Abstract: In 2005，Sun et al have proposed two secure e-mail protocols with perfect forward secrecy.Although these two protocols seem to be secure from the protective efficiency of the short-term key，yet neither of them has considered the authentication of the delivering ciphertext.If an outside attacker intercepts and modifies a delivering ciphertext，the e-mail receiver will have to accept a false plaintext.To overcome this flaw，in this paper we will make improvements on Sun et al.’s protocols such that the improved protocols have authentication，better security and practicality.

Key words: perfect forward secrecy, confidentiality, protocol, authentication, e-mail

摘要： 2005年，Sun等提出了两个完美前向安全的E-mail协议，尽管从短密钥保护的有效性来看这两个协议是安全的，但它们都不能提供密文的认证性，如果一个主动攻击者拦截或修改密文，则E-mail接受者会收到一个错误的明文。为了克服此缺点，对此两个协议作了改进，使得改进后的协议具有认证性，从而有更好的安全性和实用性。

关键词: 完美前向安全, 机密性, 协议, 认证性, E-mail