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A recent study suggests that arithmetic masking in prime fields leads to stronger security guarantees against passive physical adversaries than Boolean masking. Indeed, it is a common observation that the desired security amplification of Boolean masking collapses when the noise level in the measurements is too low. Arithmetic encodings in prime fields can help to maintain an exponential increase of the attack complexity in the number of shares even in such a challenging context. In this work, we contribute to this emerging topic in two main directions. First, we propose novel masked hardware gadgets for secure squaring in prime fields (since squaring is non-linear in non-binary fields) which prove to be significantly more resource-friendly than corresponding masked multiplications. We then formally show their local and compositional security for arbitrary orders. Second, we attempt to experimentally evaluate the performance vs. security tradeoff of prime-field masking. In order to enable a first comparative case study in this regard, we exemplarily consider masked implementations of the AES as well as the recently proposed AES- prime. AES-prime is a block cipher partially resembling the standard AES, but based on arithmetic operations modulo a small Mersenne prime. We present cost and performance figures for masked AES and AES-prime implementations, and experimentally evaluate their susceptibility to low-noise side-channel attacks. We consider both the dynamic and the static power consumption for our low-noise analyses and emulate strong adversaries. Static power attacks are indeed known as a threat for side-channel countermeasures that require a certain noise level to be effective because of the adversary’s ability to reduce the noise through intra-trace averaging. Our results show consistently that for the noise levels in our practical experiments, the masked prime-field implementations provide much higher security for the same number of shares. This compensates

Authenticated Encryption (AE) modes of operation based on Tweakable Block Ciphers (TBC) usually measure efficiency in the number of calls to the underlying primitive per message block. On the one hand, many existing solutions reach a primitive-rate of 1, meaning that each n-bit block of message asymptotically needs a single call to the TBC with output length n. On the other hand, while these modes look optimal in a blackbox setting, they become less attractive when leakage comes into play, since all these calls must then be equally well protected to maintain security. Leakage-resistant modes improve this situation, by generating ephemeral keys every constant number of calls. However, rekeying is inherently suboptimal in primitive-rate, since a TBC call can only be used either to refresh a key or to encrypt a block. Even worse, existing solutions achieving almost n bits of security for n-bit secret keys have at most a primitive-rate 2/3. Hence the question: Can we design a highly-secure TBC-based rekeying mode with “nearly optimal” primitive-rate? We answer this question positively with Multiplex, a new mode that has primitive-rate d/(d+1) given a TBC with a dn-bit tweak. Multiplex achieves n−log(n) bits of security for both (i) misuse-resilience CCA security in the blackbox setting and (ii) Ciphertext Integrity with Misuse-resistant and unbounded Leakage in encryption and decryption (CIML2). It also provides (iii) confidentiality with leakage up to the birthday bound. Furthermore, Multiplex can run d + 1 calls in parallel in each iteration. The combination of these features gives a mode of operation that inherits most of the good implementation features and flexibility of a Duplex sponge – therefore paving the way towards sound comparisons between TBC-based and permutation-based AE.

We consider the design of a tweakable block cipher from a block cipher whose inputs and outputs are of size n bits. The main goal is to achieve 2n security with a large tweak (more than n bits). Previously, Mennink at FSE’15 and Wang et al. at Asiacrypt’16 proposed constructions that can achieve 2n security. Yet, these constructions can have a tweak size up to n-bit. As evident from recent research, a tweakable block cipher with a large tweak is generally helpful as a building block for modes of operation, typical applications including MACs, authenticated encryption and full-disk encryption when sector ID is more than n bits. We begin with how to design a tweakable block cipher with 2n-bit tweak and n-bit security from two block cipher calls. For this purpose, we do an exhaustive search for tweakable block ciphers with 2n-bit tweaks from two block cipher calls, and show that all of them suffer from birthday-bound attacks. Next, we investigate the possibility to design a tweakable block cipher with 2n-bit tweak and n-bit security from three block cipher calls. We start with some conditions to build a such tweakable block cipher and propose a natural construction, called eG1, that likely meets them. After inspection, we find a weakness on eG1 which leads to a birthday-bound attack. Based on eG1, we then propose another construction, called eG2, that can avoid this weakness. We finally prove that eG2 can achieve n-bit security with 2n-bit tweak.

Embedded devices used in security applications are natural targets for physical attacks. Thus, enhancing their side-channel resistance is an important research challenge. A standard solution for this purpose is the use of Boolean masking schemes, as they are well adapted to current block ciphers with efficient bitslice representations. Boolean masking guarantees that the security of an implementation grows exponentially in the number of shares under the assumption that leakages are sufficiently noisy (and independent). Unfortunately, it has been shown that this noise assumption is hardly met on low-end devices. In this paper, we therefore investigate techniques to mask cryptographic algorithms in such a way that their resistance can survive an almost complete lack of noise. Building on seed theoretical results of Dziembowski et al., we put forward that arithmetic encodings in prime fields can reach this goal. We first exhibit the gains that such encodings lead to thanks to a simulated information theoretic analysis of their leakage (with up to six shares). We then provide figures showing that on platforms where optimized arithmetic adders and multipliers are readily available (i.e., most MCUs and FPGAs), performing masked operations in small to medium Mersenne-prime fields as opposed to binary extension fields will not lead to notable implementation overheads. We compile these observations into a new AES-like block cipher, called AES-prime, which is well-suited to illustrate the remarkable advantages of masking in prime fields. We also confirm the practical relevance of our findings by evaluating concrete software (ARM Cortex-M3) and hardware (Xilinx Spartan-6) implementations. Our experimental results show that security gains over Boolean masking (and, more generally, binary encodings) can reach orders of magnitude despite the same amount of information being leaked per share.

—We extend the study of efficient profiled attacks on masking schemes initiated by Lerman and Markowitch (TIFS, 2019) in different directions. First, we study both the profiling complexity and the online attack complexity of different profiled distinguishers. Second, we extend the range of the noise levels of their experiments, in order to cover (higher-noise) contexts where masking is effective. Third, we further contextualize the investigated distinguishers (e.g., in terms of adversarial capabilities and a priori assumptions on the leakage probability density function). Finally, we complete the list of distinguishers considered in this previous work and add expectation-maximization, soft analytical side-channel attacks and multi-layer perceptrons in our comparisons. Our results allow shedding an interesting new light on the respective strengths and weaknesses of these different statistical tools, both in the context of a side-channel security evaluation and for concrete attacks. In particular, they confirm the experimental relevance of evaluation shortcuts leveraging the masking randomness during profiling, in order to speed up the evaluation process.

Modern cryptography relies on the Kerckhoff’s principle which states that everything about a crypto-system should be public, except for the secret keys. In this thesis, I apply these principles to cryptography implementations by providing the evaluator full knowledge of the implementation, in a worst-case manner. As a result, it allows to estimate their long-term security against side-channel attacks. The first part is dedicated to the verification of the two underlying assumptions in masking proofs. More precisely, I contribute to proof-based evaluations by putting forward that i) the verification of independence thanks to leakage detection benefits from multivariate statistics ii) the noise can be estimated in a quantitative manner thanks to formal bounds on the mutual information. The second part of the thesis is dedicated to efficient methodologies to perform worst-case attacks which are a useful shortcut for evaluators since they allow to estimate, with a reduce profiling data complexity, the online complexity other attacks strategies. Concretely, I propose worst-case attacks against a recent open-source AES implementation by the French ANSSI and masked higher-order bitslice implementations. From the lessons learned from the side-channel evaluation of current proposals, I propose new designs to obtain strong side-channel protection. More precisely, the noise versus security trade-off can be improved thanks to a sound combination of masking and shuffling. Concretely, I apply masking and shuffling to ISW multiplications in order to exponentially amplify with masking the effect of shuffling., (FSA - Sciences de l'ingénieur) -- UCL, 2022

In this paper, we study the exploitation of language generation models for disinformation purposes from two viewpoints. Quantitatively, we argue that language models hardly deal with domain adaptation (i.e., the ability to generate text on topics that are not part of a training database, as typically required for news). For this purpose, we show that both simple machine learning models and manual detection can spot machine-generated news in this practically-relevant context. Qualitatively, we put forward the differences between these automatic and manual detection processes, and their potential for a constructive interaction in order to limit the impact of automatic disinformation campaigns. We also discuss the consequences of these findings for the constructive use of natural language generation to produce news.

Modern cryptography has been widely deployed in the last decades, allowing any computing device to secure its communications. Facing the strength of the cryptographic algorithms, attackers turned to their implementations. In particular, side-channel attacks have been shown to threaten the security of cryptographic embedded devices by exploiting information leaked through physical characteristics such as power consumption or electromagnetic radiation. In order to mitigate side-channel attacks, implementations use countermeasures, among which masking is the most investigated choice. In this thesis, we design and analyze masking schemes with the goal of having efficient masking schemes with strong and well-understood security. Efficiency is an important criterion for masking schemes, since they sometimes have orders of magnitude performance overheads over an implementation without countermeasures. Regarding security, our approach is to prove the security of the masking scheme as comprehensively as possible by analyzing complete masked algorithms in leakage models which are well-aligned with the characteristics of real-world leakage. We first work in the threshold probing model, which is the simplest and most widely used side-channel leakage model. Our focus is the problem of composability, that is, the possibility to build complex algorithms from small building blocks while preserving the security. Concretely, we introduce a new composable security definition, design efficient constructions that satisfy it, and use them to build more complex circuits. Next, we adapt our results from the threshold probing model to the robust probing model. This model extends the threshold probing model by taking into account physical phenomena named glitches and transitions. These phenomena that appear in concrete hardware implementations violate the independence assumption of the threshold probing model. Finally, the threshold and robust probing models fix a bound on the amount of leak, (FSA - Sciences de l'ingénieur) -- UCL, 2022

We investigate the security of the NIST Lightweight Crypto Competition’s Finalists against side-channel attacks. We start with a mode-level analysis that allows us to put forward three candidates (As- con, ISAP and Romulus-T) that stand out for their leakage properties and do not require a uniform protection of all their computations thanks to (expensive) implementation-level countermeasures. We then imple- ment these finalists and evaluate their respective performances. Our re- sults confirm the interest of so-called leveled implementations (where only the key derivation and tag generation require security against dif- ferential power analysis). They also suggest that these algorithms differ more by their qualitative features (e.g., two-pass designs to improve confi- dentiality with decryption leakage vs. one-pass designs, flexible overheads thanks to masking vs. fully mode-level, easier to implement, schemes) than by their quantitative features, which all improve over the AES and are quite sensitive to security margins against cryptanalysis.

This paper introduces and analyzes Triplex, a leakage-resistant mode of operation based on Tweakable Block Ciphers (TBCs) with 2n-bit tweaks. Triplex enjoys beyond-birthday ciphertext integrity in the presence of encryption and decryption leakage in a liberal model where all intermediate computations are leaked in full and only two TBC calls operating a long-term secret are protected with implementationlevel countermeasures. It provides beyond-birthday confidentiality guarantees without leakage, and standard confidentiality guarantees with leakage for a single-pass mode embedding a re-keying process for the bulk of its computations (i.e., birthday confidentiality with encryption leakage under a bounded leakage assumption). Triplex improves leakage-resistant modes of operation relying on TBCs with n-bit tweaks when instantiated with large-tweak TBCs like Deoxys-TBC (a CAESAR competition laureate) or Skinny (used by the Romulus finalist of the NIST lightweight crypto competition). Its security guarantees are maintained in the multi-user setting.

Hybrid Homomorphic Encryption (HHE) reduces the amount of computation client-side and bandwidth usage in a Fully Homomorphic Encryption (FHE) framework. HHE requires the usage of specific symmetric schemes that can be evaluated homomorphically efficiently. In this paper, we introduce the paradigm of Group Filter Permutator (GFP) as a generalization of the Improved Filter Permutator paradigm introduced by Méaux et al. From this paradigm, we specify Elisabeth, a family of stream cipher and give an instance: Elisabeth. After asserting the security of this scheme, we provide a Rust implementation of it and ensure its performance is comparable to state-of-the-art HHE. The true strength of Elisabeth lies in the available operations server-side: while the best HHE applications were limited to a few multiplications server-side, we used data sent through Elisabeth to homomorphically evaluate a neural network inference. Finally, we discuss the improvement and loss between the HHE and the FHE framework and give ideas to build more efficient schemes from the Elisabeth family.