Weighted Oblivious RAM, with Applications to Searchable Symmetric Encryption

From a view point of information security, researches on an encrypted search system have been done intensively. Such search systems are called searchable symmetric encryption (SSE). The main part of SSE is an encrypted index which affects security and efficiency. Until now many SSE schemes have been proposed, but most of them uses a random oracle to achieve both adaptive security and an optimal index size. The index size of adaptively secure SSE schemes without a random oracle can be much larger. In this paper, we propose a new SSE scheme which satisfies adaptive security in the standard model and has an optimal index size. Furthermore the index of our scheme consists of Bloom filters and simple arrays, that is, arrays of integers. Since Bloom filters are also implemented by an array of integers, the structure of the index is simple. Thus, unlike other SSE schemes with an optimal index size, the size does not depend on a security parameter.

Implementing and Analyzing a Recursive Technique for Building Path Oblivious RAM

Based on Sun et al. ’s multi-client symmetric searchable encryption (SSE) scheme (at ESORICS 2016), and combining Zhao’s identity-concealed authenticated encryption (CCS 2016), a new SSE scheme with multi-data-owner functionalities is proposed. By setting two key generation centers, our scheme first implements multi-data-owner SSE. In particular, compared with Sun et al. ’s scheme, the new scheme not only meets the same security requirements stated by them, but also further strengthens the securities of the same category relevant scheme by providing identity-concealment, authentication of data user to server and confidentiality of search token. The identity-concealment aims to provide privacy protection (Forward ID-Privacy) for data users by hiding their identity information, while the authentication is to resist the camouflage attack by applying certificate-based mechanism to our scheme. In particular, the confidentiality of the search token provides replay-attack-resistant by encrypting the plaintext search token generated by data user. While in other works, the adversary can employ the previously generated plaintext search tokens to force the server to perform the same search queries. Furthermore, by efficiency analysis, our scheme reaches almost the same level of efficiency as Sun et al. ’s scheme.

We point out the risks of protecting relational databases via Searchable Symmetric Encryption (SSE) schemes by proposing an inference attack exploiting the structural properties of relational databases. We also show that record-injection attacks mounted on relational databases have worse consequences than their file-injection counterparts on unstructured databases. Moreover, we discuss some techniques to reduce the effectiveness of inference attacks exploiting the access pattern leakage existing in SSE schemes.

Searchable symmetric encryption (SSE) enables users to efficiently search ciphertext in the cloud and ensures the security of encrypted data. Recent works show that forward security is an important property in dynamic SSE. Many forward secure searchable symmetric encryption (FSSE) schemes supporting single-keyword search have been proposed. Only a few SSE schemes can satisfy the forward security and support conjunctive keyword search at the same time, which are realized by adopting inefficient or complicated cryptography tools. Very recently, Hu proposed a novel construction to achieve conjunctive-keyword search, that is, using inner-product encryption (IPE) to design a conjunctive-keyword FSSE scheme. However, IPE scheme is a conceptually complex and low efficient scheme. In this paper, we use a more efficient cryptographic tool, asymmetric scalar-product-preserving encryption (ASPE), to design an efficient and secure conjunctive-keyword FSSE scheme. To improve practicality, we design our scheme to support multi-user setting. Our scheme achieves sub-linear efficiency, and can easily be used in any single-keyword FSSE scheme to obtain a conjunctive-keyword FSSE scheme supporting multi-user. Compared with the current conjunctive-keyword FSSE scheme, our scheme has a better update and search efficiency.

Symmetric Searchable Encryption (SSE) schemes enable users to search over encrypted data hosted on an untrusted server. Recently, there has been a lot of interest in forward and backward private SSE. The notion of forward privacy guarantees that updates to the encrypted structure do not reveal their association to any query made in the past. Backward privacy, on the other hand, guarantees that queries do not reveal their association to deleted documents. But strong backward private schemes are known to be inefficient in terms of both communication and computation. One avenue for improvement is leveraging the power of trusted execution environments such as Intel SGX inside the untrusted server to improve some of these inefficiencies. In this work, we propose the first SGX-supported dynamic SSE constructions that are forward-private as well as backward-private. To the best of our knowledge, while there is some work on SGX-supported Oblivious RAM (ORAM) and static SSE, there is no work on SGX-supported dynamic SSE. We propose three constructions that cover all types of backward privacy in literature that are very efficient compared to the state of the art backward private schemes. Our communication complexity is always the number of current documents matching the query and we show that there is no need for 'total obliviousness' in constructions for the strongest notion of backward privacy.

Searchable symmetric encryption (SSE) scheme allows a data owner to perform search queries over encrypted documents using symmetric cryptography. SSE schemes are useful in cloud storage and data outsourcing. Most of the SSE schemes in existing literature have been proved to leak a substantial amount of information that can lead to an inference attack. This paper presents, a novel leakage resilient searchable symmetric encryption with periodic updation (LRSSEPU) scheme that minimizes extra information leakage, and prevents an untrusted cloud server from performing document mapping attack, query recovery attack and other inference attacks. In particular, the size of the keyword vector is fixed and the keywords are periodically permuted and updated to achieve minimum leakage. Furthermore, our proposed LRSSEPU scheme provides authentication of the query messages and restricts an adversary from performing a replay attack, forged query attack and denial of service attack. We employ a combination of identity-based cryptography (IBC) with symmetric key cryptography to reduce the computation cost and communication overhead. Our scheme is lightweight and easy to implement with very little communication overhead.

In this article, we tackle for the first time the problem of dynamic memory-efficient Searchable Symmetric Encryption (SSE). In the term “memory-efficient” SSE, we encompass both the goals of local SSE, and page-efficient SSE. The centerpiece of our approach is a novel connection between those two goals. We introduce a map, called the Generic Local Transform, which takes as input a page-efficient SSE scheme with certain special features, and outputs an SSE scheme with strong locality properties. We obtain several results. (1) First, for page-efficient SSE with page size p, we build a dynamic scheme with storage efficiency \(\mathcal {O}({1})\) and page efficiency \(\widetilde{\mathcal {O}}\left( {\textrm{log}\, \textrm{log}\, (N/p)}\right) \), called \(\textsf{LayeredSSE}\). The main technical innovation behind \(\textsf{LayeredSSE}\) is a novel weighted extension of the two-choice allocation process, of independent interest. (2) Second, we introduce the Generic Local Transform, and combine it with \(\textsf{LayeredSSE}\) to build a dynamic SSE scheme with storage efficiency \(\mathcal {O}({1})\), locality \(\mathcal {O}({1})\), and read efficiency \(\widetilde{\mathcal {O}}\left( {\textrm{log}\,\textrm{log}\, N}\right) \), under the condition that the longest list is of size \(\mathcal {O}({N^{1-1/\textrm{log}\, \textrm{log}\, \lambda }})\). This matches, in every respect, the purely static construction of Asharov et al. presented at STOC 2016: dynamism comes at no extra cost. (3) Finally, by applying the Generic Local Transform to a variant of the Tethys scheme by Bossuat et al. from Crypto 2021, we build an unconditional static SSE with storage efficiency \(\mathcal {O}({1})\), locality \(\mathcal {O}({1})\), and read efficiency \(\mathcal {O}({\textrm{log}^\varepsilon N})\), for an arbitrarily small constant \(\varepsilon > 0\). To our knowledge, this is the construction that comes closest to the lower bound presented by Cash and Tessaro at Eurocrypt 2014.

Outsourcing encrypted data to the cloud demands efficient and confidentiality-preserving search techniques, yet existing Searchable Symmetric Encryption (SSE) schemes suffer from severe access and search pattern leakage, and the problem is more intractable in multi-data-owner scenarios where cross-dataset privacy-preserving search remains unsolved. This paper proposes PSSE-MDO (Privacy-Preserving SSE with Pattern Obfuscation for Multiple Data Owners), the first SSE scheme that achieves robust access and search pattern hiding for multi-data-owner settings with constant client-side overhead. Our core innovation is a cryptographic protocol enabling users to issue a single constant-sized search token (trapdoor) for searching across all data owners' encrypted indexes. The scheme integrates Inner-Product Predicate Encryption (IPE) and Somewhat Homomorphic Encryption (SHE) to probabilistically obfuscate search results for each query. A key experimental result shows that the client-side cost approximately 120 ms for search token generation—remains constant regardless of the number of data owners, in contrast to existing pattern-hiding schemes with linearly scaling costs. This validates PSSE-MDO's practicality for real-world large-scale collaborative cloud systems, marking a significant step toward practical privacy-preserving search in federated cloud environments.

With the wide spread of Cloud service, users are outsourcing their data to the third party servers. When we delegate our data to the cloud servers, there is such a potential risk that the data stored in the servers is stolen. If the data are stored in the encrypted form, the damage caused by data theft is low because the data will not leak to other parties. However, a new problem arises, that is, users are not able to operate encrypted data, which loses the merit of cloud computing. To solve such a problem, searchable symmetric encryption(SSE) is proposed. SSE is a cryptographic protocol that can execute keyword search over encrypted documents without decrypting them. Secure Indexes is one of realizations of SSE that provides constant time keyword search per document with the help of pseudo-random function and Bloom filters. However, Secure Indexes dose not support addition and deletion of documents. Tekin and Sahin proposed an improved Secure Indexes scheme that supports addition and deletion by employing Bloom filter and counting Bloom filter. The drawback of their scheme is that these operations are implemented in user side, which is not desired in the cloud computing since the aim of cloud computing is to delegate as many operations as possible. This paper proposes new SSE scheme. The proposed scheme also employ counting Bloom filter, but provide more fine-grained operations compared to the scheme by Tekin and Sahin in the sense that the proposed scheme provides not only search documents but add and delete the elements of document. Moreover, these operations are executed in the server side. The performance of the proposed SSE is comparable to the existing scheme which provides constant time for search, add and delete per document. We also implemented this SSE scheme, and verified that the scheme performed well not only theoretically but also in practice.

Storage services allow data owners to store their huge amount of potentially sensitive data, such as audios, images, and videos, on remote cloud servers in encrypted form. To enable retrieval of encrypted files of interest, searchable symmetric encryption (SSE) schemes have been proposed. However, many schemes construct indexes based on keyword-file pairs and focus on boolean expressions of exact keyword matches. Moreover, most dynamic SSE schemes cannot achieve forward privacy and reveal unnecessary information when updating the encrypted databases. We tackle the challenge of supporting large-scale similarity search over encrypted feature-rich multimedia data, by considering the search criteria as a high-dimensional feature vector instead of a keyword. Our solutions are built on carefully-designed fuzzy Bloom filters which utilize locality sensitive hashing (LSH) to encode an index associating the file identifiers and feature vectors. Our schemes are proven to be secure against adaptively chosen query attack and forward private in the standard model . We have evaluated the performance of our scheme on real-world high-dimensional datasets, and achieved a search quality of 99 percent recall with only a few number of hash tables for LSH. This shows that our index is compact and searching is not only efficient but also accurate.

The cloud-assisted intelligent systems have attracted extensive attention due to their powerful data analysis and computation capabilities. However, how to handle encrypted data remains a challenging problem in intelligent systems. A promising solution is searchable symmetric encryption (SSE), which enables a client to privately outsource their data to the cloud while preserving keyword search functionality. In practice, dynamic SSE is more practical and supports efficient data addition and deletion. Unfortunately, data update will leak some additional information which can be exploited to break data privacy. To address this issue, forward and backward secure SSE schemes are proposed to reduce the leakage of data update. That is, forward security guarantees that the newly updated documents cannot reveal the previously searched keywords, while backward security guarantees that the server cannot recover the deleted documents. However, the existing forward and backward secure SSE schemes mainly consider curious-but-honest server. How to verify the soundness and completeness of search results is still a challenge. In this paper, we propose a noninteractive verifiable dynamic SSE scheme with forward and backward security from two universal accumulators. Specifically, the server in our scheme only needs one roundtrip to return the nondeleted search results to the client, which saves the communication overhead dramatically. Besides, our scheme can achieve public verification that anyone can verify the search results but not only the client who has the private key. Finally, we give a formal security analysis and compare the proposed scheme with other related work, the results show that our scheme can achieve the desired security properties with practical efficiency.

Searchable symmetric encryption (SSE) has been proposed that enables the clients to outsource their private encrypted data onto the cloud server and later the data can be searched with limited information leakage. However, existing surveys have not covered most recent advances on SSE technique. To fill the gap, we make a survey on state-of-the-art representative SSE schemes in cloud environment. We mainly focus on dynamic SSE schemes with forward and backward privacy, two vital security elements to maintain query privacy during data update operations. Specifically, we discuss about SSE protocols based on query expressiveness, including single keyword search, conjunctive keyword search, range search, disjunctive keyword search and verifiable search. Finally, through comparison on query expressiveness, security and efficiency, we demonstrate the strengths and weaknesses of the existing SSE protocols.

Searchable symmetric encryption (SSE) enables a client to store a database on an untrusted server while supporting keyword search in a secure manner. Despite the rapidly increasing interest in SSE technology, experiments indicate that the performance of the known schemes scales badly to large databases. Somewhat surprisingly, this is not due to their usage of cryptographic tools, but rather due to their poor locality (where locality is defined as the number of non-contiguous memory locations the server accesses with each query). The only known schemes that do not suffer from poor locality suffer either from an impractical space overhead or from an impractical read efficiency (where read efficiency is defined as the ratio between the number of bits the server reads with each query and the actual size of the answer). We construct the first SSE schemes that simultaneously enjoy optimal locality, optimal space overhead, and nearly-optimal read efficiency. Specifically, for a database of size N, under the modest assumption that no keyword appears in more than N1 − 1/loglogN documents, we construct a scheme with read efficiency O(loglogN). This essentially matches the lower bound of Cash and Tessaro (EUROCRYPT ’14) showing that any SSE scheme must be sub-optimal in either its locality, its space overhead, or its read efficiency. In addition, even without making any assumptions on the structure of the database, we construct a scheme with read efficiency O(logN). Our schemes are obtained via a two-dimensional generalization of the classic balanced allocations (“balls and bins”) problem that we put forward. We construct nearly-optimal two-dimensional balanced allocation schemes, and then combine their algorithmic structure with subtle cryptographic techniques.

Although searchable encryption schemes allow secure search over the encrypted data, they mostly support conventional Boolean keyword search, without capturing any relevance of the search results. This leads to a large amount of post-processing overhead to find the most matching documents and causes an unnecessary communication cost between the servers and end-users. Such problems can be addressed efficiently using a ranked search system that retrieves the most relevant documents. However, existing state-of-the-art solutions in the context of Searchable Symmetric Encryption (SSE) suffer from either (a) security and privacy breaches due to the use of Order Preserving Encryption (OPE) or (b) non-practical solutions like using the two non-colluding servers. In this paper, we present a generic solution for multi-keyword ranked search over the encrypted cloud data. The proposed solution can be applied over different symmetric searchable encryption schemes. To demonstrate the practicality of our technique, in this paper we leverage the Oblivious Cross Tags (OXT) protocol of Cash et al. (2013) due to its scalability and remarkable flexibility to support different settings. Our proposed scheme supports the multi-keyword search on Boolean, ranked and limited range queries while keeping all of the OXT’s properties intact. The key contribution of this paper is that our scheme is resilience against all common attacks that take advantage of OPE leakage while only a single cloud server is used. Moreover, the results indicate that using the proposed solution the communication overhead decreases drastically when the number of matching results is large.