Server apparatus and program to re-encrypt ciphertext data

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.