对类型可修改的基于身份代理重加密方案的改进

类型可修改的基于身份的代理重加密方案不仅具有传统代理重加密方案的核心功能，而且密文的拥有者可以随时修改密文的类型信息，能够满足实际云存储应用中，密文类型信息需要动态转换的应用场景。对类型可修改的基于身份代理重加密方案进行分析，发现该方案存在2个安全漏洞:1）类型修改缺乏验证，攻击者可以随意修改类型标记；2）类型修改引起了新的条件性选择明文攻击问题。在分析这两个安全漏洞的基础上，提出了改进方案，并给出了安全性分析。

Further Improvement of a Dynamic Type and Identity-Based Proxy Re-Encryption Scheme

Dynamic type information of ciphertext can be modified properly so that it can be well applied in a practical cloud storage environment. In order to meet the application requirements, Liu et al proposed a dynamic type and identity-based proxy re-encryption (PRE) scheme based on Ibraimi et al's scheme. Their scheme not only keeps the traditional core function of PRE scheme, but also makes sure that the owner of ciphertext can modify the type information at any time. However, after careful security analysis it found that Liu et al.'s scheme has two security flaws. Firstly, the dynamic type information lacks of verification, the adversary can modify the type tag. Secondly, the dynamic type information causes a conditional chosen plaintext attack. Thus we further improve Liu et al.'s scheme and give the security analysis.