# US 2014/0208117 A1 · Server apparatus and program

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- Number: US 2014/0208117 A1
- Document ID: US20140208117A1
- Status: Published application
- Kind: A1
- Application number: 14/219,747
- Filed: 2014-03-19
- Published: 2014-07-24
- Earliest claimed: 2011-09-30
- Assignees: Toshiba Solutions Corporation; Kabushiki Kaisha Toshiba
- Applicants: Toshiba Solutions Corporation; Kabushiki Kaisha Toshiba
- Inventors: Ryotaro HAYASHI; Tatsuyuki MATSUSHITA; Yoshihiro FUJII; Takuya YOSHIDA; Koji OKADA
- Web page: https://patentgrep.com/patent/US20140208117A1

## CPC

- 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}
- H04L 9/0869: {involving random numbers or seeds}
- H04L 9/0894: {Escrow, recovery or storing of secret information, e.g. secret key escrow or cryptographic key storage}
- 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)}
- H04L 9/0822: {using key encryption key}
- H04L 9/0825: {using asymmetric-key encryption or public key infrastructure [PKI], e.g. key signature or public key certificates}
- 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)}
- H04L 9/0861: {Generation of secret information including derivation or calculation of cryptographic keys or passwords}
- H04L 9/14: using a plurality of keys or algorithms
- H04L 2209/76: Proxy, i.e. using intermediary entity to perform cryptographic operations
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## 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

### Claim 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 the 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 the 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 the 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.

### Claim 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 the 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 the 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 the user private key updated; 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.

### Claim 3

A program stored in a non-transitory computer-readable storage medium and 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 the user private keys of user apparatuses, the program 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 the 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 the 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.

### Claim 4

A program stored in a non-transitory computer-readable storage medium and 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 the user private keys of user apparatuses, the program having:
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 the 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 the user private key updated; 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.

### Claim 5

A program stored in a non-transitory computer-readable storage medium and 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 the user private keys of user apparatuses, the re-encryption key having been generated on the basis of an administrator public key associated with and a user public key associated with the user private key, said program 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 the updated user private key associated with the 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 user secret data updated; 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.

### Claim 6

A program stored in a non-transitory computer-readable storage medium and 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 the user private keys of user apparatuses, the re-encryption key having been generated on the basis of an administrator public key associated with and a user public key associated with the user private key, said program 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 key not updated and the user key updated; 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.