| 摘要 |
<P>PROBLEM TO BE SOLVED: To enable update of a re-encryption key even when a secret key of a user is not reported to a group administrator or even when the group administrator is absent. <P>SOLUTION: A server device comprises first calculation means, transmission/reception means, and second calculation means. When a notification for updating a re-encryption key is received from a user device, the first calculation means generates a random number, and calculates re-encryption key information on the basis of a stored re-encryption key and the generated random number. The transmission/reception means transmits the re-encryption key information to the user device, and receives re-encryption key update information calculated from the re-encryption key information on the basis of a user secret key before update and a user secret key after update, from the user device. The second calculation means calculates an updated re-encryption key on the basis of the re-encryption key update information and the random number, and also replaces the re-encryption key stored in storage means by the updated re-encryption key. <P>COPYRIGHT: (C)2013,JPO&INPIT |
