由於網路的發達，雲端運算在此基礎下漸漸成熟，許多中小企業與使用者紛紛拋棄硬體設備，轉而向雲端靠攏。然而在此半信任的環境下，資料擁有者失去了對資料的直接控制權，導致資訊安全風險增加，這時如何設計出一套有效率又安全的雲端分享系統，就是一個非常重要的課題。
許多學者提出利用公開金鑰加密來設計系統，但利用此原語(Primitives)來構造雲端資料分享方法，存在著用戶負擔過高、計算效率不佳及金鑰管理等問題。因此，我們以基於雙線性配對的無憑證公開金鑰加密系統結合代理重加密方法(Proxy re-encryption)，設計一套安全且有效的雲端分享系統，以半信任的代理實現在不看見明文的情況下，將密文的解密權限由Alice轉換給Bob。同時在不需要證書及沒有第三方金鑰託管問題的情況下保障公鑰真實性，再透過期間限制讓資料擁有者即使離線依然可以註銷接收者權限，達成可延展性(Scalability)。總結上述所說，我們提出的方法不論是在安全性、執行效能及實作可行性都非常適合應用在雲端資料分享。

Due to the development of the Internet, cloud computing has gradually matured on this basis, many SMEs and users have abandoned hardware devices and turned to the cloud. However, in this semi-trusted environment, the data owner has lost direct control over the data, leading to increased security risks. Hence, how to sharing in cloud with efficient and secure way is a very important issue.
Many scholars have proposed plenty of methods designed by Public-key Cryptography, but use this primitive to construct data sharing in cloud mothed, may cause lots of problem such as poor efficiency and key management issue. Therefore, we use a certificateless public key cryptography based on bilinear pairing combined with proxy re-encryption to design a secure and effective method, using a semi-trusted proxy converts a ciphertext for Alice into a ciphertext for Bob without seeing the underlying plaintext. At the same time, authenticity of the public key without certificate and key escrow issue, furthermore, because of period-restricted access control, data owner can revoke user even being offline which achieve scalability. To sum up, no matter what aspects such as security, efficiency and practical our method is very suitable for data sharing in cloud.

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