Confidential database-as-a-service approaches: taxonomy and survey
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paper_title: Key management for multi-user encrypted databases

paper_content:

Database outsourcing is becoming increasingly popular introducing a new paradigm, called database-as-a-service (DAS), where an organization's database is stored at an external service provider. In such a scenario, access control is a very important issue, especially if the data owner wishes to publish her data for external use.In this paper, we first present our approach for the implementation of access control through selective encryption. The focus of the paper is then the presentation of the experimental results, which demonstrate the applicability of our proposal.
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paper_title: A Quantitative Analysis of Current Security Concerns and Solutions for Cloud Computing

paper_content:

The development of cloud computing services is speeding up the rate in which the organizations outsource their computational services or sell their idle computational resources. Even though migrating to the cloud remains a tempting trend from a financial perspective, there are several other aspects that must be taken into account by companies before they decide to do so. One of the most important aspect refers to security: while some cloud computing security issues are inherited from the solutions adopted to create such services, many new security questions that are particular to these solutions also arise, including those related to how the services are organized and which kind of service/data can be placed in the cloud. Aiming to give a better understanding of this complex scenario, in this article we identify and classify the main security concerns and solutions in cloud computing, and propose a taxonomy of security in cloud computing, giving an overview of the current status of security in this emerging technology.
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paper_title: Pors: proofs of retrievability for large files

paper_content:

In this paper, we define and explore proofs of retrievability (PORs). A POR scheme enables an archive or back-up service (prover) to produce a concise proof that a user (verifier) can retrieve a target file F, that is, that the archive retains and reliably transmits file data sufficient for the user to recover F in its entirety. A POR may be viewed as a kind of cryptographic proof of knowledge (POK), but one specially designed to handle a large file (or bitstring) F. We explore POR protocols here in which the communication costs, number of memory accesses for the prover, and storage requirements of the user (verifier) are small parameters essentially independent of the length of F. In addition to proposing new, practical POR constructions, we explore implementation considerations and optimizations that bear on previously explored, related schemes. In a POR, unlike a POK, neither the prover nor the verifier need actually have knowledge of F. PORs give rise to a new and unusual security definition whose formulation is another contribution of our work. We view PORs as an important tool for semi-trusted online archives. Existing cryptographic techniques help users ensure the privacy and integrity of files they retrieve. It is also natural, however, for users to want to verify that archives do not delete or modify files prior to retrieval. The goal of a POR is to accomplish these checks without users having to download the files themselves. A POR can also provide quality-of-service guarantees, i.e., show that a file is retrievable within a certain time bound.
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paper_title: Trust mechanisms for cloud computing

paper_content:

Trust is a critical factor in cloud computing; in present practice it depends largely on perception of reputation, and self assessment by providers of cloud services. We begin this paper with a survey of existing mechanisms for establishing trust, and comment on their limitations. We then address those limitations by proposing more rigorous mechanisms based on evidence, attribute certification, and validation, and conclude by suggesting a framework for integrating various trust mechanisms together to reveal chains of trust in the cloud.
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paper_title: Enhancing Data Integrity and Privacy in the Cloud: An Agenda

paper_content:

To address users' security concerns, cloud service providers must implement strategies that enhance data integrity and privacy. New data auditing and encryption methods open a path toward these goals.
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paper_title: Authentication and integrity in outsourced databases

paper_content:

In the Outsourced Database (ODB) model, entities outsource their data management needs to a third-party service provider. Such a service provider offers mechanisms for its clients to create, store, update, and access (query) their databases. This work provides mechanisms to ensure data integrity and authenticity for outsourced databases. Specifically, this article provides mechanisms that assure the querier that the query results have not been tampered with and are authentic (with respect to the actual data owner). It investigates both the security and efficiency aspects of the problem and constructs several secure and practical schemes that facilitate the integrity and authenticity of query replies while incurring low computational and communication costs.
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paper_title: Securus: From Confidentiality and Access Requirements to Data Outsourcing Solutions

paper_content:

To preserve data confidentiality in database outsourcing scenarios, various techniques have been proposed that preserve a certain degree of confidentiality while still allowing to efficiently execute certain queries. Typically, several of those techniques have to be combined to achieve a certain degree of confidentiality. However, finding an appropriate combination is not a trivial task, as expert knowledge is required and interdependencies between the techniques exist. Securus, an approach we previously proposed, addresses this problem. Securus allows users to model their requirements regarding the information in the outsourced dataset that has to be protected. Furthermore, queries that have to be efficiently executable on the outsourced data can be specified. Based on these requirements, Securus uses Integer Linear Programming (ILP) to find a suitable combination of confidentiality enhancing techniques and generates a software adapter. This software adapter transparently applies the techniques to fulfill the specified requirements and can be used to seamlessly outsource and query the data. In this paper, we present an outline of Securus and extend our previous work by highlighting the differences to other approaches in the field. Furthermore, we show how Securus can be extended to allow for more efficient solutions if the attacker’s capabilities can be modeled by the user.
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paper_title: CryptDB: protecting confidentiality with encrypted query processing

paper_content:

Online applications are vulnerable to theft of sensitive information because adversaries can exploit software bugs to gain access to private data, and because curious or malicious administrators may capture and leak data. CryptDB is a system that provides practical and provable confidentiality in the face of these attacks for applications backed by SQL databases. It works by executing SQL queries over encrypted data using a collection of efficient SQL-aware encryption schemes. CryptDB can also chain encryption keys to user passwords, so that a data item can be decrypted only by using the password of one of the users with access to that data. As a result, a database administrator never gets access to decrypted data, and even if all servers are compromised, an adversary cannot decrypt the data of any user who is not logged in. An analysis of a trace of 126 million SQL queries from a production MySQL server shows that CryptDB can support operations over encrypted data for 99.5% of the 128,840 columns seen in the trace. Our evaluation shows that CryptDB has low overhead, reducing throughput by 14.5% for phpBB, a web forum application, and by 26% for queries from TPC-C, compared to unmodified MySQL. Chaining encryption keys to user passwords requires 11--13 unique schema annotations to secure more than 20 sensitive fields and 2--7 lines of source code changes for three multi-user web applications.
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paper_title: CryptDB: protecting confidentiality with encrypted query processing

paper_content:

Online applications are vulnerable to theft of sensitive information because adversaries can exploit software bugs to gain access to private data, and because curious or malicious administrators may capture and leak data. CryptDB is a system that provides practical and provable confidentiality in the face of these attacks for applications backed by SQL databases. It works by executing SQL queries over encrypted data using a collection of efficient SQL-aware encryption schemes. CryptDB can also chain encryption keys to user passwords, so that a data item can be decrypted only by using the password of one of the users with access to that data. As a result, a database administrator never gets access to decrypted data, and even if all servers are compromised, an adversary cannot decrypt the data of any user who is not logged in. An analysis of a trace of 126 million SQL queries from a production MySQL server shows that CryptDB can support operations over encrypted data for 99.5% of the 128,840 columns seen in the trace. Our evaluation shows that CryptDB has low overhead, reducing throughput by 14.5% for phpBB, a web forum application, and by 26% for queries from TPC-C, compared to unmodified MySQL. Chaining encryption keys to user passwords requires 11--13 unique schema annotations to secure more than 20 sensitive fields and 2--7 lines of source code changes for three multi-user web applications.
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paper_title: Efficient and Private Access to Outsourced Data

paper_content:

As the use of external storage and data processing services for storing and managing sensitive data becomes more and more common, there is an increasing need for novel techniques that support not only data confidentiality, but also confidentiality of the accesses that users make on such data. In this paper, we propose a technique for guaranteeing content, access, and pattern confidentiality in the data outsourcing scenario. The proposed technique introduces a shuffle index structure, which adapts traditional B+-trees. We show that our solution exhibits a limited performance cost, thus resulting effectively usable in practice.
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paper_title: Distributed Shuffling for Preserving Access Confidentiality

paper_content:

The shuffle index has been recently proposed for organizing and accessing data in outsourcing scenarios while protecting the confidentiality of the data as well as of the accesses to them. In this paper, we extend the shuffle index to the use of multiple servers for storing data, introducing a new protection technique (shadow) and enriching the original ones by operating in a distributed scenario. Our distributed shuffle index produces a significant increase in the protection of the system, with no additional costs.
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paper_title: Modeling and assessing inference exposure in encrypted databases

paper_content:

The scope and character of today's computing environments are progressively shifting from traditional, one-on-one client-server interaction to the new cooperative paradigm. It then becomes of primary importance to provide means of protecting the secrecy of the information, while guaranteeing its availability to legitimate clients. Operating online querying services securely on open networks is very difficult; therefore many enterprises outsource their data center operations to external application service providers. A promising direction toward prevention of unauthorized access to outsourced data is represented by encryption. However, data encryption is often supported for the sole purpose of protecting the data in storage while allowing access to plaintext values by the server, which decrypts data for query execution. In this paper, we present a simple yet robust single-server solution for remote querying of encrypted databases on external servers. Our approach is based on the use of indexing information attached to the encrypted database, which can be used by the server to select the data to be returned in response to a query without the need of accessing the plaintext database content. Our indexes balance the trade-off between efficiency requirements in query execution and protection requirements due to possible inference attacks exploiting indexing information. We investigate quantitative measures to model inference exposure and provide some related experimental results.
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paper_title: Balancing confidentiality and efficiency in untrusted relational DBMSs

paper_content:

The scope and character of today's computing environments are progressively shifting from traditional, one-on-one client-server interaction to the new cooperative paradigm. It then becomes of primary importance to provide means of protecting the secrecy of the information, while guaranteeing its availability to legitimate clients. Operating on-line querying services securely on open networks is very difficult; therefore many enterprises outsource their data center operations to external application service providers. A promising direction towards prevention of unauthorized access to outsourced data is represented by encryption. However, data encryption is often supported for the sole purpose of protecting the data in storage and assumes trust in the server, that decrypts data for query execution.In this paper, we present a simple yet robust single-server solution for remote querying of encrypted databases on untrusted servers. Our approach is based on the use of indexing information attached to the encrypted database which can be used by the server to select the data to be returned in response to a query without the need of disclosing the database content. Our indexes balance the trade off between efficiency requirements in query execution and protection requirements due to possible inference attacks exploiting indexing information. We also investigate quantitative measures to model inference exposure and provide some related experimental results.
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paper_title: Executing SQL over encrypted data in the database-service-provider model

paper_content:

Rapid advances in networking and Internet technologies have fueled the emergence of the "software as a service" model for enterprise computing. Successful examples of commercially viable software services include rent-a-spreadsheet, electronic mail services, general storage services, disaster protection services. "Database as a Service" model provides users power to create, store, modify, and retrieve data from anywhere in the world, as long as they have access to the Internet. It introduces several challenges, an important issue being data privacy. It is in this context that we specifically address the issue of data privacy.There are two main privacy issues. First, the owner of the data needs to be assured that the data stored on the service-provider site is protected against data thefts from outsiders. Second, data needs to be protected even from the service providers, if the providers themselves cannot be trusted. In this paper, we focus on the second challenge. Specifically, we explore techniques to execute SQL queries over encrypted data. Our strategy is to process as much of the query as possible at the service providers' site, without having to decrypt the data. Decryption and the remainder of the query processing are performed at the client site. The paper explores an algebraic framework to split the query to minimize the computation at the client site. Results of experiments validating our approach are also presented.
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paper_title: Secure multidimensional range queries over outsourced data

paper_content:

In this paper, we study the problem of supporting multidimensional range queries on encrypted data. The problem is motivated by secure data outsourcing applications where a client may store his/her data on a remote server in encrypted form and want to execute queries using server's computational capabilities. The solution approach is to compute a secure indexing tag of the data by applying bucketization (a generic form of data partitioning) which prevents the server from learning exact values but still allows it to check if a record satisfies the query predicate. Queries are evaluated in an approximate manner where the returned set of records may contain some false positives. These records then need to be weeded out by the client which comprises the computational overhead of our scheme. We develop a bucketization procedure for answering multidimensional range queries on multidimensional data. For a given bucketization scheme, we derive cost and disclosure-risk metrics that estimate client's computational overhead and disclosure risk respectively. Given a multidimensional dataset, its bucketization is posed as an optimization problem where the goal is to minimize the risk of disclosure while keeping query cost (client's computational overhead) below a certain user-specified threshold value. We provide a tunable data bucketization algorithm that allows the data owner to control the trade-off between disclosure risk and cost. We also study the trade-off characteristics through an extensive set of experiments on real and synthetic data.
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paper_title: Order preserving encryption for numeric data

paper_content:

Encryption is a well established technology for protecting sensitive data. However, once encrypted, data can no longer be easily queried aside from exact matches. We present an order-preserving encryption scheme for numeric data that allows any comparison operation to be directly applied on encrypted data. Query results produced are sound (no false hits) and complete (no false drops). Our scheme handles updates gracefully and new values can be added without requiring changes in the encryption of other values. It allows standard databse indexes to be built over encrypted tables and can easily be integrated with existing database systems. The proposed scheme has been designed to be deployed in application environments in which the intruder can get access to the encrypted database, but does not have prior domain information such as the distribution of values and annot encrypt or decrypt arbitrary values of his choice. The encryption is robust against estimation of the true value in such environments.
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paper_title: Order-Preserving Symmetric Encryption

paper_content:

We initiate the cryptographic study of order-preserving symmetric encryption (OPE), a primitive suggested in the database community by Agrawal et al. (SIGMOD '04) for allowing efficient range queries on encrypted data. Interestingly, we first show that a straightforward relaxation of standard security notions for encryption such as indistinguishability against chosen-plaintext attack (IND-CPA) is unachievable by a practical OPE scheme. Instead, we propose a security notion in the spirit of pseudorandom functions (PRFs) and related primitives asking that an OPE scheme look "as-random-as-possible" subject to the order-preserving constraint. We then design an efficient OPE scheme and prove its security under our notion based on pseudorandomness of an underlying blockcipher. Our construction is based on a natural relation we uncover between a random order-preserving function and the hypergeometric probability distribution. In particular, it makes black-box use of an efficient sampling algorithm for the latter.
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paper_title: Privacy Preserving Keyword Searches on Remote Encrypted Data

paper_content:

We consider the following problem: a user $\mathcal{U}$ wants to store his files in an encrypted form on a remote file server $\mathcal{S}$. Later the user $\mathcal{U}$ wants to efficiently retrieve some of the encrypted files containing (or indexed by) specific keywords, keeping the keywords themselves secret and not jeopardizing the security of the remotely stored files. For example, a user may want to store old e-mail messages encrypted on a server managed by Yahoo or another large vendor, and later retrieve certain messages while travelling with a mobile device. ::: ::: In this paper, we offer solutions for this problem under well-defined security requirements. Our schemes are efficient in the sense that no public-key cryptosystem is involved. Indeed, our approach is independent of the encryption method chosen for the remote files. They are also incremental, in that $\mathcal{U}$ can submit new files which are secure against previous queries but still searchable against future queries.
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paper_title: Dynamic searchable symmetric encryption

paper_content:

Searchable symmetric encryption (SSE) allows a client to encrypt its data in such a way that this data can still be searched. The most immediate application of SSE is to cloud storage, where it enables a client to securely outsource its data to an untrusted cloud provider without sacrificing the ability to search over it. SSE has been the focus of active research and a multitude of schemes that achieve various levels of security and efficiency have been proposed. Any practical SSE scheme, however, should (at a minimum) satisfy the following properties: sublinear search time, security against adaptive chosen-keyword attacks, compact indexes and the ability to add and delete files efficiently. Unfortunately, none of the previously-known SSE constructions achieve all these properties at the same time. This severely limits the practical value of SSE and decreases its chance of deployment in real-world cloud storage systems. To address this, we propose the first SSE scheme to satisfy all the properties outlined above. Our construction extends the inverted index approach (Curtmola et al., CCS 2006) in several non-trivial ways and introduces new techniques for the design of SSE. In addition, we implement our scheme and conduct a performance evaluation, showing that our approach is highly efficient and ready for deployment.
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paper_title: Privacy-preserving data analytics as an outsourced service

paper_content:

Two sets of privacy requirements need to be fulfilled when a company's accounting data is audited by an external party: the company needs to safeguard its data, while the auditors do not want to reveal their investigation methods. This problem is usually addressed by physically isolating data and auditors during the course of an audit. This approach however no longer works when auditing is performed remotely. We present a searchable encryption scheme for outsourcing data analytics. In our scheme the data owner needs to encrypt his data only once and ship it in encrypted form to the data analyst. The data analyst can then perform a series of queries for which he must ask the data owner for help in translating the constants in the queries. Our searchable encryption schemes allows keyword searches and range queries. Furthermore it allows queries to reuse the results of previous queries as tokens and thereby make dependent queries without interaction. Nevertheless our scheme is provably secure.
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paper_title: Practical techniques for searches on encrypted data

paper_content:

It is desirable to store data on data storage servers such as mail servers and file servers in encrypted form to reduce security and privacy risks. But this usually implies that one has to sacrifice functionality for security. For example, if a client wishes to retrieve only documents containing certain words, it was not previously known how to let the data storage server perform the search and answer the query, without loss of data confidentiality. We describe our cryptographic schemes for the problem of searching on encrypted data and provide proofs of security for the resulting crypto systems. Our techniques have a number of crucial advantages. They are provably secure: they provide provable secrecy for encryption, in the sense that the untrusted server cannot learn anything about the plaintext when only given the ciphertext; they provide query isolation for searches, meaning that the untrusted server cannot learn anything more about the plaintext than the search result; they provide controlled searching, so that the untrusted server cannot search for an arbitrary word without the user's authorization; they also support hidden queries, so that the user may ask the untrusted server to search for a secret word without revealing the word to the server. The algorithms presented are simple, fast (for a document of length n, the encryption and search algorithms only need O(n) stream cipher and block cipher operations), and introduce almost no space and communication overhead, and hence are practical to use today.
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paper_title: Distributed Shuffling for Preserving Access Confidentiality

paper_content:

The shuffle index has been recently proposed for organizing and accessing data in outsourcing scenarios while protecting the confidentiality of the data as well as of the accesses to them. In this paper, we extend the shuffle index to the use of multiple servers for storing data, introducing a new protection technique (shadow) and enriching the original ones by operating in a distributed scenario. Our distributed shuffle index produces a significant increase in the protection of the system, with no additional costs.
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paper_title: Modeling and assessing inference exposure in encrypted databases

paper_content:

The scope and character of today's computing environments are progressively shifting from traditional, one-on-one client-server interaction to the new cooperative paradigm. It then becomes of primary importance to provide means of protecting the secrecy of the information, while guaranteeing its availability to legitimate clients. Operating online querying services securely on open networks is very difficult; therefore many enterprises outsource their data center operations to external application service providers. A promising direction toward prevention of unauthorized access to outsourced data is represented by encryption. However, data encryption is often supported for the sole purpose of protecting the data in storage while allowing access to plaintext values by the server, which decrypts data for query execution. In this paper, we present a simple yet robust single-server solution for remote querying of encrypted databases on external servers. Our approach is based on the use of indexing information attached to the encrypted database, which can be used by the server to select the data to be returned in response to a query without the need of accessing the plaintext database content. Our indexes balance the trade-off between efficiency requirements in query execution and protection requirements due to possible inference attacks exploiting indexing information. We investigate quantitative measures to model inference exposure and provide some related experimental results.
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paper_title: Enhancing access privacy of range retrievals over B+-trees

paper_content:

Users of databases that are hosted on shared servers cannot take for granted that their queries will not be disclosed to unauthorized parties. Even if the database is encrypted, an adversary who is monitoring the I/O activity on the server may still be able to infer some information about a user query. For the particular case of a B+-tree that has its nodes encrypted, we identify properties that enable the ordering among the leaf nodes to be deduced. These properties allow us to construct adversarial algorithms to recover the B+-tree structure from the I/O traces generated by range queries. Combining this structure with knowledge of the key distribution (or the plaintext database itself), the adversary can infer the selection range of user queries. To counter the threat, we propose a privacy-enhancing PB+-tree index which ensures that there is high uncertainty about what data the user has worked on, even to a knowledgeable adversary who has observed numerous query executions. The core idea in PB+-tree is to conceal the order of the leaf nodes in an encrypted B+-tree. In particular, it groups the nodes of the tree into buckets, and employs homomorphic encryption techniques to prevent the adversary from pinpointing the exact nodes retrieved by range queries. PB+-tree can be tuned to balance its privacy strength with the computational and I/O overheads incurred. Moreover, it can be adapted to protect access privacy in cases where the attacker additionally knows a priori the access frequencies of key values. Experiments demonstrate that PB+-tree effectively impairs the adversary's ability to recover the B+-tree structure and deduce the query ranges in all considered scenarios.
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paper_title: Balancing confidentiality and efficiency in untrusted relational DBMSs

paper_content:

The scope and character of today's computing environments are progressively shifting from traditional, one-on-one client-server interaction to the new cooperative paradigm. It then becomes of primary importance to provide means of protecting the secrecy of the information, while guaranteeing its availability to legitimate clients. Operating on-line querying services securely on open networks is very difficult; therefore many enterprises outsource their data center operations to external application service providers. A promising direction towards prevention of unauthorized access to outsourced data is represented by encryption. However, data encryption is often supported for the sole purpose of protecting the data in storage and assumes trust in the server, that decrypts data for query execution.In this paper, we present a simple yet robust single-server solution for remote querying of encrypted databases on untrusted servers. Our approach is based on the use of indexing information attached to the encrypted database which can be used by the server to select the data to be returned in response to a query without the need of disclosing the database content. Our indexes balance the trade off between efficiency requirements in query execution and protection requirements due to possible inference attacks exploiting indexing information. We also investigate quantitative measures to model inference exposure and provide some related experimental results.
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paper_title: Two Can Keep A Secret: A Distributed Architecture for Secure Database Services

paper_content:

Recent trends towards database outsourcing, as well as concerns and laws governing data privacy, have led to great interest in enabling secure database services. Previous approaches to enabling such a service have been based on data encryption, causing a large overhead in query processing. We propose a new, distributed architecture that allows an organization to outsource its data management to {\em two} untrusted servers while preserving data privacy. We show how the presence of two servers enables efficient partitioning of data so that the contents at any one server are guaranteed not to breach data privacy. We show how to optimize and execute queries in this architecture, and discuss new challenges that emerge in designing the database schema.
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paper_title: Distributing data for secure database services

paper_content:

The advent of database services has resulted in privacy concerns on the part of the client storing data with third party database service providers. Previous approaches to enabling such a service have been based on data encryption, causing a large overhead in query processing. A distributed architecture for secure database services is proposed as a solution to this problem where data is stored at multiple servers. The distributed architecture provides both privacy as well as fault tolerance to the client. In this paper we provide algorithms for (1) distributing data: our results include hardness of approximation results and hence a heuristic greedy algorithm for the distribution problem (2) partitioning the query at the client to queries for the servers is done by a bottom up state based algorithm. Finally the results at the servers are integrated to obtain the answer at the client. We provide an experimental validation and performance study of our algorithms.
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paper_title: Preserving Privacy in Data Outsourcing

paper_content:

Privacy requirements have an increasing impact on the realization of modern applications. Commercial and legal regulations demand that privacy guarantees be provided whenever sensitive information is stored, processed, or communicated to external parties. Current approaches encrypt sensitive data, thus reducing query execution efficiency and preventing selective information release. Preserving Privacy in Data Outsourcing presents a comprehensive approach for protecting highly sensitive information when it is stored on systems that are not under the data owner's control. The approach illustrated combines access control and encryption, enforcing access control via structured encryption. This solution, coupled with efficient algorithms for key derivation and distribution, provides efficient and secure authorization management on outsourced data, allowing the data owner to outsource not only the data but the security policy itself. To reduce the amount of data to be encrypted the book also investigates data fragmentation as a possible way to protect privacy of data associations and provide fragmentation as a complementary means for protecting privacy: associations broken by fragmentation will be visible only to users authorized (by knowing the proper key) to join fragments. The book finally investigates the problem of executing queries over possible data distributed at different servers and which must be controlled to ensure sensitive information and sensitive associations be visible only to parties authorized for that. Case Studies are provided throughout the book. Privacy, data mining, data protection, data outsourcing, electronic commerce, machine learning professionals and others working in these related fields will find this book a valuable asset, as well as primary associations such as ACM, IEEE and Management Science. This book is also suitable for advanced level students and researchers concentrating on computer science as a secondary text or reference book.
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paper_title: Efficient Execution of Aggregation Queries over Encrypted Relational Databases

paper_content:

Encryption is a common method to assure privacy of stored data. In many practical situations, decrypting data before applying logic compromises privacy. The challenge is to come up with logic transformation techniques and result-mapping methods so that the exact result of applying logic to data-in-the-clear is obtained by applying the transformed logic to encrypted data and mapping the result produced. In the scope of relational aggregation queries and in the presence of logical predicates, we show how to support needed transformations and mappings.
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paper_title: Public-Key Cryptosystems Based on Composite Degree Residuosity Classes

paper_content:

This paper investigates a novel computational problem, namely the Composite Residuosity Class Problem, and its applications to public-key cryptography. We propose a new trapdoor mechanism and derive from this technique three encryption schemes : a trapdoor permutation and two homomorphic probabilistic encryption schemes computationally comparable to RSA. Our cryptosystems, based on usual modular arithmetics, are provably secure under appropriate assumptions in the standard model.
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paper_title: Aggregation Queries in the Database-As-a-Service Model

paper_content:

In the Database-As-a-Service (DAS) model, clients store their database contents at servers belonging to potentially untrusted service providers. To maintain data confidentiality, clients need to outsource their data to servers in encrypted form. At the same time, clients must still be able to execute queries over encrypted data. One prominent and fairly effective technique for executing SQL-style range queries over encrypted data involves partitioning (or bucketization) of encrypted attributes. ::: ::: However, executing aggregation-type queries over encrypted data is a notoriously difficult problem. One well-known cryptographic tool often utilized to support encrypted aggregation is homomorphic encryption; it enables arithmetic operations over encrypted data. One technique based on a specific homomorphic encryption function was recently proposed in the context of the DAS model. Unfortunately, as shown in this paper, this technique is insecure against ciphertext-only attacks. We propose a simple alternative for handling encrypted aggregation queries and describe its implementation. We also consider a different flavor of the DAS model which involves mixed databases, where some attributes are encrypted and some are left in the clear. We show how range queries can be executed in this model.
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paper_title: Replication is not needed: single database, computationally-private information retrieval

paper_content:

We establish the following, quite unexpected, result: replication of data for the computational private information retrieval problem is not necessary. More specifically, based on the quadratic residuosity assumption, we present a single database, computationally private information retrieval scheme with O(n/sup /spl epsiv//) communication complexity for any /spl epsiv/>0.
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paper_title: Software protection and simulation on oblivious RAMs

paper_content:

Software protection is one of the most important issues concerning computer practice. There exist many heuristics and ad-hoc methods for protection, but the problem as a whole has not received the theoretical treatment it deserves. In this paper, we provide theoretical treatment of software protection. We reduce the problem of software protection to the problem of efficient simulation on oblivious RAM. A machine is oblivious if thhe sequence in which it accesses memory locations is equivalent for any two inputs with the same running time. For example, an oblivious Turing Machine is one for which the movement of the heads on the tapes is identical for each computation. (Thus, the movement is independent of the actual input.) What is the slowdown in the running time of a machine, if it is required to be oblivious? In 1979, Pippenger and Fischer showed how a two-tape oblivious Turing Machine can simulate, on-line, a one-tape Turing Machine, with a logarithmic slowdown in the running time. We show an analogous result for the random-access machine (RAM) model of computation. In particular, we show how to do an on-line simulation of an arbitrary RAM by a probabilistic oblivious RAM with a polylogaithmic slowdown in the running time. On the other hand, we show that a logarithmic slowdown is a lower bound.
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paper_title: Privacy-preserving Queries over Relational Databases

paper_content:

We explore how Private Information Retrieval (PIR) can help users keep their sensitive information from being leaked in an SQL query. We show how to retrieve data from a relational database with PIR by hiding sensitive constants contained in the predicates of a query. Experimental results and microbenchmarking tests show our approach incurs reasonable storage overhead for the added privacy benefit and performs between 7 and 480 times faster than previous work.
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paper_title: Path ORAM: an extremely simple oblivious RAM protocol

paper_content:

We present Path ORAM, an extremely simple Oblivious RAM protocol with a small amount of client storage. Partly due to its simplicity, Path ORAM is the most practical ORAM scheme for small client storage known to date. We formally prove that Path ORAM requires log^2 N / log X bandwidth overhead for block size B = X log N. For block sizes bigger than Omega(log^2 N), Path ORAM is asymptotically better than the best known ORAM scheme with small client storage. Due to its practicality, Path ORAM has been adopted in the design of secure processors since its proposal.
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paper_title: Multi-cloud oblivious storage

paper_content:

We present a 2-cloud oblivious storage (ORAM) system that achieves 2.6X bandwidth cost between the client and the cloud. Splitting an ORAM across 2 or more non-colluding clouds allows us to reduce the client-cloud bandwidth cost by at least one order of magnitude, shifting the higher-bandwidth communication to in-between the clouds where bandwidth provisioning is abundant. Our approach makes ORAM practical for bandwidth-constrained clients such as home or mobile Internet connections. We provide a full-fledged implementation of our 2-cloud ORAM system, and report results from a real-world deployment over Amazon EC2 and Microsoft Azure.
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paper_title: On the Computational Practicality of Private Information Retrieval

paper_content:

We explore the limits of single-server computational private information retrieval (PIR) for the purpose of preserving client access patterns leakage. We show that deployment of non-trivial single server PIR protocols on real hardware of the recent past would have been orders of magnitude less time-efficient than trivially transferring the entire database. We stress that these results are beyond existing knowledge of mere “impracticality” under unfavorable assumptions. They rather reflect an inherent limitation with respect to modern hardware, likely the result of a communication-cost centric protocol design. We argue that this is likely to hold on non-specialized traditional hardware in the foreseeable future. We validate our reasoning in an experimental setup on modern off-the-shelf hardware. Ultimately, we hope our results will stimulate practical designs.
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paper_title: A Lattice-Based Computationally-Efficient Private Information Retrieval Protocol

paper_content:

A PIR scheme is a scheme that allows an user to get an element of a database without giving any information about what part of the database he is interested in. In this paper we present a lattice-based PIR scheme, using an NTRU-like approach, in which the computational cost is a few thousand bit-operations per bit in the database. This improves the protocol computational performance by two orders of magnitude when compared to existing approaches. Our scheme has worse communication performance than other existing protocols, but we show that practical usability of PIR schemes is not as dependent on communication performance as the literature suggests, and that a trade-off between communication and computation leads to much more versatile schemes.
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paper_title: Robust Information-Theoretic Private Information Retrieval

paper_content:

A Private Information Retrieval (PIR) protocol allows a user to retrieve a data item of its choice from a database, such that the servers storing the database do not gain information on the identity of the item being retrieved. PIR protocols were studied in depth since the subject was introduced in Chor, Goldreich, Kushilevitz, and Sudan 1995. The standard definition of PIR protocols raises a simple question - what happens if some of the servers crash during the operation? How can we devise a protocol which still works in the presence of crashing servers? Current systems do not guarantee availability of servers at all times for many reasons, e.g., crash of server or communication problems. Our purpose is to design robust PIR protocols, i.e., protocols which still work correctly even if only k out of servers are available during the protocols' operation (the user does not know in advance which servers are available). We present various robust PIR protocols giving different tradeoffs between the different parameters. These protocols are incomparable, i.e., for different values of n and k we will get better results using different protocols. We first present a generic transformation from regular PIR protocols to robust PIR protocols, this transformation is important since any improvement in the communication complexity of regular PIR protocol will immediately implicate improvement in the robust PIR protocol communication. We also present two specific robust PIR protocols. Finally, we present robust PIR protocols which can tolerate Byzantine servers, i.e., robust PIR protocols which still work in the presence of malicious servers or servers with corrupted or obsolete databases.
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paper_title: Private information retrieval

paper_content:

We describe schemes that enable a user to access k replicated copies of a database (k/spl ges/2) and privately retrieve information stored in the database. This means that each individual database gets no information on the identity of the item retrieved by the user. For a single database, achieving this type of privacy requires communicating the whole database, or n bits (where n is the number of bits in the database). Our schemes use the replication to gain substantial saving. In particular, we have: A two database scheme with communication complexity of O(n/sup 1/3/). A scheme for a constant number, k, of databases with communication complexity O(n/sup 1/k/). A scheme for 1/3 log/sub 2/ n databases with polylogarithmic (in n) communication complexity.
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paper_title: Securus: From Confidentiality and Access Requirements to Data Outsourcing Solutions

paper_content:

To preserve data confidentiality in database outsourcing scenarios, various techniques have been proposed that preserve a certain degree of confidentiality while still allowing to efficiently execute certain queries. Typically, several of those techniques have to be combined to achieve a certain degree of confidentiality. However, finding an appropriate combination is not a trivial task, as expert knowledge is required and interdependencies between the techniques exist. Securus, an approach we previously proposed, addresses this problem. Securus allows users to model their requirements regarding the information in the outsourced dataset that has to be protected. Furthermore, queries that have to be efficiently executable on the outsourced data can be specified. Based on these requirements, Securus uses Integer Linear Programming (ILP) to find a suitable combination of confidentiality enhancing techniques and generates a software adapter. This software adapter transparently applies the techniques to fulfill the specified requirements and can be used to seamlessly outsource and query the data. In this paper, we present an outline of Securus and extend our previous work by highlighting the differences to other approaches in the field. Furthermore, we show how Securus can be extended to allow for more efficient solutions if the attacker’s capabilities can be modeled by the user.
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paper_title: Path ORAM: an extremely simple oblivious RAM protocol

paper_content:

We present Path ORAM, an extremely simple Oblivious RAM protocol with a small amount of client storage. Partly due to its simplicity, Path ORAM is the most practical ORAM scheme for small client storage known to date. We formally prove that Path ORAM requires log^2 N / log X bandwidth overhead for block size B = X log N. For block sizes bigger than Omega(log^2 N), Path ORAM is asymptotically better than the best known ORAM scheme with small client storage. Due to its practicality, Path ORAM has been adopted in the design of secure processors since its proposal.
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paper_title: Multi-cloud oblivious storage

paper_content:

We present a 2-cloud oblivious storage (ORAM) system that achieves 2.6X bandwidth cost between the client and the cloud. Splitting an ORAM across 2 or more non-colluding clouds allows us to reduce the client-cloud bandwidth cost by at least one order of magnitude, shifting the higher-bandwidth communication to in-between the clouds where bandwidth provisioning is abundant. Our approach makes ORAM practical for bandwidth-constrained clients such as home or mobile Internet connections. We provide a full-fledged implementation of our 2-cloud ORAM system, and report results from a real-world deployment over Amazon EC2 and Microsoft Azure.
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Title: Confidential database-as-a-service approaches: taxonomy and survey

Section 1: Introduction
Description 1: Provide an overview of cloud computing and database-as-a-service (DaaS), highlight the importance of data confidentiality, and introduce Confidentiality Preserving Indexing (CPI) approaches.

Section 2: Taxonomy of deployment requirements
Description 2: Present a taxonomy outlining the requirements CPIs must satisfy in different deployment scenarios, focusing on functionality, protection levels, and attacker models.

Section 3: Functionality
Description 3: Discuss the importance of query execution efficiency in CPIs and introduce the types of queries and modifications that CPIs must support.

Section 4: Protection levels
Description 4: Define the different levels of data confidentiality protection that CPIs must provide and explain the concepts of computational and probabilistic record protection.

Section 5: Assumptions on attackers
Description 5: Present an attacker taxonomy based on their capabilities and knowledge, including what data they can observe and their background knowledge.

Section 6: Methodology to assess CPIs
Description 6: Describe the methodology for assessing CPIs against the taxonomy requirements, including the necessary security properties for different attacker models.

Section 7: Survey of existing CPIs
Description 7: Survey popular CPIs, categorize them based on the taxonomy, and provide an overview of their functionalities and protection guarantees.

Section 8: CPI catalog
Description 8: Present a catalog summarizing the analysis and mapping existing CPIs to deployment scenario requirements.

Section 9: Deterministic indexes
Description 9: Explain how deterministic indexes work, their applicability, and the type of attack models they protect against.

Section 10: Bucketization
Description 10: Discuss the bucketization approach for range selections, its benefits, and limitations.

Section 11: Order-preserving encryption
Description 11: Describe how order-preserving encryption schemes function and their use cases for range selections.

Section 12: Searchable encryption
Description 12: Detail searchable encryption schemes, their capabilities, and scenarios where they provide adequate protection.

Section 13: Encrypted B-Trees
Description 13: Explain encrypted B-Trees, their implementation, and the levels of protection they offer.

Section 14: Fragmentation
Description 14: Discuss methods for protecting attribute combinations through fragmentation and the different scenarios where it applies.

Section 15: Homomorphic encryption
Description 15: Describe homomorphic encryption and its application in aggregation queries.

Section 16: Oblivious RAM and private information retrieval
Description 16: Explain Oblivious RAM (ORAM) and Private Information Retrieval (PIR) techniques for achieving indistinguishable queries.

Section 17: CPI efficiency
Description 17: Provide a high-level overview of the efficiency of different CPI approaches, including their computational and transmission overhead.

Section 18: Comparative analysis and discussion
Description 18: Conduct a comparative analysis of CPIs based on functionality, protection guarantees, and efficiency, and provide recommendations on their use in various scenarios.

Section 19: Conclusion
Description 19: Summarize the study’s findings, draw key conclusions, and highlight future research directions.