Publications & Talks


Angèle Bossuat, Raphael Bost, Pierre-Alain Fouque, Brice Minaud, and Michael Reichle - Crypto 2021

Searchable Symmetric Encryption (SSE) enables a client to outsource a database to an untrusted server, while retaining the ability to securely search the data. The performance bottleneck of classic SSE schemes typically does not come from their fast, symmetric cryptographic operations, but rather from the cost of memory accesses. To address this issue, many works in the literature have considered the notion of locality, a simple design criterion that helps capture the cost of memory accesses in traditional storage media, such as Hard Disk Drives. A common thread among many SSE schemes aiming to improve locality is that they are built on top of new memory allocation schemes, which form the technical core of the constructions.

The starting observation of this work is that for newer storage media such as Solid State Drives (SSDs), which have become increasingly common, locality is not a good predictor of practical performance. Instead, SSD performance mainly depends on page efficiency, that is, reading as few pages as possible. We define this notion, and identify a simple memory allocation problem, Data-Independent Packing (DIP), that captures the main technical challenge required to build page-efficient SSE. As our main result, we build a page-efficient and storage-efficient data-independent packing scheme, and deduce the Tethys SSE scheme, the first SSE scheme to achieve at once O(1) page efficiency and O(1) storage efficiency. The technical core of the result is a new generalization of cuckoo hashing to items of variable size. Practical experiments show that this new approach achieves excellent performance.

Raphael Bost and Pierre-Alain Fouque - PETS 2019

Besides their security, the efficiency of searchable encryption schemes is a major criteria when it comes to their adoption: in order to replace an unencrypted database by a more secure construction, it must scale to the systems which rely on it. Unfortunately, the relationship between the efficiency and the security of searchable encryption has not been widely studied, and the minimum cost of some crucial security properties is still unclear.

In this paper, we present new lower bounds on the tradeoffs between the size of the client state, the efficiency and the security for searchable encryption schemes. These lower bounds target two kinds of schemes: schemes hiding the repetition of search queries, and forward-private dynamic schemes, for which updates are oblivious.

We also show that these lower bounds are tight, by either constructing schemes matching them, or by showing that even a small increase in the amount of leaked information allows for constructing schemes breaking the lower bounds.

Raphael Bost - Ph.D. Thesis

Searchable encryption aims at making efficient a seemingly easy task: outsourcing the storage of a database to an untrusted server, while keeping search features. With the development of Cloud storage services, for both private individuals and businesses, efficiency of searchable encryption became crucial: inefficient constructions would not be deployed on a large scale because they would not be usable. The key problem with searchable encryption is that any construction achieving `perfect security’ induces a computational or a communicational overhead that is unacceptable for the providers or for the users — at least with current techniques and by today’s standards.

This thesis proposes and studies new security notions and new constructions of searchable encryption, aiming improving efficiency and security. In particular, we start by considering the forward and backward privacy of searchable encryption schemes, what it implies in terms of security and efficiency, and how we can realize them. Then, we show how to protect an encrypted database user against active attacks by the Cloud provider, and that such protections have an inherent efficiency cost. Finally, we take a look at existing attacks against searchable encryption, and explain how we might thwart them. — See more

Raphael Bost and Pierre-Alain Fouque - ePrint

After the development of practical searchable encryption constructions, allowing for secure searches over an encrypted dataset outsourced to an untrusted server, at the expense of leaking some information to the server, many new attacks have recently been developed, targeting this leakage in order to break the confidentiality of the dataset or of the queries, through leakage abuse attacks.

These works helped to understand the importance of considering leakage when analyzing the security of searchable encryption schemes, but did not explain why these attacks were so powerful despite the existence of rigorous security definitions and proofs, or how they could be efficiently and provably mitigated.

This work addresses these questions by first proposing an analysis of existing leakage abuse attacks and a way to capture them in new security definitions. These new definitions also help us to devise a way to thwart these attacks and we apply it to the padding of datasets, in order to hide the number of queries’ results, and to provide provable security of some schemes with specific leakage profile against some common classes of leakage abuse attacks.

Finally, we give experimental evidence that our countermeasures can be implemented efficiently, and easily applied to existing searchable encryption schemes.

Raphael Bost, Brice Minaud and Olga Ohrimenko - CCS 2017

Using dynamic Searchable Symmetric Encryption, a user with limited storage resources can securely outsource a database to an untrusted server, in such a way that the database can still be searched and updated efficiently. For these schemes, it would be desirable that updates do not reveal any information a priori about the modifications they carry out, and that deleted results remain inaccessible to the server a posteriori. If the first property, called forward privacy, has been the main motivation of recent works, the second one, backward privacy, has been overlooked.

In this paper, we study for the first time the notion of backward privacy for searchable encryption. After giving formal definitions for different flavors of backward privacy, we present several schemes achieving both forward and backward privacy, with various efficiency trade-offs.

Our constructions crucially rely on primitives such as constrained pseudo-random functions and puncturable encryption schemes. Using these advanced cryptographic primitives allows for a fine-grained control of the power of the adversary, preventing her from evaluating functions on selected inputs, or decrypting specific ciphertexts. In turn, this high degree of control allows our SSE constructions to achieve the stronger forms of privacy outlined above. As an example, we present a framework to construct forward-private schemes from range-constrained pseudo-random functions.

Finally, we provide experimental results for implementations of our schemes, and study their practical efficiency.

Raphael Bost - CCS 2016

Searchable Symmetric Encryption aims at making possible searching over an encrypted database stored on an untrusted server while keeping privacy of both the queries and the data, by allowing some small controlled leakage to the server.
Recent works showed that dynamic schemes – in which the data is efficiently updatable – leaking some informations on updated keywords are subjects to devastating adaptative attacks breaking the queries’ privacy. The only way to thwart this attack is to design forward-private schemes whose update procedure does not leak if a newly inserted element matches previous search queries.
This work proposes Σoϕoς as a forward-private SSE scheme with performance similar to existing less secure schemes, and that is conceptually simpler (and also more efficient) than previous forward-private constructions. In particular, it only relies on trapdoor permutations and does not use an ORAM-like construction. We also explain why Σoϕoς is an optimal point of the security/performance tradeoff for SSE.
Finally, an implementation and evaluation results demonstrate its practical efficiency.

Raphael Bost and Olivier Sanders - AsiaCrypt 2016

Tweakable blockcipher (TBC) is a powerful tool to design authenticated encryption schemes as illustrated by Minematsu’s Offset Two Rounds (OTR) construction. It considers an additional input, called tweak, to a standard blockcipher which adds some variability to this primitive. More specifically, each tweak is expected to define a different, independent pseudo-random permutation.
In this work we focus on OTR’s way to instantiate a TBC and show that it does not achieve such a property for a large amount of parameters. We indeed describe collisions between the input masks derived from the tweaks and explain how they result in practical attacks against this scheme, breaking privacy, authenticity, or both, using a single encryption query, with advantage at least 1/4.
We stress however that our results do not invalidate the OTR construction as a whole but simply prove that the TBC’s input masks should be designed differently.

Raphael Bost, Pierre-Alain Fouque and David Pointcheval - ePrint

Symmetric Searchable Encryption (SSE) is a very efficient and practical way for data owners to outsource storage of a database to a server while providing privacy guarantees. Such SSE schemes enable clients to encrypt their database while still performing queries for retrieving documents matching some keyword. This functionality is interesting to secure cloud storage, and efficient schemes have been designed in the past. However, security against malicious servers has been overlooked in most previous constructions and these only addressed security against honest-but-curious servers.
In this paper, we study and design the first efficient SSE schemes provably secure against malicious servers. First, we give lower bounds on the complexity of such verifiable SSE schemes. Then, we construct generic solutions matching these bounds using efficient verifiable data structures. Finally, we modify an existing SSE scheme that also provides forward secrecy of search queries, and make it provably secure against active adversaries, without increasing the computational complexity of the original scheme.

Raphael Bost, Raluca Ada Popa, Stephen Tu and Shafi Goldwasser - NDSS 2015

Machine learning classification is used in numerous settings nowadays, such as medical or genomics predictions, spam detection, face recognition, and financial predictions. Due to privacy concerns, in some of these applications, it is important that the data and the classifier remain confidential.
In this work, we construct three major classification protocols that satisfy this privacy constraint: hyperplane decision, Naïve Bayes, and decision trees. We also enable these protocols to be combined with AdaBoost. At the basis of these constructions is a new library of building blocks for constructing classifiers securely; we demonstrate that this library can be used to construct other classifiers as well, such as a multiplexer and a face detection classifier.
We implemented and evaluated our library and classifiers. Our protocols are efficient, taking milliseconds to a few seconds to perform a classification when running on real medical datasets.


A Quick Intro to Searchable Encryption

University of Caean Security Seminar (Caen, France) — Avril 21st, 2021

Understanding and Mitigating Leakage-Abuse Attacks against Searchable Encryption

ICERM's Encrypted Search Workshop (Providence, RI) — June 10th, 2019

Security-Efficiency Tradeoffs in Searchable Encryption -- Lower Bounds and Optimal Constructions

Privacy Enhancing Technologies Symposium (Stockholm, Sweden) — July 17th, 2019
ESSA2 Workshop (Bertinoro, Italy) — July 10th, 2018

Searchable Encryption: New Constructions of Encrypted Databases (Ph.D. Defence)

Rennes, France — January 8th, 2018

Forward and Backward Private Searchable Encryption from Constrained Cryptographic Primitives

ACM CCS 2017 (Dallas, TX) — November 1st, 2017

Searchable Encryption: From Theory to Implementation

ENS (Paris, France) — June 28th, 2017

Forward Secure Searchable Encryption and Beyond

MIT (Boston, MA) — February 22nd, 2017
Brown University (Providence, RI) — February 15th, 2017

Trick or Tweak: On the (In)security of OTR's Tweaks

IACR Asiacrypt 2016 (Hanoi, Vietnam) — December 6th, 2016

Σoφoς – Forward Secure Searchable Encryption

ACM CCS 2016 (Vienna, Austria) — October 23rd, 2016
Microsoft Research (Cambridge, UK) — July 13th, 2016

Verifiable Symmetric Searchable Encryption

EMSEC's Séminaire Dromadaire (Rennes) — March 9th, 2016

Machine Learning Classification over Encrypted Data

NDSS 2015 (San Diego, CA) — February 10th, 2015
MIT (Boston, MA) — February 6th, 2015