# An Improved Ciphertext-Policy Attribute-Based Encryption Scheme in Power Cloud Access Control
## 1 Introduction  
With the fast development of cloud computing, it is economic for individuals and companies to save their data in cloud. However, the safety of outsourced data, in cloud storage, has always been a big issue. Power cloud contains a large number of users with complex system structure and different data security requirements, which needs an automatic, fine-gained access control scheme. National Institute of Standards and Technology (NIST) specifies the role-based access control (RBAC) requirements for a smart grid access control [1]. In this study, we focus on the attribute-based access control (ABAC), which is more fine-gained than RBAC. CP-ABE is an ABAC scheme and was first proposed by [PERSON] [2], who defined the basic algorithms. In the model of CP-ABE, the ciphertext corresponds to access structure, while the key corresponds to the attribute set. If, and only if, the user's attribute set satisfies the access structure, the user can decrypt the data. This scheme can meet the fine-grained access control requirement of power clouds, but it is difficult to meet the security requirement. The recent research on CP-ABE focuses on the improvement of access structures [3; 4; 5; 6; 7; 8; 9; 10] and the generation and distribution of the secret key [11; 12; 13; 14; 15; 16; 17; 18; 19]. [PERSON] [3] combined the access tree with an Ordered Binary Decision Diagram, and employed a Boolean function in the leaf node, to finish the generation of the secret key and the attribute-based encryption, with less time complexity, which reduced the computing cost. [PERSON] [4] proposed a hierarchical access structure which was based on access trees, in order to save the storage space of the access tree. However, this scheme only achieved the deduplication of access structures, rather than improve the structure of access tree. [PERSON] [5] solved the problem of a single-point performance bottleneck and the security problem of multi-agency access control. In their scheme, the verificationof the attribute set, and the access structure was shared by multiple semi-trusted third-part attribute authorities. They refined the steps of the generation and distribution of the secret key, confirmed, and traced malicious users by monitoring the key generation and the distribution process. [PERSON] [6] improved the key generation phase and proposed a new scheme in which he changed the traditional single authority model. In this model he introduced the multi-agency idea where different agencies managed different trapdoors. Users could obtain the shared secret key from multiple agencies, so it can protect the user's identifier and attribute set, to achieve the purpose of privacy protection. [PERSON] [7] applied a subset difference algorithm in the environment of fog computing, which is similar to cloud computing, in order to update the access tree after the attribute is revoked. [PERSON] [8] proposed the generation and management of the sub-attributes with 0-encoding and 1-encoding, in order to achieve the attribute comparison. [PERSON] [9] proposed a secure scheme which achieve revocation and traceability requirement with the use of a subset cover algorithm. [PERSON] [10] proposed a Privacy Preserving Constant CP-ABE (PPC-CP-ABE) scheme that reduces the ciphertext to a constant size with any given number of attributes. PPCCP-ABE applies a hidden policy construction so that the recipients' privacy is preserved efficiently. Many improvements have been made in key generation and distribution processes [11; 12; 13; 14; 15; 16; 17; 18; 19]. [PERSON] [11] proposed a scheme based on trust evaluated by the data owner or reputations generated by a number of reputation centers through the CP-ABE and Proxy Re-Encryption. [PERSON] [12] and [PERSON] [19] proposed a new way using the linear secret-sharing scheme (LSSS). They described a specific access structure by constructing an LSSS matrix from which the secret key was generated. Only when all the attribute sets satisfy the requirement of the trap doors, the secret key can be generated correctly.