| 摘要 |
PROBLEM TO BE SOLVED: To provide a key management server which lighten the burden on a user, increases convenience, and also lightens the load on the server by properly determining the order of a key management tree, the number of members of a group, a threshold for a group breakup, etc. SOLUTION: The key management server 1 groups every log<SB>4</SB>N of N users in one. Then a group key that all users in each group share and a common key having a quaternary structure that all groups having group keys share are generated, ciphered with personal keys of respective users, and distributed to user terminals 3, and the common key is updated, group by group, together at user's requests for admission or leaving received through the user terminals 3, and the updated common key is provided for a content distribution server 2. COPYRIGHT: (C)2007,JPO&INPIT
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