5. Specifically, as shown in Fig. 5(a), the issuing unit 25b defines "Applicant Not Before" in the "OtherName" extension field in the Subject Alternative Name of the public key certificate period defined in X.509 of RFC 5280. This Applicant Not Before (the applicant's initial certificate issuance date and time) indicates the first day that the site began to be continuously operated by the same applicant, i.e., the same operator.

[0044] For example, as shown by the underlined area γ in Fig. 5B, "Applicant Not Before" is set in the server guarantee. In this case, the derivation unit 35b of the client 30, which will be described later, derives the number of days that have passed since the server certificate was issued (370 days), i.e., the number of days that the site has been continuously operated by the same operator, from the difference between the date and time of "Applicant Not Before" written in area γ (February 17, 2023) and the current date and time (for example, March 5, 2023).

[0045] The initial CSR issued by the applicant does not include the old private key and does not include a signature. In this case, the Applicant Not Before date is the same as the Not Before date, which is the issuance date and time of the server certificate.

[0046] [Configuration of the Client] The client 30 includes a control unit 35 implemented by a CPU, NP, FPGA, or the like, and which executes a processing program stored in memory. The client 30 also includes a storage unit 34 implemented by a semiconductor memory element such as RAM or flash memory. The client 30 also includes a communication control unit (not shown), and communicates with the server 10, other network devices, and the like via the communication control unit. The client 30 also includes an output unit (not shown) implemented by a display device such as a liquid crystal display, a speaker, or the like.

[0047] The client 30 accesses the site operated by the server 10 via a browser by the user or mechanically via a script. It is assumed that all legitimate sites are HTTPS compatible.

[0048] As shown in FIG. 3 , the control unit 35 functions as an acquisition unit 35a, a derivation unit 35b, and a determination unit 35c. First, the acquisition unit 35a acquires the issued server certificate 14a. Specifically, the control unit 35 accesses a site operated by the server 10 via a user's browser or mechanical access using a script. At that time, the acquisition unit 35a requests a connection to the server 10 and acquires the server certificate 14a. The acquisition unit 35a also references the acquired server certificate 14a and checks information for deriving the period of time that has elapsed since the certificate was first issued to the applicant.

[0049] The derivation unit 35b uses information for deriving the period elapsed since the initial issuance of the server certificate 14a to the applicant to derive the operation period of the domain owned by the same applicant. Specifically, the derivation unit 35b derives the operation period of the domain owned by the same applicant, as illustrated in FIG. 4(b) or FIG. 5(b).

[0050] The determination unit 35c uses the derived operation period to determine whether there is a possibility that the operator has been changed or whether there is a possibility of a drop catch.

[0051] For example, if the period from the previous access to the server 10 to the current access is equal to or longer than the derived operation period, the determination unit 35c determines that the operator may have changed since the previous access. In other words, the determination unit 35c determines that a period of continuous operation by the same operator is trustworthy, but that the operator may have changed during a period exceeding that, or that a drop catch may have occurred.

[0052] If the judgment unit 35c determines that a drop catch has occurred, the judgment unit 35c may, for example, stop the mechanical script access or present a warning message to the user attempting to access the script stating that "a drop catch may have occurred."

[0053] Alternatively, the determination unit 35c determines that the operator may have changed if the derived operation period is equal to or shorter than a predetermined period. In other words, the determination unit 35c determines that the operator may have changed, that is, that a drop catch may have occurred, if the continuous operation period by the same operator is equal to or shorter than a predetermined period stipulated in accordance with an internal company policy or the like, which indicates that a drop catch may be abused and cause damage.

[0054] When the determination unit 35c determines that a drop catch has occurred, it presents a warning message to the user attempting to access the site, stating that "a drop catch may have occurred." For example, for a site that is assumed to have been in operation for a long time, a long period of time is specified as the predetermined period. If the operation period derived from this period is short, it is possible to warn the user.

[0055] [Estimation Process] Next, the estimation process performed by the estimation system 1 according to this embodiment will be described with reference to Fig. 6 and Fig. 7. Fig. 6 is a sequence diagram showing the estimation process procedure. Fig. 6 illustrates the processing procedure performed by the server 10 and the CA 20. The sequence in Fig. 6 is started, for example, when an operation input is made to instruct the start of the estimation process.

[0056] First, in the server 10, the generation unit 15a generates a new public key and a new private key for the applicant who operates the domain when updating the server certificate (step S1). Specifically, the generation unit 15a generates a pair of a new public key for the server certificate for which an update is being requested and a new private key corresponding to the new public key.

[0057] Next, the application unit 15b encrypts the new public key using the old private key corresponding to the server certificate 14a that has been in use until now, and signs it (step S2).

[0058] The application unit 15b also creates a CSR requesting renewal of the server certificate, and includes in the CSR a value of predetermined information for deriving the period of time elapsed since the initial issuance of the server certificate 14a to the applicant, as described below (step S3).

[0059] For example, the CSR may include the value of Applicant Period or Applicant Not Before as a predetermined value.

[0060] When the Applicant Period value is included in the CSR, the number of days that have passed since the current server certificate 14a became valid is confirmed, and this number of days is added to the Applicant Period value of the current server certificate 14a.

[0061] Alternatively, if the Applicant Not Before value is included in the CSR, the Applicant Not Before value of the current server certificate 14a is used as is.

[0062] Then, the application unit 15b transmits the CSR and the signature to the CA 20 (step S4).

[0063] In CA 20, verification unit 25a decrypts the applied signature with the old public key to verify the applicant. Specifically, verification unit 25a references CTLog, which is a log of previous server certificates, and obtains the old public key corresponding to the applicant's current server certificate 14a. Then, verification unit 25a verifies whether the CSR signature can be decrypted with the old public key (step S5).

[0064] The verification unit 25a also verifies whether the information included in the CSR for deriving the elapsed time since the first issuance of the certificate to the applicant is correct (step S6). For example, the verification unit 25a refers to the information in the CTLog to verify whether the values ​​of Applicant Period or Applicant Not Before included in the CSR are correct.

[0065] If these verifications are successful, the issuing unit 25b issues an updated server certificate by including information for deriving the period of time that has elapsed since the initial issuance of the certificate to the applicant, and returns the issued updated server certificate to the server 10 (step S7).

[0066] In the server 10, the application unit 15b updates the current server certificate 14a using the returned updated server certificate, thereby completing the series of processes.

[0067] Next, Fig. 7 shows an example of a processing procedure in the client 30. The flowchart in Fig. 7 starts, for example, when an operation input is made to instruct the start of the reflection processing.

[0068] First, in the client 30, when a user accesses a site operated by the server 10 via a browser or mechanically accesses the site using a script, the acquisition unit 35a requests a connection to the server 10 and acquires the server certificate 14a. The acquisition unit 35a then references the acquired server certificate 14a and acquires information for deriving the period of time that has elapsed since the certificate was first issued to the applicant (step S11).

[0069] For example, the acquisition unit 35a acquires the Applicant Period or Applicant Not Before written in the server certificate 14a as information for deriving the period of time that has elapsed since the initial issuance of the certificate to the applicant.

[0070] Next, the derivation unit 35b uses the predetermined information to derive the period of time that has elapsed since the first issuance of the server certificate 14a to the same applicant (step S12). The period of time that has elapsed corresponds to the period of time that the same applicant has operated the domain up to the present.

[0071] For example, when the Applicant Period is acquired as the predetermined information, if Applicant Period = 0, the derivation unit 35b obtains the number of days since issuance of the current server certificate 14a as the operation period. On the other hand, if Applicant Period = 0, the derivation unit 35b obtains the operation period by adding the Applicant Period and the number of days since issuance of the current server certificate 14a.

[0072] Alternatively, when Applicant Not Before is acquired as the predetermined information, the derivation unit 35b obtains the number of days that have elapsed from the date and time of Applicant Not Before to the present as the operation period.

[0073] Then, the determination unit 35c uses the derived operation period to determine whether or not there is a possibility that the operator has been changed or whether or not there is a possibility of drop catching. For example, the determination unit 35c compares the derived operation period with a predetermined period stipulated according to an internal company policy or the like during which drop catching may be abused and damage may occur (step S13).

[0074] If the derived operating period exceeds the predetermined period (step S13, Yes), the operating period is deemed reliable, and the determining unit 35c determines that there is no possibility of a change in the operator or a drop catch (step S14). In this case, the client 30 allows access by the user. This completes the process.

[0075] On the other hand, if the derived operation period is equal to or shorter than the predetermined period (No in step S13), the determination unit 35c determines that the operator may have been changed. Then, the determination unit 35c displays a warning message to the user indicating that a drop catch may have occurred (step S15). This completes the process.

[0076] [Effects] As described above, in the estimation system 1, in the server 10, the generation unit 15a generates a new public key and a new private key for the applicant who operates the domain when updating the server certificate. The application unit 15b applies to the CA 20 for updating the server certificate 14a using a signature of the new public key generated using the old private key before the update. In the CA 20, the verification unit 25a decrypts the requested signature using the old public key to verify the applicant. If the verification is successful, the issuance unit 25b issues a server certificate including information for deriving the period of time that has elapsed since the initial issuance of the server certificate to the applicant.

[0077] Specifically, the information for deriving the period of time that has elapsed since the first issuance of a certificate to an applicant is the period of time that the applicant for the certificate has remained the same, or the date and time of the first issuance of a certificate to the applicant.

[0078] Since it is guaranteed that the applicant is the same before and after the previous renewal, the period of time since the initial issuance of the derived server certificate can be interpreted as the period of continuous operation by the same operator. Therefore, a user attempting to access a domain can easily find out the period of continuous operation by the same operator by referring to the server certificate. This allows the user to easily infer a drop-catch event.

[0079] The issuing unit 25b also includes information for deriving the period of time that has elapsed since the initial issuance of the certificate to the applicant in a predetermined extension field of the certificate. By implementing the present invention within the scope of the standard specifications, it is possible to implement the present invention without affecting network devices that are not covered by the present invention.

[0080] In addition, in the client 30, the acquisition unit 35a acquires the issued server certificate 14a. The derivation unit 35b uses information for deriving the period of time elapsed since the first issuance of the certificate to the applicant to derive the operation period of the domain by the same applicant. The determination unit 35c uses the derived operation period to determine whether the operator may have changed.

[0081] Specifically, the determination unit 35c determines that the operator may have changed if the period from the previous access to the server 10 to the current access to the server 10 is equal to or longer than the derived operation period.

[0082] Alternatively, if the derived operation period is equal to or shorter than a predetermined period, the determining unit 35c determines that the operator may have been changed.

[0083] This allows users and scripts accessing the server to easily detect and respond to the possibility of a malicious drop catch, even if there is no clear definition of the period during which a malicious drop catch occurs.

[0084] [Program] A program written in a computer-executable language may be created to describe the processes executed by each device (server 10, CA 20, client 30) of the estimation system 1 according to the above embodiment. In one embodiment, the server 10 can be implemented by installing an estimation program that executes the above estimation process as package software or online software on a desired computer. For example, by executing the estimation program on an information processing device, the information processing device can function as the server 10. Other examples of information processing devices include mobile communication terminals such as smartphones, mobile phones, and PHS (Personal Handyphone Systems), as well as slate terminals such as PDAs (Personal Digital Assistants). The functions of the server 10 may also be implemented on a cloud server.

[0085] 8 is a diagram showing an example of a computer that executes an estimation program. The computer 1000 includes, for example, a memory 1010, a CPU 1020, a hard disk drive interface 1030, a disk drive interface 1040, a serial port interface 1050, a video adapter 1060, and a network interface 1070. These components are connected by a bus 1080.

[0086] The memory 1010 includes a ROM (Read Only Memory) 1011 and a RAM 1012. The ROM 1011 stores a boot program such as a BIOS (Basic Input Output System). The hard disk drive interface 1030 is connected to a hard disk drive 1031. The disk drive interface 1040 is connected to a disk drive 1041. A removable storage medium such as a magnetic disk or optical disk is inserted into the disk drive 1041. The serial port interface 1050 is connected to a mouse 1051 and a keyboard 1052, for example. The video adapter 1060 is connected to a display 1061, for example.

[0087] Here, the hard disk drive 1031 stores, for example, an OS 1091, an application program 1092, a program module 1093, and program data 1094. The various pieces of information described in the above embodiments are stored in the hard disk drive 1031 or memory 1010, for example.

[0088] The estimation program is stored in the hard disk drive 1031 as a program module 1093 in which instructions to be executed by the computer 1000 are written. Specifically, the program module 1093 in which each process executed by the server 10 described in the above embodiment is written is stored in the hard disk drive 1031.

[0089] Data used for information processing by the estimation program is stored as program data 1094, for example, in the hard disk drive 1031. Then, the CPU 1020 reads the program module 1093 and the program data 1094 stored in the hard disk drive 1031 into the RAM 1012 as necessary, and executes each of the above-described procedures.

[0090] The program module 1093 and program data 1094 related to the estimation program are not limited to being stored in the hard disk drive 1031, and may be stored in, for example, a removable storage medium and read by the CPU 1020 via the disk drive 1041. Alternatively, the program module 1093 and program data 1094 related to the estimation program may be stored in another computer connected via a network such as a LAN (Local Area Network) or a WAN (Wide Area Network), and read by the CPU 1020 via the network interface 1070.

[0091] Although the present invention has been described above as an embodiment, the present invention is not limited to the description and drawings that form part of the disclosure of the present invention. In other words, other embodiments, examples, and operational techniques that can be made by those skilled in the art based on the present invention are all included in the scope of the present invention.

[0092] DESCRIPTION OF SYMBOLS 1 Estimation system 10 Server 11 Input unit 12 Output unit 13 Communication control unit 14, 24, 34 Storage unit 14a Server certificate 15, 25, 35 Control unit 15a Generation unit 15b Application unit 20 CA (Certificate Authority) 25a Verification unit 25b Issuance unit 30 Client 35a Acquisition unit 35b Derivation unit 35c Determination unit

Claims

1. An estimation system comprising: a generation unit that generates a new public key and a new private key for an applicant who operates a domain when renewing a certificate; an application unit that applies for certificate renewal to a certification authority using a signature of the new public key generated using the old private key before renewal; a verification unit that decrypts the requested signature using the old public key to verify the applicant; and an issuance unit that issues a certificate including information for deriving the period of time that has elapsed since the certificate was first issued to the applicant if the verification is successful.

2. The estimation system according to claim 1, wherein the issuing unit includes information for deriving the period of time that has elapsed since the initial issuance of the certificate to the applicant in a predetermined extension area of ​​the certificate.

3. The estimation system described in claim 1, characterized in that the information for deriving the period of time that has elapsed since the first issuance of the certificate to the applicant is the period during which the applicant for the certificate is the same, or the date and time of the first issuance of the certificate to the applicant.

4. The estimation system described in claim 1, further comprising: an acquisition unit that acquires the issued certificate; a derivation unit that derives the operation period of the domain by the same applicant using information for deriving the elapsed period since the initial issuance of the certificate to the applicant; and a judgment unit that uses the derived operation period to determine whether or not the operator may have changed.

5. The estimation system described in claim 4, characterized in that the judgment unit judges that the operator may have changed if the period between the previous access to the server and the current access to the server is longer than the operation period.

6. The estimation system according to claim 4, characterized in that the determination unit determines that the operator may have been changed if the operation period is equal to or shorter than a predetermined period.

7. An estimation method executed by an estimation system, comprising: a generation step of generating a new public key and a new private key for an applicant who operates a domain when renewing a certificate; an application step of applying for certificate renewal to a certification authority using a signature of the new public key generated using the old private key before renewal; a verification step of decrypting the applied signature using the old public key to verify the applicant; and an issuance step of issuing a certificate including information for deriving the period of time that has elapsed since the initial issuance of the certificate to the applicant if the verification is successful.

8. An estimation program for causing a computer to execute the following steps when renewing a certificate: a generation step of generating a new public key and a new private key for an applicant who operates a domain; an application step of applying for certificate renewal to a certification authority using a signature of the new public key generated with the old private key before renewal; a verification step of decrypting the applied signature with the old public key to verify the applicant; and an issuance step of issuing a certificate including information for deriving the period of time that has elapsed since the certificate was first issued to the applicant if the verification is successful.

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