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...y one as the indicator of their search interest to retrieve the most relevant data. And each keyword in the search request is able to help narrow down the search result further. “Coordinate matching” =-=[6]-=-, i.e., as many matches as possible, is an efficient similarity measure among such multi-keyword semantics to refine the result relevance, and has been widely used in the plaintext information retriev...

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...and privacy obstacles, including various strict requirements like the data privacy, the index privacy, the keyword privacy, and many others (see Section 3.2). In the literature, searchable encryption =-=[7]-=-, [8], [9], [10], [11], [12], [13], [14], [15] is a helpful technique that treats encrypted data as documents and allows a user to securely search through a single keyword and retrieve documents of in...

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... utility, where cloud customers can remotely store their data into the cloud so as to enjoy the on-demand high-quality applications and services from a shared pool of configurable computing resources =-=[2]-=-, [3]. Its great flexibility and economic savings are motivating both individuals and enterprises to outsource their local complex data management system into the cloud. To protect data privacy and co...

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..., including various strict requirements like the data privacy, the index privacy, the keyword privacy, and many others (see Section 3.2). In the literature, searchable encryption [7], [8], [9], [10], =-=[11]-=-, [12], [13], [14], [15] is a helpful technique that treats encrypted data as documents and allows a user to securely search through a single keyword and retrieve documents of interest. However, direc...

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...ng undifferentiated results. Such ranked search system enables data users to find the most relevant information quickly, rather than burdensomely sorting through every match in the content collection =-=[5]-=-. Ranked search can also elegantly eliminate unnecessary network traffic by sending back only the most relevant data, which is highly desirable in the “pay-as-you-use” cloud paradigm. For privacy prot...

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...tacles, including various strict requirements like the data privacy, the index privacy, the keyword privacy, and many others (see Section 3.2). In the literature, searchable encryption [7], [8], [9], =-=[10]-=-, [11], [12], [13], [14], [15] is a helpful technique that treats encrypted data as documents and allows a user to securely search through a single keyword and retrieve documents of interest. However,...

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...s for all documents in the dataset. Fig. 5 shows the query time is dominated by the number of documents in dataset, and the number of keywords in the query has very slight impact on it like the trapdoor generation above. With respect to the communication cost in Query, the size of trapdoor is the same as that of subindex listed in the Tab. V, which keeps constant in the same dataset, no matter how many keywords are contained in a query. While the computation and communication cost in the query procedure is linear with the number of query keywords in other multiple-keyword search schemes [14], [16], our proposed schemes enjoy the constant overhead in the query which makes it more practical in the cloud paradigm. VI. RELATED WORK Single Keyword Searchable Encryption Traditional single keyword searchable encryption schemes [5]–[13], [22] usually build an encrypted searchable index such that its content is hidden to the server unless it is given appropriate trapdoors 9 generated via secret key(s) [2]. It is first studied by Song et al. [5] in the symmetric key setting, and improvements and advanced security definitions are given in Goh [6], Chang et al. [7] and Curtmola et al. [8]. Our ear...

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...ever. Furthermore, the keyword privacy could not be protected in the public key setting since server could encrypt any keyword with public key and then use the received trapdoor to evaluate this ciphertext. Boolean Keyword Searchable Encryption To enrich search functionalities, conjunctive keyword search [14]–[18] over encrypted data have been proposed. These schemes incur large overhead caused by their fundamental primitives, such as computation cost by bilinear map, e.g. [16], or communication cost by secret sharing, e.g. [15]. As a more general search approach, predicate encryption schemes [19]–[21] are recently proposed to support both conjunctive and disjunctive search. Conjunctive keyword search returns “all-or-nothing”, which means it only returns those documents in which all the keywords specified by the search query appear; disjunctive keyword search returns undifferentiated results, which means it returns every document that contains a subset of the specific keywords, even only one keyword of interest. In short, none of existing Boolean keyword searchable encryption schemes support multiple keywords ranked search over encrypted cloud data while preserving privacy as we propos...

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...y obstacles, including various strict requirements like the data privacy, the index privacy, the keyword privacy, and many others (see Section 3.2). In the literature, searchable encryption [7], [8], =-=[9]-=-, [10], [11], [12], [13], [14], [15] is a helpful technique that treats encrypted data as documents and allows a user to securely search through a single keyword and retrieve documents of interest. Ho...

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...uding various strict requirements like the data privacy, the index privacy, the keyword privacy, and many others (see Section 3.2). In the literature, searchable encryption [7], [8], [9], [10], [11], =-=[12]-=-, [13], [14], [15] is a helpful technique that treats encrypted data as documents and allows a user to securely search through a single keyword and retrieve documents of interest. However, direct appl...

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...like system usability, user searching experience, and easy information discovery. Although some recent designs have been proposed to support Boolean keyword search [16], [17], [18], [19], [20], [21], =-=[22]-=-, [23], [24] as an attempt to enrich the search flexibility, they are still not adequate to provide users with acceptable result ranking functionality (see Section 7). Our early works [25], [26] have ...

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...various strict requirements like the data privacy, the index privacy, the keyword privacy, and many others (see Section 3.2). In the literature, searchable encryption [7], [8], [9], [10], [11], [12], =-=[13]-=-, [14], [15] is a helpful technique that treats encrypted data as documents and allows a user to securely search through a single keyword and retrieve documents of interest. However, direct applicatio...

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...overall importance of the term within the whole collection). Among several hundred variations of the TF IDF weighting scheme, no single combination of them outperforms any of the others universally =-=[33]-=-. Thus, without loss of generality, we choose an example formula that is commonly used and widely seen in the literature (see [5, chapter 4]) for the relevance score calculation ScoreðFi;QÞ 1jFij X ...

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...isting documents, and deleting existing documents. 2.2 Threat Model The cloud server is considered as “honest-but-curious” in our model, which is consistent with related works on cloud security [28], =-=[29]-=-. Specifically, the cloud server acts in an “honest” fashion and correctly follows the designated protocol specification. However, it is “curious” to infer and analyze data (including index) in its st...

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...or example, e-mails, personal health records, photo albums, tax documents, financial transactions, and so on, may have to be encrypted by data owners before outsourcing to the commercial public cloud =-=[4]-=-; this, however, obsoletes the traditional data utilization service based on plaintext keyword search. The trivial solution of downloading all the data and decrypting locally is clearly impractical, d...

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...rivacy obstacles, including various strict requirements like the data privacy, the index privacy, the keyword privacy, and many others (see Section 3.2). In the literature, searchable encryption [7], =-=[8]-=-, [9], [10], [11], [12], [13], [14], [15] is a helpful technique that treats encrypted data as documents and allows a user to securely search through a single keyword and retrieve documents of interes...

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...uch high service-level requirements like system usability, user searching experience, and easy information discovery. Although some recent designs have been proposed to support Boolean keyword search =-=[16]-=-, [17], [18], [19], [20], [21], [22], [23], [24] as an attempt to enrich the search flexibility, they are still not adequate to provide users with acceptable result ranking functionality (see Section ...

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...[20], [21], [22], [23], [24] as an attempt to enrich the search flexibility, they are still not adequate to provide users with acceptable result ranking functionality (see Section 7). Our early works =-=[25]-=-, [26] have been aware of this problem, and provide solutions to the secure ranked search over encrypted data problem but only for queries consisting of a single keyword. How to design an efficient en...

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...ystem usability, user searching experience, and easy information discovery. Although some recent designs have been proposed to support Boolean keyword search [16], [17], [18], [19], [20], [21], [22], =-=[23]-=-, [24] as an attempt to enrich the search flexibility, they are still not adequate to provide users with acceptable result ranking functionality (see Section 7). Our early works [25], [26] have been a...

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...ing such multikeyword semantic without privacy breaches, we propose a basic idea for the MRSE using secure inner product computation, which is adapted from a secure k-nearest neighbor (kNN) technique =-=[27]-=-, and then give two significantly improved MRSE schemes in a step-by-step manner to achieve various stringent privacy requirements in two threat models with increased attack capabilities. Our contribu...

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...usability, user searching experience, and easy information discovery. Although some recent designs have been proposed to support Boolean keyword search [16], [17], [18], [19], [20], [21], [22], [23], =-=[24]-=- as an attempt to enrich the search flexibility, they are still not adequate to provide users with acceptable result ranking functionality (see Section 7). Our early works [25], [26] have been aware o...

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... þ "2 þ t0; y03 r0"3 þ t0: 8>>>>>>>>>>>>< >>>>>>>>>>>>: ð6Þ 4.6 Supporting Data Dynamics After the data set is outsourced to the cloud server, it may be updated in addition to being retrieved =-=[34]-=-. Along with the updating operation on data documents, supporting the score dynamics in the searchable index is thus of practical importance. While we consider three dynamic data operations as inserti...

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...equirements like system usability, user searching experience, and easy information discovery. Although some recent designs have been proposed to support Boolean keyword search [16], [17], [18], [19], =-=[20]-=-, [21], [22], [23], [24] as an attempt to enrich the search flexibility, they are still not adequate to provide users with acceptable result ranking functionality (see Section 7). Our early works [25]...

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...gh service-level requirements like system usability, user searching experience, and easy information discovery. Although some recent designs have been proposed to support Boolean keyword search [16], =-=[17]-=-, [18], [19], [20], [21], [22], [23], [24] as an attempt to enrich the search flexibility, they are still not adequate to provide users with acceptable result ranking functionality (see Section 7). Ou...

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...[21], [22], [23], [24] as an attempt to enrich the search flexibility, they are still not adequate to provide users with acceptable result ranking functionality (see Section 7). Our early works [25], =-=[26]-=- have been aware of this problem, and provide solutions to the secure ranked search over encrypted data problem but only for queries consisting of a single keyword. How to design an efficient encrypte...

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...elp narrow down the search result further. “Coordinate matching” [4], i.e., as many matches as possible, is an efficient principle among such multi-keyword semantics to refine the result relevance, and has been widely used in the plaintext information retrieval (IR) community. However, how to apply it in the encrypted cloud data search system remains a very challenging task because of inherent security and privacy obstacles, including various strict requirements like data privacy, index privacy, keyword privacy, and many others (see section III-B). In the literature, searchable encryption [5]–[13] is a helpful technique that treats encrypted data as documents and allows a user to securely search over it through single keyword and retrieve documents of interest. However, direct application of these approaches to deploy secure large scale cloud data utilization system would not be necessarily suitable, as they are developed as crypto primitives and cannot accommodate such high service-level requirements like system usability, user searching experience, and easy information discovery in mind. 2 Although some recent designs have been proposed to support Boolean keyword search [14]–[21] as ...

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...ments like system usability, user searching experience, and easy information discovery. Although some recent designs have been proposed to support Boolean keyword search [16], [17], [18], [19], [20], =-=[21]-=-, [22], [23], [24] as an attempt to enrich the search flexibility, they are still not adequate to provide users with acceptable result ranking functionality (see Section 7). Our early works [25], [26]...

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...result to make an estimate. As a kind of statistical information, document frequency (i.e., the number of documents containing the keyword) is sufficient to identify the keyword with high probability =-=[31]-=-. When the cloud server knows some background information of the data set, this keyword specific information may be utilized to reverseengineer the keyword. Trapdoor unlinkability. The trapdoor genera...

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...ds contained by these documents. 5 PERFORMANCE ANALYSIS In this section, we demonstrate a thorough experimental evaluation of the proposed technique on a real-world data set: the Enron Email Data Set =-=[35]-=-. We randomly select different number of e-mails to build data set. The whole experiment system is implemented by C language on a Linux Server with Intel Xeon Processor 2.93 GHz. The public utility ro...

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...ll as the data set related statistical information. As an instance of possible attacks in this case, the cloud server could use the known trapdoor information combined with document/keyword frequency =-=[30]-=- to deduce/identify certain keywords in the query. 2.3 Design Goals To enable ranked search for effective utilization of outsourced cloud data under the aforementioned model, our system design should ...

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... ;F eW2 ; . . .Þ which are the results of sequential searches. Although a few searchable encryption works, for example, [19] has been proposed to utilize private information retrieval (PIR) technique =-=[32]-=-, to hide the access pattern, our proposed schemes are not designed to protect the access pattern for the efficiency concerns. This is because any PIRbased technique must “touch” the whole data set ou...

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...evel requirements like system usability, user searching experience, and easy information discovery. Although some recent designs have been proposed to support Boolean keyword search [16], [17], [18], =-=[19]-=-, [20], [21], [22], [23], [24] as an attempt to enrich the search flexibility, they are still not adequate to provide users with acceptable result ranking functionality (see Section 7). Our early work...

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...different front, the research on top-k retrieval [31] in database community is also loosely connected to our problem. Besides, Cao et. al. proposed a privacy-preserving graph containment query scheme =-=[36]-=- which solves the search problem with graph semantics. 7 CONCLUSION In this paper, for the first time we define and solve the problem of multi-keyword ranked search over encrypted cloud data, and esta...

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...rom a shared pool of configurable computing resources [1]. Its great flexibility and economic savings are motivating both individuals and enterprises to outsource their local complex data management system into the cloud, especially when the data produced by them that need to be stored and utilized is rapidly increasing. To protect data privacy and combat unsolicited accesses in cloud and beyond, sensitive data, e.g., emails, personal health records, photo albums, tax documents, financial transactions, etc., may have to be encrypted by data owners before outsourcing to commercial public cloud [2]; this, however, obsoletes the traditional data utilization service based on plaintext keyword search. The trivial solution of downloading all the data and decrypting locally is clearly impractical, due to the huge amount of bandwidth cost in cloud scale systems. Moreover, aside from eliminating the local storage management, storing data into the cloud serves no purpose unless they can be easily searched and utilized. Thus, exploring privacy-preserving and effective search service over encrypted cloud data is of paramount importance. Considering the potentially large number of ondemand data us...

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...ct requirements like the data privacy, the index privacy, the keyword privacy, and many others (see Section 3.2). In the literature, searchable encryption [7], [8], [9], [10], [11], [12], [13], [14], =-=[15]-=- is a helpful technique that treats encrypted data as documents and allows a user to securely search through a single keyword and retrieve documents of interest. However, direct application of these a...