FIRST COMMUNICATION DEVICE, SECOND COMMUNICATION DEVICE, METHOD, AND COMPUTER PROGRAM

Abstract

Technology is provided that replaces block chains, is capable of saving accumulated data, and has low cost. A user terminal 100-1 encrypts target data (S804), and sends the data to a user terminal 100-2 (S805). The user terminal 100-2 receives the data (S901), and further encrypts the encrypted target data (S902). The user terminal 100-2 records the double-encrypted target data (S903), and transmits the data to the user terminal 100-1 (S904). The user terminal 100-1 receives the data (S806), and records the double-encrypted target data (S807).

Claims

1-8. (canceled)

9. A method executed by a first communication device in a communication system comprising the first communication device including a first encryption unit capable of encrypting predetermined data in a decryptable manner, and a second communication device including a second encryption unit capable of encrypting predetermined data in a decryptable manner according to a method that is different from a method of the first encryption unit, the first communication device and the second communication device being communicable with each other via a predetermined network, the method comprising: a first encryption step of encrypting, through the first encryption unit, target data that is data both of users of the first communication device and the second communication device intend to save, and generating first encryption target data; a first transmission step of transmitting the first encryption target data to the second communication device via the network; and a second reception step of receiving second first encryption target data from the second communication device via the network, the second first encryption target data being generated by the second communication device encrypting, through the second encryption unit, the first encryption target data received from the first communication device via the network, the steps being executed by the first communication device.

10. The method according to claim 9, wherein the first communication device automatically executes the first encryption step, the first transmission step, and the second reception step.

11. The method according to claim 9, further comprising a step of causing the first communication device to transmit the second first encryption target data to a recording device that is connected to the network and is capable of recording data, and causing the recording device to record the second first encryption target data.

12. A method executed by a second communication device in a communication system comprising the first communication device including a first encryption unit capable of encrypting predetermined data in a decryptable manner, and a second communication device including a second encryption unit capable of encrypting predetermined data in a decryptable manner according to a method that is different from a method of the first encryption unit, the first communication device and the second communication device being communicable with each other via a predetermined network, the method comprising: a first reception step of receiving first encryption target data from the first communication device via the network, the first encryption target data being generated by the first communication device encrypting, through the first encryption unit, target data that is data both of users of the first communication device and the second communication device intend to save; a second encryption step of encrypting the first encryption target data through the second encryption unit, and generating second first encryption target data; and a second transmission step of transmitting the second first encryption target data to the first communication device via the network, the steps being executed by the second communication device.

13. The method according to claim 12, wherein the second communication device automatically executes the first reception step, the second encryption step, and the second transmission step.

14. The method according to claim 12, further comprising a step of causing the second communication device to transmit the second first encryption target data to a recording device that is connected to the network and is capable of recording data, and causing the recording device to record the second first encryption target data.

15. A first communication device in a communication system that includes the first communication device, and a second communication device including a second encryption unit capable of encrypting predetermined data in a decryptable manner, the devices being communicable with each other via a predetermined network, the first communication device comprising: a first encryption unit that encrypts target data by a method different from a method of the second encryption unit, and generates first encryption target data, the target data being data both of users of the first communication device and the second communication device intend to save; a first transmission unit of transmitting the first encryption target data to the second communication device via the network; and a first reception unit that receives second first encryption target data from the second communication device via the network, the second first encryption target data being generated by the second communication device encrypting, through the second encryption unit, the first encryption target data received from the first communication device via the network.

16. A second communication device in a communication system that includes a first communication device including a first encryption unit capable of encrypting predetermined data in a decryptable manner, and a second communication device, the devices being communicable with each other via a predetermined network, the second communication device comprising: a second reception unit that receives first encryption target data from the first communication device via the network, the first encryption target data being generated by the first communication device encrypting, through the first encryption unit, target data that is data both of users of the first communication device and the second communication device intend to save; a second encryption unit that encrypts the first encryption target data by a method different from a method of the first encryption unit, and generates second first encryption target data; and a second transmission unit that transmits the second first encryption target data to the first communication device via the network.

17-29. (canceled)

30. The method according to claim 11, wherein the second first encryption target data is a block that is an aggregation of data items, and is configured to be recorded in the recording device in a state of being included in what is connected to an immediately previous block thereof, the blocks each including an operation value that is a value obtained by applying a predetermined arithmetic operation to the immediately previous block, and the first communication device generates the block including the second first encryption target data to be transmitted, and transmits the block to the recording device.

31. A method executed by the recording device subsequent to the method according to claim 11, wherein the second first encryption target data is a recorded in the recording device in a state of being included in a block that is an aggregation of data items and is connected to an immediately previous block thereof, the blocks each including an operation value that is a value obtained by applying a predetermined arithmetic operation to the immediately previous block, and the recording device generates the block, based on the second first encryption target data received from the first communication device, and connects the block to the immediately previous block.

32. The method according to claim 31, wherein the number of the second first encryption target data items included in the one block is one or more, and at predetermined timing after receiving one or more of the second first encryption target data items from the first communication device, the recording device generates the block including the one or more second first encryption target data items received after generation of the immediately previous block, and connects the block to the immediately previous block.

33. The method according to claim 11, wherein the first communication device transmits the second first encryption target data, and the target data that is an origin of the second first encryption target data, to the recording device, and the recording device records the second first encryption target data, and the target data that is the origin thereof, in a state of being associated with each other.

34. The method according to claim 30, wherein the block includes the second first encryption target data, the target data that is an origin of the second first encryption target data, and the operation value, and the first communication device generates the block including the second first encryption target data that is to be transmitted and the target data that is the origin thereof, and transmits the block to the recording device.

35. The method according to claim 31, wherein the block includes the second first encryption target data, the target data that is the origin of the second first encryption target data, and the operation value, in a state where the second first encryption target data and the target data are associated with each other, and the recording device generates the block, based on the second first encryption target data received from the first communication device and the target data that is the origin thereof, and connects the block to the immediately previous block.

36. The method according to claim 14, wherein the second first encryption target data is recorded in the recording device in a state of being included in a block that is an aggregation of data items and is connected to an immediately previous block thereof, the blocks each including an operation value that is a value obtained by applying a predetermined arithmetic operation to the immediately previous block, and the second communication device generates the block including the second first encryption target data to be transmitted, and transmits the block to the recording device.

37. A method executed by the recording device subsequent to the method according to claim 14, wherein the second first encryption target data is recorded in the recording device in a state of being included in a block that is an aggregation of data items and is connected to an immediately previous block thereof, the blocks each including an operation value that is a value obtained by applying a predetermined arithmetic operation to the immediately previous block, and the recording device generates the block, based on the second first encryption target data received from the second communication device, and connects the block to the immediately previous block.

38. The method according to claim 37, wherein the number of the second first encryption target data items included in the one block is one or more, and at predetermined timing after receiving one or more of the second first encryption target data items from the second communication device, the recording device generates the block including the one or more second first encryption target data items received after generation of the immediately previous block, and connects the block to the immediately previous block.

39. The method according to claim 14, wherein the second communication device transmits the second first encryption target data, and the target data that is an origin of the second first encryption target data, to the recording device, and the recording device records the second first encryption target data, and the target data that is the origin thereof, in a state of being associated with each other.

40. The method according to claim 36, wherein the block includes the second first encryption target data, the target data that is an origin of the second first encryption target data, and the operation value, and the second communication device generates the block including the second first encryption target data that is to be transmitted and the target data that is the origin thereof, and transmits the block to the recording device.

41. The method according to claim 37, wherein the block includes the second first encryption target data, the target data that is the origin of the second first encryption target data, and the operation value, in a state where the second first encryption target data and the target data are associated with each other, and the recording device generates the block, based on the second first encryption target data received from the second communication device and the target data that is the origin thereof, and connects the block to the immediately previous block.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0089] FIG. 1 is a diagram schematically showing an overall configuration of a communication system according to a first embodiment;

[0090] FIG. 2 is a perspective view showing an external appearance of a user terminal in the communication system shown in FIG. 1;

[0091] FIG. 3 is a diagram showing a hardware configuration of the user terminal in the communication system shown in FIG. 1;

[0092] FIG. 4 is a block diagram showing functional blocks generated in the user terminal in the communication system shown in FIG. 1;

[0093] FIG. 5 is a diagram showing a flow of processes to be executed in the communication system shown in FIG. 1;

[0094] FIG. 6 is a diagram showing an example of an image displayed on a display of the user terminal shown in FIG. 2;

[0095] FIG. 7 is a diagram schematically showing an overall configuration of a communication system according to a second embodiment;

[0096] FIG. 8 is a diagram showing an example of an image displayed on a display of the user terminal according to the second embodiment;

[0097] FIG. 9 is a block diagram showing functional blocks generated in a recording device in the communication system of Modification Example 1;

[0098] FIG. 10 is a diagram schematically showing data included in blocks recorded in a recording unit of the recording device in the communication system of Modification Example 1; and

[0099] FIG. 11 is a block diagram showing functional blocks generated in the user terminal in the communication system of Modification Example 2.

DESCRIPTION OF EMBODIMENTS

[0100] First and second embodiments of the present invention will be described below. In the description of each embodiment and modification example, the same element is denoted by the same reference numeral and duplication description will not be presented in some cases. If there is no contradiction, each embodiment and modification example can be combined with another embodiment and modification example.

First Embodiment

[0101] FIG. 1 schematically shows an overall configuration of a communication system according to a first embodiment.

[0102] Normally, the communication system is configured to include a plurality of user terminals 100-1 to 100-N (which may also hereinafter simply be described as user terminal 100.). These are all connectable to a network 400.

[0103] The network 400 is the Internet in this embodiment without being limited thereto.

[0104] The user terminal 100 is an example of a communication device (or a first communication device and a second communication device) of the invention of the present application in a broader sense.

[0105] Although not necessary, such a communication system includes a recording device 300 connected to the network 400. In many cases, multiple recording devices 300 are provided. In some cases, many such devices are provided.

[0106] Normally, the user terminal 100 is possession of each user. The user terminal 100 includes a computer. The user terminal 100 is a mobile phone, a smartphone, a tablet, a notebook computer, a desktop computer, or the like. These may be general-purpose ones. The smartphone is, for example, an iPhone that is manufactured and sold by Apple Japan, LLC. An example of the tablet is an iPad that is manufactured and sold by Apple Japan, LLC. Hereinafter, the description is made assuming that the user terminal is the smartphone, but is not limited thereto.

[0107] The user terminals 100 are at least required to be capable of communicating with each other via the network 400, and capable of performing processes of encrypting and decrypting data, which will be described later. The user terminal 100 in this embodiment is a smartphone. Accordingly, it is a matter of course that this terminal can communicate with another user terminal 100 via the network 400. The functions of encrypting and decrypting data can be acquired by a computer program described later.

[0108] A configuration of the user terminal 100 will be described below. Each of the user terminals 100-1 to 100-N has the same configuration in relation to the invention of the present application.

[0109] An example of external appearance of the user terminal 100 is shown in FIG. 2.

[0110] The user terminal 100 includes a display 101. The display 101 is used to display still images or moving images, and a publicly known or well-known display can be used. The display 101 is, for example, a liquid crystal display. The user terminal 100 also includes an input device 102. The input device 102 is used for the user to perform a desired input on the user terminal 100. A publicly known or well-known input device can be used as the input device 102. The input device 102 of the user terminal 100 in this embodiment is a button-type input device, but is not limited thereto, and a numeric keypad, a keyboard, a trackball, a mouse, or the like can also be used. When the user terminal 100 is, for example, a notebook computer or a desktop computer, the input device 102 may be a keyboard, a trackball, a mouse or the like. Moreover, when the display 101 is a touch panel, the display 101 also has a function of the input device 102, which applies to this embodiment.

[0111] Although every detail of data input through the input device 102 will be described later, the data is selection information and start information, for example.

[0112] A configuration of hardware of the user terminal 100 is shown in FIG. 3.

[0113] The hardware includes a CPU (central processing unit) 111, a ROM (read only memory) 112, a RAM (random access memory) 113, and an interface 114, which are interconnected by a bus 116.

[0114] The CPU 111 is an arithmetic operation device that performs arithmetic operations. The CPU 111, for example, executes a computer program recorded in the ROM 112 to execute processing to be described below. It should be noted that the computer program used herein includes at least a computer program for causing the user terminal 100 to function as a communication device (or the first communication device and the second communication device) of the invention of the present application. The computer program may be pre-installed in the user terminal 100, or may be post-installed. The computer program may be installed in the user terminal 100 via a predetermined recording medium (not shown) such as a memory card, or may be installed via a network such as the Internet.

[0115] The ROM 112 records computer programs and data necessary for the CPU 111 to execute processing to be described below. The computer program recorded in the ROM 112 is not limited thereto. For example, when the user terminal 100 is the smartphone, a computer program and data are recorded, for example, for executing telephone calls or e-mails, which are required to cause the user terminal 100 to function as the smartphone. The user terminal 100 can also browse websites based on data received via the network 400, and is mounted with a publicly known web browser for enabling the browsing of the websites.

[0116] The RAM 113 provides a work area necessary for the CPU 111 to perform processing.

[0117] The interface 114 performs data exchange between the CPU 111 and the RAM 113 connected via the bus 116 and the outside. The display 101 and the input device 102 described above are connected to the interface 114. The operation content input from the input device 102 is input to the bus 116 from the interface 114, and image data to be described below is output to the display 101 from the interface 114. The interface 114 is also connected to a transmission and reception unit, which is not shown.

[0118] The transmission and reception unit transmits and receives data via the network 400 that is the Internet. Such communication may also be performed in a wired manner, but when the user terminal 100 is the smartphone, the communication is wirelessly performed in general. As long as it is possible, the transmission and reception unit may have a publicly known or well-known configuration. The data received by the transmission and reception unit from the network 400 is received through the interface 114, and the data passed from the interface 114 to the transmission and reception unit is sent to the outside, for example, to another user terminal 100 by the transmission and reception unit via the network 400. The data that may be sent from the transmission and reception unit to another device via the network 400 is target data or encrypted target data, which are to be described later. This similarly applies to data that may be received by the transmission and reception unit from another device via the network 400.

[0119] Functional blocks as shown in FIG. 4 are generated within the user terminal 100 when the CPU 111 executes the computer program. It should be noted that while the following functional blocks may be generated with functions of the above-described computer program alone for causing the user terminal 100 to function as the communication device (or the first communication device and the second communication device) of the invention of the present application, the following functional blocks may be generated in cooperation with the above-described computer program and an OS and other computer programs installed in the user terminal 100.

[0120] A control unit 120 to be described below is generated in the user terminal 100 in relation to functions of the invention of the present application. A main control unit 121, a data input/output unit 122, a first recording unit 123, and an encryption unit 124 are in the control unit 120.

[0121] The control unit 120 executes information processing as will be described below.

[0122] The main control unit 121 performs overall control within the control unit 120. For example, the main control unit 121 may accept selection information and start information, which are to be described below, from the data input/output unit 122. The selection information is information for selecting target data that is to be shared among some user terminals 100 including the user terminal 100 having the main control unit 121 and be recorded and saved, and for specifying the user terminals 100 that share, record and save the target data. Upon acceptance of the selection information, the main control unit 121 reads the target data selected by the selection information, from the first recording unit 123. Upon acceptance of the start information, the main control unit 121 executes an after-mentioned process for starting an encryption step which is to be described later.

[0123] When the target data is read from the first recording unit 123, the main control unit 121 may send the target data to the encryption unit 124 in some cases. The main control unit 121 may receive the target data or encrypted target data from the data input/output unit 122 in some cases. Upon receipt thereof, the main control unit 121 sends the data to the encryption unit 124 or the first recording unit 123.

[0124] Note that a user ID unique to each user terminal 100 is recorded in the main control unit 121 of the user terminal 100. Use of the user ID can identify each user terminal 100. The user ID is, for example, numerals, alphabetic characters, symbols, or a combination of them. Although not close to reality, this embodiment assumes that symbols 100-1 to 100-N, which are assigned to the respective user terminals 100, are the user IDs of the user terminals 100. Usage of the user ID will be described later.

[0125] The main control unit 121 may receive the encrypted target data from the encryption unit 124 in some cases. Upon receipt of the encrypted target data, the main control unit 121 sends the data to the data input/output unit 122.

[0126] The data input/output unit 122 inputs and outputs data from and to the control unit 120.

[0127] Specifically, the data input/output unit 122 accepts the selection information and the start information input from the input device 102 described above via the interface 114. Upon acceptance of the data from the input device 102, the data input/output unit 122 sends the selection information and the start information to the main control unit 121.

[0128] The target data or the encrypted target data may be sent from the main control unit 121 to the data input/output unit 122. When the data is sent, the data input/output unit 122 sends the data to the transmission and reception unit via the interface 114.

[0129] The target data or the encrypted target data sent from another device via the network 400 and received by the transmission and reception unit may be sent via the interface 114 to the data input/output unit 122. Upon receipt of the data, the data input/output unit 122 sends the data to the main control unit 121.

[0130] The first recording unit 123 is for recording data. Data that can be recorded in the first recording unit 123 is, for example, target data, or after-mentioned data obtained by encrypting the target data. As described later, other data, for example, key data, may be recorded in the first recording unit 123 in some cases.

[0131] The encryption unit 124 encrypts and decrypts data. Methods of encryption and decryption performed by the encryption unit 124 may be the same, for example, always be performed using the same key and the same algorithm, or may not be the same. For example, the encryption method performed by the encryption unit 124 may vary every time a process of encrypting certain data is performed. In a case where the encryption method performed by the encryption unit 124 is changed, this may be achieved by any of three modes including change in key used for encryption, change in algorithm used for encryption, and change in key and algorithm used for encryption. Irrespective of whether the encryption method performed by the encryption unit 124 is changed or not, when the encryption unit 124 decrypts data, arithmetic operations required to decrypt encrypted data are performed by the encryption unit 124.

[0132] The user terminals 100 include the respective encryption units 124. The encryption method executed by the encryption unit 124 of each user terminal 100 is unique. For example, in a case where the encryption method executed by the encryption units 124 of the user terminals 100 is fixed to a single method, the algorithm used for the encryption process by the encryption units 124 of the user terminals 100 may be the same while the keys used for encryption processes by the encryption units 124 of the user terminals 100 may be different from each other. In the case where the encryption methods executed by the encryption units 124 of the user terminals 100 vary, for example, the case where the algorithm used for encryption by the encryption units 124 of the user terminals 100 is always fixed and is the same among all the encryption units 124 and the key used for encryption by the encryption unit 124 of each user terminal 100 changes every time data is encrypted, even with the same key possibly happening to be used between a certain user terminal 100 and another user terminal 100, if the probability is low to an ignorable extent (for example, the occurrence probability of the same key is a ten-thousandth or less), the encryption method executed by the encryption unit 124 of each user terminal 100 can be assumed to be unique.

[0133] The encryption unit 124 may receive the target data or encrypted target data from the main control unit 121 in some cases. If the data is sent from the main control unit 121 to generate multi-encrypted target data, the encryption unit 124 further encrypts the received data and then returns the encrypted data to the main control unit 121. The encryption unit 124 may receive the encrypted target data from the main control unit 121. To restore the data to the target data, if it has been sent from the main control unit 121, the encryption unit 124 applies a decryption process to the data and returns the decrypted data to the main control unit 121.

[0134] A configuration of the recording device will be described below.

[0135] The recording device 300 is a typical database connectable to the network 400. The hardware configuration may be the same as that of a computer or a server for constructing a typical database, and has no specific features. Although not shown, the recording device 300 includes a CPU, a ROM, a RAM, an HDD that is an example of a large-capacity recording medium, and an interface, which are connected to each other by a bus.

[0136] In view of hardware, the recording device 300 includes a recording unit as a functional block that is made up of a part of an HDD and is not shown. The multi-encrypted target data sent from the user terminal 100 is recorded in such a recording unit. As described later, the multi-encrypted target data may be recorded in the recording unit in a state viewable from other devices (for example, user terminals 100) connected to the network 400.

[0137] Although the number of recording devices 300 may be one, the number is typically more than one. For the node of block chain of bitcoin, the recording device 300 may be installed voluntarily by a party participating in the structure of the communication system, or installed by a party that is an administrator of the communication system or similar to such an administrator.

[0138] A method of use and operations of the communication system described above are described mainly with reference to FIG. 5.

[0139] In such a communication system, target data that is data that all of users of multiple user terminals 100 intend to save is saved in the user terminals 100.

<Case where Two User Terminals 100 Save Target Data>

[0140] First, a case where two users intend to save the target data and two user terminals 100 of these users are used to save the target data is described. Although not limited thereto, the following description is made assuming that the user terminal 100-1 and the user terminal 100-2 are the two user terminals 100 in this case.

[0141] First, at least one of the user terminal 100-1 and the user terminal 100-2 is in a state of holding the target data. The data that both the users of the user terminal 100-1 and the user terminal 100-2 intend to save (for example, record it in the user terminal 100-1 and the user terminal 100-2 in a state incapable of being falsified) is, typically, contract data specifying a contract concluded between the two users. For example, if the user of the user terminal 100-1 transfers a virtual currency to the user of the user terminal 100-2, i.e., data on the virtual currency is transmitted from the user terminal 100-1 to the user terminal 100-2, such transaction record data serves as the contract data. It is a matter of course that the contract specified by the contract data is not necessarily accompanied by transmission and reception of data between the user terminals 100, and may be a land sales contract, a marriage contract or the like. Alternatively, one of the user terminal 100-1 and the user terminal 100-2, for example, the user terminal 100-1, may be a web server for allowing a third party to browse webpages, and such a contract may be a contract that pertains to a copyright and is concluded before allowing the user of the user terminal 100-2 to download a certain program.

[0142] It is a matter of course that the target data is not limited to that pertaining to a contract. Alternatively, the data may be data specifying information that the user terminal 100-1 and the user terminal 100-2 intend to keep in confidence between these two parties, intend not to be falsified, and intend to record correctly. For example, if the user of the user terminal 100-1 is a doctor and the user of the user terminal 100-2 is a patient, data on a prescription or a medical certificate issued by the doctor to the patient is the data concerned. For example, if the user of the user terminal 100-1 is an attorney at law and the user of the user terminal 100-2 is his/her client, data on advice or a written expert opinion provided by the attorney to the client is the data concerned. At least a part of the target data may include a computer program. If a predetermined condition is satisfied, the program may be appropriately, automatically executed by a certain computer. According to this, the communication system can support smart contract that is being realized through Ethereum (trademark) or the like.

[0143] However, all the above target data items are only examples. The target data item may be any data item only if all of the users of user terminals 100 intend to save the data.

[0144] In any case, the target data is generated by a publicly known or well-known method (S801). The target data may be generated in the user terminal 100-1, generated in the user terminal 100-2, generated simultaneously, for example, in the user terminal 100-1 and the user terminal 100-2, or generated in a device other than the user terminal 100-1 and the user terminal 100-2 and then sent through transmission or the like via the network 400 to the user terminal 100-1 and the user terminal 100-2. Further alternatively, the target data is not necessarily newly generated data, for example, may be data having previously been held by at least one of the user terminal 100-1 and the user terminal 100-2.

[0145] Irrespective of the type of the obtaining step, one of the user terminal 100-1 and the user terminal 100-2 is in a state of holding the target data. In this embodiment, it is assumed that the target data exists only in the user terminal 100-1, in any way.

[0146] The target data is, for example, in a state of being recorded in the first recording unit 123 of the user terminal 100-1. Here, the user of the user terminal 100-1 inputs selection information that is a combination of information specifying the target data to be saved, and a user intending to share and save the target data or data specifying his/her user terminal 100 (S802). When the selection information is input, for example, an image for prompting the user to input the selection information is displayed on the display 101 of the user terminal 100-1, as shown in FIG. 6. Data on such an image is generated by the main control unit 121 of the user terminal 100-1. Such image data is sent from the main control unit 121 to the display 101 via the data input/output unit 122 and the interface 114, thereby displaying the image as shown in FIG. 6 on the display 101.

[0147] The image for prompting the user to input the selection information includes: characters of Target data, and a landscape rectangular frame that is for allowing the target data to be input and is arranged horizontally next to the characters; characters of Sharing destination(s) 1 to 3, and landscape rectangular frames that are for input associated respectively with the characters of Sharing destination(s) 1 to 3 and are arranged horizontally next to the characters; characters of Multicast, and buttons that are assigned YES and NO and are arranged horizontally next to the characters of Multicast; and a button that is assigned characters of OK and disposed lowermost.

[0148] The frame displayed horizontally next to the characters of Target data is for allowing information for selecting the target data to be written therein. Typically, the target data seems to be a file on certain data. In this case, the user writes the file name of the target data in the frame, for example. Such writing is performed by operating the input device 102.

[0149] The frames prepared horizontally next to the characters of sharing destination(s) 1 to 3 are for allowing information to be written therein, the information being for identifying the user terminal 100 that saves the target data with the user terminal 100-1. The sharing destinations 1 to 3 each correspond to the information for identifying one user terminal 100. In the example of FIG. 6, three user terminals 100 at the maximum can be designated as user terminals 100 that save the target data together with the user terminal 100-1. Such designation of the user terminals 100 can be performed based on the user ID, for example. Alternatively, the designation may be performed based on the name or a nickname of each user or any identifier assigned to each user terminal 100. In this embodiment, the user terminal 100 that saves the target data together with the user terminal 100-1 is only the user terminal 100-2. The user of the user terminal 100-1 writes it only to the sharing destination 1 on the display 101. Such writing is performed by operating the input device 102. Note that in this embodiment, the frames where the sharing destinations are to be written are preliminarily prepared as the three sharing destinations 1 to 3. However, these are not limited thereto. For example, such a frame is, first, indicated on the display 101 as only a single frame. The frame may be added by the user through operation of the input device 102 as required. Alternatively, the user may be allowed to input the required number of frames, and frames as many as the number designated by the user may be indicated on the display 101.

[0150] The input of the selection information has thus been described so far. In this embodiment, although not necessarily, in addition to the selection information, the following input about whether multicast is required or not is performed. Note that input about the necessity of multicast may be considered to correspond to a part of input of the selection information. It may be configured to always execute multicast, or not to execute multicast at all. In such a case, there is no need to ask the user for the necessity of multicast. The after-mentioned input and the indication on the display 101 for prompting the user to input it can be omitted.

[0151] The buttons assigned the characters of YES and NO positioned horizontally next to the characters of Multicast are for allowing the user to click any of the buttons to select whether multicast is performed or not. Here, Multicast means a process of causing the recording device 300 to record the multi-encrypted target data, described later. Such a process will be described later in detail. If the user selects the button assigned the characters of YES, multicast is performed. If the user selects the button assigned the characters of NO, multicast is not performed. Such selection is performed by operating the input device 102. Although not limited thereto, this embodiment assumes that the user selects the button assigned the characters of YES.

[0152] After completion of the entire input described above, the user clicks the button assigned the characters of OK. When input to the button assigned the characters of OK is performed, input of the selection information described above and the information about selection of whether multicast is performed or not are executed. Input to the button assigned the characters of OK also serves as input of the start information (S803).

[0153] When the user of the user terminal 100-1 clicks the button assigned the characters of OK, the selection information, the information on the necessity of multicast, and the start information are generated. These pieces of information are sent from the input device 102 to the data input/output unit 122 of the control unit 120 through the interface 114 and then sent to the main control unit 121.

[0154] Upon receipt of the selection information, the main control unit 121 reads, from the first recording unit 123, data specified by the information written in the frame indicated horizontally next to the characters of Target data on the display 101 among the pieces of the selection information, as the target data. Such data is plaintext data. This is hereinafter dealt with as the target data.

[0155] The main control unit 121 sends the target data read from the first recording unit 123 to the encryption unit 124. The encryption unit 124 having received the data encrypts the received target data (S804). Although not limited thereto, the algorithm used by the encryption unit 124 in this embodiment to encrypt the target data or the encrypted target data is always the same. On the other hand, although not limited thereto, the encryption unit 124 in this embodiment generates a new key in advance every time the target data or the encrypted target data is encrypted, and executes an encryption process using the generated new key. The encryption unit 124 encrypts the target data using the fixed algorithm and the key that varies every time the encryption process is performed, and obtains the encrypted target data.

[0156] The encryption unit 124 sends the encrypted target data and the key used for the encryption, to the main control unit 121. The main control unit 121 temporarily saves the key. The main control unit 121 adds the information written in the frame indicated horizontally next to the characters of the sharing destination 1 on the display 101 among the pieces of selection information, that is, the information for identifying the user terminal 100-2 that is the user terminal 100 of the opposite user sharing the multi-encrypted target data together with the user terminal 100-1, and the information for identifying the user terminal 100-1, to the encrypted target data. Although not limited thereto, in this embodiment, the user ID of the user terminal 100-2 is used as the former piece of information assigned to the encrypted target data, and user ID of the user of the user terminal 100-1, which is held by the main control unit 121, is used as the latter. The main control unit 121 sends the target data assigned the two user IDs to the data input/output unit 122. Such data is sent from the data input/output unit 122 to the transmission and reception unit via the interface 114.

[0157] The transmission and reception unit transmits the encrypted target data assigned the two user IDs, via the network 400, to the user terminal 100-2, which is the opposite user terminal 100 sharing the multi-encrypted target data together with the user terminal 100-1 (S805). The destination user terminal 100-2 can be identified by the user ID of the opposite user terminal 100-2, the user ID being assigned to the target data.

[0158] The encrypted target data assigned the two user IDs is transmitted via the network 400, and received by the transmission and reception unit of the user terminal 100-2 (S901).

[0159] The target data that has been encrypted by the user terminal 100-1 and assigned the two user IDs is sent from the transmission and reception unit of the user terminal 100-2 to the main control unit 121 via the interface 114 and the data input/output unit 122. The main control unit 121 sends the encrypted target data in the data to the encryption unit 124.

[0160] The encryption unit 124 further encrypts the encrypted target data (S902). The target data has thus been encrypted by the encryption unit 124 of the user terminal 100-1 and the encryption unit 124 of the user terminal 100-2 sequentially, or continuously in this order. That is, the thus obtained data is the multi-encrypted (double-encrypted) target data. Similar to the case of the user terminal 100-1, the encryption unit 124 of the user terminal 100-2 is assumed to perform encryption using the fixed algorithm and the varying key. However, the encryption algorithms used by the user terminal 100-1 and the user terminal 100-2 are not necessarily the same as each other.

[0161] The double-encrypted target data, accompanied by the key used for encrypting the data by the encryption unit 124, is sent from the encryption unit 124 to the main control unit 121. The main control unit 121 records the double-encrypted target data, with the key used for encrypting the data, in the first recording unit 123 (S903). The double-encrypted target data recorded in the first recording unit 123 may be assigned at least the user ID of the user of the user terminal 100-1 between the two user IDs described above. Meanwhile, the main control unit 121 assigns the two user IDs described above to the double-encrypted target data again. The double-encrypted target data assigned these two user IDs is sent from the main control unit 121 to the transmission and reception unit via the data input/output unit 122 and the interface 114.

[0162] The transmission and reception unit transmits the double-encrypted target data assigned the two user IDs to the user terminal 100-1 via the network 400 (S904). The destination user terminal 100-1 can be identified by the user ID of the opposite user terminal 100-1, the user ID being assigned to the target data.

[0163] The encrypted target data assigned the two user IDs is transmitted via the network 400, and received by the transmission and reception unit of the user terminal 100-1 (S806).

[0164] The double-encrypted target data assigned the two user IDs is sent from the transmission and reception unit of the user terminal 100-1 to the main control unit 121 via the interface 114 and the data input/output unit 122.

[0165] The main control unit 121 records, in the first recording unit 123, the double-encrypted target data together with the key used for encrypting the target data at the first time by the encryption unit 124 of the user terminal 100-1 (this key is provided from the encryption unit 124 and held by the main control unit 121 as described above) (S807). The double-encrypted target data recorded in the first recording unit 123 may be assigned at least the user ID of the user of the user terminal 100-2 between the two user IDs described above.

[0166] Thus, the double-encrypted target data is in a state of being shared between the user terminal 100-1 and the user terminal 100-2. The double-encrypted target data cannot be solely decrypted, of course by a third party, and also by the user terminal 100-1 and the user terminal 100-2, which are parties concerned having generated the double-encrypted target data. If the double-encrypted target data is falsified in any way, the falsification is found based on the fact that the double-encrypted target data cannot be decrypted even by cooperation of the user terminal 100-1 and the user terminal 100-2. Consequently, the double-encrypted target data, and the plaintext target data that is the origin thereof accordingly, are shared between the user terminal 100-1 and the user terminal 100-2 in a state of being saved in an unfalsifiable manner.

[0167] Meanwhile, the main control unit 121 multicasts the double-encrypted target data assigned again with the two user IDs described above (S808). The multicast process is executed by the main control unit 121 based on the fact that the user has selected the button assigned the characters of YES as described above. To perform multicast, the main control unit 121 transmits data items obtained by removing the key from the data recorded by the main control unit 121 in the first recording unit 123 of the user terminal 100-1, to the transmission and reception unit via the data input/output unit 122 and the interface 114. The data items sent to the transmission and reception unit are sent from the transmission and reception unit to at least one of the recording devices 300 via the network 400. The recording device 300 serving as the destination of these data items may be a device selected by the user of the user terminal 100-1 from among the recording devices 300, a device randomly selected from among the recording devices 300, a device selected by the user terminal 100-1 according to a certain rule, or all the recording devices 300.

[0168] The recording device 300, having received the double-encrypted target data assigned the two user IDs, records the received data in the recording unit, not shown. The data may be recorded in the recording unit in a state viewable by a third party via the network 400, which applies to this embodiment. Note that what is in the state viewable by a third party may be only the multi-encrypted target data. The number of recording devices 300 may be only one. If multiple devices reside, multiple (or many) multi-encrypted target data items are recorded in each recording device 300, with the passage of time. In this case, the multi-encrypted target data items recorded in each recording device 300 are not necessarily the same. The multi-encrypted target data items recorded in each recording device 300 may be all the multiple generated multi-encrypted target data items or some of them. The multi-encrypted target data items recorded in multiple recording devices 300 that record only some of items among the multiple generated multi-encrypted target data items may be partially redundant. Alternatively, multi-encrypted target data items different from each other may be recorded.

[0169] As a result, the double-encrypted target data assigned the two user IDs is in a state of being shared and saved by the user terminal 100-1 and the user terminal 100-2, which are the two user terminals 100 serving as parties concerned having generated the data, and by a certain number of recording devices 300, for example. Note that the multicast process can be performed only with the double-encrypted target data. Accordingly, the process may be performed by another device holding the double-encrypted target data. For example, instead of the user terminal 100-1, the user terminal 100-2 may perform the multicast process.

[0170] Among the steps described above, the processes from S803 pertaining to input of the start information to S807 pertaining to reception of the double-encrypted target data, or the processes from S803 pertaining to input of the start information to S808 pertaining to multicast may be automatically performed by the user terminal 100-1 and the user terminal 100-2, and by the recording device 300 in some cases; this applies to this embodiment, although not limited thereto. This embodiment assumes that the selection information and the start information are input by the user of the user terminal 100-1. Alternatively, for example, all the processes described above may be automatically executed by the user terminal 100-1 and the user terminal 100-2, and by the recording device 300 in some cases, when the target data is generated. In this case, the input of the selection information and the start information by the user may be omitted.

[0171] The double-encrypted target data shared by the user terminal 100-1, the user terminal 100-2 and the recording device 300 is not necessarily used at all thereafter. As described later, for example, if the necessity of verifying the content of the target data arises in the future, the data can be used.

[0172] In this case, the double-encrypted target data is decrypted and restored to the plaintext target data. The decrypted plaintext target data has the same content as the target data having not been double-encrypted yet has. Accordingly, verification of the past target data based on the decrypted target data causes no error. There is no doubt on the authenticity of the target data.

[0173] Note that in this example, what is sent from the user terminal 100-1 or the user terminal 100-2 to the recording device 300 during multicast is the double-encrypted target data assigned the two user IDs. Here, the target data that is the origin of the double-encrypted target data can be added to the data sent from the user terminal 100-1 or the user terminal 100-2 to the recording device 300 during multicast. In this case, what is recorded in the recording unit, which is of the recording device 300 but is not shown, includes the double-encrypted target data assigned the two user IDs, and the target data that is the origin of the double-encrypted target data. The double-encrypted target data, and the target data that is the origin of the double-encrypted target data are recorded in association with each other or as a pair. The double-encrypted target data and the target data that is the origin of the double-encrypted target data, which belong to the same pair or different pairs, are accumulated in the recording unit of each recording device 300. In this case, at least one of the double-encrypted target data and the target data that is the origin thereof may be in a state of being disclosed to other devices via the network 400. If only the user terminal 100-1 holds the plaintext target data during the process of generating the target data (S801), basically only the user terminal 100-1 between the user terminal 100-1 and the user terminal 100-2 can transmit, in addition to the double-encrypted target data, the target data that is the origin thereof, to the recording device 300. If these data items are transmitted from the user terminal 100-2 to the recording device 300, the plaintext target data may be transmitted from the user terminal 100-1 to the user terminal 100-2, or the target data encrypted by the user terminal 100-1 for allowing the user terminal 100-2 to perform decryption may be transmitted from the user terminal 100-1 to the user terminal 100-2 at appropriate timing before the user terminal 100-2 transmits the double-encrypted target data and the target data that is the origin thereof to the recording device 300.

<Case where Two User Terminals 100 Decrypt Saved Target Data>

[0174] The double-encrypted target data is decrypted as follows, for example.

[0175] The party intending to verify the double-encrypted target data is, typically, the user of the user terminal 100-1 or the user of the user terminal 100-2.

[0176] As described above, the user terminal 100-1 and the user terminal 100-2 hold the double-encrypted target data in a state of being recorded in their first recording units 123. Accordingly, the user may select the double-encrypted data item that the user intends to decrypt, from among data items recorded in the first recording unit 123, exchange the data item between the user terminal 100-1 and the user terminal 100-2, and achieve decryption. Such a process is, typically, executed as an inverse process where the double-encrypted target data is adopted as the target data. Although this embodiment is not limited thereto, the execution applies to this embodiment.

[0177] For example, it is assumed that the user of the user terminal 100-2 intends to decrypt the target data double-encrypted with the user terminal 100-1. The user of the user terminal 100-2 selects the double-encrypted target data, using the input device 102 of the user terminal 100-2, for example. As described above, the selected double-encrypted target data is assigned the key used at the time of encryption of the data and the user IDs of the user terminal 100-1 and the user terminal 100-2. The main control unit 121 reads all the data items from the first recording unit 123.

[0178] The main control unit 121 transmits the key and the double-encrypted target data among the read data items, to the encryption unit 124. The encryption unit 124 decrypts the double-encrypted target data, using the fixed algorithm and the key provided from the main control unit 121. Accordingly, the double-encrypted target data is decrypted, and the state returns to the state where the target data is encrypted only by the encryption unit 124 of the user terminal 100-1. The encryption unit 124 sends this data together with the double-encrypted data, to the main control unit 121.

[0179] The main control unit 121 assigns these data items the user IDs of the user terminal 100-1 and the user terminal 100-2, and sends them to the transmission and reception unit via the data input/output unit 122 and the interface 114. These data items are sent from the transmission and reception unit to the user terminal 100-1 via the network 400.

[0180] The user terminal 100-1 receives the data items at the transmission and reception unit. The data that is in the state where the target data encrypted only by the encryption unit 124 of the user terminal 100-1 and is assigned the two user IDs and the double-encrypted target data, is sent from the transmission and reception unit of the user terminal 100-1 to the main control unit 121 via the interface 114 and the data input/output unit 122.

[0181] The main control unit 121, having received the data, searches the data items recorded in the first recording unit 123 for the same data as the double-encrypted data. When the same data as the double-encrypted data received from the user terminal 100-2 is found in the first recording unit 123, the main control unit 121 reads the key assigned to the double-encrypted data, from the first recording unit 123. This key is the key used by the encryption unit 124 of the user terminal 100-1 for encrypting the double-encrypted data. The main control unit 121 transmits, to the encryption unit 124, this key, and the data that has been transmitted from the user terminal 100-2 and is in the state of the target data encrypted only by the encryption unit 124 of the user terminal 100-1.

[0182] Upon receipt of the data in the state of the target data encrypted only by the encryption unit 124 of the user terminal 100-1 from the main control unit 121, the encryption unit 124 decrypts the data using the fixed algorithm and the key provided from the main control unit 121. Accordingly, the data in the state of the target data encrypted only by the encryption unit 124 of the user terminal 100-1 returns to the plaintext target data.

[0183] The encryption unit 124 sends the target data to the main control unit 121. The main control unit 121 records the target data in the first recording unit 123 of the user terminal 100-1. Accordingly, the state comes into a state where target data can be verified.

[0184] If required, the target data is sent from the user terminal 100-1 to the user terminal 100-2. Accordingly, the user terminal 100-1 and the user terminal 100-2 come into the state of sharing the target data.

[0185] As evidenced by the above description, the transmission of the double-encrypted target data from the user terminal 100-2 to the user terminal 100-1 is for identifying the key to be used by the encryption unit 124 of the user terminal 100-1 to decrypt the data in the state of the target data encrypted only by the encryption unit 124 of the user terminal 100-1. Typically, multi-encrypted target data items other than the double-encrypted target data item generated in corporation with the user terminal 100-2 is recorded in the first recording unit 123 of the user terminal 100-1. Furthermore, the pieces of content and the number of the multi-encrypted target data items that are recorded in the first recording units 123 of many user terminals 100 or recorded in the recording units of the multiple recording devices 300, may vary during operation of the communication system. In such situations, when the user terminal 100-1 decrypts the above-described data received from the user terminal 100-2, the user terminal 100-1 is required to identify the key to be used to decrypt the data. One example to be used for identifying such a key is the aforementioned double-encrypted target data transmitted from the user terminal 100-2 to the user terminal 100-1.

[0186] It is therefore necessary to send the data that is in the state of the target data encrypted only by the encryption unit 124 of the user terminal 100-1, from the user terminal 100-2 to the user terminal 100-1. If the identification of the key described above can be achieved by another method, the double-encrypted target data is not required to be transmitted from the user terminal 100-2 to the user terminal 100-1. For example, it is assumed that when the double-encrypted target data is generated, the identifier for identifying the data is generated by, for example, the user terminal 100-2, the main control unit 121 of the user terminal 100-2 assigns the identifier to the double-encrypted target data and records it in the first recording unit 123 while the main control unit 121 of the user terminal 100-1 having received the identifier from the user terminal 100-2 also assigns the identifier to the double-encrypted target data and records it in the first recording unit 123. In this case, when the user terminal 100-2 having decrypted the double-encrypted data sooner sends, to the user terminal 100-1, the data that is in the state of the target data encrypted only by the encryption unit 124 of the user terminal 100-1, the user terminal 100-2 can transmit the identifier instead of the double-encrypted data. The main control unit 121 of the user terminal 100-1 having received these data items can use, as a query, the identifier instead of the double-encrypted target data; the query is for searching the keys recorded in the first recording unit 123, for the key to be used by the encryption unit 124 to decrypt the data that is in the state of the target data encrypted only by the encryption unit 124 of the user terminal 100-1.

[0187] Note that the example described above assumes that the user terminal 100-1 and the user terminal 100-2 save the key in the first recording units 123. This is because the key used to double-encrypt the target data is allowed to be identified when the double-encrypted target data is decrypted. If the key can be identified by another method, the data to be recorded in the first recording unit 123 in association with the double-encrypted target data is not necessarily the key data itself and may be any data item allowing the key to be identified. For example, it is assumed that the encryption units 124 of the user terminal 100-1 and the user terminal 100-2 change the key on the basis of the date and time of the encryption. According to such an assumption, both the encryption units 124 can regenerate the key used for the encryption process, only with information on the date and time of generation of the key. In such a case, the first recording unit 123 may adopt, as data on the date and time of the generation of the key, the data recorded in association with the double-encrypted target data instead of the key data. Accordingly, instead of supply of the key data from the main control unit 121, supply of the data on the date and time allows both the encryption units 124 to generate a key that is the same as the key that would have been supplied from the main control unit 121. Consequently, the decryption process can be achieved in a manner indistinguishable from the case described above.

[0188] Furthermore, if the key used by both the encryption units 124 is always the same, there is no need at all to identify the key used when the target data was double-encrypted in the past, for decryption by the both the encryption units 124. In such a case, data, such as the key data or the above-described data on the date and time allowing the key to be identified, is not required to be recorded in the first recording unit 123.

[0189] In the above example, decryption of the double-encrypted target data is started from the user terminal 100-2 that encrypted the double-encrypted target data at the last time. This is because when certain data is encrypted by different methods multiple times, the original data cannot typically be restored unless the decryption is performed in the inverted order. That is, in the above example, the target data is encrypted by the encryption unit 124 of the user terminal 100-1 and is further encrypted by the encryption unit 124 of the user terminal 100-2. In this case, the data is restored to the original target data, if the double-encrypted target data is decrypted by the encryption unit 124 of the user terminal 100-2 and is further decrypted by the encryption unit 124 of the user terminal 100-1. However, the data is not restored to the original target data if the data is decrypted by the encryption unit 124 of the user terminal 100-1 first and is further decrypted by the encryption unit 124 of the user terminal 100-2.

[0190] Accordingly, for example, even if the user of the user terminal 100-1 intends to decrypt the double-encrypted target data described above, decryption of the double-encrypted target data is required to be started by the encryption unit 124 of the user terminal 100-2 first. The user terminal 100-1 requests the user terminal 100-2 to decrypt the double-encrypted target data while specifying the double-encrypted target data intended to be decrypted. The user terminal 100-1 may send, to the user terminal 100-2, the double-encrypted target data recorded in the first recording unit 123 of the user terminal 100-1 as the data for identifying the double-encrypted target data. If the above-described identifier commonly held by the user terminal 100-1 and the user terminal 100-2 exists, the identifier may be sent to the user terminal 100-2. The flow of the process of decrypting the double-encrypted target data to be started from the user terminal 100-2 is the same as that in the case described above.

[0191] Note that when the double-encrypted target data is decrypted in the communication system in this embodiment, preparation should be made such that the user terminals 100 having performed encryption to achieve the double-encrypted target data and the order of the encryption can be grasped in the case of need of decryption. In consideration of such points, based on the order of data items that are the user IDs of the user terminal 100-1 and the user terminal 100-2 assigned to the double-encrypted target data in the first recording units 123 of the user terminal 100-1 and the user terminal 100-2, the order of encryption by the user terminal 100-1 and the user terminal 100-2 is required to be grasped. Otherwise, data indicating the user terminals 100 having performed encryption to achieve the double-encrypted target data and the order of the encryption is required to be assigned alternatively.

[0192] Likewise, when the user, the administrator or the like of each recording device 300 intends to decrypt multi-encrypted target data recorded in the recording unit of the recording device 300, the user or the administrator specifies the multi-encrypted target data on which he/she intends to request decryption, and requests the user terminal 100 that encrypted the multi-encrypted target data at the last time, to decrypt the multi-encrypted target data. When a certain user, administrator or the like intends to decrypt the double-encrypted target data described above, the user, administrator or the like specifies the double-encrypted target data on which he/she intends to request decryption, and requests the user terminal 100-2, which is the user terminal 100 that encrypted the double-encrypted target data at the last time, to decrypt the double-encrypted target data. The flow of the process of decrypting the double-encrypted target data to be started from the user terminal 100-2 is the same as that in the case described above. Alternatively, when a third party monitoring the multi-encrypted target data that is recorded in the recording unit of the recording device 300 intends to decrypt certain multi-encrypted target data, the third party may also specify the multi-encrypted target data on which the third party intends to request decryption, and request the user terminal 100 that encrypted the multi-encrypted target data at the last time, to decrypt the multi-encrypted target data.

<Case where Three or More User Terminals 100 Save Target Data>

[0193] A case where three user terminals 100 save the target data is described. Although not limited thereto, the following description is made assuming that a user terminal 100-1, a user terminal 100-2 and a user terminal 100-3 are the three user terminals 100 in this case.

[0194] In this case, at least one of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3 is in a state of holding the target data. The target data in this case is, typically, based on a three-party agreement among the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3. However, the target data is not limited thereto.

[0195] The target data may be generated in any one of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3, generated for example simultaneously in at least two of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3, or generated in another device other than the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3. The target data is not necessarily newly generated data, and may be data that at least one of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3 have previously held.

[0196] In this case, it is assumed that the target data exists only in the user terminal 100-1.

[0197] Hereafter, the target data is encrypted by the encryption unit 124 of the user terminal 100-1 first, and is subsequently encrypted by the encryption units 124 of the user terminal 100-2 and the user terminal 100-3. Similar to the case where the target data is double-encrypted with the user terminal 100-2, for example, the user ID can be used for selection of the user terminal 100-2 and the user terminal 100-3, which are user terminals 100 to encrypt the target data together with the user terminal 100-1, and for transmission of the encrypted target data to the user terminal 100-2 and the user terminal 100-3.

[0198] While the device that performs encryption first is freely selected between the encryption unit 124 of the user terminal 100-2 and the encryption unit 124 of the user terminal 100-3, the target data is encrypted by all the encryption units 124 of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3 as a result. It is simple that for example, the user terminal 100-1 including the encryption unit 124 to perform encryption first determines the order of encryption in the encryption units 124 of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3. Alternatively, the order may be determined by the user terminal 100-2 or the user terminal 100-3. Further alternatively, the order may be determined collectively by the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3 (or the users thereof), or determined randomly. The situations similarly apply to a case of two user terminals 100, and also to four or more user terminals 100.

[0199] Multi-encrypted (triple-encrypted) target data achieved through encryption sequentially by all the encryption units 124 of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3 is recorded in all the first recording units 123 of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3. Similar to the description of <Case where two user terminals 100 save target data>, data to be herein recorded together with the triple-encrypted target data in the three first recording units 123 is data required by the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3 to decrypt the triple-encrypted target data thereafter. For example, in the three first recording units 123 of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3, the user IDs of the three user terminals 100 are recorded together with the triple-encrypted target data. Similar to the above case, it is also preferable that these three user IDs be arranged so as to indicate the order of encryption of the triple-encrypted target data by the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3.

[0200] At least one of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3 that share the triple-encrypted target data can perform a multicast process. In this case, the data including the triple-encrypted target data identical to that recorded in the first recording units 123 of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3, is recorded in at least one of the recording units of the recording devices 300.

[0201] Not only the triple-encrypted target data but also the target data that is the origin thereof is adopted as multicast targets, and the recording unit of the recording device 300 can be brought into a state where plaintext target data can also be written; this applies to the case of two user terminals 100 and also to the case of three user terminals 100.

[0202] Similar to the description of <Case where two user terminals 100 decrypt saved target data>, the triple-encrypted target data recorded in the first recording units 123 of the user terminal 100-1, the user terminal 100-2 and the user terminal 100-3 is decrypted by the encryption units 124 of the user terminals 100 in an inverted order of encryption of the target data that is the origin of the triple-encrypted target data. Accordingly, the triple-encrypted target data is restored to the original target data.

[0203] This also applies to handling of the triple-encrypted target data recorded in the recording device 300.

[0204] In the above example, the number of user terminals 100 that encrypt the target data is three. Likewise, the description applies also to the case where the number is four, five or M.

[0205] In the case where M user terminals 100 save certain target data, the target data or encrypted data thereof is encrypted by all the encryption units 124 of the M user terminals 100 while being transmitted and received between the M user terminals 100. Accordingly, M-fold multiple-encrypted target data is shared among the M user terminals 100, and is recorded in the first recording units 123 of the M user terminals 100. Data to be recorded in the first recording units 123 of the M user terminals 100 together with the M-fold multiple-encrypted target data is data required for decryption, as described above.

[0206] Similar to the case described above, the M-fold multiple-encrypted target data recorded in the first recording units 123 of the M user terminals 100 is decrypted by the encryption units 124 of the user terminals 100 in an inverted order of encryption of the target data that is the origin of the M-fold multiple-encrypted target data. Accordingly, the M-fold multiple-encrypted target data is restored to the original target data.

[0207] This also applies to handling of the M-fold multiple-encrypted target data recorded in the recording device 300.

Second Embodiment

[0208] A communication system of a second embodiment is described.

[0209] The communication system in the second embodiment is substantially identical to the communication system in the first embodiment. Similar to the case of the first embodiment, the communication system in the second embodiment includes many user terminals 100 connected to the network 400, and many, at least a plurality of, recording devices 300.

[0210] As shown in FIG. 7, the communication system in the second embodiment is different from the communication system in the first embodiment in that the communication system in the second embodiment includes many, or a plurality of, auxiliary encryption terminals 200, which do not exist in the case of the first embodiment. In view of the configuration, the second embodiment and the first embodiment are different only in this point.

[0211] The configurations and operations of the auxiliary encryption terminal 200-1 to the auxiliary encryption terminal 200-N may be the same in consideration of the relationship with the invention of the present application; this applies to this embodiment.

[0212] The auxiliary encryption terminal 200 corresponds to an auxiliary communication device in the invention of the present application. It can be assumed that the auxiliary encryption terminal 200 has only some of functions of the user terminal 100. To put it plainly, the user terminal 100 has a function of determining target data, a function of executing one encryption process during execution of multiple encryption of the target data, a function of saving multi-encrypted target data, and a function of executing one decryption process during decryption of the multi-encrypted target data. Among these functions, the functions necessary for the auxiliary encryption terminal 200 are only the function of executing one encryption process during multiple encryption of the target data, and the function of executing one decryption process during decryption of the multi-encrypted target data.

[0213] As described in the first embodiment, the target data is what the users of the multiple user terminals 100 intend to save. That is, the user of the user terminal 100 can be a party motivated to save the target data, and typically, a party having an interest on the content of the target data. On the contrary, the auxiliary encryption terminal 200 or its user or administrator is not necessarily motivated to save the target data. In the first embodiment, multi-encryption of the target data is performed by the multiple user terminals 100 whose users have the statuses of parties concerned. In the second embodiment, not only the user terminals 100 but also the auxiliary encryption terminals 200 participate in multiple encryption of the target data. To allow participation of multi-encryption of the target data, the auxiliary encryption terminal 200 requires the function of executing one encryption process during multi-encryption of the target data, as described above. To decrypt the multi-encrypted target data with participation by not only the multiple user terminals 100 but also the auxiliary encryption terminals 200, the multi-encrypted target data cannot be restored to the plaintext target data unless not only the multiple user terminals 100 but also the auxiliary encryption terminals 200 participate. To achieve this, the auxiliary encryption terminal 200 requires the function of one decryption process during decryption of the multi-encrypted target data, as described above.

[0214] For example, the auxiliary encryption terminal 200 may be a device managed by a user having a public status, such as of a notary public. Alternatively, the auxiliary encryption terminal 200 may be the user terminal 100 itself, and the function of the auxiliary encryption terminal 200 described later may be achieved by the user terminal 100. The recording device 300 may also have the functions of the auxiliary encryption terminal 200. The following description is made assuming that in this embodiment, only users having a status such as of a notary public, have the auxiliary encryption terminals 200.

[0215] The hardware configuration of the auxiliary encryption terminal 200 may be the same as that of the user terminal 100. This applies to this embodiment. The functions required by the auxiliary encryption terminal 200 are some of the functions of the user terminal 100, as described above. Accordingly, the functional blocks generated in the auxiliary encryption terminal 200 may be some of the functional blocks generated in the user terminal 100. Although not limited thereto, in this embodiment, the functional blocks show in FIG. 4 that are the same as those residing in the user terminal 100 are generated in the auxiliary encryption terminal 200.

[0216] The functions of the functional blocks generated in the auxiliary encryption terminal 200 are the same as the functions of the functional blocks generated in the user terminal 100. Note that the main control unit 121 is not required to deal with the selection information and the start information. Accordingly, the main control unit 121 of the auxiliary encryption terminal 200 does not have the function of dealing with the selection information and the start information that the main control unit 121 of the user terminal 100 has, the function having been described in the first embodiment. As described above, the auxiliary encryption terminal 200 requires the function of executing one encryption process during multi-encryption of the target data during multi-encryption of the target data, and the function of executing one decryption process during decryption of the multi-encrypted target data. Similar to the case of the user terminal 100, such functions are guaranteed by the encryption unit 124 of the auxiliary encryption terminal 200. That is, the encryption unit 124 of the auxiliary encryption terminal 200 has a function of encrypting data, and a function of decrypting data encrypted by the encryption unit 124. Similar to the user terminal 100 described in the first embodiment, the encryption unit 124 of the auxiliary encryption terminal 200 may always perform decryption and encryption by the same method. In some cases, for example, the methods of encryption and decryption may be changed every time encryption and decryption of target data or the like are performed (e.g., the key is changed). Note that the processes of encryption and decryption executed by the encryption unit 124 of each auxiliary encryption terminal 200 and the encryption unit 124 of each user terminal 100 are unique to each other.

[0217] A method of using the communication device and operations thereof in the second embodiment are described.

[0218] For example, a case where two user terminals 100 save the target data is described. Similar to the case of the first embodiment, the devices saving the target data are the user terminal 100-1 and the user terminal 100-2.

[0219] Similar to the case of the first embodiment, first, at least one of the user terminal 100-1 and the user terminal 100-2 is in the state of holding the target data. Similar to the first embodiment, the second embodiment also assumes that the target data exists only in the user terminal 100-1.

[0220] The target data is, for example, in a state of being recorded in the first recording unit 123 of the user terminal 100-1. Here, the user of the user terminal 100-1 inputs selection information that is a combination of information specifying the target data to be saved, and a user intending to share and save the target data or data that identifies his/her user terminal 100. When the selection information is input, for example, an image for prompting the user to input the selection information is displayed on the display 101 of the user terminal 100-1, as shown in FIG. 8. The image for prompting the user to input the selection information includes: characters of Target data, and a landscape rectangular frame corresponding thereto; characters of Sharing destination(s) 1 to 3, and landscape rectangular frames corresponding thereto; characters of Multicast, and buttons assigned characters of YES and NO; and a button assigned characters of OK. All of them, including the functions assigned thereto, are the same as those in the first embodiment. In addition to those described above, the image in the second embodiment further includes characters of Notary public 1 and Notary public 2, and landscape rectangular frames respectively arranged horizontally next to them.

[0221] The frames residing next to the characters of Notary public 1 and Notary public 2 are for allowing the user of the user terminal 100-1 to select whether not only the user terminal 100 but also the auxiliary encryption terminal 200 owned by the user having a status such as of a notary public are allowed to participate in the step of multi-encrypting the target data when the target data is encrypted. If one auxiliary encryption terminal 200 is allowed to participate in the step of multi-encrypting the target data, the user of the user terminal 100-1 inputs information for specifying the auxiliary encryption terminal 200 of the notary public (e.g., the user ID identifying the notary public) in the frame right to the characters of Notary public 1. Alternatively, if two auxiliary encryption terminals 200 are allowed to participate in the step of multi-encrypting the target data, the user inputs information for specifying the auxiliary encryption terminals 200 of the two notaries public in the frames right to the characters of Notary public 1 and Notary public 2, respectively. It is a matter of course that to allow more auxiliary encryption terminals 200 to participate in the step of multi-encrypting the target data, more frames for allowing writing for notaries public may be prepared, or the number of frames can be increased based on the intention of the user. The aforementioned information on whether to allow the auxiliary encryption terminal 200 of the notary public to participate in the step of multi-encrypting the target data or not, and on which auxiliary encryption terminal 200 is allowed to participate in the step if the participation is allowed, constitutes a part of the selection information. Although not limited thereto, this embodiment assumes that the user ID of the notary public that is information for selecting the auxiliary encryption terminal 200-1 is written in the frame right to the Notary public 1.

[0222] After completion of the entire input described above, the user clicks the button assigned the characters of OK. When input to the button assigned the characters of OK is performed, input of the selection information described above and input of the information about selection of whether multicast is performed or not are executed. Input to the button assigned the characters of OK also serves as input of the start information.

[0223] When the user of the user terminal 100-1 clicks the button assigned the characters of OK, the selection information, the information on the necessity of multicast, and the start information are generated. These pieces of information are sent from the input device 102 to the data input/output unit 122 of the control unit 120 through the interface 114 and then sent to the main control unit 121. The processes thereafter are substantially similar to those in the first embodiment. In particular, the processes in the case where no auxiliary encryption terminal 200 is allowed to participate in the process of multi-encrypting the target data are not different from those in the first embodiment at all.

[0224] Upon receipt of the selection information, the main control unit 121 reads, from the first recording unit 123, data specified by the information written in the frame indicated horizontally next to the characters of Target data on the display 101 among the pieces of the selection information, as the target data. Such data is plaintext data. This is hereinafter dealt with as the target data.

[0225] The main control unit 121 sends the target data read from the first recording unit 123 to the encryption unit 124. The encryption unit 124 having received the data encrypts the received target data. The target data encrypted by the encryption unit 124 is subsequently sent to one of the user terminal 100-2 and the auxiliary encryption terminal 200-1, encrypted by the one of the user terminal 100-2 and the auxiliary encryption terminal 200-1, then sent to the other of the user terminal 100-2 and the auxiliary encryption terminal 200-1, and encrypted by the encryption unit 124 of the other of the user terminal 100-2 and the auxiliary encryption terminal 200-1. The thus generated multi-encrypted (triple-encrypted) target data is transmitted and received between the user terminal 100-1, the user terminal 100-2 and the auxiliary encryption terminal 200-1, irrespective of the path of the transmission and reception. Resultantly, the data is shared among the user terminal 100-1, the user terminal 100-2 and the auxiliary encryption terminal 200-1. The triple-encrypted target data is recorded in the first recording units 123 of the user terminal 100-1, the user terminal 100-2 and the auxiliary encryption terminal 200-1, for example. The data for specifying information required to decrypt the multi-encrypted target data thereafter as described in the first embodiment is recorded together with the multi-encrypted target data, in the three first recording units 123. However, as described above, the auxiliary encryption terminal 200 participates in the process of multi-encrypting the target data, and the process of decrypting the multi-encrypted target data, but is not required to save the target data. Accordingly, if the data for specifying the information required to decrypt the multi-encrypted target data is recorded in the first recording unit 123 of the auxiliary encryption terminal 200, the multi-encrypted target data is not necessarily recorded.

[0226] Similar to the case of the first embodiment, the multicast process is performed as required. In the second embodiment, the multicast process is not necessarily performed by the user terminal 100-1 and the user terminal 100-2. If the multi-encrypted target data is held by the auxiliary encryption terminal 200-1, the process may be performed by the auxiliary encryption terminal 200-1.

[0227] The method of using the thus generated multi-encrypted target data is similar to the case of the first embodiment.

[0228] However, not only the user terminal 100-1 and the user terminal 100-2 but also the auxiliary encryption terminal 200-1 is required to participate in the step of decrypting the multi-encrypted target data in this case.

[0229] Note that the second embodiment assumes that at least one auxiliary encryption terminal 200 allowed to participate in the step of multi-encrypting the target data is determined by the user terminal 100 that is to encrypt the target data first. However, such an auxiliary encryption terminal 200 is not necessarily determined by the first user terminal 100, and may be determined through cooperation with another user terminal 100 or multiple user terminals 100. At least one auxiliary encryption terminal 200 participating in the step of multi-encrypting the target data can be determined such that, for example, the users of the user terminals 100 participating in the step of multi-encrypting the target data determine by themselves at least one auxiliary encryption terminal 200 allowed to participate in the step of multi-encrypting the target data, and the necessity of participation. Accordingly, the user of each user terminal 100 can allow the auxiliary encryption terminal 200 trusted by the user to participate in the step of multi-encrypting the target data. For example, when each user terminal 100 participates in the process for multi-encrypting the target data, the user terminal 100 can select the auxiliary encryption terminal 200 necessarily allowed to participate in the process. Alternatively, when multiple user terminals 100 try to save the target data, at least one auxiliary encryption terminal 200 to participate in the process for multi-encrypting the target data may be automatically determined irrespective of the intention of the user of each user terminal 100.

[0230] Note that the method of using and the operations of the communication system in the second embodiment similarly apply to a case where the number of user terminals 100 intending to save the target data is three or more.

[0231] For example, it is assumed that the number of user terminals 100 intending to save the target data is four, and the user of each user terminal 100 sets one different auxiliary encryption terminal 200 so as to participate in the step of multi-encrypting the target data. Accordingly, provided that a certain user terminal 100 starts the first encryption process for multi-encrypting the target data, the target data is subsequently, further encrypted by three user terminals 100 and four auxiliary encryption terminals 200. The resultantly multi-encrypted target data is eight-fold multiple-encrypted target data. In this case, the order of encryption processes of the three user terminals 100 and the four auxiliary encryption terminals 200 is freely determined. There are 7! possible such orders. The order may be determined by the user terminal 100 that is to execute the encryption process first, determined by cooperation among the four user terminals 100, determined by cooperation among the four user terminals 100 and the four auxiliary encryption terminals 200, or further alternatively, determined by another method.

Modification Example 1

[0232] Modification Example 1 is achieved by adding some modification to the first embodiment. However, Modification Example 1 is not different from the first embodiment in points that are not specifically referred to. Also in the communication system of Modification Example 1, many user terminals 100 and recording devices 300 connectable to the network 400 are used. In the first embodiment, multiple or many recording devices 300 are thus adopted. In Modification Example 1, at least one recording device 300, which can be a single, is adopted.

[0233] The difference between Modification Example 1 and the first embodiment is the difference of the recording device 300. Similar to the case of the first embodiment, double-encrypted (or multi-encrypted, which similarly applies hereinafter in Modification Example 1) target data is recorded in the recording device 300 in Modification Example 1. Unlike the case of the first embodiment, the double-encrypted target data is recorded in a state of being included in a block chain.

[0234] The recording device 300 in Modification Example 1 is configured basically as with the recording device 300 in the first embodiment. In particular, the hardware configuration may be the same as that of the first embodiment. Meanwhile, the recording device 300 in Modification Example 1 requires a block chain, or a function of generating a new block. In the recording device 300 in Modification Example 1, for example, functional blocks are generated as shown in FIG. 9. Such functional blocks are generated in a large-capacity recording medium by the CPU executing a computer program recorded in the large-capacity recording medium.

[0235] A control unit 320 that includes a data input/output unit 321, a main control unit 322, a block generation unit 323, and a recording unit 324 is generated in the recording device 300.

[0236] The data input/output unit 321 receives data input into the control unit 320, or outputs data from the control unit 320 to the outside. The data input/output unit 321 is configured to send data received from a transmission and reception unit, described later, to the main control unit 322, and is configured to send data received from the main control unit 322 to the transmission and reception unit. The data input/output unit 321 in Modification Example 1 is at least configured to exchange data with the transmission and reception unit that resides outside of the control unit 320. As with the transmission and reception unit in the user terminal 100 or the auxiliary encryption terminal 200, the transmission and reception unit in the recording device 300 has a function of communicating with another device connected to the network 400 via the network 400. The data input from the transmission and reception unit into the data input/output unit 321 is the double-encrypted target data assigned the two user IDs sent from the user terminal 100 by the multicast process. When the double-encrypted target data is required to be verified, a request for transmission of the double-encrypted target data is sent by a device of a third party to the recording device 300, in some cases. Data on such a request is also sent from the transmission and reception unit to the data input/output unit 321. Meanwhile, data sent from the data input/output unit 321 to the transmission and reception unit is the double-encrypted data assigned the two user IDs. As described above, when the target data is required to be verified by the third party monitoring the target data, data on the request for transmission from the recording device 300 to the device of the third party is transmitted in some cases. When the data on such a request is received by the recording device 300, data corresponding to the request is transmitted from the data input/output unit 321 to the transmission and reception unit, as described later. The data is transmitted from the transmission and reception unit to the device of the third party.

[0237] The main control unit 322 has a function of overall control of the functional blocks in the control unit 320. Upon receipt of the double-encrypted data assigned the two user IDs from the data input/output unit 321, the main control unit 322 transmits the received data to the block generation unit 323. When transmission of the double-encrypted target data serving as a data verification target is requested by the device of the third party, for example, the user terminal 100 of a user unrelated to generation of the target data serving as a data verification target, the main control unit 322 reads the double-encrypted target data identified by the request, and the user ID assigned thereto, from the recording unit 324. The main control unit 322 transmits the data read from the recording unit 324 to the data input/output unit 321.

[0238] The block generation unit 323 has a function of generating a new block and recording the block in the recording unit 324. The method of the block generation unit 323 generating a new block can conform to a publicly known or well-known method. For example, a method of generating a new block in a block chain of bitcoin can be adopted as a method executed by the block generation unit 323 to generate a new block. The block generation unit 323 transmits the generated new block to the recording unit 324. The block generation unit 323 records, in the recording unit 324, the generated new block in a state of being connected to the immediately previous block.

[0239] The recording unit 324 records the block generated by the block generation unit 323. The blocks recorded in the recording unit 324 are sequentially connected in the order of their generation. That is, a block chain is recorded in the recording unit 324.

[0240] A method of using the communication system and operations thereof in Modification Example 1 are described.

[0241] All the processes up to and including multicast of the communication system in Modification Example 1 are not different from those in the first embodiment, irrespective of whether the number of user terminals 100 saving the target data is two or three.

[0242] It is assumed that multicast is started from a certain user terminal 100, and the double-encrypted target data assigned the user IDs is transmitted to a certain recording device 300.

[0243] Accordingly, the recording device 300 receives, at its transmission and reception unit, the double-encrypted target data assigned the user IDs. Note that in the following description, the double-encrypted (multi-encrypted) target data is called encryption target data. The encryption target data assigned the user IDs is sent from the transmission and reception unit to the data input/output unit 321, and then is sent from the data input/output unit 321 to the main control unit 322.

[0244] Upon receipt of the encryption target data, the main control unit 322 sends the data to the block generation unit 323. The block generation unit 323, having received the data, generates a new block. Data on blocks are schematically illustrated in FIG. 10. Each block B includes a recording space S that records encryption target data C, and an operation value H that is a value obtained by applying a certain operation to the immediately previous block. For example, the operation value H is a hash value obtained by applying a hash operation to the immediately previous block.

[0245] For example, it is assumed that the blocks to the n-th block B.sub.n have already been generated, and the block generation unit 323 newly generates the (n+1)-th block B.sub.n+1. The blocks B from B.sub.1 to B.sub.n are connected sequentially in the generated order, and constitute what is called a block chain. To generate the block B.sub.n+1 newly, the block generation unit 323 reads data on the block B.sub.n from the recording unit 324 and applies, for example, a hash operation to the data on the block B.sub.n, thereby obtaining an operation value H.sub.n+1 that is a hash value. The block generation unit 323 writes, in the recording space S.sub.n+1, the encryption target data assigned the user IDs received from the main control unit 322, and adds the operation value H.sub.n+1 to the recording space S.sub.n+1, thereby generating data on the new block B.sub.n+1. The block generation unit 323 then records, in the recording unit 324, the new block B.sub.n+1 in a state of being connected to the block B.sub.n.

[0246] Also in Modification Example 1, the processes described in <Case where two user terminals 100 decrypt saved target data> in the first embodiment can be executed basically in a similar manner as that in the first embodiment. When the user terminal 100 having participated in the generation of the encryption target data decrypts the encryption target data, the user terminal 100 may decrypt the encryption target data held by the user terminal 100 concerned as with the case in the first embodiment. The encryption target data recorded in the recording unit 324 of the recording device 300 is not used.

[0247] Meanwhile, when the user terminal 100 or the like having participated in the generation of the encryption target data decrypts the encryption target data, a request for transmitting the encryption target data is sent by the user terminal 100 or the like to the recording device 300. It is a matter of course that such a request includes information specifying the encryption target data to be transmitted. Such a request is received by the transmission and reception unit of the recording device 300. Such a request is sent from the transmission and reception unit to the data input/output unit 321 and then to the main control unit 322. The main control unit 322 reads the encryption target data specified by the request, from the block chain recorded in the recording unit 324. Such encryption target data is sent to the transmission and reception unit via the data input/output unit 321, and is then to the user terminal 100 or the like in a manner similar to that in the first embodiment. The request of the decryption issued by the user terminal 100 or the like to the user terminal 100 capable of decrypting the encryption target data, and the processes thereafter are the same as those in the first embodiment.

[0248] Note that the above description on Modification Example 1 assumes that the number of encryption target data items included in one recording space S of the block B is one. Alternatively, multiple encryption target data items may be included in one recording space S of the block B. For example, instead of the block generation unit 323 generating a new block B every time the encryption target data is received from the main control unit 322, for example, the block generation unit 323 may generate a new block B every time a predetermined number of encryption target data items are received from the main control unit 322 or every time a predetermined time period elapses after generation of the immediately previous block B, and may write all the encryption target data items assigned the user IDs received from the main control unit 322 after generation of the previously generated block B, in the recording space S of the new block B. In the latter case, the number of encryption target data items written in the recording space S is sometimes zero or one. However, such a configuration allows the number of encryption target data items included in the recording space S of the block B to be more than one.

[0249] As described in the first embodiment, not only the encryption target data but also the original target data is sometimes sent from the user terminal 100 to the recording device 300. In this case, not only the encryption target data assigned the user IDs, but also the encryption target data and the original target data, are recorded in the recording space S of each block. In this case, both the data items are in a state of being associated with each other. In some cases, multiple encryption target data items are recorded in the recording space S as described above. In these cases, such association is necessary.

Modification Example 2

[0250] A communication system according to Modification Example 2 is rarely different from the communication system in Modification Example 1. Modification Example 2 is not different from the Modification Example 1 in points that are not specifically referred to.

[0251] Similar to the case in Modification Example 1, the communication system in Modification Example 2 also includes many user terminals 100 and one or more recording devices 300, which can be connected to the network 400. Similar to the case in Modification Example 1, also in the recording device 300 in Modification Example 2, the encryption target data is recorded in the recording device 300 in a state of being included in the blocks constituting the block chain.

[0252] In the communication system in Modification Example 2, a new block including the encryption target data is generated by the user terminal 100. In this point, the communication system in Modification Example 2 is different from the communication system in Modification Example 1 where the new block is generated by the recording device 300.

[0253] The configuration of the user terminal 100 in Modification Example 2 is described. In view of the hardware configuration, the user terminal 100 in Modification Example 2 is the same as the user terminal 100 in Modification Example 1. The difference is in functional blocks generated therein. The functional blocks generated in the user terminal 100 in Modification Example 2 are described.

[0254] The functional blocks generated in the user terminal 100 in Modification Example 2 are rarely different from the functional blocks generated in the user terminals 100 according to Modification Example 1 and the first embodiment. The functional blocks generated in the user terminal 100 in Modification Example 2 include a block generation unit 125 (FIG. 11) that does not exist in the case of Modification Example 1.

[0255] Similar to the block generation unit 323 that is in the recording device 300 in Modification Example 1 described in Modification Example 1, such a block generation unit 125 has a function of generating a new block to be connected to the block chain. It is described later how the block generation unit 125 generates a block.

[0256] The recording device 300 in Modification Example 2 is configured basically similar to that of the recording device 300 in the first embodiment. The encryption target data assigned the user IDs is recorded in the recording unit, similar to the recording device 300 in the first embodiment. However, in Modification Example 2, as described above, the encryption target data is recorded in a state of being included in the recording space of the blocks constituting the block chain; in this point, the example is different from the first embodiment.

[0257] Next, a method of using the communication system and operations thereof in Modification Example 2 are described.

[0258] Similar to the case in Modification Example 1, the method of using the communication system and operations thereof in Modification Example 2 is not different from the first embodiment only in that the encryption target data is shared by the user terminals 100 of multiple users intending to save the target data.

[0259] One of the user terminals 100 sharing the encryption target data in the communication system in Modification Example 2 tries to perform multicast, similar to the user terminal 100 in the first embodiment or Modification Example 1.

[0260] Similar to the first embodiment, the main control unit 121 of the user terminal 100 that tries to perform multicast in the communication system in Modification Example 2 reads the encryption target data assigned the user IDs from the first recording unit 123. Meanwhile, the main control unit 121 generates a request for transmitting the latest one of the blocks constituting the block chain recorded in the recording unit of the recording device 300 from the recording device 300 serving as a multicast target, and transmits the request to the recording device 300 via the data input/output unit 122, the transmission and reception unit, and the network 400. The recording device 300 having received the request sends the latest block data to the user terminal 100 via the network 400. In a case where the number of recording devices 300 serving as multicast targets is more than one, data on the block is transmitted from the recording devices 300 to the user terminal 100. The latest blocks included in the block chain recorded in the recording units of the recording devices 300 are different among the recording devices 300 in some cases. In these cases, the data items on the blocks transmitted from the recording devices 300 are different from each other.

[0261] The data on the blocks is received by the transmission and reception unit of the user terminal 100, and is sent to the main control unit 121 via the data input/output unit 122. Accordingly, the main control unit 121 of the user terminal 100 in Modification Example 2 is in a state of saving the encryption target data assigned the user IDs and the data on the immediately previous block among the blocks constituting the block chain. Such a state is the same as the state of the block generation unit 323 in the recording device 300 in Modification Example 1 immediately before generation of the new block. That is, the block generation unit 125 in Modification Example 2 can generate a new block by the same method as that of the block generation unit 323 in Modification Example 1. The block generation unit 125 in Modification Example 2 thus generates data on the new block. Upon acceptance of the data items on the blocks from the recording devices 300 from the main control unit 121, the block generation unit 125 generates data items on the new blocks of the recording devices 300 on the basis of the data items on the blocks.

[0262] After generation of each data item on the new block, the block generation unit 125 sends it to the main control unit 121. If multiple data items on the new blocks are generated, all of them are sent to the main control unit 121. Under control of the main control unit 121, the data items on the new blocks are sent to the respective recording devices 300, for example.

[0263] Each recording device 300 connects the new block sent from the user terminal 100, to the latest block of the block chain recorded in the corresponding recording unit, and records the connected block. The thus updated block chain can be the same as that in Modification Example 1; this applies to Modification Example 2.

[0264] In Modification Example 2, the processes described in <Case where two user terminals 100 decrypt saved target data> in the first embodiment are the same as the processes described in Modification Example 1.

Modification Example 3

[0265] Modification Example 3 is a communication system obtained by modifying the recording device 300 according to the second embodiment to the recording device 300 in Modification Example 1.

[0266] The recording device 300 in Modification Example 3 generates a new block on the basis of the encryption target data assigned the user IDs accepted not only from the user terminal 100 but also from the auxiliary encryption terminal 200, and connects the block to the immediately previous end block of the block chain, in some cases. The user terminal 100 and the auxiliary encryption terminal 200 send the encryption target data assigned the user IDs, and the target data that is the origin of the encryption target data, to the recording device 300, in some cases. Irrespective of reception from which one of the user terminal 100 and the auxiliary encryption terminal 200, the recording device 300 records, in its recording unit, the user ID, the encryption target data, and the target data that is the origin of the encryption target data, in a state of being associated with each other.

[0267] The operations of and the method of using the communication system in Modification Example 3 conform to those in Modification Example 1. Accordingly, the description thereof is omitted.

Modification Example 4

[0268] Modification Example 4 is a communication system obtained by modifying the user terminal 100 and the auxiliary encryption terminal 200 according to the second embodiment to the user terminal 100 in Modification Example 2. The recording device 300 is similar to the recording device 300 in Modification Example 2.

[0269] Similar to the case in Modification Example 2, the user terminal 100 or the auxiliary encryption terminal 200 in Modification example 4 generates a new block immediately before multicast. The data on block is the same as that generated in Modification Example 2. The new block is sent to each recording device 300 from the user terminal 100 or the auxiliary encryption terminal 200. Similar to the case in Modification Example 2, each recording device 300 connects the new block received from the user terminal 100 or the recording device 300, to what is positioned at the end among the blocks constituting the block chain recorded in the recording unit.

[0270] The operations of and the method of using the communication system in Modification Example 4 conform to those in Modification Example 2. Accordingly, the description thereof is omitted.