JP4200965B2 - Cryptographic mail server and its program - Google Patents

Description

  The present invention relates to a server capable of communicating by encrypting an electronic mail, and more particularly to an electronic signature for the electronic mail.

  Encrypt e-mail to prevent it from being decrypted by a third party, add e-signature to e-mail, and confirm that it has been tampered with in the middle of the e-mail from the sender It has been proposed to do so (Patent Documents 1 and 2).

In order to add an electronic signature, it is necessary to authenticate the public key necessary for verifying the signature from the certificate authority and to publish the public key together with the certificate. Note that authenticating the public key is sometimes proved, and the authentication data is simply called a certificate. By the way, if each client has to individually authenticate its own public key, for example, immediately after joining the LAN, an unauthenticated client cannot perform an electronic signature. On the other hand, if a shared certificate determined by each mail server is used by all clients, the receiving side cannot verify from which client the e-mail is sent, and only the mail server can be verified.
JP 2002-368823 A JP 2003-198632 A

SUMMARY OF THE INVENTION An object of the present invention is to enable a client who has not acquired an individual certificate for an electronic signature to sign an electronic mail.
An additional object of the present invention is to enable electronic signatures even when individual certificates expire.
Another additional problem of the present invention is to enable separate use of individual certificates and shared certificates (claim 3).

  The encrypted mail server according to the present invention includes a certificate shared between clients with respect to an electronic signature in an encrypted mail server provided with an electronic mail encryption means and decryption means, and an electronic signature means and an electronic signature verification means. A storage means for storing the individual certificate of the client, and the electronic signature means gives an electronic signature based on the shared certificate to an e-mail from a client that does not have the individual certificate. It is characterized by doing so.

The individual certificate may be a certificate for each client or a certificate for a group of clients that is narrower than the entire client. In addition, an electronic signature using an individual certificate is referred to as an individual electronic signature, and an electronic signature using a shared certificate is sometimes referred to as a shared electronic signature.
Preferably, when the storage unit checks the expiration date of the shared certificate and the individual certificate and detects that the individual certificate has expired, the electronic certificate is generated by the shared certificate. Is granted.
Preferably, a client having an individual certificate can select whether to use a shared certificate or an individual certificate.

  A program for an encrypted mail server according to the present invention is an encrypted mail server program comprising an electronic mail encrypting means and a decrypting means, and an electronic signature means and an electronic signature verification means. In order to store a shared certificate and a client's individual certificate, an electronic signature using a shared certificate is attached to an e-mail from a client that does not have an individual certificate using an electronic signature means. And an instruction to do so.

  According to the present invention, even a client that does not have an individual certificate can perform an electronic signature using a shared certificate (claims 1 to 4).

According to the second aspect of the present invention, when an individual certificate expires, an electronic signature can be performed using a shared certificate before obtaining a certificate reissue.
According to the third aspect of the present invention, the client can use the electronic signature properly according to the importance of the electronic mail.

  In the following, an optimum embodiment for carrying out the present invention will be shown.

  The encryption mail server 2 of an Example is shown in FIGS. Reference numeral 4 denotes a mail agent, which transmits and receives electronic mail using protocols such as SMTP, POP, and IMAP. Reference numeral 6 denotes a Web server for transmitting / receiving Web e-mail as an HTML document by HTTP or the like to a client in the LAN 24, for example. Further, the Web server 6 sends its setting data as an HTML document or the like to the personal computer of the administrator of the encrypted mail server 2 so that the administrator can edit the setting data from his personal computer or the like. .

  Reference numeral 8 denotes an encryption unit, which performs encryption using public key encryption or private key encryption, and has already coped with main types of encryption algorithms performed in transmission / reception of electronic mail. An electronic signature unit 10 adds an electronic signature to an electronic mail to be transmitted. For example, an electronic signature certificate is sent prior to transmission of the electronic signature. Then, the message digest is calculated for the body of the email to be signed or the body and attached file using the private key corresponding to the public key described in the certificate, and the message digest processed with the private key is used as the electronic signature. Append. When the range of data to be signed is narrow, for example, when there are only a few lines of text, it is not necessary to process the entire text with a secret key and use a message digest.

  There are two types of electronic signatures: an electronic signature using a shared certificate for clients in the LAN 24 of the encrypted mail server 2 and an electronic signature using an individual certificate for each client. In the signature strength, it is assumed that an electronic signature with an individual certificate for each client is strong, and a shared electronic signature for the client of the encrypted mail server 2 is weak. The individual electronic signature may be an individual client unit or a group of clients, and may be a group unit smaller than the entire client of the encrypted mail server 2.

  A client that can use a plurality of certificates can select which electronic signature is to be used by specifying a default value or an option by appropriate settings or for each electronic mail to be transmitted. For example, specify a certificate for each destination, or specify the type of certificate by keywords such as “important” or “emergency” appearing in the header or text, or in the subject field. Also, for example, separately from the transmission destination, the certificate used last time can be used for the next transmission unless otherwise specified. Each client can select whether to encrypt an electronic mail, whether to give an electronic signature, and whether to perform both encryption and electronic signature.

  When receiving the encrypted electronic mail, the decryption unit 12 decrypts it into plain text. When the verification unit 14 receives an electronic mail with an electronic signature, the verification unit 14 processes the electronic signature using the sender's public key, and verifies, for example, whether or not it matches the value of the message digest. If these values match, it can be confirmed that the sender is the owner of the public key and that the part to which the electronic signature is given has not been altered in the transmission process.

  The certificate database 16 stores an electronic signature certificate shared between clients of the encrypted mail server 2 and individual certificates of the clients, and manages whether the certificate has expired or has expired. Further, the verification unit 14 checks whether or not the certificate stored in the certificate database 16 has been revoked from the homepage of the certificate authority. In response to an inquiry to the certificate database 16, if the certificate has expired or the like, if there is a certificate that can be substituted for that, the data of the substitute certificate is output. For example, when the user's personal certificate has expired, the shared certificate data is output as an alternative certificate. The client database 18 stores data related to the client of the encrypted mail server 2, and in particular, it is possible to use certificates that can be used, such as whether only a shared certificate can be used or whether individual certificates can be used. The range and the criteria for which certificate to use preferentially are stored.

  The destination database 20 stores, for each e-mail transmission destination, the necessity of encryption, the public key for encryption of the transmission destination and the certificate of the public key for electronic signature, and other public keys for encryption. And the signature algorithm of the public key for verifying the electronic signature are stored. These certificates, for example, are stored in the received e-mail, for example, and periodically check the certificate authority's homepage to check whether the certificate is valid. The mail box 22 stores outgoing mail and received mail for each client.

  Reference numeral 26 denotes an Internet facsimile machine that performs Internet facsimile in a format such as an electronic mail in addition to a G3 facsimile, and transmits and receives an electronic mail by a protocol such as SMTP, IMAP, or POP. The encrypted mail server 2 may be integrated with, for example, the Internet facsimile machine 26 and a part thereof. A personal computer 28 is an example of a client.

  The LAN 24 is connected to an external mail server 32 via a router 30, and the Internet facsimile machine 26 and the encrypted mail server 2 operate as a POP client of the external mail server 32, and independently send e-mail as an SMTP server or the like. Send and receive. The mail server 32 communicates with a mail server 33 on the transmission destination side via a WAN such as the Internet, and transmits / receives an electronic mail to / from an encrypted mail server 3 similar to its own device via the router 31.

  FIG. 2 shows an example of the certificate database 16. Each column shows one certificate, the public key and the corresponding private key, the name of the certificate authority that issued the certificate, the certificate expiration date, and Describes the algorithm of electronic signature. In addition to this, an encryption public key or secret key, an algorithm thereof, and the like may be stored. For each certificate, whether the certificate is shared between the clients of the encrypted mail server or individual certificates is described, and the strength of the signature is described in ranks such as A and B in each individual certificate. For example, it is assumed that the number of clients of the encrypted mail server 2 is 100 in total, 10 of them share the ID2 certificate, and only a specific client among them has the ID3 certificate. Then, the certificate of ID3 is rank B, higher than rank A of the certificate of ID2, and the shared certificate of ID1 is the weakest rank. That is, the smaller the number of clients sharing a certificate, the stronger the certificate. In addition, the owner of each certificate is stored, and the owner of the shared certificate is the entire client of the encrypted mail server 2.

  FIG. 3 shows an example of the client database 18. A local address, a local account, and a local password are stored for each client. Remember account and password. For each client, the ID of a certificate that can be used for electronic signatures or encrypted reception is stored. For example, the client in the left column of FIG. 3 can use only the shared certificate, The client can use certificates 1, 2, and 3.

  The default column stores which certificate is used when there is no individual designation in the case of a client that can use a plurality of certificates. Data prioritizing the default value is stored in the option column, and for example, a certificate to be used can be defined for each destination, or for each appropriate keyword in the header or text. Alternatively, it stores that the same certificate that was used last time should be used as the default value. Each client can select whether an electronic signature is required when requesting transmission of an e-mail, and the type of electronic signature can be selected from an electronic signature for clear text (clear digital signature) or an electronic signature for ciphertext. You can specify whether the certificate used for electronic signature is shared or individual. This designation has priority over the type of certificate determined in the client database 18.

  FIG. 4 shows an example of the destination database 20. For each mail address that is a destination, a default value of necessity of encryption of the e-mail to be transmitted, a public key certificate number and public key of the destination side, its algorithm, The certificate authority that issued the certificate and the expiration date of the certificate are stored. Further, a public key of the electronic signature on the destination side and a signature algorithm are also stored. Providing the destination database 20 facilitates the verification of the electronic signature attached to the received electronic mail and eliminates the need to acquire the public key one by one before sending the electronic mail.

  FIG. 5 shows an algorithm related to encryption of an outgoing e-mail and an electronic signature in the embodiment. When an e-mail to be transmitted is received from a client such as an Internet facsimile machine (step 1), is it specified that the e-mail is required to be encrypted and is specified as a destination that requires encryption in the destination database 20? Whether or not the encryption is specified to be necessary, for example, in either or both, the electronic mail is encrypted (step 3). For example, in this case, a public key for encryption of a transmission destination outside the LAN is acquired from the destination database 20 or from the transmission destination itself, and a secret key is exchanged using this to encrypt an electronic mail. If encryption is not required, step 3 is omitted.

  Next, the electronic mail received from the client checks whether or not an electronic signature is required (step 4). Whether or not the electronic signature is necessary is described in the destination database 20 or the client database 18 with a default value or the like, even if there is no designation from the client, for a specific destination (transmission destination) or a specific client, You may make it carry out an electronic signature. When an electronic signature is required, the type of certificate that can be used is determined from the client database 18 and which certificate is to be used according to the description in the header of the email received from the client and the destination. Determine (step 5). However, in the case of a client that can use only a shared certificate, there is one type of certificate. In the case of a client that can use a plurality of certificates, one of these certificates can be selected. Furthermore, the certificate database 16 checks the expiration date of the certificate. If the certificate has expired, a lower-ranked certificate, for example, a shared certificate is used.

  If a certificate for each client can be used, an electronic signature is performed based on the certificate (step 6). If a shared certificate is used, an electronic signature is made based on the certificate (step 7). The type of electronic signature may be an electronic signature for plain text or an electronic signature for ciphertext. Although FIG. 5 shows that encryption is performed first and an electronic signature is added later, the encryption may be performed after the electronic signature is applied first. Subsequently, the source address is replaced with the global address from the client's local address (step 8), and an e-mail is requested to be sent to an external mail server via the router by POP (step 9), or SMTP or the like. To send the e-mail directly to the WAN side.

In the embodiment, the following effects can be obtained.
(1) Even a client who does not have an individual certificate can use a shared certificate for electronic signature.
(2) Although it is easy for the client's individual certificate to expire, in such a case, an electronic signature can be temporarily created using a shared certificate.
(3) For clients that can use multiple certificates, it is possible to select which certificate to use.

The block diagram which shows the structure of the encryption mail server of an Example, and the system structure using this The figure which shows typically the certificate database used in the Example The figure which shows the client database used in an Example typically The figure which shows typically the destination database used in an Example The flowchart which shows the algorithm at the time of transmission in an Example

Explanation of symbols

2, 3 Cryptographic mail server 4 Mail agent 6 Web server 8 Encryption unit 10 Digital signature unit 12 Decryption unit 14 Verification unit 16 Certificate database 18 Client database 20 Destination database 22 Mailbox 24 LAN
26 Internet facsimile machine 28 Personal computer 30, 31 Router 32, 33 Mail server

Claims (4)

In an encrypted mail server provided with an electronic mail encryption means and decryption means, an electronic signature means and an electronic signature verification means,
For electronic signatures, a storage means is provided for storing a certificate shared between clients and an individual certificate of the client.
The cryptographic mail server, wherein the electronic signature means adds an electronic signature based on the shared certificate to an electronic mail from a client having no individual certificate. When the storage unit checks the expiration date of the shared certificate and the individual certificate and detects that the individual certificate has expired, the electronic certificate is given by the shared certificate. The encrypted mail server according to claim 1, wherein the encrypted mail server is configured as described above. 3. The encrypted mail server according to claim 1, wherein a client having an individual certificate can select whether to use a shared certificate or an individual certificate. In a program for an encrypted mail server provided with an electronic mail encryption means and decryption means, an electronic signature means and an electronic signature verification means,
For electronic signatures, a storage instruction for storing a certificate shared between clients and a client's individual certificate;
A program for an encrypted mail server, characterized in that an instruction for giving an electronic signature by a shared certificate to an electronic mail from a client having no individual certificate is provided.

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