Patent Publication Number: US-2007124586-A1

Title: Dedicated communication system and dedicated communicating method

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a dedicated communication system and a dedicated communicating method, and more specifically to a dedicated communication system and a dedicated communicating method capable of realizing communications by delivering electronic mail for a guarantee of anonymity, VoIP (Voice over IP), etc.  
      2. Description of the Related Art  
      Recently, there are an increasing number of problems of spam mail by revealing personal information such as a personal mail address, etc. to a malicious third party. Preventive means against them can be mail filtering. However, the mail filtering often gets into a vicious circle with a spammer, and becomes ineffective, and mail which is to be correctly delivered can be filtered out and may not be delivered. Lately, there has been a method of generating a plurality of temporary dedicated mail addresses to be used for the respective communication partners, thereby designating the revealing source of addresses, rejecting the reception of the corresponding address, thus preventing spam mail.  
      Conventionally, a plurality of mail addresses can be freely generated using an extended mail address such as qmail, etc. However, in this method, a transmitter can be estimated and an original address can be revealed to a spammer. On the other hand, Address Guard of Yahoo (registered trademark) and privango of NTT (registered trademark) are used with an alias with the original user name concealed. According to JP2003-141042A, a random address is generated, and a dedicated address is issued to the original address.  
      JP2004-228832A describes the technology of preventing spam mail from being transmitted to a destination by transmitting inquiry information to a transmitter and deleting it when no response is received.  
      JP2003-186805A describes the technology of storing a conversion table indicating the correspondence among mail, an encryption key, and an ID, and encrypting a mail address using the conversion table.  
      JP2004-15180A describes the technology of providing an address conversion server for storing each address of a number of members associated with a unique reception permitted domain name, and not transmitting mail to a member when a source domain name does not match a reception permitted domain name.  
      JP2004-23592A describes the technology of concealing the original mail address by converting a mail address.  
     SUMMARY OF THE INVENTION  
      The above-mentioned technologies of Address Guard, privango, and JP2003-141042A have the feature of high anonymity, but it is necessary to manage all conversion table of dedicated addresses by a mail server. To suppress the pressure of the conversion table by an increasing number of zombie addresses that never appear again, it is necessary for a user to manually generate a dedicated address, and explicitly manage the dedicated address to use it for a specific communication partner.  
      In the case of the above-mentioned JP2004-228832A, it is necessary to inquire about each piece of mail. Therefore, it is difficult to process mail when a large volume of spam mail is transmitted. In the case of JP2003-186805A, it is necessary to manage a conversion table. JP2004-15180A has no effect when an address has not been registered in advance by a member. In the case of JP2004-23592A, it is necessary to manage an address book corresponding to the conversion table.  
      (Problem 1)  
      In the above-mentioned representative conventional technology relating to the dedicated address, the address of an alias name is used to conceal the original address, but a user manually generates the name, manages the conversion table of an original address and a dedicated address, and manages the correspondence between the dedicated address and a communication partner. That is, it is necessary in the conventional technologies to hold in a mail server, etc. the conversion table between a generated address and its original address. If a dedicated address is automatically generated for each communication partner, the number of dedicated addresses that will never be used again explosively increases, and the mail server has to continuously manage the conversion table.  
      Therefore, the advantage of the present invention is to provide a dedicated mail system and dedicated mail transmitting method capable of eliminating the necessity to manage the conversion table of an original address and a dedicated address and manage the correspondence between the dedicated address and the communication partner on the user and mail server sides by incorporating a conversion table and communication partner information into a mail address, encrypting them, and guaranteeing the confidentiality.  
      (Problem 2)  
      In the above-mentioned representative conventional technology relating to the dedicated address, the Address Guard and the privango generate a dedicated address using an alias for replacement of the address of a user. However, since the alias corresponds one to one to the address of the user, the reveal of the correspondence threatens all the other dedicated addresses to be revealed, and newly generated dedicated addresses are also threatened to be revealed.  
      Therefore, the advantage of the present invention is to provide a dedicated communication system and a dedicated communicating method capable of reducing the influence of the possible reveal of the correspondence between one dedicated address and the address of a user by encrypting and using the address of the user without using an alias corresponding one to one to the address of a user.  
      The dedicated communication system according to a first aspect of the present invention is a dedicated communication system which converts a communication address received from a communication source into a dedicated address and transmits the dedicated address to a communication destination, and includes: encryption means for encrypting communication partner information about a communication partner together with a communication source address; and dedicated address generation means for generating a dedicated address based on an encryption result from the encryption means. The communication source address is replaced with the dedicated address generated by the dedicated address generation means, and is transmitted to the communication destination. With the configuration, communications can be performed with the original address concealed from a communication partner, and it is not necessary for a mail server to manage the dedicated address. Since it is also not necessary for the user to manage the dedicated address, the anonymity can be guaranteed in almost the same method as the conventional technology in communications. The “address” is a broad concept including an SIP (session initiation protocol) address, etc. in addition to the electronic mail address.  
      The dedicated communication system according to a second aspect of the present invention is based on the first aspect, and the dedicated address generation means adds a serial number of a private key used in the encryption by the encryption means when the dedicated address is generated. If a dedicated address is generated with a serial number such as version information, etc. added, and communications are performed, then the damage of the possible reveal of an encrypted private key for any reason can be minimized, and the influence of the reveal can be avoided in the future.  
      The dedicated communication system according to a third aspect of the present invention is based on the first or second aspect, and the dedicated address generation means compresses the communication partner information when the dedicated address is generated. With the configuration, a dedicated address can be shorter than in the case of it being not compressed when it is generated.  
      The dedicated communicating method according to a fourth aspect of the present invention is a dedicated communicating method for converting a communication address received from a communication source into a dedicated address and transmits the dedicated address to a communication destination, and includes: an encrypting step of encrypting communication partner information about a communication partner together with a communication source address; a dedicated address generating step of generating a dedicated address based on an encryption result obtained in the encrypting step; and a step of replacing the communication source address with the dedicated address generated in the dedicated address generating step, and transmitting the address to the communication destination. With the configuration, communications can be performed with the original address concealed from a communication partner, and it is not necessary for a mail server to manage the dedicated address. Since it is also not necessary for the user to manage the dedicated address, the anonymity can be guaranteed in almost the same method as the conventional technology in communications.  
      The dedicated communicating method according to a fifth aspect of the present invention is based on the fourth aspect, and in the dedicated address generating step, the serial number of the private key used in the encryption in the encrypting step is added when the dedicated address is generated. If a dedicated address is generated with a serial number such as version information, etc. added, and communications are performed, then it is not necessary to continuously use the same private key, and the damage of the possible reveal of an encrypted private key for any reason can be minimized, and the influence of the reveal can be avoided in the future.  
      The dedicated communicating method according to a sixth aspect of the present invention is based on the fourth or fifth aspect, and in the dedicated address generating step, the communication partner information is compressed when the dedicated address is generated. With the configuration, a dedicated address can be shorter when it is generated.  
      According to the present invention, a dedicated address is generated for each communication partner for use in communications, thereby performing the communications while concealing the original address from each communication partner. It is not necessary for a mail server, etc. to manage the dedicated address. Since it is also not necessary for a user to manage a dedicated address, the anonymity can be guaranteed in almost the same method as the conventional technology in communications.  
      Furthermore, if a dedicated address is generated with a serial number such as version information, etc. added, and communications are performed, and the damage of the possible reveal of an encrypted private key for any reason can be minimized, and the influence of the reveal can be avoided in the future. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram showing an example of the configuration of the dedicated mail system according to a mode for embodying the present invention;  
       FIG. 2  shows the sequence of an example of the operation performed when a user transmits mail in the dedicated mail system shown in  FIG. 1 ;  
       FIG. 3  shows the related information which can be set in the dedicated address use request field;  
       FIG. 4  shows the sequence of an example of the operation performed when encryption is performed in the dedicated mail system shown in  FIG. 1 ;  
       FIG. 5  shows an example of the format of an option;  
       FIG. 6  shows an example of the format of an option for a valid term;  
       FIG. 7  shows an example of the format of an option of adding communication partner information;  
       FIG. 8  shows an example of a format of an option for a ringing tone;  
       FIG. 9  shows an example of the configuration of the header shown in  FIG. 10 ;  
       FIG. 10  shows the address converting process in the dedicated mail system shown in  FIG. 1 ;  
       FIG. 11  shows the sequence of an example of the operation performed when another user returns mail in the dedicated mail system shown in  FIG. 1 ; and  
       FIG. 12  shows the sequence of an example of the operation performed when a decoding process is performed in the dedicated mail system shown in  FIG. 1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The mode for embodying the present invention is explained by referring to the attached drawings. In each of the figures referred to in the following explanation, the corresponding portions are identified by the same reference numerals.  
      (First Dedicated Mail System: Basic Concept)  
      In the dedicated mail system according to the present invention, an address conversion device (hereinafter appropriately referred to as an “MAC”: Mail Address Converter) is arranged near the mail delivery device (hereinafter appropriately referred to as an “MTA”: Mail Transfer Agent) in order to solve the above-mentioned problem 1. A mail delivery device can also function as an address conversion device. A mail delivery device is classified as a device explained as a mail server in the conventional technology.  
      A transmitter A uses a mail client device (hereinafter appropriately referred to as an “MUA” (Mail User Agent). It is obtained by reforming an existing mail client device, and has a setting of a selection as to whether or not a dedicated address is used. When a dedicated address is used, the mail client device describes an instruction to use a dedicated address and related information in the mail header when mail is transmitted to the mail delivery device (hereinafter referred to as a dedicated address use request field). The related information can be “using the contents of a To field as communication partner information”, etc.  
      The mail delivery device which receives mail analyzes the header. When it confirms the use of a dedicated address, it notifies the address conversion device of the information about the communication partner B and the information (hereinafter referred to as encryption information) required to generate a private key for encryption.  
      The address conversion device generates or acquires a private key, encrypts an address of transmitter A and communication partner information, and returns a part of the dedicated address to the mail delivery device.  
      Since it is hard to understand at a glance whose address a dedicated address is, the mail delivery device can add alias information regardless of the delivery. Furthermore, the mail delivery device adds information about the mail delivery device itself (information after “@”), adds to the header the identifier (hereinafter referred to as a dedicated address identifier, for example, an underline “_”) indicating the dedicated address, thereby generating a dedicated address. The address is replaced with the address of the transmitter A described in the transmitter information such as a From field, etc., and a dedicated address use request field is removed from the mail header. Then, the mail delivery device delivers the mail to the communication partner B described in the destination field such as a To field.  
      No special system or device but a conventional mail system is required for the communication partner B. The communication partner B can return mail to the dedicated address of the transmitter by a return button, etc.  
      The mail transmitted by the communication partner B reaches the mail delivery device of the original transmitter A. The mail delivery device checks the destination address, and if the address refers to the dedicated address identifier, then an encrypted portion is retrieved and passed to the address conversion device. The address conversion device decodes the address of transmitter A, that is, the original destination and the communication partner information, from the portion, and returns it to the mail delivery device.  
      The mail delivery device can verify the address of the communication partner B, that is, the transmitter of the mail if necessary. Then, the dedicated address described as the destination in the mail header is rewritten to the original destination address of the transmitter A, and the information that the mail has been transmitted to the dedicated address (hereinafter referred to as a dedicated address use request field) is added to the mail header and delivered to a corresponding mail box.  
      Thus, mail can be communicated with the communication partner B with the original address of the transmitter A concealed. At this time, it is not necessary for any of the mail client device, the mail delivery device, and the address conversion device to manage the conversion table between the original address of the transmitter A and the dedicated addresses. Furthermore, it is necessary for no device to manage the correspondence between a dedicated address and a communication partner B.  
      Although the mail delivery device has to hold encryption information, it holds the information commonly for all subordinate users. Furthermore, it is also possible that the mail delivery device does not transmit the encryption information to the address conversion device for each encryption and decryption, but the address conversion device temporarily stores the information.  
      In addition, when whole mail address of the communication partner B is encrypted and incorporated as communication partner information in the dedicated address, the dedicated address can be too long. Therefore, a hash value is calculated with an appropriate length from the mail address of the communication partner B, represented by an irreversible code, and used as the communication partner information. That is, the communication partner information can be compressed and the address length can be suppressed. The determination can be described in the dedicated address use request field.  
      When mail is received from the communication partner B, it can be verified whether or not the communication partner information refer to the communication partner B. It can be specified in the dedicated address use request field. The information can be incorporated into the encrypted dedicated address.  
      It is not necessary that the communication partner information always refers to the mail address of the communication partner B, but the any information can be used. For example, when the communication partner B also uses a dedicated address, the transmitter A first uses any information as communication partner information, generates a dedicated address as the communication partner information not to be verified, and publishes it to the communication partner B. To the address, the communication partner B generates a dedicated address and transmits mail.  
      When a dedicated address is shared in a group, all group members can share the dedicated address by generating a dedicated address which is set as communication partner information not to be verified and uses the identification information (mailing address, etc.) about the group as the communication partner information without generating a dedicated address for each group member.  
      Furthermore, various information can be optionally added to the encrypted contents in addition to the address of the transmitter A and the communication partner information about the communication partner B. For example, an option of setting a valid term can be added. Additionally, when a dedicated address is used for a plurality of communication partners, there are an option of describing communication partners in addition to the communication partner information, an option of setting a tone to be raised when mail is received from the communication partner B, etc. The options can be set as a daisy chain.  
      (Second Dedicated Mail System: Countermeasure Against Reveal)  
      To solve the above-mentioned problem 2, the conversion algorithm of the address conversion device in the first dedicated mail system is partially specified.  
      The mail delivery device of the first dedicated mail system holds encryption information for each mail delivery device. The address conversion device encrypts the address of the transmitter A (contents before “@”) and the communication partner information based on the generated private key. Thus, when the communication partner B receives a dedicated address, the original address of the transmitter A is concealed. In the first dedicated mail system, the transmitter A can add an alias of any characters, but since it does not correspond one to one to the address, the second problem does not occur. However, when a private key is revealed, the original address is also revealed, and the problem similar to that of the problem 2 occurs.  
      To solve the problem, in the second dedicated mail system, the encryption information is specified as a combination of seed information for generating random number (hereinafter referred to as seed information) or a private key itself and its version information. The mail delivery device can recognize a private key from the version information. The implementing method can be, for example, managing a private key for each version and using a private key corresponding to version information. Otherwise, the method can be generating a private key using the n-th random number generated from the seed information when the version information is “n”.  
      The address conversion device adds version information after performing an encrypting process based on a private key. When a decoding process is performed, a corresponding private key is first detected from the version information, and then the decoding process is performed.  
      Thus, it is not necessary to constantly use the same private key, and although a private key is revealed, the range of the influence is limited to the use of the same version. Then, by changing the version information, there is no influence of the reveal on a newly generated dedicated address. The seed information and the private key information are stored in the mail delivery device and shared by all subordinate users, but the version information can be separately assigned to each user by the mail delivery device, and the user can also directly specify the version information. The case in which version information is used is described above, but the present invention is not limited to the case, and any serial number can be used.  
      In the conventional technology, when an original address corresponding to an alias is once revealed, there is the risk that the original address will be revealed in the future because it is necessary to use the same alias for a dedicated address to be newly generated. However, in the second dedicated mail system, the possibility can be suppressed.  
      A mail address is exclusively explained above, but an address such as an SIP address, etc. which has a style similar to that of the mail address and is used for identification of a person can also be converted and managed in the same way.  
     EMBODIMENTS  
      An embodiment of the dedicated mail system according to the present invention is explained by referring to the attached drawings. In the present embodiment, the function of a mail delivery device and the function of an address conversion device are implemented in the same device.  
      (Example of System Configuration)  
       FIG. 1  is a block diagram showing an example of the configuration of the dedicated mail system according to the present invention. In  FIG. 1 , the dedicated mail system according to present embodiment includes: a mail client device  10  such as a mobile terminal, etc., a mail delivery device  20 , an address conversion device  30 , another mail delivery device  40 , and a communication partner&#39;s mail client device  50  such as a notebook-size computer, etc. A well-known SMTP (simple mail transfer protocol)/POP (post office protocol), etc. are used for communications between the mail client device  10  and the mail delivery device  20 . A well-known SMTP is used for communications between the mail delivery device  20  and the mail delivery device  40 . The communications between the mail delivery device  40  and the communication partner&#39;s mail client device  50  are performed by a well known SMTP/POP, etc.  
      The mail client device  10  includes a mail transmission/reception unit  11  for transmitting and receiving mail, a mail user interface  12  for performing a mail transmitting and receiving operation, etc., an address book  13  for management of a mail address of a communication partner, and a mailbox  14  for storing transmitted and received mail. The address book  13  and the mailbox  14  are stored in semiconductor memory or a well known storage device. The communication partner&#39;s mail client device  50  has a configuration similar to the configuration of the mail client device  10 .  
      The mail delivery device  20  includes a mail transmission/reception unit  21  for transmitting and receiving mail, a mail determination unit  22  for assorting and filtering mail, a mailbox  23  for storing transmitted and received mail, a filter information management unit  24  for managing filter information, and an encryption information management unit  25  for managing encryption information. The mail delivery device  40  has the same configuration as that of the mail delivery device  20  except the filter information management unit  24  and the encryption information management unit  25 .  
      The address conversion device  30  includes an address conversion unit  31  for converting a mail address, a private key generation unit  32  for generating a private key, and an encryption information management unit  33  for managing encryption information.  
      In the present embodiment, an operation example will be described for a case that a transmitter A as a user who uses the mail client device  10  transmits mail to a communication partner B as a user who uses the communication partner&#39;s mail client device  50 , and the communication partner B returns mail to the transmitter A.  
      (Transmitting Mail)  
      An example of an operation of transmitting mail by the dedicated mail system according to the present mode for embodying the present invention is explained below by referring to  FIG. 2 .  FIG. 2  shows the sequence of an example of the operation performed when a transmitter A transmits mail to a communication partner B using the mail client device  10 .  
      In  FIG. 2 , the transmitter A makes a setting of transmitting mail using a dedicated address through the mail user interface  12  or the address book  13 . For example, there is a method of the transmitter A indicating a check mark in a check box using the mail user interface  12 , or a method of describing the presence/absence of a use of a dedicated address for each communication partner using the address book  13 , and automatically indicating the correspondence between them.  
      When the transmitter A issues an instruction to transmit mail, the mail transmission/reception unit  11  of the MUA  10  adds a “dedicated address use request field” to a mail header (step S 101 ). For example, the following setting is made using the field name X-encode-DEA.  
      X-encode-DEA: yes; Expire=Dec. 2002 12: 52:35+0900: Verify=off  
      Plural pieces of related information can be set in the “dedicated address use request field”. For example, the information shown in  FIG. 3  can be set.  
      The mail transmission/reception unit  11  transmits mail to the MTA  20  (step S 102 ).  
      Upon receipt of mail by the mail transmission/reception unit  11 , the MTA  20  confirms the contents of the mail header by the mail determination unit  22 . At this time, it confirms the presence/absence of the “dedicated address use request field” (step S 103 ). If there is no dedicated address use request field as a result of the confirmation, the mail is normally transmitted as is (steps S 103  to S 110 ), and stored in the mailbox (step S 111 ). Thus, the communication partner B can normally receive mail from the transmitter A (step S 112 ).  
      On the other hand, if there is a dedicated address use request field as a result of the confirmation in step S 103 , the related information is read and control is passed to the process of generating a dedicated address.  
      In generating a dedicated address, an original mail address  300  of the transmitter A is first obtained from the From field of the mail header, and divided into an ID ( 3001 ) and a host name ( 3002 ). Unless otherwise specified in the related information, the address of the communication partner B in the To field is used as communication partner information. Then, the ID, the communication partner information, and the related information are passed to the address conversion unit  31  of the MAC  30  (steps S 103  and S 104 ). In this example, the functions of the MTA  20  and the MAC  30  are implemented in the same device.  
      Then, the address conversion unit  31  performs an encrypting process. The encryption sequence (step S 105 ) of the address conversion unit  31  is described later. The address conversion unit  31  returns encrypted data  304  to the mail determination unit  22  (step S 106 ).  
      Upon receipt of the data  304 , the mail determination unit  22  generates a dedicated address (step S 107 ). In the present example, an underline “_” as an identifier indicating a dedicated address is added to the head of the data, and an at mark “@” and a host name  3002  are added to the end of the data. When an alias is specified in the related information a period “.” and an alias character string are inserted between the data  304  and the at mark “@”. Thus, a dedicated address is generated. The mail delivery device  20  provides a normal mail address with a restriction such that the leading character is a character other than the underline“_”.  
      The mail determination unit  22  replaces the original mail address  300  of the transmitter A described in the From field of the original mail with a dedicated address (step S 108 ), and deletes the “dedicated address use request field” in the header (step S 109 ). Then, the mail is delivered to the mail delivery device  40  of the communication partner B through the mail transmission/reception unit  21  (step S 110 ), and stored in the mailbox (step S 111 ). Thus, the communication partner B can receive from the transmitter A the mail concealed by the dedicated address (step S 112 ).  
      (Encryption Sequence)  
      Next, the encryption sequence by the address conversion unit  31  (step S 105 ) used in the sequence shown in  FIG. 2  is explained by referring to  FIG. 4 .  
      The address conversion unit  31  acquires the ID ( 3001 ) of the transmitter A added to the encryption request, the communication partner information, and the related information from the mail determination unit  22  (step S 201 ). The acquired related information is described in the “dedicated address use request field” including a hash system, an encryption system, possibility of a verification, key version information, valid term, etc.  
      The address conversion unit  31  determines according to related information whether or not a hash is used (step S 202 ). In this example, when a hash is used and the communication partner information is equal to or more than a predetermined number of bytes (for example, 6 bytes), a desired hash value of the communication partner information is calculated. When a desired hash value is calculated, the length of the communication partner information is calculated from the length of an added option (step S 203 ). In this example, at least 6 bytes are used for a desired hash value, and the length of the desired hash value is determined such that the entire length of the data  301  can be the length of a multiple of a predetermined number−1 (for example, 7, 15, 23, . . . , 8n−1 when “Blowfish” is used in the analysis algorithm) when data  301  is generated. In this example, the desired hash length is 9 bytes. Next, a hash algorithm is selected from the related information, and the hash value of the communication partner information is calculated (step S 204 ). For example, when SHA-1 is used, the hash value is 20 bytes. In the leading bytes, a desired hash length is used as new communication partner information.  
      When the communication partner information is determined, it is added to the ID ( 3001 ) and communication partner information  3013 , and other necessary related information is combined as an option, and the data  301  is generated (step S 205 ). In step S 202  described above, when a hash is not used, the data  301  is generated as is. An option to be combined is described in a format as shown in  FIG. 5 . That is, it is described in the format constituted by a “type of option”, a “length of an option”, and “contents of an option”.  
      The option can be, for example, a valid term option  3014  ( FIG. 6 ), a communication partner information additional option  3015  ( FIG. 7 ), and a ringing tone option  3016  ( FIG. 8 ) as shown in FIGS.  6  to  8 . These options can be described after the communication partner information  3014  shown in  FIG. 5 .  
      Back in  FIG. 4 , the address conversion unit  31  checks the encryption algorithm according to the related information (step S 206 ). If the encryption is required, a private key is requested to the encryption information management unit  25  (the encryption information management unit  33  has the same function) according to the encryption system and the version information (step S 207 ). The encryption information management unit  25  first checks whether or not there is a private key in the cache (temporary storage of a key) (step S 208 ). If there is a private key in the cache, the private key is returned to the address conversion unit  31  (steps S 208  to S 211 ).  
      On the other hand, if there is no private key in the cache, the seed information and the version information in the encryption information management unit  25  are passed to the private key generation unit  32 , and the encryption information management unit  25  acquires the private key generated by the private key generation unit  32  (step S 209 ). The private key acquired by the encryption information management unit  25  is temporarily stored in the cache (step S 210 ), and the private key is passed to the address conversion unit  31  (steps S 210  to S 211 ).  
      The seed information is managed by the encryption information management unit  25  of the MTA  20 , and shared by all subordinate users. The version information can be separately assigned to all subordinate users. The transmitter A can explicitly specify the version information in the dedicated address use request field.  
      Then, the address conversion unit  31  encrypts the data  301  using an acquired private key, and generates data  302  (step S 212 ). If the encryption is not required in step S 206 , the data  301  is used as the data  302  as is (step S 213 ).  
      Then, a 2-byte header  3031  is added to the generated data  302 , there by generating data  303  (step S 214 ). The header  3031  has a format as shown in  FIG. 9 , and stores necessary related information.  
      Back in  FIG. 4 , the data  303  is a byte string, and cannot be represented in a character string. Therefore, the data is encoded by the well known Base  64  and converted into a character string (step S 215 ). Since a character which cannot be represented in a mail address or URL is not used, “+” is converted into “−”, and “/” is converted into “_” after the coding by Base  64 . Thus, data  304  can be generated.  
      The address conversion unit  31  returns the data  304  to the mail determination unit  22  (step S 216 ).  
      (Data in the Converting Process)  
      The type, etc. of data generated in the above-mentioned converting process is shown in  FIG. 10 . That is, the mail address  300  of the transmitter A is analyzed into the ID ( 3001 ) and the host name ( 3002 ), thereby generating the data  301 . The private key and the version information as a serial number are input and encrypted. Thus, the encrypted data  302  is generated. Furthermore, the data  303  is generated by adding the key version information, the encryption system, and the header  3031  indicating the presence/absence of the hash use.  
      The data  303  is encoded by the well known Base  64 , “+” is converted into “−”, and “/” is converted into “_”, thereby generating the data  304 . The underline “_” is added to the head of the data  304 , the host name  302  and the at mark “@” are added to the end of the data, a period “.” and an alias character string are inserted between the data  304  and the at mark “@” when the alias is specified in the related information, thus data  305  as a dedicated address is generated.  
      (Reception of Mail)  
       FIG. 11  shows a sequence of the operation of the transmitter A receiving returned mail to a dedicated address from the communication partner B.  
      In  FIG. 11 , the communication partner B first transmits mail from the mail client device  50  to the dedicated address of the transmitter A (step S 401 ). The mail is delivered to the MTA  20  through the mail delivery device  40  (step S 402 ).  
      The mail determination unit  22  of the MTA  20  acquires the data  305  from the To field of the mail header. It is checked whether or not the leading character of the data  305  is the underline “_” as an identifier of the dedicated address (step S 403 ). If the leading character is an underline “_”, it is recognized as a dedicated address, and the reading operation is started.  
      The reading operation of a dedicated address is performed as follows. First, unnecessary information for the reading operation is removed from the data  305 , and the data  304  is obtained (steps S 403  to S 404 ). Then, the data  304  is passed to the address conversion unit  31 , and the address conversion unit  31  is instructed to decode the data (step S 405 ). Based on the decoding sequence (step S 406 ) described later, the address conversion unit  31  acquires the ID ( 3001 ), the communication partner information, and the related information, and passes them to the mail determination unit  22  (step S 407 ).  
      The mail determination unit  22  then compares a black list for each ID ( 3001 ) stored in the filter information management unit with the data  304  in the To field (step S 408 ). If it is described in the black list, the mail is discarded (steps S 408  to S 415 ), and error mail is returned to the communication partner B (steps S 416  and S 417 ). The black list is dedicated addresses discarded by the transmitter A. When a dedicated address is revealed by a spammer, etc, the black list is used to stop the use of the dedicated address. The valid term of the dedicated address is checked (step S 408 ), the mail is discarded when the validity expires (steps S 408  to S 415 ), and error mail is returned to the communication partner B (steps S 416  and S 417 ).  
      When the black list check and the valid term check are satisfied, the decoded related information is checked, and the verification possibility flag is confirmed (steps S 408  to S 409 ). If the flag is not set (when the flag is OFF), the verification sequence is bypassed (steps S 408  to S 414 ).  
      When the flag is set (when the flag is ON), the communication partner information  3013  obtained by decoding is compared with the data in the From field. If the use of a hash is indicated according to the related information, the hash value in the From field is calculated with the specified algorithm, and it is confirmed whether or not the matching is obtained in the range of the same length as the communication partner information  3013  (steps S 410  to S 411 ). If no hash is used, the communication partner information  3013  is compared with the From field as is (steps S 410  to S 412 ). If the comparison result indicates a non-matching result, it is determined that the address is illegally used, the mail is discarded, and error mail is transmitted to the communication partner B (steps S 413  to S 416 , and S 417 ).  
      If the verification indicates a matching result, it is determined that the received mail is transmitted from an authorized communication partner B, and the dedicated address in the To field is replaced with the original mail address  300  of the transmitter A (steps S 413  to S 414 ). Simultaneously, the “dedicated address destination notification field” is added to the mail header. The field is described as follows, for example.  
      X-DEA-decoded: yes; dea=#FABu-02s0v7uqXX3fugvm0r3  
      The information before the at mark “@” is described for the dea option of the field in the used dedicated address.  
      Afterwards, the mail determination unit  22  stores the mail in the mailbox of the transmitter A (step S 418 ), and the transmitter A can receive the mail by the mail client device  10  (step S 419 ).  
      Thus, the transmitter A can communicate with the communication partner B while concealing its own original mail address  300 .  
      In step S 403 , when the leading character of the data  305  starts with a character other than an underline “_”, it is recognized as a user address, and is stored in the mailbox (steps S 403  to S 418 ). Thus, the transmitter A can receive mail in the MUA  10  (step S 419 ).  
      (Decoding Sequence)  
       FIG. 12  shows a sequence showing an example of the process contents of the decoding sequence (step S 406 ) by the address conversion unit  31  used in the sequence shown in  FIG. 11 .  
      In  FIG. 12 , the address conversion unit  31  acquires the data  304  from the mail determination unit  22  (step S 501 ). Since the data  304  is encoded with an arrangement to the well known Base  64 , the decoding of the Base  64  is performed after first converting “−” into “+” and “_” into “/”, to thereby obtain the data  303  (step S 502 ). Then, the encryption system, the hash system, the possibility of the verification, and the version information are acquired as related information from the header  3031  of the data  303 , and the data  302  of the byte string excluding the two leading bytes is acquired (step S 503 ).  
      Next, when the data  302  is encrypted, the address conversion unit  31  request the encryption information management unit  25  (having the same function as the encryption information management unit  33 ) for a private key according to the encryption system acquired from the header  3031  and the version information (steps S 504  to S 505 ). The encryption information management unit  25  first checks whether or not there is a private key in the cache (temporary storage of a key) (step S 506 ). If there is a private key in the cache, the private key is returned to the address conversion unit  31  (steps S 506  to S 509 ).  
      On the other hand, if there is no private key in the cache, the seed information and the version information in the encryption information management unit  25  are passed to the private key generation unit  32 , and the encryption information management unit  25  acquires the private key generated by the private key generation unit  32  (step S 507 ). The private key acquired by the encryption information management unit  25  is temporarily stored in the cache (step S 508 ), and the private key is passed to the address conversion unit  31  (steps S 508  to S 509 ).  
      The address conversion unit  31  decodes the data  302  using the acquired private key, and acquires the data  301  (step S 510 ). If it is determined in step S 504  that encryption is not used, the data  302  is used as the data  301  as is (steps S 504  to S 511 ).  
      The address conversion unit  31  acquires ID ( 3001 ), the communication partner information  3013  and the related information including the option information and valid term (step S 512 ), and passes the information to the mail determination unit  22  (step S 513 ). Thus, the information such as an ID, etc. can be restored from the dedicated address.  
      In the above-mentioned sequence, the transmitter A automatically generates a dedicated address from the original mail address  300 , and discloses it to the communication partner B, thereby communicating with the communication partner B while concealing the original mail address  300 . It is not necessary for the mail delivery device  20  and the transmitter A to manage the conversion between the original mail address  300  and the dedicated address, and the MTA  20  only has to manage the seed information by the encryption information management unit  25 . Furthermore, it is not necessary for the MTA  20  and the transmitter A to manage the correspondence that a dedicated address is for the communication partner B.  
      Since the version information can be optionally changed by dividing the encryption information into the seed information and the version information, the damage of the possible reveal of the information for guarantee of anonymity can be minimized, and the influence of the reveal can be avoided in the future more correctly than in the conventional technology.  
      (Mail Delivering Method)  
      In the above-mentioned dedicated mail system, the following dedicated mail transmitting method is realized. That is, it is a dedicated mail transmitting method for transmitting an address of mail received from a mail source to a mail destination after converting the address into a dedicated address, and realized by: an encrypting step of encrypting communication partner information about a communication partner together with a communication source address; a dedicated address generating step of generating a dedicated address based on an encryption result obtained in the encrypting step; and a step of converting the mail source address into a dedicated address generated in the dedicated address generating step, and transmitting it to the mail destination. Thus, the original address can be concealed from a communication partner during the communications, and it is not necessary for a mail server to manage the dedicated address. Since it is also not necessary for a user to manage the dedicated address, the anonymity can be guaranteed during the communication in almost the same method as in the conventional technology.  
      In the above-mentioned dedicated address generating step, a serial number of a private key used in the encryption in the encrypting step can be added when the dedicated address is generated. If a dedicated address is generated with a serial number such as version information, etc. added, and communications are performed, then the damage of the possible reveal of an encrypted private key for any reason can be minimized, and the influence of the reveal can be avoided in the future.  
      In the above-mentioned dedicated address generating step, the communication partner information can be compressed when the dedicated address is generated. Thus, a dedicated address can be prevented from being too long when it is generated.  
      The present invention can be used when data transmission and reception are performed with the anonymity guaranteed.