Abstract: Cloud computing has emerged as one of the most influential paradigm in the IT industry in recent years. Since this new computing technology requires users to entrust their valuable data to cloud providers, there have been increasing security and privacy concerns on outsourced data .Several schemes employing attribute based encryption(ABE) have been proposed for access control of out sourced data in cloud computing; however most of them suffer from inflexibility in implementing complex access control policies. In order to realize scalable flexible and fine grained access control of outsourced data in cloud computing,proposed the use of Hierarchical Attribute Set Based Encryption (HASBE)by extending cipher text policy Attribute Set Based Encryption(ASBE) with a hierarchical structure of users .The proposed scheme not only achieves scalability due to its hierarchical structure, but also inherits flexibility and fine grained access control in supporting compound attributes of ASBE. In addition HASBE employs multiple value assignments for access expiration time to deal with user revocation more efficiently than existing schemes. The proposed scheme shows that it is efficient and flexible in dealing with access control for outsourced data in cloud computing with comprehensive experiments. One major problem of existing HASBE schemes is bulky, linearly increasing cipher text. In the CP-ABE schemes used, the size of a cipher text proliferates linearly with respect to the number of included attributes. In this paper, propose a novel PP-CP-ABE construction, named Privacy Preserving Constant-size Cipher text Policy Attribute Based Encryption (PP-CP-ABE), which enforces hidden access policies with wildcards and incurs constant-size conjunctive headers, regardless of the number of attributes. Based on this PP-CP-ABE, provide a new construction named as Privacy Preserving Attribute Based Broadcast Encryption (PP-AB-BE). Compared with existing CP-ABE constructions, PP-CP-ABE significantly reduces the ciphertext size from linear to constant and supports expressive access policies. Thus, PP-CP-ABE can be used in many communication constrained environments.

Keywords: Attribute-based encryption (ABE), privacy-preserving, ciphertext-policy, constant cipher text length, broadcast encryption.