To protect privacy of users, sensitive data need to be encrypted before outsourcing to cloud, which makes effective data retrieval a very tough task. In this paper, we proposed a novel order-preserving encryption(OPE) based ranked search scheme over encrypted cloud data, which uses the encrypted keyword frequency to rank the results and provide accurate results via two-step ranking strategy. The first step coarsely ranks the documents with the measure of coordinate matching, i.e., classifying the documents according to the number of query terms included in each document. In the second step, for each category obtained in the first step, a fine ranking process is executed by adding up the encrypted score. Extensive experiments show that this new method is indeed an advanced solution for secure multi-keyword retrieval.
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