Patent Publication Number: US-2002004899-A1

Title: Secure mail proxy system, method of managing security, and recording medium

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a secure mail proxy system and a method of managing security for ensuring the security of electronic-mail, and to a recording medium in which a program is recorded.  
       [0003] 2. Description of the Related Art  
       [0004] As systems for ensuring the security of electronic-mail, mail clients are widely used that are equipped with security capabilities such as: S/MIME (Secure Multipurpose Internet Mail Extension; Developed by RSA Data Security Inc.) for transmitting encrypted mail messages in MIME format; and PGP (Pretty Good Privacy; an encryption program developed by PGP Inc. in which the mail content is encrypted using a public key of the transmission partner and then transmitted).  
       [0005] One method typically used to realize effective functioning of security involves installing beforehand one&#39;s own secret key as well as the transmission partner&#39;s digital identification in the terminal that one is using.  
       [0006] However, systems of the prior art for ensuring the security of electronic-mail have the following problems:  
       [0007] The range of terminals that receive mail has increased from PC (personal computer) terminals of the prior art to terminals such as portable telephones, portable information terminals, and FAX (facsimile), and this range has further been augmented by terminals not having mail clients equipped with security functions, and as a result, mail security could not be ensured on the Internet.  
       [0008] In addition, the incorporation of security functions on the terminal side has been problematic in portable telephones, which have quickly become popular, and this weakness has been an important factor in preventing the use of the portable telephones for business.  
       SUMMARY OF THE INVENTION  
       [0009] The present invention was achieved in view of the above-described problems, and has as an object the provision of a system and method, as well as a recording medium, that can ensure the security of electronic-mail on the Internet regardless of whether security functions are incorporated on the client side.  
       [0010] In the present invention for realizing the above-described object, a proxy server is arranged between a mail server and the Internet for carrying out processing relating to security of electronic-mail. This proxy server is provided with a means for encrypting and decrypting electronic-mail, attaching signatures, and detecting falsification, and thus can ensure security of electronic-mail on the Internet regardless of the type of mail server, mail client or user terminal that is used by the user and regardless of whether mail security functions are incorporated in the mail server, mail client, or user terminal.  
       [0011] In the present invention, a proxy server is arranged between a mail server and the Internet for carrying out processing relating to the security of electronic-mail. Ordinary-text mail that has not been encrypted or not bearing a signature is transmitted to a mail server from a mail client that is connected to a LAN, this mail server detects whether or not the address of this mail is in the LAN, and sends only mail having an address outside the LAN to a proxy server as ordinary text without alteration. The proxy server includes means for encrypting ordinary-text mail that has been received from a mail server such that only the mail recipient can decrypt the mail; and means for attaching the signature of the mail originator to the mail and transmitting the encrypted mail with attached signature to the Internet.  
       [0012] The proxy server further includes: means for, when encrypted mail with attached signature has been transmitted in by way of the Internet addressed to a mail server, checking whether or not the mail has been subjected to falsification, and if the mail has not been subjected to falsification, decrypting the encrypted mail to ordinary text and transmitting to the mail server; and means for, if mail has been subjected to falsification, rejecting the reception of the mail to prevent entry of the mail into the LAN.  
       [0013] The user uses the mail client to request the mail server for mail that has been received, and receives ordinary text mail from the mail server.  
       [0014] The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015]FIG. 1 is a block diagram showing the system configuration of the first embodiment of the present invention.  
     [0016]FIG. 2 is a block diagram showing an example of the construction of a proxy server in the first embodiment of the present invention.  
     [0017]FIG. 3 is a flow chart for explaining operations when sending mail from a mail client in the first embodiment of the present invention.  
     [0018]FIG. 4 is a flow chart for explaining operations when encrypted mail with attached signature has been received from the Internet in the first embodiment of the present invention.  
     [0019]FIG. 5 is a schematic view of an example of combinations of electronic-mail addresses and secret keys that are stored in the secret key storage unit in the first embodiment of the present invention.  
     [0020]FIG. 6 is a schematic view of an example of combinations of electronic-mail addresses and public keys that are stored in the public key storage unit in the first embodiment of the present invention.  
     [0021]FIG. 7 is a block diagram showing the system configuration of the second embodiment of the present invention.  
     [0022]FIG. 8 is a block diagram showing the system configuration of the third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0023] Next, regarding an embodiment of the present invention, a proxy server for carrying out processing relating to the security of electronic-mail is arranged between the Internet and a mail server on a LAN (Local Area Network). This proxy server ensures the security of electronic-mail on the Internet regardless of the type of mail server, mail client or user terminal that is used by the user and regardless of whether security functions are incorporated in the mail server, mail client, or user terminal by performing encryption and decryption of electronic-mail as well as by attaching signatures and detecting falsification.  
     [0024] In FIG. 1, a user uses mail client  3  that is connected to LAN  1  to transmit ordinary-text mail that has not been encrypted or provided with a signature to mail server  2 .  
     [0025] Mail server  2  checks whether or not the address of electronic-mail (hereinafter referred to as simply “mail”) is within LAN  1 , and sends only mail addressed to destinations outside LAN  1  to proxy server  4  as ordinary text without alteration.  
     [0026] Proxy server  4  encrypts the ordinary-text mail that is received from mail server  2  such that only the mail recipient can decrypt the mail, attaches the signature of the mail sender, and sends the encrypted mail with attached signature to Internet  5 .  
     [0027] When encrypted mail with attached signature addressed to mail server  2  is transmitted in from Internet  5 , proxy server  4  checks whether or not the mail has been falsified.  
     [0028] If the mail has not been falsified, proxy server  4  decrypts the encrypted mail, and after converting it to ordinary-text mail, sends it to mail server  2 .  
     [0029] If the mail has been subjected to falsification, proxy server  4  rejects the reception of the mail to prevent the entry of the falsified mail into LAN  1 .  
     [0030] The user uses mail client  3  to request the mail that has been received at mail server  2  and receives the ordinary-text mail from mail server  2 .  
     [0031] Next regarding a more detailed explanation of this embodiment with reference to the accompanying drawings, FIG. 1 is a block diagram showing the system architecture of the secure mail proxy system of the first embodiment of the present invention. Referring to FIG. 1, the first embodiment of the present invention is provided with: LAN  1 , which is a local area network such as Ethernet; mail server  2 , which is an information processor that is connected to LAN  1 ; mail client  3 , which operates on a device such as a personal computer, portable telephone, portable information terminal, or FAX; proxy server, which is an information processor that intermediates between mail server  2  and Internet  5 ; and Internet  5 .  
     [0032]FIG. 2 is a block diagram showing an example of the construction of proxy server  4  in the first embodiment of the present invention. Referring now to FIG. 2, proxy server  4  includes data processor  41  that operates under program control, and storage device  42  that stores information.  
     [0033] Storage device  42  is provided with secret key storage section  421  and public key storage section  422 .  
     [0034] Secret key storage section  421  stores combinations of electronic-mail addresses (hereinafter referred to as simply “mail addresses”) and corresponding secret keys. The secret keys are used when attaching a sender&#39;s signature to electronic-mail, and when decrypting encrypted mail that has been transmitted to a mail address in LAN  1 .  
     [0035] Public key storage section  422  stores combinations of electronic-mail addresses and corresponding public keys. Public keys are used when encrypting electronic-mail such that the mail can be read only by the user of the electronic-mail address that is designated in the address of the electronic-mail, and when checking whether or not electronic-mail has been falsified.  
     [0036] Data processor  41  is provided with: mail encryption means  411 , mail decryption means  412 , mail signature attaching means  413 , mail signature checking means  414 , and data communication means  415 .  
     [0037] Mail encryption means  411  obtains the public key that corresponds to the electronic-mail address of an electronic-mail destination from public key storage section  422 , and encrypts ordinary-text mail using the public key.  
     [0038] Mail decryption means  412  obtains the secret key that corresponds to the electronic-mail address of the electronic-mail destination from secret key storage section  421  and decrypts the encrypted electronic-mail using the secret key.  
     [0039] Mail signature attaching means  413  obtains the secret key that corresponds to the electronic-mail address of the electronic-mail originator from secret key storage section  421 , calculates the electronic-mail message digest (hash value) and, after encrypting these values with the secret key, attaches them to the electronic-mail as the sender&#39;s signature.  
     [0040] Mail signature checking means  414  obtains, from public key storage section  422 , the public key that corresponds to the electronic-mail address of the originator of received electronic-mail, uses the public key to decrypt the signature that is attached to the electronic-mail, and checks whether or not the electronic-mail has been falsified by comparing the values of the signature with the electronic-mail message digest (hash values).  
     [0041] Data communication means  415  receives ordinary-text mail from mail server  2  and transmits encrypted mail with attached signature to Internet  5 , and further, receives encrypted mail with attached signature from Internet  5  and transmits ordinary-text mail to mail server  2 .  
     [0042] In the first embodiment of the present invention, the processing and functions of mail encryption means  411 , mail decryption means  412 , mail signature attaching means  413 , mail signature checking means  414 , and data communication means  415  are realized by a program that is executed by data processor  41 . In this case, the proxy server according to the present invention can be operated by reading the program from a recording medium (magnetic disk, magnetic tape, optical disk, or semiconductor memory) that stores the program to data processor  41  and then executing the program.  
     [0043] Referring now to FIGS.  1  to  6 , a detailed explanation is next presented regarding the operation of the first embodiment of the present invention.  
     [0044]FIG. 3 is a flow chart for explaining operations when sending electronic-mail from mail client  3  in the first embodiment of the present invention. Explanation will begin with the transmission of electronic-mail from mail client  3 .  
     [0045] The user creates electronic-mail using mail client  3  and sends the mail to mail server  2  as ordinary text (Step A 1 ).  
     [0046] Mail server  2  checks whether or not the destination of the mail transmitted from mail client  3  is within LAN  1  (Step A 2 ), sends the ordinary-text mail to proxy server  4  if addressed to a destination outside LAN  1  (Step A 3 ), and if addressed to a destination within LAN  1 , sends the electronic-mail as ordinary text without alteration to mail server  2  that is connected to LAN  1  (Step A 4 ).  
     [0047] Proxy server  4  receives the ordinary-text mail from mail server  2  by means of data communication means  415 , and by means of mail encryption means  411 , obtains the public key that corresponds to the mail address of the destination of the electronic-mail from public key storage section  422 , and encrypts the ordinary-text mail using the public key (Step A 5 ).  
     [0048]FIG. 6 schematically shows an example of combinations of electronic-mail addresses and public keys that are stored in public key storage section  422 .  
     [0049] If the mail address of the mail destination is “usuzuki@abc.com”, “111 . . . 001” is used as the corresponding public key in encryption.  
     [0050] By means of mail signature attaching means  413 , proxy server  4  next obtains the secret key that corresponds to the electronic-mail address of the mail originator from secret key storage section  421 , calculates the message digest (hash values) of the electronic-mail, and, after encrypting these values using the secret key, attaches them as the signature of the mail sender (Step A 6 ).  
     [0051]FIG. 5 shows an example of the combinations of electronic-mail addresses and secret keys that are stored in secret key storage section  421 . If the electronic-mail address of the mail sender is “t-azuma@nec.co.jp”, “101 . . . 001” is used as the corresponding secret key in the signature.  
     [0052] Finally, proxy server  4  sends the encrypted mail with attached signature to Internet  5  by means of data communication means  415  (Step A 7 ).  
     [0053]FIG. 4 is a flow chart for explaining the operation when receiving encrypted mail with attached signature from Internet  5  in the first embodiment of the present invention. The operations when receiving encrypted mail with attached signature from Internet  5  are next explained.  
     [0054] Proxy server  4  receives encrypted mail with attached signature from Internet  5  by means of data communication means  415  (Step B 1 ).  
     [0055] By means of mail signature checking means  414 , proxy server  4  obtains the public key that corresponds to the mail address of the mail originator from public key storage section  422 , decrypts the signature that is attached to the electronic-mail using the public key (Step B 2 ), and detects whether or not the electronic-mail has been falsified by comparing the values of the signature and the electronic-mail message digest (hash values) (Step B 3 ).  
     [0056] In the example of FIG. 6, when the mail address of the mail originator is “u-suzuki@abc.com”, “111 . . . 001” is used as the corresponding public key for decrypting the signature.  
     [0057] If the electronic-mail has not been falsified, proxy server  4  uses mail decryption means  412  to obtain the secret key that corresponds to the mail address of the electronic-mail destination and decrypts the encrypted electronic-mail using the secret key (Step B 4 ).  
     [0058] In the example shown in FIG. 5, if the mail address of the mail recipient is “t-azuma@nec.co.jp”, “101 . . . 001” is used as the corresponding secret key in the decryption of the encrypted message.  
     [0059] The message that has been decrypted to ordinary text is then sent to mail server  2  in LAN  1  by data communication means  415  (Step B 5 ).  
     [0060] In a case in which the electronic-mail has been falsified, however, proxy server  4  rejects the reception of the mail to prevent the falsified mail from entering LAN  1  (Step B 6 ).  
     [0061] Mail server  2  receives the ordinary-text mail from proxy server  4  (Step B 7 ), and returns the ordinary-text mail to mail client [ 3 ] when there is a request from mail client  3  (Step B 9 ).  
     [0062] The user uses mail client  3  to request mail server  2  for mail that has been received (Step B 8 ), and receives ordinary-text mail from mail server  2  (Step B 10 ).  
     [0063] Explanation next regards another embodiment of the present invention.  
     [0064]FIG. 7 is a block diagram showing the construction of the second embodiment of the present invention. Referring to FIG. 7, the second embodiment of the present invention may use any one or all of, for example, public line network  61 , radio communication network  62 , and CATV network  63  as a means for connecting mail client  3  to LAN  1  rather than connecting mail client  3  directly to LAN  1  as in the above-described embodiment.  
     [0065] A dial-up connection form is one example in which mail client  3  is connected to LAN  1  by way of public line network  61  using an Internet connection service provider (ISP).  
     [0066] As an example of connection to LAN  1  by way of radio communication network  62 , connection is realized from a portable telephone by way of a portable telephone dealer that offers an Internet connection service.  
     [0067] As an example of a connection to LAN  1  by way of CATV (cable TV), connection is realized by way of a CATV company that offers an Internet connection service.  
     [0068] Next, regarding the third embodiment of the present invention, we refer to FIG. 8, which is a block diagram showing the construction of the third embodiment of the present invention. Referring to FIG. 8, the present embodiment includes key management server  7  and directory server  8 , and proxy server  4  is not provided with private key storage section  421  and public key storage section  422 .  
     [0069] Key management server  7  is a server provided exclusively for managing combinations of electronic-mail addresses and secret keys as shown in FIG. 5, and directory server  8  is provided exclusively for managing combinations of electronic-mail addresses and public keys, as shown in FIG. 6.  
     [0070] In this embodiment, mail encryption means  411  and mail signature checking means  414  of proxy server  4  acquire public keys from directory server  8 .  
     [0071] In addition, mail decryption means  412  and mail signature attaching means  413  acquire secret keys from key management server  7 .  
     [0072] Other than the acquisition of public keys and secret keys from directory server  8  and key management server  7 , respectively, the processing procedure of proxy server  4  in the third embodiment of the present invention is similar to the procedures shown in FIG. 3 and FIG. 4.  
     [0073] As described in the foregoing explanation, the following effects can be obtained by the present invention:  
     [0074] As the first effect, the present invention can ensure mail security on the Internet without incorporating special software or devices in a terminal that transmits and receives mail.  
     [0075] The effect of the present invention to ensure security is particularly notable in systems that employ, as mail client terminals, the portable telephones and portable information terminals that have rapidly come into wide use. The present invention is effective both because of the great variety of devices to be treated and because of the huge number of units already in use.  
     [0076] The invention is effective because processing that is necessary for ensuring mail security in the present invention is allotted not to user-side terminals, but rather, to a proxy server that is arranged at the connection point with the Internet. The effect of the present invention is also notable because threats to security are far less serious inside the point at which an in-house LAN connects to the Internet than on the Internet itself, and security functions can be concentrated at the point of connection with the Internet.  
     [0077] The second effect of the present invention is a great reduction in management costs for ensuring security. This effect is particularly notable for a user that employs a plurality of terminals because security need not be established at each terminal.  
     [0078] The invention is effective because, in the present invention, the management of secret keys and public keys that are necessary for ensuring security is centralized at the proxy server and security settings are not required for each client.  
     [0079] It is to be understood, however, that although the characteristics and advantages of the present invention have been set forth in the foregoing description, the disclosure is illustrative only, and changes may be made in the arrangement of the parts within the scope of the appended claims.