隨著現今許多資料開始數位化，如相片、影片、書籍等等，使用者的個人電腦可能因儲存空間的不足，而傾向於將資料儲存於雲端儲存設備中。然而，使用者並非直接將檔案上傳至雲端，而是會將檔案加密後才將此檔案上傳，但這樣造成了使用者在處理檔案分享時許多負擔。使用代理人重加密的系統能讓授權人 (delegator) 藉由一位代理人 (proxy) 的幫助下，減輕授權人的運算負擔，當代理人獲得由授權人產生之被授權人 (delegatee) 的重加密金鑰 (re-encryption key) 後，代理人能夠將授權人的密文轉換成針對被授權人的密文，並且密文經由代理人的轉換過程，並不會洩漏給代理人關於該密文的任何明文資訊。由於重加密金鑰的能力太過於強大，代理人能夠將授權人所有的密文都轉換給被授權人，造成了授權人在權限管理的不便。在現實的應用狀況，授權人可能只希望將某一份密文轉換給被授權人，而不是授權人的所有密文，這樣的缺點使代理人重加密系統變得不切實際。
在本論文中，我們檢驗現有的代理人重加密系統的安全性，並且提出一個讓授權人妥善較好管理代理人轉換能力的系統，該系統能抵抗金鑰揭露攻擊 (key exposure attack) ，且解密授權人的密文與轉換給被授權人的密文都只需要同一種解密演算法 (universal decryption) 就可以完成解密。

Proxy re-encryption schemes enable users to share their digital content with other
parties in a more ecient way. In existing proxy re-encryption schemes, the security
issues of key exposure attack and unauthorized re-encryption and the performance
challenges have been extensively considered for years. In this thesis, we point out
some security
aws that occurred in certain famed proxy re-encryption schemes and
propose a secret data system based on an improved proxy re-encryption scheme with
access control. In addition, the proposed scheme can withstand an adversary from
chosen-ciphertext attack.

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