Even though the content of the queries and the documents are encrypted, during this interaction the server learns which documents are accessed, i.e., the access pattern, and which queries are equal, i.e., the search pattern. Most existing SSE schemes [7], [9], [12], [21]-[23], [33] allow such leakage for performance considerations. However, recent studies [8], [20], [39] demonstrated that, with some prior knowledge of the outsourced database or a subset of the queries, an honest-butcurious server can recover the underlying keywords of queries with high accuracy, which violates the client's privacy. There are different techniques that allow enhancing the privacy properties of SSE schemes, but they incur an utility cost which is typically a combination of communication overhead, extra computational complexity, and local client storage requirements. Certain schemes, like those based on Oblivious RAM (ORAM) [19] or Private Information Retrieval (PIR) [11], can fully hide the access pattern when reading a document from a database. However, they incur a large communication and computation overhead, respectively, and are not specifically designed towards securely searching over an encrypted database (except for TWORAM [17]). A recent framework by Chen et al. [10] protects access-pattern leakage in SSE by obfuscating the index of the database before outsourcing it. This way, the server only learns obfuscated access patterns, making it harder to successfully carry out attacks on the client's privacy from such leakage. However, despite its efficiency, this framework cannot hide search patterns since the access pattern for each keyword is determined after outsourcing. This search pattern leakage allows different practical attacks [20], [27], [31] to perform remakably well regardless of the access pattern obfuscation (see Sect. IX). Motivated by this vulnerability of Chen et al.'s scheme [10], in this work we propose OSSE (Obfuscated SSE), a new SSE scheme that protects both the access and search patterns. The main idea behind OSSE is that it produces a fresh obfuscation per query, instead of just once when outsourcing the database, thus making it hard for the server to decide whether or not two queries are for the same keyword. Our scheme allows to perform queries on the encrypted database and receive the matched documents in the same communication round (TWORAM requires at least four rounds [17]). Under some reasonable assumptions on the query and database distribution, OSSE achieves a lower communication overhead than TWORAM (e.g., only a small constant when the keyword distribution is uniform). Our scheme relies on computationheavy cryptographic techniques and thus its computational cost is considerable (e.g., it can require 30 minutes to run a query Abstract-Searchable Symmetric Encryption (SSE) allows a data owner to securely outsource its encrypted data to a cloud server while maintaining the ability to search over it and retrieve matched documents. Most existing SSE schemes leak which documents are accessed per query...
