Private information retrieval (PIR) schemes allow a user to retrieve the ith bit of an n-bit data string x, replicated in k?2 databases (in the information-theoretic setting) or in k?1 databases (in the computational setting), while keeping the value of i private. The main cost measure for such a scheme is its communication complexity. In this paper we introduce a model of symmetrically-private information retrieval (SPIR), where the privacy of the data, as well as the privacy of the user, is guaranteed. That is, in every invocation of a SPIR protocol, the user learns only a single physical bit of x and no other information about the data. Previously known PIR schemes severely fail to meet this goal. We show how to transform PIR schemes into SPIR schemes (with information-theoretic privacy), paying a constant factor in communication complexity. To this end, we introduce and utilize a new cryptographic primitive, called conditional disclosure of secrets, which we believe may be a useful building block for the design of other cryptographic protocols. In particular, we get a k-database SPIR scheme of complexity O(n1/(2k?1)) for every constant k?2 and an O(logn)-database SPIR scheme of complexity O(log2n·loglogn). All our schemes require only a single round of interaction, and are resilient to any dishonest behavior of the user. These results also yield the first implementation of a distributed version of (n1)-OT (1-out-of-n oblivious transfer) with information-theoretic security and sublinear communication complexity.

Protecting Data Privacy in Private Information Retrieval Schemes.

Mit seiner Publikation “One Size Fits All: An Idea Whose Time Has Come and Gone?“ [St05] hat Michael Stonebraker 2005 die Datenbankszene provoziert. Er vertritt dort die These, dass die eingeführten und in vielen Kontexten eingesetzten „traditionellen“ Datenbanksysteme zwar für viele datenzentrische Anwendungen eingesetzt werden, aber keineswegs die optimalen Systeme für viele dieser Anwendungen (OLAP, Stream Processing, XML Datenbanken, wissenschaftliche Datenbanken, Textsuche) sind. In einer späteren Publikation [St07] stellte er sogar die These auf, dass es keine Anwendungen gibt, für die die traditionellen Datenbanksysteme die beste Alternative sind. Er sagte voraus, dass sich in der kommerziellen Welt eine Anzahl unabhängiger spezialisierter „neuer“ Datenbanksysteme aufsplitten werde, die bestenfalls durch ein gemeinsames Frontend verbunden sind.

"One Size Fits All": An Idea Whose Time Has Come and Gone?

We extend the searchable symmetric encryption (SSE) protocol of [Cash et al., Crypto’13] adding support for range, substring, wildcard, and phrase queries, in addition to the Boolean queries supported in the original protocol. Our techniques apply to the basic single-client scenario underlying the common SSE setting as well as to the more complex Multi-Client and Outsourced Symmetric PIR extensions of [Jarecki et al., CCS’13]. We provide performance information based on our prototype implementation, showing the practicality and scalability of our techniques to very large databases, thus extending the performance results of [Cash et al., NDSS’14] to these rich and comprehensive query types.

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Rich Queries on Encrypted Data: Beyond Exact Matches.

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Rich Queries on Encrypted Data: Beyond Exact Matches

We analyse the security of database encryption schemes supporting range queries against persistent adversaries. The bulk of our work applies to a generic setting, where the adversary's view is limited to the set of records matched by each query (known as access pattern leakage). We also consider a more specific setting where rank information is also leaked, which is inherent inherent to multiple recent encryption schemes supporting range queries. We provide three attacks. First, we consider full reconstruction, which aims to recover the value of every record, fully negating encryption. We show that for dense datasets, full reconstruction is possible within an expected number of queries N log N + O(N), where N is the number of distinct plaintext values. This directly improves on a quadratic bound in the same setting by Kellaris et al. (CCS 2016). Second, we present an approximate reconstruction attack recovering all plaintext values in a dense dataset within a constant ratio of error, requiring the access pattern leakage of only O(N) queries. Third, we devise an attack in the common setting where the adversary has access to an auxiliary distribution for the target dataset. This third attack proves highly effective on age data from real-world medical data sets. In our experiments, observing only 25 queries was sufficient to reconstruct a majority of records to within 5 years. In combination, our attacks show that current approaches to enabling range queries offer little security when the threat model goes beyond snapshot attacks to include a persistent server-side adversary.

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Improved Reconstruction Attacks on Encrypted Data Using Range Query Leakage

Protected database search systems cryptographically isolate the roles of reading from, writing to, and administering the database. This separation limits unnecessary administrator access and protects data in the case of system breaches. Since protected search was introduced in 2000, the area has grown rapidly; systems are offered by academia, start-ups, and established companies. 
However, there is no best protected search system or set of techniques. Design of such systems is a balancing act between security, functionality, performance, and usability. This challenge is made more difficult by ongoing database specialization, as some users will want the functionality of SQL, NoSQL, or NewSQL databases. This database evolution will continue, and the protected search community should be able to quickly provide functionality consistent with newly invented databases. 
At the same time, the community must accurately and clearly characterize the tradeoffs between different approaches. To address these challenges, we provide the following contributions: 
1) An identification of the important primitive operations across database paradigms. We find there are a small number of base operations that can be used and combined to support a large number of database paradigms. 
2) An evaluation of the current state of protected search systems in implementing these base operations. This evaluation describes the main approaches and tradeoffs for each base operation. Furthermore, it puts protected search in the context of unprotected search, identifying key gaps in functionality. 
3) An analysis of attacks against protected search for different base queries. 
4) A roadmap and tools for transforming a protected search system into a protected database, including an open-source performance evaluation platform and initial user opinions of protected search.

SoK: Cryptographically Protected Database Search

Protected database search systems cryptographically isolate the roles of reading from, writing to, and administering the database. This separation limits unnecessary administrator access and protects data in the case of system breaches. Since protected search was introduced in 2000, the area has grown rapidly, systems are offered by academia, start-ups, and established companies. However, there is no best protected search system or set of techniques. Design of such systems is a balancing act between security, functionality, performance, and usability. This challenge is made more difficult by ongoing database specialization, as some users will want the functionality of SQL, NoSQL, or NewSQL databases. This database evolution will continue, and the protected search community should be able to quickly provide functionality consistent with newly invented databases. At the same time, the community must accurately and clearly characterize the tradeoffs between different approaches. To address these challenges, we provide the following contributions:1) An identification of the important primitive operations across database paradigms. We find there are a small number of base operations that can be used and combined to support a large number of database paradigms.2) An evaluation of the current state of protected search systems in implementing these base operations. This evaluation describes the main approaches and tradeoffs for each base operation. Furthermore, it puts protected search in the context of unprotected search, identifying key gaps in functionality.3) An analysis of attacks against protected search for different base queries.4) A roadmap and tools for transforming a protected search system into a protected database, including an open-source performance evaluation platform and initial user opinions of protected search.

We consider a setting where users store their encrypted documents on a remote server and can selectively share documents with each other. A user should be able to perform keyword searches over all the documents she has access to, including the ones that others shared with her. The contents of the documents, and the search queries, should remain private from the server.

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Multi-Key Searchable Encryption, Revisited

We consider a scenario where a server holds a huge database that it wants to make accessible to a large group of clients. After an initial setup phase, clients should be able to read arbitrary locations in the database while maintaining privacy (the server does not learn which locations are being read) and anonymity (the server does not learn which client is performing each read). This should hold even if the server colludes with a subset of the clients. Moreover, the run-time of both the server and the client during each read operation should be low, ideally only poly-logarithmic in the size of the database and the number of clients. We call this notion Private Anonymous Data Access (PANDA). PANDA simultaneously combines aspects of Private Information Retrieval (PIR) and Oblivious RAM (ORAM). PIR has no initial setup, and allows anybody to privately and anonymously access a public database, but the server’s run-time is linear in the data size. On the other hand, ORAM achieves poly-logarithmic server run-time, but requires an initial setup after which only a single client with a secret key can access the database. The goal of PANDA is to get the best of both worlds: allow many clients to privately and anonymously access the database as in PIR, while having an efficient server as in ORAM.

Private Anonymous Data Access

This work presents the results of a three-year project that assessed nine different privacy-preserving data search systems. We detail the design of a software assessment framework that focuses on low system footprint, repeatability, and reusability. A unique achievement of this project was the automation and integration of the entire test process, from the production and execution of tests to the generation of human-readable evaluation reports. We synthesize our experiences into a set of simple mantras that we recommend following in the design of any assessment framework.

Ariel Hamlin

Papers

SoK: Cryptographically Protected Database Search

SoK: Cryptographically Protected Database Search

Multi-Key Searchable Encryption, Revisited

Private Anonymous Data Access

Automated Assessment of Secure Search Systems