JP2007096685A - Method and device for encryption in bilateral information transmission by way of network - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a device for encryption for realizing encryption with one-time-key in a simple configuration. <P>SOLUTION: In the encryption method when information is transmitted to a specified transmission destination by way of network, a transmission history containing at least previous transmission date/time of information is preserved for each transmission destination to which any information has been transmitted. A history information representing at least previous transmission date/time is extracted from the transmission history preserved corresponding to the transmission destination which is an address of the information to be transmitted. An encryption key is generated based on the extracted history information, the information to be transmitted is encrypted using the encryption key. The information that is encrypted with a transmission destination as an address is sent out to a network. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention provides an encryption method and encryption for preventing leakage of information to a third party in bilateral information transmission in which, for example, a mail specifying a destination is delivered via a public network such as the Internet Relates to the device.

Various information exchanged via the Internet, not limited to e-mails, is exposed to the risk that the contents will be leaked to others at various stages, such as the process of being transmitted over the network and the stage where it is stored on the server. Yes.
Various encryption techniques such as a common key method and a public key method are widely used as a method for protecting information transmission via a network from such danger. In addition, in a network in which synchronization is established, such as a network composed of communication devices, an encryption method that uses information transmission time as a one-time key (one-time key) has been proposed (see Patent Document 1).
Japanese Patent Laid-Open No. 2002-223312

The above-described conventional encryption method is a technique originally proposed for realizing extremely high security, and therefore requires a complicated operation system for key management and the like.
For example, the common key method requires a mechanism for the sender to securely and reliably deliver the encryption key used for encryption to the receiver, while the public key method requires a public key corresponding to the sender's private key. A certification body that guarantees the validity of the key and distributes it is necessary.

Further, in these encryption methods using a fixed key, once the encryption key is leaked, the information exchanged thereafter is completely defenseless.
Although an encryption method using a one-time key is superior in security in this respect, a mechanism for safely delivering an encryption key that changes every time information is transmitted and received is required. In addition, in the encryption method using the transmission time as a one-time key, the transmission of the encryption key as described above is unnecessary, but instead, it is essential that the transmission terminal and the reception terminal are completely synchronized. Therefore, a synchronization system for that is indispensable.

In order to introduce an encryption method that requires such a complicated operation system, it is naturally necessary for the user to pay a cost commensurate with the safety realized by the operation system. However, such costs are somewhat burdensome for general users who want to exchange personal e-mails.
On the other hand, of course, there is a need and need to keep the contents of such personal mail confidential from a third party. For this reason, there is a need for an encryption method and an encryption apparatus that can be operated with a simple mechanism.

  An object of the present invention is to provide an operable encryption method and encryption apparatus capable of realizing encryption with a one-time key with a simple mechanism.

The encryption method according to the present invention includes a history storage procedure, an extraction procedure, an encryption key creation procedure, an encryption procedure, and a sending procedure.
The principle of the encryption method according to the present invention is as follows.
In the encryption method when information is transmitted to the specified destination via the network, the history saving procedure is a transmission including at least the transmission date and time when the information was transmitted last time for each destination that has transmitted the information so far. Save history. In the extraction procedure, history information indicating at least the previous transmission date and time is extracted from the transmission history stored corresponding to the transmission destination that is the destination of the information to be transmitted. In the encryption key creation procedure, an encryption key is created based on the extracted history information. In the encryption procedure, information to be transmitted is encrypted using an encryption key. In the sending procedure, the encrypted information is sent to the network with the destination as the destination.

The operation of the encryption method configured as described above is as follows.
For example, the history information extracted from the transmission history stored corresponding to the destination of the mail is different each time because the transmission history of the mail transmitted immediately before is newly included for each transmission. . Therefore, a different encryption key is created for each transmission based on this history information, and mail encrypted using this encryption key is sent to the destination via the network. As a result, encryption can be performed using a different encryption key each time information is transmitted, so even if a previously used encryption key is leaked, the encrypted information can be safely transmitted via the network. Can be delivered to the destination.

The transmission date and time when the encrypted mail is transmitted in this way is saved as a new transmission history, and then when a mail is transmitted to the same destination, a part of the history information for generating the encryption key Used as
The mail encrypted in this way is restored to a plain text mail by using an encryption key created based on the history information indicating the transmission date and time extracted from the previous mail stored in the destination. In other words, it can be restored only by using the previous transmission history saved in the destination.

A first encryption apparatus according to the present invention includes a history storage unit, an extraction unit, a key generation unit, an encryption unit, and a transmission unit.
The principle of the first encryption device according to the present invention is as follows.
In the encryption method for transmitting information to a specified destination via the network, the history storage means transmits at least the transmission date and time at which the information was transmitted last time for each destination that has transmitted the information so far. Save history. The extraction unit extracts history information indicating at least the previous transmission date and time from the transmission history stored in the history storage unit corresponding to the transmission destination that is the destination of the information to be transmitted. The key creation means creates an encryption key based on the extracted history information. The encryption means encrypts information to be transmitted using an encryption key. The transmission means transmits the encrypted information with the transmission destination as the destination to the network.

The operation of the first encryption apparatus configured as described above is as follows.
For example, when sending an email, an encryption key is created by the key creation means based on the history information extracted from the history storage means by the extraction means, and this encryption key is subjected to encryption processing by the encryption means. . Since the history information described above newly includes the transmission history of the mail sent immediately before each transmission, the mail to be transmitted is encrypted using a different encryption key each time and sent to the network by the transmission means. Is done. Then, the transmission date and time when this encrypted mail is transmitted is stored in the history storage means as a new transmission history, and the history information for creating the encryption key when the next mail is transmitted to the same destination. Used as part of

The mail encrypted in this way is restored to a plain text mail by using an encryption key created based on the history information indicating the transmission date and time extracted from the previous mail stored in the destination. In other words, it can be restored only by using the previous transmission history saved in the destination.
The second encryption device according to the present invention comprises mail storage means, history extraction means, key creation means, encryption means, and transmission means.

The principle of the second encryption device according to the present invention is as follows.
In an encryption method for transmitting mail to a designated destination via a network, the mail storage means stores at least the mail sent last time for each destination that has sent mail so far. The history extraction unit extracts history information indicating each transmission date and time from header information included in at least one mail stored in the mail storage unit in correspondence with a transmission destination that is a destination of the mail to be transmitted. The key creation means creates an encryption key based on the extracted history information. The encryption means encrypts information to be transmitted using an encryption key. The transmission means transmits the encrypted information with the transmission destination as the destination to the network.

The operation of the second encryption apparatus configured as described above is as follows.
For example, when sending an email, an encryption key is created by the key creation unit based on the history information extracted from the email storage unit by the history extraction unit, and this encryption key is subjected to an encryption process by the encryption unit. The Since the history information described above newly includes the transmission history of the mail sent immediately before each transmission, the mail to be transmitted is encrypted using a different encryption key each time and sent to the network by the transmission means. Is done. Each time this encrypted mail is sent, the mail before being encrypted together with the header information including the transmission date and time of this mail is newly stored in the mail storage means, and then the mail is sent to the same destination. When transmitting, information indicating the transmission date and time included in the header information of this mail is used as a part of history information for creating an encryption key.

The mail encrypted in this way is restored to a plain text mail by using an encryption key created based on the history information indicating the transmission date and time extracted from the previous mail stored in the destination. In other words, it can be restored only by using the previous transmission history saved in the destination.
A third encryption apparatus according to the present invention is configured such that, in the above-described second encryption apparatus, the history extraction means includes the latest date and time extraction means, selection means, and history information collection means.

The principle of the third encryption apparatus according to the present invention is as follows.
In the history extraction unit provided in the second encryption device described above, the latest date and time extraction unit extracts the latest date and time information indicating the transmission date and time of the mail last transmitted from the mail storage unit. The selection unit selects at least one of the emails stored corresponding to the destination corresponding to the email storage unit based on the latest date and time information. The history information collection unit extracts transmission date information from the header information of each of at least one mail selected by the selection unit, and uses the at least one transmission date information for encryption key generation processing by the key generation unit.

The operation of the third encryption apparatus configured as described above is as follows.
Based on the last transmission date and time extracted from the mail storage unit by the latest date and time extraction unit, the selection unit selects a number indicating at least one email stored corresponding to the corresponding transmission destination. Then, at least one transmission date / time information is extracted from the header information of the mail indicated by this number by the history information collecting means, and the history information consisting of these transmission date / time information is subjected to encryption key creation processing by the key creation means. Provided. In other words, the history information is extracted from the mail corresponding to a different number each time according to the last transmission date and used for the encryption key creation process, so that the mail stored in the mail storage means is more effectively used. Can be used for creation.

As described above, according to the encryption method and the first encryption device of the present invention, a one-time key can be obtained with a very simple configuration by using a transmission history stored on the transmission side. In addition, it is possible to generate an encryption key that can be decrypted only by the reception history stored on the receiving side.
Accordingly, it is possible to protect the privacy by applying the encryption method to personal mail exchange at a low cost, and concealing information to be exchanged from a third party.

Further, in the second encryption device, the transmission date and time information included in the header information added to the mail is used as it is as transmission history information, so that the configuration of the means for extracting the history information can be further simplified. .
On the other hand, in the third encryption device, by utilizing the accumulation of mails exchanged closely by changing the combination of mails for extracting history information based on the latest mail transmission date and time, It is possible to strongly protect privacy.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 shows a first embodiment of an encryption apparatus according to the present invention. FIG. 2 is a flowchart showing the mail transmission operation by the encryption apparatus.
The reception processing unit 201 shown in FIG. 1 receives information indicating a mail text and a destination together with a mail transmission request from a user. First, header information including transmission date and time information is added to the mail text, which is suitable for network transmission. A transmission mail in the form is created (see step 301 in FIG. 2). This transmission mail is transferred to the transmission mail storage unit 202 via the transmission mail holding unit 206 shown in FIG. 1, and is stored for each destination as described later.

In addition, the application determination unit 203 illustrated in FIG. 1 receives the transmission mail and the destination information from the reception processing unit 201, and applies encryption to the transmission mail based on the information held in the destination list 204. Determine whether or not.
In the destination list 204 shown in FIG. 1, for example, application information indicating the necessity of encryption processing is stored in correspondence with each mail address indicating each destination having a history of mail exchange. . Therefore, the application determination unit 203 refers to the destination list 204 based on the destination information received from the reception processing unit 201 (see step 302 in FIG. 2), and encrypts the outgoing mail based on the corresponding application information. Whether or not to do so can be determined (see step 303 in FIG. 2).

When the application information corresponding to the designated destination indicates that the encryption process is necessary (affirmative determination in step 303 shown in FIG. 2), the application determination unit 203 shown in FIG. The transmitted mail is passed to the encryption processing unit 210, and the transmitted mail is subjected to the encryption processing described below.
In the encryption processing unit 210 shown in FIG. 1, the transmission date and time extraction unit 211 selects the corresponding destination from the mails stored in the transmission mail storage unit 202 corresponding to the destination information received from the application determination unit 203. The last mail transmitted last is searched, information indicating the transmission date and time is extracted from the header information included in the previous mail (see step 304 in FIG. 2), and passed to the encryption key creation unit 212 shown in FIG. .

The encryption key creation unit 212 creates an encryption key based on the received information (step 305 shown in FIG. 2). Any algorithm can be used for creating this encryption key as long as it creates a sufficiently long encryption key from information indicating the transmission date and time.
Using the encryption key created in this way, the encryption unit 213 shown in FIG. 1 performs an encryption process on the outgoing mail received from the application determination unit 203 described above (step 306). Then, the encrypted mail obtained by this encryption processing is transferred to the transmission processing unit 205 and used for transmission processing to the network (step 307 in FIG. 2).

  Thereafter, when the history management unit 207 illustrated in FIG. 1 receives a notification that the above-described mail transmission processing by the transmission processing unit 205 has been completed, the history management unit 207 notifies the transmission mail storage unit 202 to the transmission mail holding unit 206. Instructing to store the stored transmission mail corresponding to the destination, and in response to this, the newly transmitted mail is stored as the last mail corresponding to the corresponding destination (step 309 in FIG. 2). , The process ends.

  After the encrypted mail is transmitted as described above, the transmission mail stored in the transmission mail storage unit 206 is stored in the transmission mail storage unit 202, so that the next mail is transmitted to the same destination. Sometimes, the transmission date / time information indicated by the header information included in the transmitted mail is used for the encryption key creation process. As described above, since the transmission date and time of the immediately preceding mail changes every time a new mail is transmitted, the encryption key generation unit 212 can easily generate a one-time key by using this for the generation of the encryption key. it can.

  When the destination associated with the application information indicating that the encryption process is not applied is specified in the destination list 204 described above, a negative determination is made in step 303 shown in FIG. The outgoing mail is passed to the outgoing call processing unit 205 as it is and sent to the network as plain text (step 308). After that, as in the case of the encrypted mail described above, the outgoing mail held in the outgoing mail holding section 206 corresponds to the destination corresponding to the outgoing mail storage section 202 in accordance with the instruction from the history management section 207. Are stored (step 309), and the mail transmission process ends.

In this way, it is possible to store mail even for a destination to which application of encryption processing is not instructed, and to prepare for a case where encryption processing is required later.
Next, a method for restoring the mail encrypted with the one-time key created as described above at the mail destination will be described.
FIG. 3 shows a configuration example of a decoding apparatus according to the present invention.

In the received mail storage unit 224 shown in FIG. 3, the mail received so far is stored for each transmission source in the same manner as the above-described transmitted mail.
In FIG. 3, newly arrived mail via the network is passed to the application determining unit 222 via the mail receiving unit 221, and this received mail is encrypted based on the information stored in the transmission source list 223. Whether or not is applied is determined.

  In the transmission source list 223 shown in FIG. 3, for example, application information indicating whether or not the above-described encryption is applied is stored corresponding to each mail address indicating the transmission source of the mail received so far. Has been. The application determining unit 222 refers to the application information based on the transmission source information received together with the received mail from the mail receiving unit 221. If it is indicated that the encryption process is not applied, the application determining unit 222 If it is output as it is, and it is shown that the encryption process is applied, the received mail is passed to the decryption processing unit 225 for the decryption process described below.

  In the decryption processing unit 225 illustrated in FIG. 3, the transmission date and time extraction unit 226 accumulates corresponding to the transmission source corresponding to the received mail accumulation unit 224 based on the transmission source information received from the application determination unit 222 described above. The latest received mail is searched from the received mail, and the transmission date / time information indicated by the header information included in the latest received mail is extracted and passed to the encryption key creation unit 227.

Since the transmission date and time extracted by the transmission date and time extraction unit 226 is the same as the transmission date and time used for generating the encryption key in the encryption device described above, the encryption key generation unit 227 is provided in the encryption device described above. By creating an encryption key according to the same algorithm as the encryption key creation unit 212, the encryption key used for encrypting the mail can be reproduced.
Therefore, the original plaintext mail can be restored by passing the encryption key obtained by the encryption key creation section 227 to the decryption section 228 and subjecting it to decryption processing of the encrypted received mail.

In addition, the received mail restored in this way and the received mail to which encryption processing has not been applied are passed to the received mail storage unit 224 by the storage management unit 229, and the latest received mail corresponding to each transmission source. And is used for subsequent encryption key restoration processing.
As described above, according to the encryption device shown in FIG. 1 and the decryption device shown in FIG. 3, based on the information indicating the history of the exchange of mails respectively stored on the transmission side and the reception side of the mail, The encryption process using the one-time key can conceal the exchange of individual mails from a third party and protect the privacy of the sender and the receiver.

  Since the mail exchange history accumulated on the sender and receiver is information that cannot be easily understood by a third party, the encryption key is created based on this information and transmitted via the network. It is possible to encrypt information such as e-mail that is difficult for a third party to decipher, thus ensuring the security of information transmission between two parties via a public network such as e-mail exchange via the Internet. can do.

On the other hand, the mail exchange history stored on the sender side and the receiver side is almost certainly left in the sender and receiver. The cost required when applying the method can be kept extremely low.
As described above, when the mail to be exchanged is stored in the transmitted mail storage unit 202 (received mail storage unit 224), the digest of the body of the last transmitted (received) mail is used. It is also possible to create an encryption key.

On the other hand, only the information used to create the encryption key (for example, the transmission date and time indicated by the header of the mail) is stored as history information independently of the mail itself, and the encryption processing unit 210 shown in FIG. It is also possible to use for this processing.
(Second Embodiment)
FIG. 4 shows a second embodiment of the encryption apparatus according to the present invention.

4 that are the same as those shown in FIG. 1 are denoted by the same reference numerals as those shown in FIG. 1 and description thereof is omitted.
The destination list 208 shown in FIG. 4 is applied with encryption processing together with application information indicating whether or not to apply encryption processing corresponding to each mail address indicating a destination to which mail has been sent so far. For example, encryption level information indicating the degree of necessity of encryption processing in a plurality of stages is held. Also, the application determining unit 203 shown in FIG. 4 displays the new transmission mail together with the new transmission mail when the application information stored in the destination list 208 indicates that the encryption processing is to be applied to the new transmission mail. The corresponding encryption level information is transferred to the encryption processing unit 230 and used for encryption processing.

  In the encryption processing unit 230 illustrated in FIG. 4, the immediately preceding date and time extraction unit 231 transmits the transmission mail stored in the transmission mail storage unit 202 corresponding to the corresponding destination based on the destination information received from the application determination unit 203. The latest transmission mail is searched from among them, transmission date information (hereinafter referred to as immediately preceding date information) included in the header information of this transmission mail is extracted, The above encryption level information is passed.

  For example, the extraction target determination unit 232 illustrated in FIG. 4 generates a plurality of different numbers based on the immediately preceding date and time information, and selects a number corresponding to the above-described encryption level information from these numbers. As a result of the selection, a transmission mail that is a target for extracting transmission date and time information used for generating an encryption key from among a large number of transmission mails stored in the transmission mail storage unit 202 corresponding to the corresponding destination is determined.

The history information collection unit 233 shown in FIG. 4 extracts the transmission date / time information from the header information of each transmission mail indicated by the number determined as described above, and sends the transmission date / time information to the encryption key generation unit 234. Passed to the encryption key creation process.
In this way, according to the encryption level information indicating that higher security is required, the extraction target determining unit 232 determines the outgoing mail corresponding to many numbers as the history information extraction target. Thus, it is possible to create an encryption key that is less likely to be estimated than when an encryption key is created using a single transmission date and time, and use it for encryption processing. That is, a difference in tightness for each destination can be reflected in the strength of the encryption process.

The number of transmission mails indicated by the above-described encryption level in the order of transmission date and time is extracted from the history information, the transmission date and time information is extracted from the header information of these transmission mails, and the encryption key creation unit 234 It can also be used for encryption key creation processing.
(Third embodiment)
FIG. 5 shows a third embodiment of the encryption apparatus according to the present invention.

5 that are the same as those shown in FIG. 1 or FIG. 3 are denoted by the same reference numerals as those shown in FIG. 1 and FIG. 3, and description thereof is omitted.
In FIG. 5, the authentication information sent from the client (not shown) is encrypted by the encryption device 240 and then passed to the decryption device 245 corresponding to the server (not shown) via the network. The decryption device 245 restores the original authentication information and passes it to the server.

  In the encryption device 240 shown in FIG. 5, the authentication request reception unit 241 adds date / time information indicating the date / time when the request is received to the authentication information passed from the client, and then passes it to the encryption processing unit 210. The encryption processing unit 210 encrypts the authentication information including the date and time information described above based on the authentication history information held in the request history holding unit 243, and sends it to the network via the encrypted information sending unit 242.

  The request history holding unit 243 illustrated in FIG. 5 is added to each authentication information by the authentication request receiving unit 241 as request history information indicating the date and time when the client described above has requested authentication to the server. Information indicating the date and time is stored. In this case, for example, in response to a new authentication request, the encryption processing unit 210 extracts the last request history information held in the request history holding unit 243, and based on the last request history information The authentication information is encrypted using the created encryption key, and is sent to the sending process by the encrypted information sending unit 242.

  In this way, when the notification from the encrypted information transmission unit 242 indicates that the encrypted authentication information has been transmitted, the request history management unit 244 receives the new request described above from the authentication request reception unit 241. The date / time information added to the authentication information is received, the date / time information is stored in the request history holding unit 243 as new request history information, and is used for the subsequent encryption processing related to the authentication information.

  As described above, the request history holding unit 243 stores a new request history every time an authentication request is made, and this is used for encryption processing of authentication information at the time of the next authentication request. Accordingly, since the authentication information from the client is encrypted by the encryption processing unit 210 of the encryption device 240 using a different encryption key each time, the client authentication information can be safely transmitted via a public network such as the Internet. Can be delivered to the server side.

The encrypted authentication information that has reached the decryption device 245 on the server device side through the network in this way is passed to the decryption processing unit 225 via the encryption information receiving unit 246, and the authentication history holding unit 247. Is decrypted on the basis of the information held in the authentication information and passed to the server via the authentication information sending unit 248.
In the authentication history holding unit 247 shown in FIG. 5, information indicating the date and time added to each authentication information received from each client so far is stored as authentication history information for each client. In this case, for example, in response to the input of new encrypted authentication information, the decryption processing unit 225 extracts the last authentication history information held in correspondence with the client corresponding to the authentication history holding unit 247. Then, the encrypted authentication information is decrypted using the encryption key created based on the last authentication history information, and the obtained authentication information is subjected to a sending process by the authentication information sending unit 248.

In addition, after the authentication information restored from the encrypted authentication information is sent to the server in this way, the authentication history management unit 249 sends the date and time information added to the newly restored authentication information to the authentication information. The date / time information is stored in the authentication history holding unit 247 as new authentication history information, and is used for subsequent decryption processing related to the authentication information.
In this way, according to restoration of new authentication information, the date and time information added to the authentication information in the encryption device 240 is accumulated in the authentication history holding unit 247 of the decryption device 245, and the next encryption authentication is performed. By using the information restoration process, it is possible to solve a complicated key management problem when using a one-time key with an extremely simple configuration. In other words, a complicated system for the use of a one-time key, that is, a system for passing the encryption key itself independently of the encrypted message, or a transmission / reception device for using the transmission date of the encrypted message itself as a one-time key It is not necessary to construct a system that synchronizes both.

Note that the information added to the authentication information by the request receiving unit 241 illustrated in FIG. 5 is not limited to information indicating the date and time when the request is received. For example, a random number with an appropriate number of digits generated in response to the request, Any information that is different for each request and suitable for generating an encryption key may be used.
Further, by applying the technique described in the second embodiment, an encryption key is created using request history information stored in the request history holding unit 243 corresponding to a plurality of requests, and this encryption key is used. It is also possible to encrypt the authentication information.

As described above, according to the encryption method and the encryption apparatus according to the present invention, the encryption process using the one-time key can be realized with a very simple apparatus configuration. However, it is very effective as an encryption technology when exchanging personal e-mails etc. via a public network.
It can also be used to protect authentication information passed from the client to the server during remote access. In such an application, it is particularly useful that the encryption method and the encryption apparatus according to the present invention use different encryption keys each time information is transmitted. This is because, when authentication information is encrypted using a one-time key, replay attacks that use encrypted authentication information that has been illegally acquired during transmission over the network are completely eliminated. Because it can.

It is a figure which shows 1st Embodiment of the encryption apparatus concerning this invention. It is a flowchart showing mail transmission operation. It is a figure which shows the structural example of the decoding apparatus concerning this invention. It is a figure which shows 2nd Embodiment of the encryption apparatus concerning this invention. It is a figure which shows 3rd Embodiment of the encryption apparatus concerning this invention.

Explanation of symbols

201 reception processing unit 202 transmission mail storage unit 203 application determination unit 204, 208 destination list 205 transmission processing unit 206 transmission mail holding unit 207 history management unit 210, 230 encryption processing unit 211 transmission date and time extraction unit 212, 227, 234 encryption key Creation unit 213 Encryption unit 221 Mail reception unit 222 Application determination unit 223 Transmission source list 224 Received mail storage unit 225 Decryption processing unit 226 Transmission date and time extraction unit 228 Decryption unit 229 Accumulation management unit 231 Immediate date and time extraction unit 232 Extraction target determination Unit 233 history information collection unit 240 encryption device 241 authentication request reception unit 242 encryption information transmission unit 243 request history storage unit 244 request history management unit 245 decryption device 246 encryption information reception unit 247 authentication history storage unit 248 authentication information transmission Part 249 Authentication history management part

Claims (4)

In the encryption method when transferring information to the specified destination via the network,
For each destination that has sent information so far, save a transmission history that includes at least the date and time the information was sent last time,
Extract history information indicating at least the previous transmission date from the transmission history stored corresponding to the transmission destination that is the destination of the information to be transmitted,
Create an encryption key based on the extracted history information,
Encrypt information to be transmitted using the encryption key,
The encryption method, wherein the encrypted information is sent to a network with the destination as a destination. In the encryption method when transferring information to the specified destination via the network,
History storage means for storing a transmission history including at least the transmission date and time when information was transmitted last time for each destination to which information has been transmitted;
Extraction means for extracting history information indicating at least the previous transmission date and time from the transmission history stored in the history storage means corresponding to the transmission destination of the information to be transmitted;
Key creation means for creating an encryption key based on the extracted history information;
Encryption means for encrypting information to be transmitted using the encryption key;
An encryption apparatus comprising: a transmission unit configured to send the encrypted information to a network with the transmission destination as a destination. In the encryption method when mail is transmitted to the specified destination via the network,
Email storage means to store at least the last email sent to each destination that has sent email before,
History extracting means for extracting history information indicating each transmission date and time from header information included in at least one mail stored in the mail storage means corresponding to the destination of the mail to be transmitted;
Key creation means for creating an encryption key based on the extracted history information;
Encryption means for encrypting information to be transmitted using the encryption key;
An encryption apparatus comprising: a transmission unit configured to send the encrypted information to a network with the transmission destination as a destination. The encryption device according to claim 3,
The history extraction means
Latest date and time extraction means for extracting the latest date and time information indicating the transmission date and time of the mail sent last from the mail storage means;
Selection means for selecting at least one of a plurality of mails stored in correspondence with the destination corresponding to the mail storage means based on the latest date and time information;
History information collection means for extracting transmission date / time information from the header information of each of at least one mail selected by the selection means, and using the at least one transmission date / time information as history information for encryption key creation processing by the key creation means; An encryption device characterized by comprising.

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