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A user stores his personal files in a cloud, and retrieves them wherever and whenever he wants. For the sake of protecting the user data privacy and the user queries privacy, a user should store his personal files in an encrypted form in a cloud, and then sends queries in the form of encrypted keywords. However, a simple encryption scheme may not work well when a user wants to retrieve only files containing certain keywords using a thin client. First, the user needs to encrypt and decrypt files frequently, which depletes too much CPU capability and memory power of the client. Second, the service provider couldn't determine which files contain keywords specified by a user if the encryption is not searchable. Therefore, it can only return back all the encrypted files. A thin client generally has limited bandwidth, CPU and memory, and this may not be a feasible solution under the circumstances. In this paper, we investigate the characteristics of cloud computing and propose an efficient privacy preserving keyword search scheme in cloud computing. It allows a service provider to participate in partial decipherment to reduce a client's computational overhead, and enables the service provider to search the keywords on encrypted files to protect the user data privacy and the user queries privacy efficiently. By proof, our scheme is semantically secure.