Abstract: Attribute based encryption provides flexible and fine-grained access control for outsourced data stored in the cloud.The traditional attribute based ciphertext policy encryption scheme(CP-ABE),whose access policy often appears in the form of plaintext,is very easy to expose users’ sensitive privacy information.In addition,due to the addition of attributes,the related calculation and storage costs in the encryption,decryption and search stages are linear with the number of attributes,and policy hiding will also increase the subsequent calculation costs.These are difficult to meet the actual needs of secure and efficient searchable encryption with privacy protection in cloud environment.To solve the above problems,a searchable encryption scheme supporting both policy hiding and constant ciphertext length is proposed.Based on the multi-valued wildcard and gate strategy,the scheme realizes the constant length of the ciphertext,and has a fixed encryption,decryption and search overhead,reducing users’ computing overhead and the storage overhead of the ciphertext in the cloud.The attributes in the access policy are completely hidden by encryption,and the bloom filter is used to judge whether the user has the relevant attributes in the access policy during the search,which not only protects users’ privacy,but also improves the computing efficiency.The scheme meets the IND-CPA safety under the assumption of q-BDHE.Security analysis and experimental results show that the scheme is safe,efficient and feasible.It is an efficient keyword search scheme,and has a good application prospect in cloud environment and Internet of Things.

Key words: Attribute based encryption, Policy hiding, Constant ciphertext, Keyword search, Cloud environment