Cloud computing offers computing services such as data storage and computing power and relieves its users of the burden of their direct management. While being extremely convenient, therefore immensely popular, cloud computing instigates concerns of privacy of outsourced data, for which conventional encryption is hardly a solution as the data is meant to be accessed, used and processed in an efficient manner. Multi keyword ranked search over encrypted data (MRSE) is a special form of secure searchable encryption (SSE), which lets users to privately find out the most similar documents to a given query using document representation methods such as tf-idf vectors and metrics such as cosine similarity. In this work, we propose a secure MRSE scheme that makes use of both a new secure k-NN algorithm and somewhat homomorphic encryption (SWHE). The scheme provides data, query and search pattern privacy and is amenable to access pattern privacy. We provide a formal security analysis of the secure k-NN algorithm and rely on IND-CPA security of the SWHE scheme to meet the strong privacy claims. The scheme provides speedup of about two orders of magnitude over the privacy-preserving MRSE schemes using only SWHE while its overall performance is comparable to other schemes in the literature with weaker forms of privacy claims. We present implementations results including one from the literature pertaining to response times, storage and bandwidth requirements and show that the scheme facilitates a lightweight client implementation
