本論文提出了一個免憑證加密系統的方案，並且將此方案實作出來，使得此方案更具實用性。此方案主要架構為免憑證加密系統，利用此系統的特性消除傳統公開金鑰密碼系統中需要公開金鑰憑證認證的麻煩，也不會產生基於身分認證加密系統的私鑰託管問題，有效的結合了兩項系統的優點，並且提高了這兩種系統的安全性及方便性。本論文的協定中，在基於身分認證加密系統的公鑰部分還加入了階級以及時間戳記的概念，用以限制接收方取得部份私鑰的能力，並且也將接收方的部分公鑰加入其中，來增加部份私鑰的安全性。另外此協定也加入了提早解密金鑰的部分，可讓傳送方在傳出密文後更改解密時間，而不需要重新使用新的公鑰加密資訊，便可提早讓接收方取得相對應的明文資訊。 / In this paper, we propose a new certificateless public key encryption system, and implement it for securing e-mail systems. Certificateless cryptography, which is in contrast to traditional public key crypto-systems, does not require the use of certificates to guarantee the authenticity of public key. It does rely on the use of a trusted third party (TTP) who is in possession of a master key, just like the identity-based public key cryptography. However, certificateless public key crypto-system does not suffer from the key escrow property, whereas, it is a problem in the identity-based public key crypto-systems. Moreover, in our system, we add some new properties like level and time-stamp to limit the ability of receivers and to promote the safety of the system. Time-stamp ensures that the ciphertext cannot be decrypted before the indicated time and a level ensures that only the user with the corresponding identity and level can decrypt the ciphertext. In addition, a new feature is also introduced which is called the time-release encryption. Time-release encryption allows the encrypter to publish a release key so that the ciphertext can be decrypted by the receiver before the time indicated in the time-stamp when necessary.