Server apparatus and program to re-encrypt ciphertext data

• US 9,426,131 B2
• Filed: 03/19/2014
• Issued: 08/23/2016
• Est. Priority Date: 09/30/2011
• Status: Active Grant

• Current Assignee: Toshiba Solutions Corporation (Toshiba Corporation)
• Original Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation), Toshiba Solutions Corporation (Toshiba Corporation)
• Inventors: Hayashi, Ryotaro, Matsushita, Tatsuyuki, Fujii, Yoshihiro, Yoshida, Takuya, Okada, Koji
• Primary Examiner(s): EDWARDS, LINGLAN E
• Application Number:
  US14/219,747
• Publication Number:
  US 20140208117A1
• Time in Patent Office: 888 Days
• US Class Current: 1/1
• CPC Class Codes: H04L 2209/76 Proxy, i.e. using intermediary entity to perform cryptographic operations network architectures or network communication protocols using hop-by-hop encryption H04L63/0464
• CPC Class Codes: H04L 63/0471 applying encryption by an intermediary, e.g. receiving clear information at the intermediary and encrypting the received information at the intermediary before forwarding
• CPC Class Codes: H04L 9/0819 Key transport or distribution, i.e. key establishment techniques where one party creates or otherwise obtains a secret value, and securely transfers it to the other(s) network architectures or network communication protocols for key distribution in a packet data network H04L63/062
• CPC Class Codes: H04L 9/0822 using key encryption key
• CPC Class Codes: H04L 9/0825 using asymmetric-key encryption or public key infrastructure [PKI], e.g. key signature or public key certificates
• CPC Class Codes: H04L 9/0833 involving conference or group key network architectures or network communication protocols for key management in group communication in a packet data network H04L63/065
• CPC Class Codes: H04L 9/0861 Generation of secret information including derivation or calculation of cryptographic keys or passwords
• CPC Class Codes: H04L 9/0869 involving random numbers or seeds
• CPC Class Codes: H04L 9/0894 Escrow, recovery or storing of secret information, e.g. secret key escrow or cryptographic key storage
• CPC Class Codes: H04L 9/14 using a plurality of keys or algorithms

Abstract

A server apparatus according to an embodiment generates a random number on receiving from a user apparatus a notification showing that a re-encryption key should be updated, and calculates re-encryption key data on the basis of the re-encryption key stored and the random number generated. The server apparatus transmits the re-encryption key to the user apparatus and receives, from the user apparatus, the user private key not updated yet and re-encryption key updating data calculated from the re-encryption key data on the basis of the user private key updated. The server apparatus calculates the re-encryption key updated, on the basis of the re-encryption key updating data and the random number, and replaces the re-encryption key stored in the storage device with the updated re-encryption key.

Claims

1. A server apparatus configured to re-encrypt ciphertext data obtained by encrypting plain text data with an administrator public key, without decrypting the ciphertext data and to store a re-encryption key for obtaining re-encrypted text data able to be decrypted with user private keys of user apparatuses, the server apparatus comprising:

a storage device which stores the re-encryption key generated on the basis of an administrator private key associated with the administrator public key, and a user public key associated with a user private key;

a transmitting/receiving device which transmits the re-encryption key to a user apparatus on receiving from the user apparatus a notification showing that the re-encryption key should be updated, and receives an updated re-encryption key calculated from the re-encryption key on the basis of the user private key not updated and an updated user private key; and

a replacing device which replaces the re-encryption key stored in the storage device with the updated re-encryption key.

2. A server apparatus configured to re-encrypt ciphertext data obtained by encrypting plain text data with an administrator public key, without decrypting the ciphertext data and to store a re-encryption key for obtaining re-encrypted text data able to be decrypted with user private keys of user apparatuses (A, B, C, . . . ), the server apparatus comprising:

a storage device which stores the re-encryption key generated on the basis of an administrator private key associated with the administrator public key and a user public key associated with a user private key;

a first calculating device which generates a random number on receiving from the user apparatus a notification showing that the re-encryption key should be updated, and calculates re-encryption key data on the basis of the re-encryption key stored and the random number generated;

a transmitting/receiving device which transmits the re-encryption key to a user apparatus and receives, from the user apparatus, the user private key not updated yet and re-encryption key updating data calculated from the re-encryption key data on the basis of an updated user private key; and

a second calculating device which calculates the re-encryption key updated, on the basis of the re-encryption key updating data and random number, and replaces the re-encryption key stored in the storage device with the updated re-encryption key.

3. A non-transitory computer-readable storage medium storing program code designed to be executed by the processor of a server apparatus storing a re-encryption key for re-encrypting ciphertext data obtained by encrypting plain text data with an administrator public key, without decrypting the ciphertext data, thereby obtaining re-encrypted text data able to be decrypted with user private keys of user apparatuses, the program code comprising:

a first program code for causing the processor to write, in a storage device of the server apparatus, the re-encryption key generated on the basis of an administrator private key associated with the administrator public key, and a user public key associated with a user private key;

a second program code for causing the processor to transmit the re-encryption key to a user apparatus on receiving from the user apparatus a notification showing that the re-encryption key should be updated, and to receive an updated re-encryption key calculated from the re-encryption key on the basis of the user private key not updated and an updated user private key; and

a third program code for causing the processor to replace the re-encryption key stored in the storage device with the updated re-encryption key.

4. A non-transitory computer-readable storage medium storing program code designed to be executed by the processor of a server apparatus storing a re-encryption key for re-encrypting ciphertext data obtained by encrypting plain text data with an administrator public key, without decrypting the ciphertext data, thereby obtaining re-encrypted text data able to be decrypted with user private keys of user apparatuses, the program code comprising:

a first program code for causing the processor to write, in a storage device of the server apparatus, the re-encryption key generated on the basis of an administrator private key associated with the administrator public key, and a user public key associated with a user private key;

a second program code for causing the processor to generate a random number on receiving from the user apparatus a notification showing that the re-encryption key should be updated, and to calculate re-encryption key data on the basis of the re-encryption key stored and the random number generated;

a third program code for causing the processor to transmit the re-encryption key to a user apparatus and to receive, from the user apparatus, the user private key not updated yet and re-encryption key updating data calculated from the re-encryption key data on the basis of an updated user private key; and

a fourth program code for causing the processor to calculate the re-encryption key updated, on the basis of the re-encryption key updating data and random number, and to replace the re-encryption key stored in the storage device with the updated re-encryption key.

5. A non-transitory computer-readable storage medium storing program code designed to be executed by the processor of a server apparatus storing a re-encryption key for re-encrypting ciphertext data obtained by encrypting plain text data with an administrator public key, without decrypting the ciphertext data, thereby obtaining re-encrypted text data able to be decrypted with user private keys of user apparatuses, the re-encryption key having been generated on the basis of an administrator private key associated with the administrator public key and a user public key associated with a user private key, said program code comprising:

a first program code for causing the processor to write, in a storage device of a user apparatus, the user private key not updated yet and associated with the user public key not updated yet and also an updated user private key associated with an updated user public key;

a second program code for causing the processor to transmit, to the server apparatus, a notification showing that the re-encryption key should be updated, and to receive, from the server apparatus, re-encryption key data calculated on the basis of the re-encryption key and a random number;

a third program code for causing the processor to calculate updated re-encryption key data from the re-encryption key data, on the basis of the re-encryption key data received, the user private key not updated yet and the updated user private key; and

a fourth program code for causing the processor to transmit, to the server apparatus, the updated re-encryption key data so calculated,

wherein the updated re-encryption key data is data that the server apparatus uses to calculate the updated re-encryption key on the basis of the random number; and

the updated re-encryption key has replaced the re-encryption key not updated yet, in the server apparatus, and is stored in the server apparatus.

6. A non-transitory computer-readable storage medium storing program code designed to be executed by the processor of a server apparatus storing a re-encryption key for re-encrypting ciphertext data obtained by encrypting plain text data with an administrator public key of a group administrator apparatus, without decrypting the ciphertext data, thereby obtaining re-encrypted text data able to be decrypted with user private keys of user apparatuses, the re-encryption key having been generated on the basis of an administrator private key associated with the administrator public key and a user public key associated with a user private key, said program code comprising:

a first program code for causing the processor to write the administrator private key in a storage device of the group administrator apparatus;

a second program code for causing the processor to generate the re-encryption key on the basis of the user public key and administrator private key received from the server apparatus; and

a third program code for causing the processor to transmit the re-encryption key to the server apparatus,

wherein the re-encryption key is data from which the server apparatus calculates re-encryption key data on the basis of a random number;

the re-encryption key data is data from which the user apparatus calculates new re-encryption key data on the basis of the user private key not updated and an updated user private key;

the new re-encryption key data is data from which an updated re-encryption key is calculated on the basis of the random number by the server apparatus; and

the updated re-encryption key has replaced the re-encryption key not updated yet, in the server apparatus, and is stored in the server apparatus.