JP4832752B2 - Encryption mail server - Google Patents

Description

  The present invention relates to a server that encrypts and communicates electronic mail, and more particularly to decryption of encrypted electronic mail and verification of an electronic signature.

  Encrypt e-mail to prevent it from being decrypted by a third party, or add an e-signature to the e-mail so that it can be confirmed that the e-mail has not been tampered with in the middle (Patent Documents 1 and 2).

By the way, the inventor has paid attention to the following problems when receiving an encrypted electronic mail or an electronic mail with an electronic signature. In other words, if the ciphertext is decrypted by the mail server or the electronic signature is verified and transferred to the client, the fact that it was encrypted or the fact that the electronic signature was applied is not described. For this reason, it becomes unknown on the client side that it has been encrypted or has been digitally signed. To avoid this, if the client side performs decryption or verification of the electronic signature, the burden on the client increases.
JP 2002-368823 A JP 2003-198632 A

An object of the present invention is to make it clear on the client side that there is no need to decrypt the e-mail on the client side, and that the e-mail has been securely transmitted by encrypted communication.
Another object of the present invention is to reduce risk by storing plain text obtained by decrypting an encrypted electronic mail in a local mailbox.

The encrypted mail server of the present invention performs encryption and decryption of electronic mail, electronic signature and verification thereof, and can be freely connected to a client via a LAN in an encrypted mail server having a local mailbox. In addition, a decryption means for decrypting an encrypted email from outside the LAN, and a comment indicating that it has been received in ciphertext are added as a comment.
By providing a transfer means for transferring the e-mail to the client in plain text, when receiving an encrypted e-mail, a keyword obtained from the plain text decrypted by the decryption means and a comment indicating that the e-mail has been received in cipher text; Is stored in the local mailbox together with the encrypted email as a parameter, the plaintext is erased, and a response to the mail check from the client is made based on the parameter and the header of the encrypted email. When requesting downloading of encrypted electronic mail, and decodes the plaintext by the decoding means, characterized in that so as to transfer to the client by the pre-Symbol transfer means.

Preferably, when the electronic signature of the e-mail from outside the LAN is verified, the transfer means adds the verification result as a comment and transfers it to the client.
Preferably, an encryption unit for encrypting a plaintext unsigned e-mail received from a client and an e-signature unit for enciphering the e-mail are provided. The sent e-mail is transmitted outside the LAN.

In this invention, since the encrypted mail server decrypts the received encrypted electronic mail, the client does not need to decrypt it.

Furthermore, in the present invention, since it is not necessary to store plain text obtained by decrypting the encrypted electronic mail in the local mailbox, it is safe even if unauthorized access is made to the local mailbox.

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

  1 to 9 show the encrypted mail servers 2 and 3 of the embodiment and their modifications. Reference numeral 4 denotes a mail agent, which transmits and receives electronic mails inside and outside the LANs 24 and 25 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. . The configurations of the encrypted mail servers 2 and 3 are the same, and the reference numerals 24 and 25 are used to distinguish between the LAN on the transmission side and the LAN on the reception side.

  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, a 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 is processed with the private key. Add as 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 give priority to keywords such as “important” and “emergency” that appear in the header and body, and descriptions in the subject field, over the destination. 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 can be used, for example, whether only a shared certificate can be used or whether an individual certificate can be used. The scope of the certificate and the criteria of which certificate is used 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 local mail box 22 stores outgoing mail and incoming mail for each client.

  23 is a comment section, which receives an encrypted electronic mail, and when decrypted by the decryption section 12, adds a comment to the effect that it has been received as an encrypted electronic mail to a plain text electronic mail to be transferred to a client in the LAN 24. To do. When an electronic mail with an electronic signature is received from outside the LAN 24, the verification result of the electronic signature, that is, information specifying the sender such as the name of the sender, and the fact that no alteration has been made are commented. It is added to the e-mail to be transferred to the client in the LAN 24. If verification of the electronic signature fails, an electronic mail that may have been tampered with in the communication process is added as a comment.

  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 servers 2 and 3 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. The Internet facsimile machine 26 and the encrypted mail servers 2 and 3 operate as POP clients of the external mail servers 32 and 33 and are unique as SMTP servers. Send and receive emails. 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 electronic mail to / from, for example, the encrypted mail server 3 via the router 31. The LANs 24 and 25 installed on the ground side, not a wireless LAN, are regarded as a safe environment, and e-mails are transmitted and received in plain text in the LANs 24 and 25, and an electronic signature is unnecessary. Encryption and electronic signatures at the time of transmission to the outside of the LANs 24 and 25 are performed by the encryption mail servers 2 and 3, and decryption of electronic mails received from outside the LAN 24 and verification of electronic signatures are also performed by the encryption mail servers 2 and 3. Do. For this reason, the client does not need to allocate resources for encryption communication or electronic signature.

  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, its algorithm, or the like may be described. 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, and a global mail address and a global address for transmission / reception with the outside of the LAN are also stored. 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 for necessity of encryption of the e-mail to be sent, a public key certificate number and a public key on the destination (destination) side And the algorithm, the certificate authority that issued the certificate, and the certificate expiration date 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 example of a public key certificate 40 for encryption and electronic signature. For example, a certificate version, a certificate number, and an algorithm used for signature are described. Then, the name of the certificate authority that issued the certificate, the start date and the last date of the certificate expiration date are described, and the certificate owner's information, that is, the encryption and electronic signature described in the certificate are disclosed Information on the person who uses the key and information on the client is described. A public key and an algorithm are described for both encryption and electronic signature.

FIG. 6 and FIG. 7 show, as a reference example , processing when adding an encrypted electronic mail and the verification result of the electronic signature as a comment. Here, as shown in FIG. 7, an encrypted e-mail or electronic signature is sent from a client such as the Internet facsimile machine 26 to the receiving side encrypted mail server 3 via the transmitting side mail server such as the encrypted mail server 2. Suppose you send an e-mail with The encrypted mail server 3 performs decryption and verification of the electronic signature, adds corresponding comments 52 and 53 to the header or body of the electronic mail, and transfers them to the client's Internet facsimile machine 27 or the like in plain text.

  In the algorithm of FIG. 6, the encrypted mail server on the receiving side checks whether there is a new mail from a mail server outside the LAN (step 1), and if there is a new mail (step 2), downloads the e-mail (step 3). ). If the e-mail is encrypted (step 4), the e-mail is decrypted in step 5, and in step 6, a comment 52 such as “This e-mail is encrypted (received securely)” Is added. Note that Steps 5 and 6 are omitted in the e-mail received in plain text.

  In step 7, the presence or absence of an electronic signature is checked. If an electronic signature exists, the electronic signature is verified in step 8. As the comment 53 when the verification result is OK, “signature verification OK; signer XUZ; signature expiration date; received by the signer as sent without being falsified” or the like is added. If the verification fails, a warning is added as a comment that there is a possibility that the e-mail has been altered on the communication path. If the electronic signature is not applied, steps 8 and 9 are omitted. If the electronic signature is applied to the ciphertext, Step 7 to Step 9 are executed before Step 4 to Step 6. The e-mail that has been decrypted and verified is stored in the local mailbox (step 10).

FIG. 8 and FIG. 9 show an algorithm according to an embodiment in which measures against unauthorized access to an encrypted mail server from outside the LAN or the like are added. This algorithm is the same as the algorithm of FIG. 6 (steps 1 to 9) until the verification result of the electronic signature is added as a comment in step 9. However, since the decryption in step 5 is for obtaining parameters, for example, only the text is decrypted and the attached file need not be decrypted. The header is normally sent without being encrypted.

  In order to eliminate the need to store the decrypted email in the local mailbox, it is checked whether or not the email is encrypted in step 11. In the case of an encrypted e-mail, in step 12, a comment such as being sent in ciphertext, a signer of the electronic signature or not being tampered with in the middle, the presence or absence of an attached file, and its data length , Send date and other keywords, etc. as parameters. In step 13, the decrypted data is erased, and the encrypted electronic mail and parameters are stored in the local mailbox (step 14). As a result, even if unauthorized access to the local mailbox is performed, it is possible to prevent access to the plain text of the electronic mail.

Subsequently, in the algorithm of FIG. 9, the presence / absence of a mail check from the client is checked in step 21, and if the mail check is performed, a response is made based on the data in the local mailbox (step 22). That is, when a plain text electronic mail is stored in the local mailbox, a response is made based on the plain text, and when a cipher text is stored, a response is made with an unencrypted portion such as a header or a parameter. As a result, the client side can know the length of the electronic mail, the presence / absence of an attached file, the transmission date, the presence / absence of encryption, the presence / absence of an electronic signature, the verification result, the sender, and the like. In step 23, it is determined whether or not to download the mail. When the mail is downloaded, if it is an encrypted mail (step 24), the electronic mail is decrypted and transmitted to the client (step 25 and step 26). In the case of plain text mail, it may be transferred to the client as it is (step 26). In the case of a command other than the mail download command, processing is performed in accordance with the command requested by the client (step 27).

In the embodiment, the following effects can be obtained.
(1) Since encryption and decryption of electronic mail, electronic signature and verification thereof can be performed by an encrypted mail server, the client need not have such ability.
(2) If multiple certificates are available for the electronic signature to be sent to the e-mail to be sent, the client can select the certificate to be used.
(3) Risk can be reduced by storing plain text decrypted from encrypted email in the local mailbox.

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 Diagram showing certificate authority certificate Flow chart showing the algorithm at the time of reception in the reference example The figure which shows typically the transmission process from the Internet facsimile machine in the reference example to the Internet facsimile machine The flowchart which shows the algorithm at the time of reception in an Example Flowchart showing the algorithm following FIG.

Explanation of symbols

2, 3 Cryptographic mail server 4 Mail agent 6 Web server 8 Encryption unit 10 Electronic signature unit 12 Decryption unit 14 Verification unit 16 Certificate database 18 Client database 20 Destination database 22 Local mailbox 23 Comment units 24, 25 LAN
26, 27 Internet facsimile machine 28 Personal computer 30, 31 Router 32, 33 Mail server 40 Certificate 50 E-mail 51 Attached files 52, 53 Comments

Claims (1)

In an encrypted mail server that performs encryption and decryption of electronic mail, electronic signature and verification thereof, and a local mailbox,
The e-mail server can be freely connected to the client via the LAN, decryption means for decrypting the encrypted e-mail from outside the LAN, and a comment indicating that the e-mail is received in the ciphertext are added as a comment. By providing a transfer means to transfer to the client in plain text,
When the encrypted email is received, the keyword obtained from the plaintext decrypted by the decryption means and the comment indicating that the encrypted text is received as parameters are stored together with the encrypted email in the local mailbox, and Clear plaintext,
In response to a mail check from the client based on the parameters and the header of the encrypted email, when the client requests download of the encrypted email,
It decodes the plaintext by the decoding means, and characterized in that the pre-Symbol transfer means to forward to the client, encrypted mail server.

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