有效率且安全的群組金鑰管理方法-用於付費電視系統且支援頻繁的金鑰更新

Abstract

在現在的時代裡，付費電視已經變成一個普遍的訂閱服務。為了防止沒有付錢的人非授權的存取電視內容，付費電視的供應商通常會對每一個頻道的內容加密，並把對應的金鑰分配給合法的使用者，如此一來，只有合法的使用者才可以正確解密。用來維持和分配一個共有的解密金鑰給眾多的使用者的方法，通稱為群組金鑰管理。 在這篇論文，我們提出了一個很適合付費電視系統且安全有效率的樹狀架構群組金鑰管理方法。之前的樹狀架構有以下優點，每個使用者只需要存O(logN)個密鑰，每一次群組金鑰更新時伺服器只需傳送O(logN)個訊息，N 為使用者的總數。除了之前的這些優點外，我們的方法還有另外兩個特點：(1)當有使用者加入或離開時，其他的使用者只需要計算一次就可以取得群組金鑰。(2)為了使離線的使用者重新上線時可快速取得最新的金鑰，伺服器只需要在佈告欄存O(N)個公開訊息，而一個離線的使用者只需要解密O(logN)次就可以更新最新的金鑰和群組金鑰，所需的解密次數與離線時間有多少次更新無關。在付費電視系統，這些特點不只最小化群組金鑰更新的延遲時間，並使系統在頻繁的金鑰更新之下更為實際。在最後，我們有討論如何將我們的群組金鑰管理方法用於多個頻道的服務上。

Pay-TV has become a popular subscribed-based service in recent years. To prevent unauthorized access from non-paid users over a broadcast channel, the TV server usually encrypts each TV program to a ciphertext such that only the legal members can decrypt it. The way of maintaining the common decryption key of a TV program to a dynamic subscription group of members is called the group key management. In this paper, we propose a secure and efficient tree-based group key management scheme that is very suitable for Pay-TV systems. In addition to possessing the advantages of the former tree-based scheme, such as O(logN) communication cost for each group key update and O(logN) secret key for each member, our scheme has two distinct features, where N is the total number of members. (1) Each member only needs to decrypt one ciphertext or compute one hash value to get the group key from the rekey messages for each member leaving/joining. (2) To handle the key update for reconnected members who have missed the group key updates in the off-line period of time, the server only needs to store O(N) public tokens on the bulletin and each off-line member only needs O(logN) decryptions for getting the newest group key, which are independent of the number of group key updates. In Pay-TV systems, these features not only minimize the delay time for each group key update, but also let the system more practical even if the key update frequency is very high, such as, the Pay-Per-View TV service. Finally, we have a discussion of applying our GKM scheme to a multi-program service.