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The security of our data stores is underestimated in current practice, which resulted in many large-scale data breaches. To change the status quo, this paper presents the design of ShieldDB, an encrypted document database. ShieldDB adapts the searchable encryption technique to preserve the search functionality over encrypted documents without having much impact on its scalability. However, merely realising such a theoretical primitive suffers from real-world threats, where a knowledgeable adversary can exploit the leakage (aka access pattern to the database) to break the claimed protection on data confidentiality. To address this challenge in practical deployment, ShieldDB is designed with tailored padding countermeasures. Unlike prior works, we target a more realistic adversarial model, where the database gets updated continuously, and the adversary can monitor it at an (or multiple) arbitrary time interval(s). ShieldDB's padding strategies ensure that the access pattern to the database is obfuscated all the time. Additionally, ShieldDB provides other advanced features, including forward privacy, re-encryption, and flushing, to further improve its security and efficiency. We present a full-fledged implementation of ShieldDB and conduct intensive evaluations on Azure Cloud., Comment: Accepted version of our work published in IEEE Transactions on Knowledge and Data Engineering (TKDE, 2021)

Searchable Encryption (SE) enables a data owner to outsource encrypted data to an untrusted server while preserving keyword search functionality. Typically, the server learns whether or not a query has been performed more than once, which is called the search pattern. However, such kind of information leakage might be leveraged to break query privacy. To reduce such type of leakage and provide strong privacy guarantee, Wang et al. proposed a novel SE scheme based on the Paillier encryption scheme in INFOCOM'15. Unfortunately, their scheme cannot perform keyword search successfully, because the additive homomorphic property is not sufficient for their construction. In this paper, we first show that why their scheme fails to return the correct search result, and then propose a new SE scheme by adopting a special additive homomorphic encryption scheme to achieve the multiplicative homomorphic property efficiently. Furthermore, we enhance the security on the user side. Specifically, we use random polynomials with an appropriate degree to guarantee that the user cannot learn anything other than the desired search result. Finally, we present a formal security analysis and implement our scheme on a real-world database, which demonstrates that our construction can achieve the desired security properties with good performance.

This paper presents two dynamic symmetric searchable encryption schemes for geometric range search. Our constructions are the first to provide forward/backward security in the context of SSE-based schemes supporting geometric range search. Besides, we define a security notion called content privacy. This security notion captures the leakages that are critical in the context of geometric range search but not considered by forward/backward security. Content privacy eliminates the leakage on the updated points of the database during both search and update. Due to the inherent leakages associated with range queries, none of the existing related works can support content privacy, whereas the design of our constructions avoids such leakages. When compared to the state-of-the-art schemes, our constructions provide a higher level of security and practical efficiency supported by our experimental results.

Ring confidential transaction (RingCT) protocol is widely used in cryptocurrency to protect the privacy of both users' identities and transaction amounts. Most recently, a new RingCT protocol was proposed by leveraging cryptographic accumulators, which can achieve a constant-size output theoretically but still far from being practical due to the heavy zero-knowledge associated with the accumulator. In this work, we revisit the design of ring confidential transaction protocol and put forward a more efficient privacy-preserving payment protocol, which is built upon a extended version of one-out-of-many proof and a special multi-signature. Compared with previous works, the new protocol is not only more practical, but also does not suffer from a trusted setup. Besides, we show that the protocol satisfies the security requirements provided that the underlying cryptographic primitives are secure in the random oracle model. We implement our new payment protocol in Java, and the experimental results show that it is efficient enough to be used in practice.

In this work, we introduce a new mechanism for constructing multi-client searchable encryption (SE). By tactfully leveraging the RSA-function, we propose the first multi-client SE protocol that successfully avoids per-query interaction between data owner and client. Therefore, our approach significantly reduces the communication cost by eliminating the need for data owner to authorize client queries at all times. To be compatible with the RSA-based approach, we also present a deterministic and memory-efficient ‘keyword to prime’ hash function, which may be of independent interest. Further, to improve efficiency, we put forward a more generic construction from set-constrained PRFs. The construction not only inherits the merits of our first protocol, but also achieves an enhanced security (against untrusted clients), where colluding attack among clients is also taken into account. Both protocols are instantiated via the recent representative SE protocol by Cash et al. with support of boolean queries. At last, we implement our proposed protocols and comprehensively evaluate their performance to demonstrate their practicability and scalability.

Due to its capabilities of searches and updates over the encrypted database, the dynamic searchable symmetric encryption (DSSE) has received considerable attention recently. To resist leakage abuse attacks, a secure DSSE scheme usually requires forward and backward privacy. However, the existing forward and backward private DSSE schemes either only support single keyword queries or require more interactions between the client and the server. In this paper, we first give a new leakage function for range queries, which is more complicated than the one for single keyword queries. Furthermore, we propose a concrete forward and backward private DSSE scheme by using a refined binary tree data structure. Finally, the detailed security analysis and extensive experiments demonstrate that our proposal is secure and efficient, respectively.

DualRing is a novel generic construction introduced by Yuen et al. (CRYPTO’21), which can transform a special kind of (Type-T*) canonical identification scheme to a ring signature scheme. Compared with the classical approaches, this method can get a shorter signature. In this paper, we construct a new middle-product learning with errors (MPLWE)-based ring signature scheme by using this framework. Specifically, we propose a new MPLWE-based identification scheme, which is compatible with the DualRing, then we obtain a ring signature scheme by using DualRing framework. We also show how to achieve linkability from this ring signature by using a collision resistant hash function. In the end, we provide available parameter options for our (linkable) ring signature scheme. Under these parameters, the signature size of our linkable ring signature is $2-40 \times $ shorter (depending on the ring size) than the previous MPLWE-based scheme by Das et al. (Africacrypt’19).

E-medical records are sensitive and should be stored in a medical database in encrypted form. However, simply encrypting these records will eliminate data utility and interoperability of the existing medical database system because encrypted records are no longer searchable. Moreover, multiple authorities could be involved in controlling and sharing the private medical records of clients. However, authorizing different clients to search and access records originating from multiple authorities in a secure and scalable manner is a nontrivial matter. To address the above issues, we propose an authorized searchable encryption scheme under a multi-authority setting. Specifically, our proposed scheme leverages the RSA function to enable each authority to limit the search capability of different clients based on clients’ privileges. To improve scalability, we utilize multi-authority attribute-based encryption to allow the authorization process to be performed only once even over policies from multiple authorities. We conduct rigorous security and cost analysis, and perform experimental evaluations to demonstrate that the proposed scheme introduces moderate overhead to existing searchable encryption schemes.

The immutability of blockchain is crucial to the security of many blockchain applications, while it is still desired or even legally obliged to allow for redacting the contents of blockchain for some scenarios. In this work, we revisit the conflict between the immutability and redaction of blockchain, and put forward a new fine-grained redactable blockchain with a semi-trusted regulator, who follows our protocol but has a tendency to abuse his power. To the best of our knowledge, it is the first blockchain that not only supports the supervision of blockchain content, but also allows users themselves to manage their own data. To this end, we introduce a new variant of chameleon-hash function, named stateful Chameleon Hash with Revocable Subkey, which is important for building our redactable blockchain and may be of independent interest. We also propose a black-box construction from standard chameleon-hash functions, and prove its security properties under our proposed security notions. At last, we provide a proof-of-concept implementation. The evaluation results demonstrate that our redactable blockchain is practical and can be adopted with small additional overhead compared to the immutable blockchain.

The primitive of Searchable Symmetric Encryption (SSE) allows a server to perform search directly over encrypted documents. Recently, forward secure SSE (FSSE) has attracted significant attention due to its secure property for document update. That is, it can prevent the server from learning whether the newly updated document contains previously queried keywords. Most of the existing FSSE schemes are designed for single keyword search. For the case of conjunctive keyword search, prior dedicated FSSE constructions either require inefficient tools or suffer from significant leakage (i.e., it leaks the documents matching each queried keywords). To deal with this challenge, we propose a general framework to design efficient conjunctive keyword FSSE schemes without significant leakage: a basic scheme FOXT-B and an enhanced one FOXT-E. Specifically, in our basic scheme, we combine XSet structure in the Oblivious Cross-Tags protocol (CRYPTO 2013) with a single keyword FSSE to support conjunctive keyword search. However, this simple combination is limited to support document update. That is, the user cannot perform search until all the keyword-document pairs in a document have been updated. To support flexible keyword-document update, we design a new XSet for our enhanced scheme, in which the user can perform search at any time. Finally, we present a formal security analysis and implement our schemes on a real-world database, which demonstrates that our proposed schemes can achieve the desired security goals with practical efficiency.

In this work, we introduce the first practical post-quantum verifiable random function (VRF) that relies on well-known (module) lattice problems, namely Module-SIS and Module-LWE. Our construction, named LB-VRF, results in a VRF value of only 84 bytes and a proof of around only 5 KB (in comparison to several MBs in earlier works), and runs in about 3 ms for evaluation and about 1 ms for verification.

Order-revealing encryption (ORE) is a cryptographic primitive that enables ciphertext comparison while leaking nothing about the underlying plaintext beyond their lexicographic ordering. However, how to achieve efficient and secure ciphertext comparison for multi-user settings is still a challenging problem. In this work, we propose an efficient multi-client order-revealing encryption scheme (named m-ORE) by introducing a new token-based comparison method. Specifically, data owner is enabled to delegate token generation ability to some authorized users without revealing his secret key, and then each authorized user can perform comparison on ciphertexts from multiple data owners by generating the associated comparison tokens. Benefiting from our new method, m-ORE can not only reduce ciphertext size but also improve comparison efficiency, compared with the state-of-the-art (Cash et al. Asiacrypt 2018). Further, we present a non-interactive multi-client range query scheme by extending m-ORE. Finally, we show a formal security analysis and implement our scheme. The evaluation result demonstrates that m-ORE outperforms the scheme by Cash et al. in terms of both query and storage cost while achieving the same level of security.

Network Function Virtualisation (NFV) advances the adoption of composable software middleboxes. Accordingly, cloud data centres become major NFV vendors for enterprise traffic processing. Due to the privacy concern of traffic redirection to the cloud, secure middlebox systems (e.g., BlindBox) draw much attention; they can process encrypted packets against encrypted rules directly. However, most of the existing systems supporting pattern matching based network functions require the enterprise gateway to tokenise packet payloads via sliding windows. Such tokenisation induces a considerable communication overhead, which can be over 100$\times$ to the packet size. To overcome this bottleneck, in this paper, we propose the first bandwidth-efficient encrypted pattern matching protocol for secure middleboxes. We resort to a primitive called symmetric hidden vector encryption (SHVE), and propose a variant of it, aka SHVE+, to achieve constant and moderate communication cost. To speed up, we devise encrypted filters to reduce the number of accesses to SHVE+ during matching highly. We formalise the security of our proposed protocol and conduct comprehensive evaluations over real-world rulesets and traffic dumps. The results show that our design can inspect a packet over 20k rules within 100 $\mu$s. Compared to prior work, it brings a saving of $94\%$ in bandwidth consumption., Comment: Accepted version of our work published in IEEE Transactions on Dependable and Secure Computing (TDSC, 2021)

Searchable encryption (SE) is one of the key enablers for building encrypted databases. It allows a cloud server to search over encrypted data without decryption. Dynamic SE additionally includes data addition and deletion operations to enrich the functions of encrypted databases. Recent attacks exploiting the leakage in dynamic operations drive the rapid development of SE schemes revealing less information while performing updates; they are also known as forward and backward private SE. Newly added data is no longer linkable to queries issued before, and deleted data is no longer searchable in queries issued later. However, those advanced SE schemes reduce the efficiency of SE, especially in the communication cost between the client and server. In this paper, we resort to the hardware-assisted solution, aka Intel SGX, to ease the above bottleneck. Our key idea is to leverage SGX to take over most tasks of the client, i.e., tracking keyword states along with data addition and caching deleted data. However, handling large datasets is non-trivial due to the I/O and memory constraints of SGX. We further develop batch data processing and state compression techniques to reduce the communication overhead between the SGX and untrusted server and minimise the memory footprint within the enclave. We conduct a comprehensive set of evaluations on both synthetic and real-world datasets, which confirm that our designs outperform the prior art.

In this paper, we propose the most efficient blockchain ring confidential transaction protocol (RingCT3.0) for protecting the privacy of the sender’s identity, the recipient’s identity and the confidentiality of the transaction amount. For a typical 2-input transaction with a ring size of 1024, the ring signature size of our RingCT3.0 protocol is 98% less than the ring signature size of the original RingCT1.0 protocol used in Monero. Taking the advantage of our compact RingCT3.0 transcript size, privacy-preserving cryptocurrencies can enjoy a much lower transaction fee which will have a significant impact on the crypto-economy.

We revisit the method of designing public-key puncturable encryption schemes and present a generic conversion by leveraging the techniques of distributed key-distribution and revocable encryption. In particular, we first introduce a refined version of identity-based revocable encryption, named key-homomorphic identity-based revocable key encapsulation mechanism with extended correctness. Then, we propose a generic construction of puncturable key encapsulation mechanism from the former by merging the idea of distributed key-distribution. Compared to the state-of-the-art, our generic construction supports unbounded number of punctures and multiple tags per message, thus achieving more fine-grained revocation of decryption capability. Further, it does not rely on random oracles, not suffer from non-negligible correctness error, and results in a variety of efficient schemes with distinct features. More precisely, we obtain the first scheme with very compact ciphertexts in the standard model, and the first scheme with support for both unbounded size of tags per ciphertext and unbounded punctures as well as constant-time puncture operation. Moreover, we get a comparable scheme proven secure under the standard DBDH assumption, which enjoys both faster encryption and decryption than previous works based on the same assumption, especially when the number of tags associated with the ciphertext is large.

In this work, we investigate how to protect public key encryption from both key-leakage attacks and tampering attacks. First, we formalize the notions of chosen ciphertext (CCA) security against key-leakage and tampering attacks. To this goal, we then introduce the concept of key-homomorphic hash proof systems and present a generic construction of public key encryption based on this new primitive. Our construction, compared with previous works, realizes leakage-resilience and tampering-resilience simultaneously but completely independently, so it can tolerate a larger amount of bounded-memory leakage and be instantiated with more flexibility. Moreover, it allows for an unbounded number of affine-tampering queries, even after the challenge phase. With slight adaptations, our construction also achieves CCA security against subexponentially hard auxiliary-input leakage attacks and a polynomial of affine-tampering attacks. Thus, to the best of our knowledge, we get the first public key encryption scheme secure against both auxiliary-input leakage attacks and tampering attacks.

Along with the development of Internet of Things (IoT) technology and Vehicular Ad-Hoc Networks (VANETs), security and effectiveness of road utilization have also been greatly improved. Currently, many certificateless signature schemes have been proposed to improve the communication speed while protecting the communication contents and real identities of the vehicles. Among others, some schemes realize batch verification or aggregation of multiple signatures. However, they either incur high calculation costs or cannot execute the operation of batch verification successfully. In this paper, we propose an efficient and privacy-preserving certificateless public key signature scheme based on the blockchain. Firstly, our scheme provides support for batch verification and aggregation of the signatures, and has lower calculation cost compared with the previous ones. Secondly, our scheme adds two blockchains to the structure in order to protect the identity privacy of the vehicles. Utilizing the immutability and openness of the blockchain, all nodes in VANETs can verify whether the identity of the vehicle is legal. Therefore, the scheme can greatly reduce the risk of privacy disclosure, and the pseudo-identities can be generated by the vehicles themselves. Thirdly, our scheme can realize traceability of the vehicles, which means the real identities of the vehicles can be revealed if they commit an illegal behavior. The experimental results show that our scheme can greatly reduce the computational cost of the vehicles and provide the functions of batch verification and aggregation. Therefore, the proposed scheme can simultaneously improve security and efficiency of the signature schemes for VANETs.

As a special type of fault injection attacks, Related-Key Attacks (RKAs) allow an adversary to manipulate a cryptographic key and subsequently observe the outcomes of the cryptographic scheme under these modified keys. In the real life, related-key attacks are already practical enough to be implemented on cryptographic devices. To avoid cryptographic devices suffering from related-key attacks, it is necessary to design a cryptographic scheme that resists against such attacks. This paper proposes an efficient RKA-secure Key Encapsulation Mechanism (KEM), in which the adversary can modify the secret key sk to any value f ( sk ), as long as, f is a polynomial function of a bounded degree d . Especially, the polynomial-RKA security can be reduced to a hard search problem, namely d -extended computational Bilinear Diffie-Hellman (BDH) problem, in the standard model. Our construction essentially refines the security of Haralambiev et al.’s BDH-based KEM scheme from chosen-ciphertext security to related-key security. The main technique applied in our scheme is the re-computation of the public key in the decryption algorithm so that any (non-trivial) modification to the secret key can be detected.

The authors present a new general construction of public key encryption (PKE) based on the restricted subset membership (RSM) assumption, which can achieve the bounded-memory leakage resilient security and the auxiliary-input leakage resilient security simultaneously. The construction is BHHO-type, as Brakerski et al. work, but the message space is much larger and the proof is more concise benefiting from the RSM assumption. Instantiating the construction with the QR assumption, the authors get the first QR-based auxiliary-input secure PKE with a larger message space than {0,1}. Moreover, the authors generalise the Goldreich–Levin theorem to large rings. This theorem helps to improve the construction to achieve the same security level with fewer public parameters and shorter ciphertexts compared with Brakerski et al. work. For the bounded-memory leakage resilient security, the construction can achieve leakage rate of 1 − o(1) and avoid the dependence between the message length and the amount of leakage. Based on the general construction, the authors also can achieve both bounded-memory leakage resilient chosen ciphertext attack (CCA) security and the auxiliary-input leakage resilient CCA security via the well-known Naor–Yung paradigm.

Most cryptocurrencies have successfully provided anonymity in a permissionless environment. However, the pattern of transfers is open to publicity. To face this issue, Monero was proposed to provide untraceability from ring signatures by introducing mixins to obfuscate addresses. By temporal analysis, however, the transfer pattern can still be partially revealed in a stochastic approach due to inappropriate selections of mixins. Thereby, each flow of coins can be traced with high probability which disobeys the untraceability principle of Monero. In this work, we propose a hard-to-trace protocol based on Monero where each transaction output is assembled into a fixed ring set. In this way, inappropriate mixins are forbidden, and thereby the temporal analysis is resisted. Apart from the traceability issue, Monero is also challenged due to its growing difficulty of block assembly. To guarantee the privacy, "key images" with a considerable size have to be stored by each miner to verify transactions and assemble blocks. As blockchain grows, the number of key images increases and a significant burden has already been caused, making the block assembly of Monero inefficient to most miners. Aimed at a more practical block assembly, our protocol allows key image truncations to facilitate transaction verifications.

With features such as decentralization, consistency, tamper resistance, non-repudiation, and pseudonym, blockchain technology has the potential to strengthen the Internet of Things (IoT) significantly, thus opening an intriguing research area in the integration of blockchain and IoT. However, most existing blockchain schemes were not dedicated to the IoT ecosystem and hence could not meet the specific requirements of IoT. This paper aims to fix the gap. Inspired by Chainspace, a blockchain platform which could be applicable in IoT, VChain is proposed, a novel blockchain scheme suitable for IoT which is more secure, concrete, and practical compared with Chainspace. Specifically, in VChain, a two-layer BFT-based consensus protocol with HoneyBadger BFT protocol is proposed and a collective signature scheme as building blocks. The designs above allow for supporting faulty-shards-tolerance and asynchronous network model, which could not be sustained in Chainspace, and keeping high efficiency as well. Moreover, the sharding strategy presented in VChain, different from that in RapidChain, which adopts the energy-consuming PoW mechanism for sharding, is environmentfriendly and thus makes VChain fit for IoT well. Last but not least, VChain also inherits the merits of Chainspace to separate the execution and verification of smart contracts for privacy.

In this paper, we propose GraphSE\textsuperscript2, an encrypted graph database for online social network services to address massive data breaches. GraphSE\textsuperscript2 ~preserves the functionality of social search, a key enabler for quality social network services, where social search queries are conducted on a large-scale social graph and meanwhile perform set and computational operations on user-generated contents. To enable efficient privacy-preserving social search, GraphSE\textsuperscript2 ~provides an encrypted structural data model to facilitate parallel and encrypted graph data access. It is also designed to decompose complex social search queries into atomic operations and realise them via interchangeable protocols in a fast and scalable manner. We build GraphSE\textsuperscript2 ~with various queries supported in the Facebook graph search engine and implement a full-fledged prototype. Extensive evaluations on Azure Cloud demonstrate that GraphSE\textsuperscript2 ~is practical for querying a social graph with a million of users.

We revisit the problem of constructing public key encryption (PKE) secure against both key-leakage and tampering attacks. First, we present an enhanced security against both kinds of attacks, namely strong leakage and tamper-resilient chosen-ciphertext (sLTR-CCA) security, which imposes only minimal restrictions on the adversary’s queries and thus captures the capability of the adversary in a more reasonable way. Then, we propose a generic paradigm achieving this security on the basis of a refined hash proof system (HPS) called public-key-malleable HPS. The paradigm can not only tolerate a large amount of bounded key-leakage, but also resist an arbitrary polynomial of restricted tampering attacks, even depending on the challenge phase. Moreover, the paradigm with slight adaptations can also be proven sLTR-CCA secure with respect to subexponentially hard auxiliary-input leakage. In addition, we instantiate our paradigm under certain standard number-theoretic assumptions, and thus, to our best knowledge, obtain the first efficient PKE schemes possessing the strong bounded/auxiliary-input leakage and tamper-resilient chosen-ciphertext security in the standard model.

Group signatures are considered as one of the most prominent cryptographic primitives to ensure privacy. In essence, group signatures ensure the authenticity of messages while the author of the message remains anonymous. In this study, we propose a dynamic post-quantum group signature (GS) extending the static G-Merkle group signature (PQCRYPTO 2018). In particular, our dynamic G-Merkle (DGM) allows new users to join the group at any time. Similar to G-Merkle scheme, our DGM only involves symmetric primitives and makes use of a One-Time Signature scheme (OTS). Each member of the group receives a certain amount of OTS key pairs and can ask the Manager \(\mathcal {M}\) for more if needed. Our DGM also provides an innovative way of signing revocation by employing Symmetric Puncturable Encryption (SPE) recently appeared in (ACM CCS 2018). DGM provides a significantly smaller signature size than other GSs based on symmetric primitives and also reduces the influence of the number of group members on the signature size and on the limitations of the application of G-Merkle.

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.

Dynamic Searchable Symmetric Encryption (DSSE) enables a client to perform updates and searches on encrypted data which makes it very useful in practice. To protect DSSE from the leakage of updates (leading to break query or data privacy), two new security notions, forward and backward privacy, have been proposed recently. Although extensive attention has been paid to forward privacy, this is not the case for backward privacy. Backward privacy, first formally introduced by Bost et al., is classified into three types from weak to strong, exactly Type-III to Type-I. To the best of our knowledge, however, no practical DSSE schemes without trusted hardware (e.g. SGX) have been proposed so far, in terms of the strong backward privacy and constant roundtrips between the client and the server.

We consider the need for security while providing services that are comparable to that of traditional applications to fully exploit cloud services to its fullest potential. While Dynamic Searchable Symmetric Encryption (DSSE) supports such needs, we want to be able to protect against file-injection attacks. Hence, we require forward privacy and a scheme which allows for a wide range of searching capabilities. We propose an extension, based on the RSA problem, to a DSSE scheme that supports range queries allowing the scheme to also support multiple clients. Furthermore, we describe how we can further manage clients using Attribute-Based Encryption (ABE) such that clients cannot decrypt ciphertexts that fall outside of their access rights.

Social applications are becoming one of the most popular applications for users to share data and communicate online. These applications deal with a lot of personal data, e.g., users' locations, interests and documents stored on the remote cloud storage servers. Therefore, we need to pay a deeper attention to data confidentiality and privacy. To address the problem of data confidentiality, existing solutions usually count the security requirement of data owner for data sharing in social applications. However, on the side of the data consumer or member, we want to securely and efficiently get our own interested data. Both the data owner and the member are two roles of users in data sharing applications, and there are little existing research efforts to investigate the implementation of achieving both of their requirements at the same time. In this paper, we propose DASS, a privacy-preserving DAta Sharing Scheme to comprehensively satisfy users' security requirements for social applications. Our solution consists of a fine-grained access control scheme, a dynamic social attribute management model, and a multi-user searchable encryption scheme. We have described our scheme and provided performance evaluation. Validations are done to demonstrate that our scheme is secure, fine-grained and efficient.

Compared with traditional X-ray imaging systems based on polycapillary X-ray optics or a pinhole, coded aperture imaging system has the advantages in simple structure, high sensitivity, and strong expandability, which make it possess the potential applications in X-ray fluorescence imaging. In this work, a new coded aperture X-ray imaging system based on a novel imaging model which decomposes the mask projections into a superposition of two separable functions is designed and proposed for high-resolution X-ray imaging. The performance of the system is demonstrated by using the Geant4 package. To reduce the computational complexity of calibration and image reconstruction, a separable mask with 90 × 90 pixels is used. The mask is designed by selecting the central part of the original rank 463 modified uniformly redundant arrays. The mask is made of platinum foil with a pixel pitch of 25 microns. To study the effect of mask thickness on system performance, the mask thickness is varied from 25 to 200 microns. The active area of the Si detector employed in the system is 2 mm × 2 mm, divided into 80 × 80 pixels, each with a size of 25 μm × 25 μm. The field of view of the system is equal to the area of the detector, which is 2 mm × 2 mm. The detector is parallel to and center-aligned with the mask with a fixed distance of 2.0 mm. The images are reconstructed by using the fast iterative shrinkage-thresholding algorithm. The high-quality reconstructed images of different energy line sources and complex shaped objects are obtained. The simulation and analysis results indicate that for the near-field imaging, unlike imaging systems based on the conventional convolution model, the system has the performance that is not affected by the aperture collimation effect. The spatial resolution of the imaging system is about 65 microns. The calibrated matrices used have an important influence on the image quality. The quality of the reconstructed image is affected by the energy of X-rays used during calibration and the energy of X-rays emitted from the object; the smaller the difference between these two energy values, the higher the quality of the reconstructed images will be. The three-dimensional reconstruction results show that the system can correctly estimate the distance between the object and the system from a single two-dimensional projection. The axial spatial resolution of the system is about 1.1 mm.