Threshold cryptography system and threshold cryptography method

Abstract

Ensure that decryption is performed even if the users are distributed across different locations when a decryption request occurs. [Solution] Dealer device 1 distributes multiple distributed secret keys to multiple users and registers these users in a specific record on the DDNS server 3. When each user device 4 receives a distributed secret key, it reflects the user's decryption status in the aforementioned specific record. Subsequently, when a decryption request occurs, user device 4 sends a distributed secret key for that user to dealer device 1, refers to the aforementioned specific record, identifies the user among the multiple users who is allowed to decrypt in the specific record, and sends a consent request. When another user device 4 receives a consent request, if that user is allowed to decrypt, it sends a distributed secret key to dealer device 1. Then, when dealer device 1 receives a predetermined number of distributed secret keys or more, it reconstructs the secret key.

Description

TECHNICAL FIELD

[0001] The present invention relates to a threshold cryptography system and a threshold cryptography method.

BACKGROUND ART

[0002] In some systems, when a user consents to decryption, the user causes a card reader connected to a processing device to read his or her own IC card, whereby the decryption consent is transmitted to a dealer. Then, if the number of users who have consented to decryption is equal to or greater than a predetermined threshold, the encrypted information is decrypted (see, for example, Patent Document 1).

PRIOR ART DOCUMENTS

PATENT DOCUMENTS

[0003]

PATENT DOCUMENT 1

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0004] However, in the above-described system, when the user is not at the installation location of the card reader, the user cannot cause the card reader to read the IC card, so that decryption consent cannot be obtained immediately, and decryption may not be performed immediately.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to obtain a threshold cryptography system and a threshold cryptography method that perform decryption even when users are dispersed, as long as decryption conditions are satisfied when a decryption request is generated.

MEANS FOR SOLVING THE PROBLEMS

[0006] The threshold encryption system according to the present invention comprises a dealer device and a plurality of user devices. The dealer device comprises a private key distribution unit that generates a plurality of distributed private keys and distributes them to a plurality of users and registers the plurality of users in a specific record of a predetermined DDNS server, and a decryption processing unit that, upon receiving the distributed private keys from a predetermined number or more of the plurality of user devices, reconstructs a private key from the received distributed private keys. Each of the plurality of user devices includes a distributed secret key receiving unit that receives the distributed secret key for the user of the user device, a record registration unit that, upon receiving the distributed secret key, reflects in the specific record of the DDNS server whether the user can decrypt or not, and a decryption request processing unit that, when a decryption request occurs in the user device, (a1) sends the distributed secret key for the user to the dealer device, (a2) obtains a user list of the plurality of users by referring to the specific record of the DDNS server, sends a consent request to users among the plurality of users included in the user list who are marked as decryptable in the specific record, and (b) when the consent request is received from another user device, sends the distributed secret key for the user to the dealer device if the user of the user device has marked as decryptable.

[0007] The threshold encryption method according to the present invention includes the steps of: generating a plurality of distributed secret keys using a dealer device; distributing the plurality of distributed secret keys to a plurality of users using the dealer device; registering the plurality of users in a specific record of a predetermined DDNS server; receiving the distributed secret keys for the users of the user device using a user device; when the user device receives the distributed secret keys, reflecting whether the users can decrypt the data in the specific record of the DDNS server; and when a decryption request occurs in the user device, (a) sending the distributed secret keys for the users of the user device to the dealer device. (b) The steps of (b) obtaining a user list of the plurality of users by referring to the specific record of the DDNS server, and sending a consent request to a user among the plurality of users included in the user list who is decryptable in the specific record; (c) when the user device receives the consent request from another user device, if the user of that user device has decrypted it, the user device sends a distributed secret key for that user to the dealer device; and when the dealer device receives a predetermined number or more of the plurality of distributed secret keys, the steps of reconstructing a secret key from the received distributed secret keys. [Effects of the Invention]

[0008] According to the present invention, a threshold cryptography system and method are obtained in which, when a decryption request occurs, decryption is performed even if the users are distributed, provided that the decryption conditions are met.

[0009] The above or other objects, features, and advantages of the present invention will become even more apparent from the following detailed description in conjunction with the accompanying drawings. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a block diagram showing the configuration of a threshold cryptography system according to an embodiment of the present invention. [Figure 2]Figure 2 is a sequence diagram illustrating the operation of the threshold cryptography system shown in Figure 1. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described below with reference to the figures.

[0012] Figure 1 is a block diagram showing the configuration of a threshold encryption system according to an embodiment of the present invention. The threshold encryption system shown in Figure 1 comprises a dealer device 1, a DDNS (Dynamic Domain Name System) server 3 connected to a network 2 such as the Internet, and a plurality of user devices 4 capable of data communication with the dealer device 1 and the DDNS server 3 via the network 2.

[0013] Dealer equipment 1 consists of servers and terminal equipment, while user equipment 4 consists of servers, terminal equipment (such as personal computers and smartphones), and electronic devices (such as image forming machines like multifunction printers).

[0014] The dealer device 1 includes a communication device 11, a storage device 12, and a processing unit 13.

[0015] The communication device 11 is, for example, a network interface connected to network 2, and is a device that performs data communication with other devices connected to network 2 (such as a DDNS server 3 and a user device 4).

[0016] Furthermore, the storage device 12 is a device capable of storing various data and programs necessary for the processing described later. As the storage device 12, a non-volatile, high-capacity storage medium such as non-volatile memory or a hard disk drive is used.

[0017] The arithmetic processing device 13 includes a computer or the like, and operates as various processing units by executing software processing on the computer. Here, the arithmetic processing device 13 operates as a secret key generation unit 21, an encryption unit 22, a secret key distribution unit 23, and a decryption processing unit 24. The computer includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like, loads a program from the ROM, the storage device 12, or the like into the RAM, and executes the program on the CPU to operate as the above-described processing units.

[0018] The secret key generation unit 21 generates a secret key for decrypting certain data (data file) (by a public key cryptosystem or a common key cryptosystem).

[0019] The encryption unit 22 encrypts the data (data file). In the case of the common key encryption scheme, encryption is performed with the generated secret key. In the case of the public key encryption scheme, encryption is performed with a public key corresponding to the generated secret key.

[0020] The secret key distribution unit 23 generates a plurality of distributed secret keys corresponding to the generated secret key according to an existing threshold cryptosystem, distributes the generated keys to a plurality of users, and registers the plurality of users in a specific record of a predetermined DDNS server 3.

[0021] In this embodiment, for each of the plurality of users described above, the secret key distribution unit 23 transmits the encrypted data and the distributed secret key for the user via the communication device 11 according to a predetermined communication protocol.

[0022] When receiving distributed secret keys from a predetermined number or more of user devices 4 among the plurality of user devices 4, the decryption processing unit 24 reconstructs the secret key from the received distributed secret keys.

[0023] Further, the user device 4 includes a communication device 41, a storage device 42, and an arithmetic processing device 43.

[0024] The communication device 41 is, for example, a wireless or wired network interface connected to the network 2, and is a device that performs data communication with other devices (such as the DDNS server 3 and the dealer device 1) connected to the network 2.

[0025] In addition, the storage device 42 is a device capable of storing various data and programs required for the processes described later. As the storage device 42, a nonvolatile large-capacity storage medium such as a nonvolatile memory or a hard disk drive is used.

[0026] The arithmetic processing device 43 includes a computer or the like, and operates as various processing units by executing software processing on the computer. Here, the arithmetic processing device 43 operates as a distributed secret key receiving unit 51, a record registration unit 52, and a decryption request processing unit 53. The computer includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like, loads a program from the ROM, the storage device 12 or the like into the RAM, and executes the program with the CPU, thereby operating as the above-described processing units.

[0027] The distributed secret key receiving unit 51 receives a distributed secret key for the user of the user device 4. The distributed secret key is transmitted and received according to a specific communication protocol.

[0028] In this embodiment, the distributed secret key receiving unit 51 receives the encrypted data and the distributed secret key for the user via the communication device 11 in accordance with a predetermined communication protocol, and stores the encrypted data and the distributed secret key for the user in the storage device 42.

[0029] When the distributed secret key is received, the record registration unit 52 uses the communication device 41 to reflect the decryption availability of the user in a specific record of the DDNS server 3.

[0030] For example, the display device of the user device 4 displays the file name of the encrypted data, and a dialog box for inputting whether decryption is permitted or not is displayed. When the input device of the user device 4 detects a user operation indicating whether decryption is permitted or not, the record registration unit 52 determines whether decryption is permitted or not based on that user operation, and as described above, reflects the user's decryption status in a specific record of the DDNS server 3, and stores the data indicating the user's decryption status in the storage device 42.

[0031] When a decryption request occurs in the user device 4, the decryption request processing unit 53 (a) sends a distributed secret key for that user to the dealer device 1, and (b) refers to a specific record in the DDNS server 3 to obtain a user list of the multiple users mentioned above (i.e., users to whom a distributed secret key has been issued), and sends a consent request to users among the multiple users included in the user list who are allowed to decrypt in a specific record.

[0032] Furthermore, when the decryption request processing unit 53 receives a consent request from another user device 4, if the user of that user device 4 has given permission for decryption, it automatically sends the distributed secret key for that user to the dealer device 1. For example, it is determined whether the user of that user device 4 has given permission for decryption by referring to the data mentioned above and the records of the DDNS server 3.

[0033] Here, the private key distribution unit 23 in the dealer device 1 registers a TXT record representing each of the multiple users mentioned above with the DDNS server 3. The TXT record includes a status value indicating whether the user can decrypt. At the time of registration by the private key distribution unit 23, the status value of the TXT record is initially set to "unknown". When the distributed private key is received, the record registration unit 52 identifies whether the user can decrypt based on the user operation of the user device 4 and updates the status value to a value indicating whether decryption is possible.

[0034] In this embodiment, the status value described above is one of several values, including the aforementioned undetermined (unknown), a first value indicating decryption is possible (here, OK), a second value indicating decryption is not possible (here, NG), and a third value indicating pending (here, Pending). When the decryption request processing unit 53 receives a consent request from another user device 4, and the status value for the user of that user device 4 is the third value (Pending), if a user operation indicating decryption is possible by that user is detected, the decryption request processing unit 53 sends the distributed secret key for that user to the dealer device 1. On the other hand, in this case, if a user operation indicating decryption is not possible by that user is detected, the decryption request processing unit 53 does not send the distributed secret key for that user to the dealer device 1.

[0035] For example, the private key distribution unit 23 of the dealer device 1 registers the following TXT record for users to whom a distributed private key has been issued.

[0036] TXT "name=Taro Tanaka" "department=xxx" "mail=tarotanaka@example.com" "status=unknown"

[0037] Here, the TXT record includes the user's name (field name), department (field department), contact information (email address) (field mail), and status value (field status).

[0038] Then, if the user grants permission for decryption, the decryption request processing unit 53 updates the status value of the TXT record as follows.

[0039] TXT "name=Taro Tanaka" "department=xxx" "mail=tarotanaka@example.com" "status=OK"

[0040] On the other hand, if the user determines that decryption is not possible, the decryption request processing unit 53 updates the status value of the TXT record as follows.

[0041] TXT "name=Taro Tanaka" "department=xxx" "mail=tarotanaka@example.com" "status=NG"

[0042] Furthermore, if the user has set the decryption status to pending, the decryption request processing unit 53 updates the status value of the TXT record as follows.

[0043] TXT "name=Taro Tanaka" "department=xxx" "mail=tarotanaka@example.com" "status=Pending"

[0044] Next, the operation of the threshold encryption system described above will be explained. Figure 2 is a sequence diagram illustrating the operation of the threshold encryption system shown in Figure 1.

[0045] In the dealer device 1, the secret key generation unit 21 generates a secret key for a certain data (data file), the encryption unit 22 encrypts the data (data file), and the secret key distribution unit 23 generates multiple distributed secret keys corresponding to the generated secret key for a predetermined number of users (step S1).

[0046] Then, the private key distribution unit 23 registers the aforementioned multiple users in a specific record of the DDNS server 3 (step S2), and sends the aforementioned distributed private key and encrypted data file (encrypted file) to the multiple users (step S3). For example, by referring to user registration data stored in advance, the username, affiliation, contact information, etc. are identified, the aforementioned record is registered, and the distributed private key and encrypted file are sent to that contact.

[0047] In each user's device 4 for multiple users, the distributed secret key receiving unit 51 receives the distributed secret key and encrypted file for that user and stores them in the storage device 42.

[0048] When the distributed secret key and encrypted file are received, the record registration unit 52 determines whether the user can decrypt the encrypted file based on user actions indicating whether the user can decrypt it, and uses the communication device 41 to reflect the user's decryption status in a specific record on the DDNS server 3 (step S4).

[0049] Subsequently, when a decryption request for an encrypted file is detected in any of the user devices 4 (step S5), the decryption request processing unit 53 in that user device 4 sends the distributed secret key for that user along with the decryption request to the dealer device 1 (step S6), retrieves a user list of the multiple users mentioned above by referring to a specific record in the DDNS server 3 (step S7), and sends a consent request to each of the multiple users included in that user list who are allowed to decrypt in the specific record (step S8). Here, a consent request is also sent to each of the users whose decryption status is pending in the specific record.

[0050] In this user list, each user's contact information is included, and a consent request is sent to that contact.

[0051] Decryption requests are generated by applications or user operations. Furthermore, if the user of user device 4 has designated the encrypted file as undecryptable, the distributed secret key may not be sent to dealer device 1 along with the decryption request, or neither the distributed secret key nor the decryption request may be sent to dealer device 1.

[0052] When a consent request is received on another user device 4, the decryption request processing unit 53 of that user device 4 automatically and immediately transmits the distributed secret key for that user to the dealer device 1, provided that the user of that user device 4 has given permission for decryption (step S9).

[0053] On the other hand, if the user of the user device 4 has reserved the option of decryption, the decryption request processing unit 53 will, based on the user's operation regarding decryption, send a distributed secret key for that user to the dealer device 1 if the user has approved decryption (step S9).

[0054] Then, the decryption processing unit 24 of the dealer device 1, after receiving the decryption request and the distributed secret key, determines within a predetermined time whether it has received distributed secret keys from a predetermined number or more of the user devices 4 among the multiple user devices 4. If it has received a predetermined number or more of distributed secret keys, it reconstructs the secret key from the received distributed secret keys (step S10).

[0055] The decryption processing unit 24 sends the regenerated secret key to the user device 4 that sent the decryption request (step S11). Upon receiving the secret key, the decryption request processing unit 53 of the user device 4 uses the secret key to decrypt the encrypted file (step S12). In this case, the encrypted file is decrypted by the user device 4, but the decryption processing unit 24 of the dealer device 1 may decrypt the encrypted file and send the decrypted file to the user device 4.

[0056] As described above, according to the above embodiment, the dealer device 1 (a) generates multiple distributed secret keys and distributes them to multiple users, registers multiple users in a specific record of the DDNS server 3, and (b) when it receives distributed secret keys from a predetermined number or more of the multiple user devices, it reconstructs a secret key from the received distributed secret keys. Each user device 4 receives the distributed secret key for its user, and upon receiving the distributed secret key, it reflects whether the user can decrypt in a specific record of the DDNS server 3. Subsequently, when a decryption request occurs in a user device 4, the user device 4 sends the distributed secret key for that user to the dealer device 1, refers to a specific record of the DDNS server 3 to obtain a user list of multiple users, and sends a consent request to users among the multiple users included in that user list who are allowed to decrypt in a specific record. Also, when a consent request is received in another user device 4, if the user of that user device 4 has allowed decryption, that user device 4 sends the distributed secret key for that user to the dealer device 1.

[0057] This means that when a decryption request occurs, if the decryption conditions (consent from a predetermined number of users) are met, decryption will be performed even if the users are distributed amongst themselves.

[0058] Furthermore, various changes and modifications to the embodiments described above will be obvious to those skilled in the art. Such changes and modifications may be made without deviating from the spirit and scope of the subject matter and without diminishing the intended advantages. In other words, such changes and modifications are intended to be included in the claims. [Industrial applicability]

[0059] The present invention can be applied, for example, to threshold cryptography systems. [Explanation of Symbols]

[0060] 1. Dealer equipment 3 DDNS servers 4. User devices 23. Private Key Distribution Unit 24 Decryption Processing Unit 51 Distributed Secret Key Receiving Unit 52 Record Registration Section 53 Decryption Request Processing Unit

Claims

1. Dealer equipment and Equipped with multiple user devices, The dealer device includes a private key distribution unit that generates multiple distributed private keys and distributes them to multiple users, and registers the multiple users in a specific record of a predetermined DDNS server, and a decryption processing unit that, upon receiving the distributed private keys from a predetermined number or more of the multiple user devices, reconstructs a private key from the received distributed private keys. Each of the aforementioned user devices includes: a distributed secret key receiving unit that receives the distributed secret key for the user of the user device; a record registration unit that, upon receiving the distributed secret key, reflects in the specific record of the DDNS server whether the user can decrypt; and a decryption request processing unit that, when a decryption request occurs in the user device, (a1) transmits the distributed secret key for the user to the dealer device; (a2) obtains a user list of the aforementioned users by referring to the specific record of the DDNS server, and sends a consent request to users among the aforementioned users included in the user list who are deemed to be decryptable in the specific record; and (b) when the consent request is received from another user device, transmits the distributed secret key for the user to the dealer device if the user of that user device has indicated that they can decrypt. A threshold cryptography system characterized by the following.

2. The aforementioned private key distribution unit registers a TXT record representing each of the multiple users with the DDNS server. The TXT record includes a status value indicating whether the user can decrypt it or not. The record registration unit identifies whether the user can decrypt based on the user's operation and updates the status value to a value indicating whether decryption is possible or not. A threshold cryptography system according to claim 1, characterized by the following:

3. The aforementioned status value is one of a plurality of values, including a first value indicating decryption is possible, a second value indicating decryption is not possible, and a third value indicating that it is pending. When the decryption request processing unit receives the consent request from the other user device, and the status value for the user of that user device is the third value, if a user operation allowing decryption by that user is detected, it transmits a distributed secret key for that user to the dealer device. A threshold cryptography system according to claim 1, characterized by the following:

4. The steps include generating multiple distributed secret keys using a dealer device, The steps include: distributing the multiple distributed secret keys to multiple users using the dealer device; The steps include registering the multiple users in a specific record of a designated DDNS server, The steps include: receiving the distributed secret key for the user of the user device via the user device; When the distributed secret key is received by the user device, the user reflects in the specific record of the DDNS server whether or not the user can decrypt it. In the user device, when a decryption request occurs, the steps include: (a) transmitting the distributed secret key for the user of the user device to the dealer device; (b) obtaining a user list of the multiple users by referring to the specific record of the DDNS server; and sending a consent request to the users among the multiple users included in the user list who are decryptable in the specific record; In the user device, when the consent request is received from another user device, if the user of that user device has enabled decryption, the user device sends a distributed secret key for that user to the dealer device. In the aforementioned dealer device, when a predetermined number or more of the plurality of distributed secret keys are received, the steps include: reconstructing a secret key from the received distributed secret keys; A threshold encryption method characterized by comprising the following features.

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