JP5768543B2 - Electronic signature system, signature server, signer client, electronic signature method, and program - Google Patents

Description

  The present invention relates to an electronic signature system, a signature server, a signer client, an electronic signature method, and a program.

  An electronic signature using a public key cryptosystem is widely used as a digital signature scheme for an electronic document. To execute an electronic signature, a private key and an application for executing the electronic signature are required. However, when it becomes necessary to change the cryptographic algorithm used for the electronic signature to a new algorithm due to compromise of the cryptographic algorithm, etc. It is necessary to modify all client applications that perform digital signatures.

An example of an electronic signature system is described in Patent Document 1 and Patent Document 2.
Patent Document 1 discloses a system that collectively manages a client's private key in a database of a server instead of a client and executes an electronic signature on the server.
Patent Document 2 discloses a system that transmits a secret key from a client to a server each time an electronic signature is performed, and executes the electronic signature on the server.

JP 2004-200740 A JP 2009-2112747 A

  However, in the system of Patent Document 1, protection of the secret key depends on server management and operation. Therefore, there is a high risk when the client's private key is leaked due to the vulnerability of the server. Further, in order to confirm that the server is a secret key holder, a mechanism for authenticating the client is required separately. Furthermore, since it is necessary to monitor the management state of the secret key in addition to the strict management of the server and the database, the operation load of the entire system increases.

  In the system of Patent Document 2, although the above problem is solved, there is a possibility that the signature is executed by an incorrect combination of the electronic signature target document, the private key, and the password in the server. The reason is that the signature document sent to the server by the client, the private key, and the password can only be secured by the sequence within the same SSL session. This is because, when the same session is not used, the relationship between the document to be signed on the server, the private key, and the password cannot be secured.

  Further, in the system of Patent Document 2, there is a possibility that a signature is executed on a signature target document that is not intended by the client in the server. The reason is that there are no restrictions on the documents that are signed on the server, so if a malicious third party replaces the document to be signed with a document different from the document intended by the client due to unauthorized entry into the server, etc. This is because it cannot be detected.

  The present invention transmits a private key from a client to a server each time an electronic signature is performed, and in a system that executes an electronic signature on the server, ensures the association between a document to be signed and the private key, thereby providing a signature designated by the client. One object of the present invention is to provide a system that can securely digitally sign a target document.

  An electronic signature system according to the present invention includes a signature server that gives an electronic signature to a signature target document, and a signer client that transmits the signature target document and a private key used for giving the electronic signature to the signature server. When the signer client transmits the document to be signed to the signing server, the signer client transmits a first time stamp token including an identifier to the signing server via the time stamp server, and transmits the secret to the signing server. When transmitting a key, the second time stamp token including the identifier is transmitted to the signing server via the time stamp server, and the signing server includes the first time stamp token and the second time stamp. Only when the identifiers included in the token match, the signature target document is digitally signed using the private key. It is those that confer.

  According to the present invention, in the signature server, it is possible to reliably associate the signature target document with the private key, and to securely perform the electronic signature on the signature target document designated by the signer client.

1 is a block diagram showing a configuration of an electronic signature system according to Embodiment 1 of the present invention. The figure explaining the function of the certification authority system by Embodiment 1 of this invention. The figure explaining the function of a signer client by Embodiment 1 of this invention. The figure explaining the function of the signature server by Embodiment 1 of this invention. The figure explaining the function of the time stamp server by Embodiment 1 of this invention. The figure explaining operation | movement of the electronic signature system by Embodiment 1 of this invention. The figure explaining the function of a signer client by Embodiment 2 of this invention. The figure explaining the function of the signature server by Embodiment 2 of this invention. The figure explaining operation | movement of the electronic signature system by Embodiment 2 of this invention.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a configuration of an electronic signature system 100 according to Embodiment 1 of the present invention.
As shown in FIG. 1, an electronic signature system 100 includes a certificate authority system 1, a signer client 2, a signature server 3, and a time stamp server 4, and a communication network 5 such as the Internet for connecting them together. .

The certificate authority system 1 includes a registration unit 11 and an issuing unit 12. The function of each part will be described with reference to FIG.
The registration unit 11 registers the user in response to the certificate issuance request from the signer client 2 and notifies the issuing unit 12 of the certificate issuance request.
The issuing unit 12 has a function of issuing a public key certificate in response to a certificate issuance request and a function of distributing the public key certificate to the signer client 2.

  The signer client 2 stores the public key certificate distributed from the certificate authority system 1. Note that the secret key corresponding to the public key certificate may be generated by the secret key management unit 22 of the signer client 2 or may be generated by the issuing unit 12 of the certificate authority system 1.

The signer client 2 includes a transmission / reception unit 21, a secret key management unit 22, and a signature target document management unit 23. The function of each part will be described with reference to FIG.
The secret key management unit 22 has a function of managing a secret key and a public key (public key certificate) and a function of encrypting a secret key from an input password by a password-based encryption method (PKCS # 5). The secret key management unit 22 encrypts the secret key using the input password, and then transmits the password and the encrypted secret key to the signature server 3 via the transmission / reception unit 21.

  The signature target document management unit 23 has a function of setting a nonce acceptable value, a function of selecting a signature target document, and a function of generating and storing a nonce (identifier).

  The transmission / reception unit 21 transmits a nonce acceptable value, a signature target document, an encrypted secret key, and a password to the signature server 3. The transmission / reception unit 21 has a function of generating a time stamp request and transmitting it to the time stamp server 4 and a function of receiving signature data from the signature server 3.

  Using the nonce received from the signature target document management unit 23, the transmission / reception unit 21 generates a time stamp request for the signature target document and transmits it to the time stamp server 4. In addition, using the nonce stored in the signature target document management unit 23, that is, using the same nonce as the time stamp request for the signature target document, a time stamp request corresponding to the secret key and password is generated, and the time stamp server 4 to send. For the generation of the time stamp request and the time stamp token described below, for example, the method described in “RFC3161,“ Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP) ”, 2001” should be used. Can do.

The signature server 3 includes a transmission / reception unit 31, a secret key acquisition unit 32, and a signature execution unit 33. The function of each part will be described with reference to FIG.
The transmission / reception unit 31 has a function of receiving a nonce acceptable value, a signature target document, an encrypted secret key, and a password from the signer client 2 and receiving a time stamp token from the time stamp server 4.

  The secret key acquisition unit 32 has a function of extracting the secret key using the encrypted secret key and password.

  The signature execution unit 33 has a function of verifying the validity of the time stamp token, a function of acquiring and storing a nonce from the time stamp token, and a function of executing a signature by accessing a secret key.

  The signature execution unit 33 extracts and stores a nonce included in the time stamp token for the document to be signed. Next, matching between the encrypted secret key and the nonce included in the time stamp token for each password is performed.

  Specifically, the nonce acceptable value is set as the number of nonce match confirmations, and the number of nonce match confirmations is subtracted every time match confirmation is performed. Further, when the nonce matches, the nonce acceptable value is subtracted. Only when the nonce match confirmation number = the nonce acceptable number = 0, the secret key acquisition unit 32 extracts the secret key from the encrypted secret key and password, and the signature execution unit 33 executes the signature. The secret key acquisition unit 32 erases the secret key after executing the signature.

The time stamp server 4 includes a transmission / reception unit 41 and a signature execution unit 42. The function of each part will be described with reference to FIG.
The transmission / reception unit 41 receives the time stamp request transmitted from the signer client 2. The signature execution unit 42 generates a time stamp token corresponding to the time stamp request. Further, the transmission / reception unit 41 transmits the generated time stamp token to the signature server 3.

  In addition, without using a nonce, include the identifier that identifies the document to be signed in the extension field of the time stamp request, and include the same identifier for the encrypted private key and password corresponding to it. The relationship between the document, the encrypted secret key, and the password may be determined.

Next, the operation of the electronic signature system 100 will be described in detail with reference to FIG.
The signer client 2 sets nonce acceptable value = 2 (step S1), and selects a signature target document (step S2). Next, a nonce that is a random number is generated and stored (step S3).

  The signer client 2 creates a time stamp request for the signature target document using this nonce and transmits it to the time stamp server 4 (steps S4 and S23). Further, the signature target document and the nonce acceptable value are transmitted to the signature server 3 (steps S5 and S12).

  Next, the signer client 2 inputs a password in order to encrypt the secret key for signing the document to be signed (step S6), and encrypts the secret key using the password-based encryption method (PKCS # 5 format). (Step S7). Further, a time stamp request for each of the encrypted private key and password is created using the same nonce as used when generating the time stamp request for the signature target document in step S4, and the time stamp server 4 (Steps S8, S10, S26, S29). Further, the encrypted secret key and password are transmitted to the signature server 3 (steps S9, S11, S13, S14).

  The signature server 3 verifies the validity of the time stamp token using the signature target document transmitted from the signer client 2 and the time stamp token for the signature target document transmitted from the time stamp server 4. The nonce included in the token is saved (steps S15, S24, S25).

  Next, the signature server 3 sets the nonce acceptable number = 2 to the nonce match confirmation number, and the encrypted secret key transmitted from the signer client 2 and the encrypted secret transmitted from the time stamp server 4. Whether the nonce included in the time stamp token after verifying the validity of the time stamp token using the time stamp token for the key matches the nonce included in the time stamp token for the signature target document Is confirmed (steps S16, S27, S28). After the confirmation process, the nonce match confirmation number is subtracted to be 1. Further, when it is confirmed that the nonce matches, the nonce acceptable number is subtracted to be 1.

  Next, after verifying the validity of the time stamp token using the password transmitted from the signer client 2 and the time stamp token for the password transmitted from the time stamp server 4, it is included in the time stamp token. It is checked whether the nonce and the nonce included in the time stamp token for the signature target document match (steps S17, S30, and S31). After the confirmation process, the nonce match confirmation number is subtracted to zero. Furthermore, when it is confirmed that the nonce matches, the nonce acceptable number is subtracted to 0.

  The signature server 3 takes out the secret key using the encrypted secret key and password only when the nonce match confirmation number = the nonce acceptable number = 0 (step S18). Further, the signature server 3 executes a signature on the document to be signed (step S19), and transmits the signature data to the signer client 2 (steps S21 and S22). Further, the private key is deleted after the signature is executed (step S20).

  As described above, according to the present embodiment, when the signer client 2 transmits the signature target document, the secret key, and the password, the signer client 2 transmits the time stamp token including the same nonce to the signature server 3, and Since the server 3 executes the signature after confirming that the nonce included in each time stamp token is the same, the correct signature target document and private key transmitted by the signer client 2 in the signature server 3. , And a combination of passwords can be used for electronic signatures.

  In addition, the signer client 2 transmits a nonce acceptable value to the signature server 3, and the signature server 3 performs the signature only when the nonce acceptable value matches the number of times that the nonce matches. Since the signature is executed for the target document, the signature is surely executed only for the signature target document intended by the client.

Embodiment 2. FIG.
FIG. 7 is a diagram for explaining the function of the signer client 2 of the electronic signature system 100 according to the second embodiment of the present invention.
In the second embodiment, the signature target document management unit 24 of the signer client 2 has a function of setting a time stamp acceptance allowable value together with a nonce acceptable value. The function for selecting other signature target documents and the function for generating and storing a nonce are the same as those in the first embodiment.

  The transmitting / receiving unit 25 transmits a nonce acceptable value, a time stamp acceptable value, a signature target document, an encrypted secret key, and a password to the signature server 3. The function of generating a time stamp request and transmitting it to the time stamp server 4 and the function of receiving signature data from the signature server 3 are the same as in the first embodiment. The function of the secret key management unit 22 is the same as that of the first embodiment.

FIG. 8 is a diagram illustrating the function of the signature server 3 of the electronic signature system 100 according to the second embodiment.
In the second embodiment, the transmission / reception unit 34 of the signature server 3 receives the nonce acceptable value, the time stamp acceptance allowable value, the signature target document, the encrypted secret key, and the password received from the signer client 2. The function of receiving a time stamp token from the time stamp server 4 is the same as in the first embodiment.

  The signature execution unit 35 has a function of verifying the validity of the time stamp token, a function of acquiring time information from the time stamp token, a function of acquiring and storing a nonce from the time stamp token, and a signature by accessing the private key It has a function to execute.

  The signature execution unit 35 extracts time information included in the time stamp token for the document to be signed, and calculates a difference value from the time when the validity verification of the time stamp token is performed. Next, a difference value between the time information included in the time stamp token for the encrypted secret key and the time when the validity verification of the time stamp token is performed is calculated. Further, a difference value between the time information included in the time stamp token for the password and the time when the validity verification of the time stamp token is performed is calculated.

  Next, the nonce included in the time stamp token for the signature target document is extracted and stored. Next, a match with the nonce included in the time stamp token for the encrypted secret key is confirmed. Next, a match with the nonce included in the time stamp token for the password is confirmed.

  Specifically, the nonce acceptable value = nonce match confirmation number is set, and the nonce match confirmation number is subtracted every time the difference value is calculated and the nonce match is confirmed. Further, when the nonce matches, the nonce acceptable value is subtracted. Only when the total difference value is equal to or less than the time stamp acceptance allowable value and the nonce match confirmation number = the nonce acceptable number = 0, the secret key acquisition unit 32 encrypts the secret key and the password. The secret key is extracted from the signature, and the signature execution unit 33 executes the signature. The secret key acquisition unit 32 erases the secret key after executing the signature.

  Note that the timing at which a time stamp request is transmitted may be increased, and a time stamp request may be made for a combination of difference value calculation locations or a combination of allowable value setting locations. Further, the time stamp server 4 that accepts the time stamp request may control the acceptance allowable value, and determine whether or not the signature is possible based on the presence or absence of the time stamp token.

Next, the operation of the electronic signature system 100 according to the second embodiment will be described in detail with reference to FIG.
The signer client 2 sets a nonce acceptable value = 2 and a time stamp acceptable value T 1 (step S101), and selects a signature target document (step S2). Next, a nonce that is a random number is generated and stored (step S3).

  The signer client 2 creates a time stamp request for the signature target document using this nonce and transmits it to the time stamp server 4 (steps S4 and S23). Further, the signature target document, the nonce acceptable value, and the time stamp acceptable value T are transmitted to the signature server 3 (steps S105 and S12).

  Next, the signer client 2 inputs a password in order to encrypt the secret key for signing the document to be signed (step S6), and encrypts the secret key using the password-based encryption method (PKCS # 5 format). (Step S7). Further, a time stamp request for each of the encrypted private key and password is created using the same nonce as used when generating the time stamp request for the signature target document in step S4, and the time stamp server 4 (Steps S8, S10, S26, S29). Further, the encrypted secret key and password are transmitted to the signature server 3 (steps S9, S11, S13, S14).

  The signature server 3 verifies the validity of the time stamp token using the signature target document transmitted from the signer client 2 and the time stamp token for the signature target document transmitted from the time stamp server 4. Save the nonce contained in the token. Further, the difference value T_D1 = T_G1-T_V1 is calculated from the time T_V1 when the validity of the time stamp token is verified and the time information T_G1 acquired from the time stamp token (steps S115, S24, and S25).

  Next, the signature server 3 sets the nonce acceptable number = 2 to the nonce match confirmation number, and the encrypted secret key transmitted from the signer client 2 and the encrypted secret transmitted from the time stamp server 4. Whether the nonce included in the time stamp token after verifying the validity of the time stamp token using the time stamp token for the key matches the nonce included in the time stamp token for the signature target document Confirm. After the confirmation process, the nonce match confirmation number is subtracted to be 1. Further, when it is confirmed that the nonce matches, the nonce acceptable number is subtracted to be 1. Further, the difference value T_D2 = T_G2-T_V2 is calculated from the time T_V2 when the validity of the time stamp token is verified and the time information T_G2 acquired from the time stamp token (steps S116, S27, S28).

  Next, after verifying the validity of the time stamp token using the password transmitted from the signer client 2 and the time stamp token for the password transmitted from the time stamp server 4, it is included in the time stamp token. It is confirmed whether the nonce and the nonce included in the time stamp token for the signature target document match. After the confirmation process, the nonce match confirmation number is subtracted to zero. Furthermore, when it is confirmed that the nonce matches, the nonce acceptable number is subtracted to 0. Further, the difference value T_D3 = T_G3-T_V3 is calculated from the time T_V3 when the validity of the time stamp token is verified and the time information T_G3 acquired from the time stamp token (steps S117, S30, and S31).

  Next, the signature server 3 encrypts the secret only when the total difference value (T_D1 + T_D2 + T_D3) is equal to or smaller than the time stamp acceptance allowable value T and the nonce match confirmation number = the nonce acceptable number = 0. The secret key is extracted using the key and password (steps S118 and S18). Further, the signature server 3 executes a signature on the document to be signed (step S19), and transmits the signature data to the signer client 2 (steps S21 and S22). Further, the private key is deleted after the signature is executed (step S20).

  As described above, according to the present embodiment, the signer client 2 transmits the time stamp acceptance allowable value T to the electronic signature server 3, and the signature server 3 verifies the validity of each time stamp token. The difference value is calculated from the time acquired from the time stamp token, and the electronic signature is executed only when the sum of the difference values is equal to or smaller than the time stamp acceptance allowable value T. The key and password combination can be verified more reliably.

A part or all of the above embodiment can be described as in the following supplementary notes, but is not limited thereto.
(Supplementary note 1) a signature server for giving an electronic signature to a signature target document;
The signing server comprises a signer client that transmits the document to be signed and a secret key used for giving the electronic signature,
The signer client
When sending the document to be signed to the signing server, a first time stamp token including an identifier is sent to the signing server via the time stamp server,
When transmitting the private key to the signing server, a second time stamp token including the identifier is transmitted to the signing server via the time stamp server;
The signing server
An electronic signature system that applies an electronic signature to the signature target document using the private key only when the identifiers included in the first time stamp token and the second time stamp token match.

(Supplementary note 2) The electronic signature system according to supplementary note 1, wherein
The signer client sends an identifier acceptable value to the signing server;
The signature server assigns an electronic signature to the signature target document using the private key only when the identifier acceptable value matches the number of time stamp tokens including the same identifier. .

(Appendix 3) The electronic signature system according to Appendix 2,
The signer client transmits a time stamp acceptance allowable value to the electronic signature server,
The signing server includes a difference between a time stamped on the first time stamp token and a time when an identifier included in the first time stamp token is verified, and a time stamped on the second time stamp token. An electronic signature is attached to the signature target document using the private key only when the total value of the difference between the times when the identifiers included in the second time stamp token are confirmed is equal to or less than the time stamp acceptance allowable value. An electronic signature system.

(Supplementary note 4) The electronic signature system according to supplementary note 1, wherein
The signer client encrypts the private key with a password and then sends it to the signing server, and sends the password to the signing server,
When transmitting the password to the signing server, a third time stamp token including the identifier is transmitted to the signing server via the time stamp server;
The signing server
Obtaining the secret key using the password only when the identifiers included in the first timestamp token, the second timestamp token, and the third timestamp token match, and An electronic signature system that gives an electronic signature to the signature target document using the secret key.

(Supplementary Note 5) A signature server for giving an electronic signature to a signature target document,
Receiving from the signer client the document to be signed and the private key used to give the electronic signature;
When receiving the document to be signed, a first time stamp token including an identifier is received from a time stamp server;
A transmission / reception unit that receives a second time-stamp token including the identifier from the time-stamp server when receiving the secret key;
A signature execution unit that gives an electronic signature to the signature target document using the private key only when the identifiers included in the first time stamp token and the second time stamp token match. Signing server.

(Appendix 6) A signer client that requests a signature server to give an electronic signature to a signature target document,
Sending the document to be signed to the signing server and sending a first time stamp token including an identifier to the signing server via the time stamp server;
A signer having a transmission / reception unit that transmits a secret key used to give the electronic signature to the signature server and transmits a second time-stamp token including the identifier to the signing server via the time-stamp server. client.

(Supplementary note 7) The signer client transmits the document to be signed to the signing server and transmits a first time stamp token including an identifier to the signing server through the time stamp server.
The signer client transmits a secret key used to give the electronic signature to the signing server, and transmits a second time stamp token including the identifier to the signing server via the time stamp server;
An electronic signature that gives an electronic signature to the signature target document using the private key only when the signature server includes the identifier included in the first time stamp token and the second time stamp token. Signature method.

(Supplementary note 8) A program for causing a computer to function as a signature server for giving an electronic signature to a signature target document,
The computer,
Receiving from the signer client the document to be signed and the private key used to give the electronic signature;
When receiving the document to be signed, a first time stamp token including an identifier is received from a time stamp server;
A transmission / reception unit that receives a second time-stamp token including the identifier from the time-stamp server when receiving the secret key;
A signature execution unit that gives an electronic signature to the signature target document using the private key only when the identifiers included in the first time stamp token and the second time stamp token match. A program to function.

(Supplementary Note 9) A program for causing a signature server to function as a signer client that requests a signature server to give an electronic signature to a signature target document,
The computer,
Sending the document to be signed to the signing server and sending a first time stamp token including an identifier to the signing server via the time stamp server;
A program that functions as a transmission / reception unit that transmits a secret key used to give the electronic signature to the signature server and transmits a second time stamp token including the identifier to the signature server via the time stamp server.

  1 Certificate Authority System, 2 Signer Client, 3 Signature Server, 4 Timestamp Server, 5 Communication Network, 11 Registration Unit, 12 Issuing Unit, 21,25 Transmission / Reception Unit, 22 Secret Key Management Unit, 23, 24 Signature Target Document Management , 31, 34 Transmission / reception unit, 32 Private key acquisition unit, 33, 35 Signature execution unit, 41 Transmission / reception unit, 42 Signature execution unit, 100 Electronic signature system

Claims (9)

A signature server for giving an electronic signature to a document to be signed;
The signing server comprises a signer client that transmits the document to be signed and a secret key used for giving the electronic signature,
The signer client
When sending the document to be signed to the signing server, a first time stamp token including an identifier is sent to the signing server via the time stamp server,
When transmitting the private key to the signing server, a second time stamp token including the identifier is transmitted to the signing server via the time stamp server;
The signing server
An electronic signature system that applies an electronic signature to the signature target document using the private key only when the identifiers included in the first time stamp token and the second time stamp token match. The electronic signature system according to claim 1,
The signer client sends an identifier acceptable value to the signing server;
The signature server assigns an electronic signature to the signature target document using the private key only when the identifier acceptable value matches the number of time stamp tokens including the same identifier. . The electronic signature system according to claim 2,
The signer client, before Symbol station name server, and sends the time stamp accepted allowable value,
The signing server includes a difference between a time stamped on the first time stamp token and a time when an identifier included in the first time stamp token is verified, and a time stamped on the second time stamp token. An electronic signature is attached to the signature target document using the secret key only when the total value of the difference between the times when the identifiers included in the second time stamp token are verified is equal to or less than the time stamp acceptance allowable value. An electronic signature system. The electronic signature system according to claim 1,
The signer client encrypts the private key with a password and then sends it to the signing server, and sends the password to the signing server,
When transmitting the password to the signing server, a third time stamp token including the identifier is transmitted to the signing server via the time stamp server;
The signing server
Obtaining the secret key using the password only when the identifiers included in the first timestamp token, the second timestamp token, and the third timestamp token match, and An electronic signature system that gives an electronic signature to the signature target document using the secret key. A signing server that gives an electronic signature to a document to be signed,
Receiving from the signer client the document to be signed and the private key used to give the electronic signature;
When receiving the document to be signed, a first time stamp token including an identifier is received from a time stamp server;
A transmission / reception unit that receives a second time-stamp token including the identifier from the time-stamp server when receiving the secret key;
A signature execution unit that gives an electronic signature to the signature target document using the private key only when the identifiers included in the first time stamp token and the second time stamp token match. Signing server. A signer client that requests a signature server to give an electronic signature to a document to be signed,
Sending the document to be signed to the signing server and sending a first time stamp token including an identifier to the signing server via the time stamp server;
A signer having a transmission / reception unit that transmits a secret key used to give the electronic signature to the signature server and transmits a second time-stamp token including the identifier to the signing server via the time-stamp server. client. A signer client sends a document to be signed to a signing server and sends a first time stamp token including an identifier to the signing server via the time stamp server;
The signer client transmits a secret key used to give the electronic signature to the signing server, and transmits a second time stamp token including the identifier to the signing server via the time stamp server;
An electronic signature that gives an electronic signature to the signature target document using the private key only when the signature server includes the identifier included in the first time stamp token and the second time stamp token. Signature method. A program for causing a computer to function as a signature server for giving an electronic signature to a document to be signed,
The computer,
Receiving from the signer client the document to be signed and the private key used to give the electronic signature;
When receiving the document to be signed, a first time stamp token including an identifier is received from a time stamp server;
A transmission / reception unit that receives a second time-stamp token including the identifier from the time-stamp server when receiving the secret key;
A signature execution unit that gives an electronic signature to the signature target document using the private key only when the identifiers included in the first time stamp token and the second time stamp token match. A program to function. A program that causes a signature server to function as a signer client that requests a signature server to give an electronic signature to a signature target document,
The computer,
Sending the document to be signed to the signing server and sending a first time stamp token including an identifier to the signing server via the time stamp server;
A program that functions as a transmission / reception unit that transmits a secret key used to give the electronic signature to the signature server and transmits a second time stamp token including the identifier to the signature server via the time stamp server.

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