Local Differential Privacy (LDP) mechanisms consist of (locally) adding controlled noise to data in order to protect the privacy of their owner. In this paper, we introduce a new cryptographic primitive called LDP commitment. Usually, a commitment ensures that the committed value cannot be modified before it is revealed. In the case of an LDP commitment, however, the value is revealed after being perturbed by an LDP mechanism. Opening an LDP commitment therefore requires a proof that the mechanism has been correctly applied to the value, to ensure that the value is still usable for statistical purposes. We also present a security model for this primitive, in which we define the hiding and binding properties. Finally, we present a concrete scheme for an LDP staircase mechanism (generalizing the randomized response technique), based on classical cryptographic tools and standard assumptions. We provide an implementation in Rust that demonstrates its practical efficiency (the generation of a commitment requires just a few milliseconds).On the application side, we show how our primitive can be used to ensure simultaneously privacy, usability and traceability of medical data when it is used for statistical studies in an open science context. We consider a scenario where a hospital provides sensitive patients data signed by doctors to a research center after it has been anonymized, so that the research center can verify both the provenance of the data (i.e. verify the doctors’ signatures even though the data has been noised) and that the data has been correctly anonymized (i.e. is usable even though it has been anonymized).

As messaging applications are becoming increasingly popular, it is of utmost importance to analyze their security and mitigate existing weaknesses. This paper focuses on one of the most acclaimed messaging applications: Signal. Signal is a protocol that provides end-to-end channel security, forward secrecy, and post-compromise security. These features are achieved thanks to a key-ratcheting mechanism that updates the key material at every message. Due to its high security impact, Signal's key-ratcheting has recently been formalized, along with an analysis of its security. In this paper, we revisit Signal, describing some attacks against the original design and proposing SAID: Signal Authenticated and IDentity-based. As the name indicates, our protocol relies on an identity-based setup, which allows us to dispense with Signal's centralized server. We use the identity-based long-term secrets to obtain persistent and explicit authentication, such that SAID achieves higher security guarantees than Signal. We prove the security of SAID not only in the Authenticated Key Exchange (AKE) model (as done by previous work), but also in the Authenticated and Confidential Channel Establishment (ACCE) model, which we adapted and redefined for SAID and asynchronous messaging protocols in general into a model we call identity-based Multistage Asynchronous Messaging (iMAM). We believe our model to be more faithful in particular to the true security of Signal, whose use of the message keys prevents them from achieving the composable guarantee claimed by previous analysis.

TLS (Transport Layer Security) is a widely deployed protocol that plays a vital role in securing Internet traffic. Given the numerous known attacks for TLS 1.2, it was imperative to change and even redesign the protocol in order to address them. In August 2018, a new version of the protocol, TLS 1.3, was standardized by the IETF (Internet Engineering Task Force). TLS 1.3 not only benefits from stronger security guarantees, but aims to protect the identities of the server and client by encrypting messages as soon as possible during the authentication. In this paper, we model the privacy guarantees of TLS 1.3 when parties execute a full handshake or use a session resumption, covering all the handshake modes of TLS. We build our privacy models on top of the one defined by Hermans et al. for RFIDs (Radio Frequency Identification Devices) that mostly targets authentication protocols. The enhanced models share similarities to the Bellare-Rogaway AKE (Authenticated Key Exchange) security model and consider adversaries that can compromise both types of participants in the protocol. In particular, modeling session resumption is non-trivial, given that session resumption tickets are essentially a state transmitted from one session to another and such link reveals information on the parties. On the positive side, we prove that TLS 1.3 protects the privacy of its users at least against passive adversaries, contrary to TLS 1.2, and against more powerful ones.

Touch screens have become ubiquitous in the past few years, like for instance in smartphones and tablets. These devices are often the entry door to numerous information systems, hence having a secure and practical authentication mechanism is crucial. In this paper, we examine the complexity of different authentication methods specifically designed for such devices. We study the widely spread technology to authenticate a user using a Personal Identifier Number code (PIN code). Entering the code is a critical moment where there are several possibilities for an attacker to discover the secret. We consider the three attack models: a Bruteforce Attack (BA) model, a Smudge Attack (SA) model, and an Observation Attack (OA) model where the attacker sees the user logging in on his device. The aim of the intruder is to learn the secret code. Our goal is to propose alternative methods to enter a PIN code. We compare such different methods in terms of security. Some methods require more intentional resources than other, this is why we performed a psychological study on the different methods to evaluate the users' perception of the different methods and their usage.

The MapReduce programming paradigm allows to process big data sets in parallel on a large cluster of commodity machines. The MapReduce users often outsource their data and computations to a public cloud provider. We focus on the fundamental problem of matrix multiplication, and address the inherent security and privacy concerns that occur when outsourcing to a public cloud. Our goal is to enhance the two state-of-the-art algorithms for MapReduce matrix multiplication with privacy guarantees such as: none of the nodes storing an input matrix can learn the other input matrix or the output matrix, and moreover, none of the nodes computing an intermediate result can learn the input or the output matrices. To achieve our goal, we rely on the well-known Paillier's cryptosystem and we use its partially homomorphic property to develop efficient algorithms that satisfy our problem statement. We develop two different approaches called Secure-Private (SP) and Collision-Resistant-Secure-Private (CRSP), and compare their trade-offs with respect to three fundamental criteria: computation cost, communication cost, and privacy guarantees. Finally, we give security proofs of our protocols.

Distance-bounding protocols have been introduced to thwart relay attacks against contactless authentication protocols. In this context, verifiers have to authenticate the credentials of untrusted provers. Unfortunately, these protocols are themselves subject to complex threats such as terrorist-fraud attacks, in which a malicious prover helps an accomplice to authenticate. Provably guaranteeing the resistance of distance-bounding protocols to these attacks is complex. The classical solutions assume that rational provers want to protect their long-term authentication credentials, even with respect to their accomplices. Thus, terrorist-fraud resistant protocols generally rely on artificial extraction mechanisms, ensuring that an accomplice can retrieve the credential of his partnering prover, if he is able to authenticate. We propose a novel approach to obtain provable terrorist-fraud resistant protocols that does not rely on an accomplice being able to extract any long-term key. Instead, we simply assume that he can replay the information received from the prover. Thus, rational provers should refuse to cooperate with third parties if they can impersonate them freely afterwards. We introduce a generic construction for provably secure distance-bounding protocols, and give three instances of this construction: (1) an efficient symmetric-key protocol, (2) a public-key protocol protecting the identities of provers against external eavesdroppers, and finally (3) a fully anonymous protocol protecting the identities of provers even against malicious verifiers that try to profile them.

Ring and group signatures allow their members to anonymously sign documents in the name of the group. In ring signatures, members manage the group themselves in an ad-hoc manner while in group signatures, a manager is required. Moreover, k-times traceable group and ring signatures [1] allow anyone to publicly trace two signatures from a same user if he exceeds the a priori authorized number of signatures. In [2], Canard et al. give a 1-time traceable ring signature where each member can only generate one anonymous signature. Hence, it is possible to trace any two signatures from the same user. Some other works generalize it to the k-times case, but the traceability only concerns two signatures. In this paper, we define the notion of k-times full traceable ring signature (k-FTRS) such that all signatures produced by the same user are traceable if and only if he produces more than k signatures. We construct a k-FTRS called Ktrace. We extend existing formal security models of k-times linkable signatures to prove the security of Ktrace in the random oracle model. Our primitive k-FTRS can be used to construct a k-times veto scheme or a proxy e-voting scheme that prevents denial-of-service caused by cheating users.

International audience

Contactless communications have become omnipresent in our daily lives, from simple access cards to electronic passports. Such systems are particularly vulnerable to relay attacks, in which an adversary relays the messages from a prover to a verifier. Distance-bounding protocols were introduced to counter such attacks. Lately, there has been a very active research trend on improving the security of these protocols, but also on ensuring strong privacy properties with respect to active adversaries and malicious verifiers.