允許否定條件之隱藏式存取控制

Abstract

現今的網路環境中，越來越多網路使用者存取儲存在第三方檔案伺服器的資料。在此情況下，除了維持資料的隱密性，保護存取權限和政策的隱私性也相當重要。針對此問題，以往的研究者提出了隱藏式憑證和隱藏式政策分別用於隱藏使用者的存取權限和資料提供者的存取政策。然而若要同時達成此兩者，以往的研究必須仰賴可信賴的仲裁者或是需要使用者和資料提供者之間進行多回合的信任協商。在此論文中，我們提出了一個新的密碼系統，將存取權限隱藏於密文之內，只有符合條件的使用者能成功地解回原文件。我們的系統能在單次的加密解密程序間，同時隱藏使用者屬性和資料提供者指定的存取政策，且不需要可信賴的仲裁者。除此之外，我們的機制更允許資料提供者於存取政策內設定否定的條件。最後，我們使用PBC 函式庫實作了此系統，並討論其於各種情境下之效能。

In today's open environment, increasing numbers of network users access data stored in third-parties such as file servers on the Internet. Preserving data con dentiality is important; however, preserving privacy of access right and policy is imperative when the right and policy are as con dential as the data itself. To address this problem, previous researchers have proposed concepts of hidden credential and hidden policy to hide user's access privileges and content provider's policies respectively. To achieve both features simultaneously, other researchers either rely on a trusted mediator or require multiple rounds of online trust negotiations between two parties. In this paper, we present a novel cryptosystem which hides access policies in ciphertext and allows only users with satis ed attributes to retrieve corresponding plaintext successfully. By using our technique, both attributes and access policies can be kept secret through an o ine encrypt-decrypt process; furthermore, the trust dependency is reduced since no trusted mediator is required. Besides, our construction permits access policies constituted of both positive and negative attributes. Finally, we implement our method using Pairing-based Cryptography (PBC) Library, and discuss its performance under various contexts.