International Association
for Cryptologic Research

Targeted Denial-of-Service (DoS) attacks have been a practical concern for permissionless blockchains. Potential solutions, such as random sampling, are adopted by blockchains. However, the associated security guarantees have only been informally discussed in prior work. This is due to the fact that existing adversary models are either not fully capturing this attack or giving up certain design choices (as in the sleepy model or asynchronous network model), or too strong to be practical (as in the mobile Byzantine adversary model).

This paper provides theoretical foundations and desired properties for consensus protocols that resist against targeted DoS attacks. In particular, we define the Mobile Crash Adaptive Byzantine (MCAB) model to capture such an attack. In addition, we identify and formalize two properties for consensus protocols under the MCAB model, and analyze their trade-offs. As case studies, we prove that Ouroboros Praos and Algorand are secure in our MCAB model, giving the first formal proofs supporting their security guarantee against targeted DoS attacks, which were previously only informally discussed. We also illustrate an application of our properties to secure a streamlined BFT protocol, chained Hotstuff, against targeted DoS attacks.

In this work we demonstrate for the first time that a full FHE bootstrapping operation can be proven using a SNARK in practice. We do so by designing an arithmetic circuit for the bootstrapping operation and prove it using plonky2. We are able to prove the circuit on an AWS C6i.metal instance in about 20 minutes. Proof size is about 200kB and verification takes less than 10ms. As the basis of our bootstrapping operation we use TFHE's programmable bootstrapping and modify it in a few places to more efficiently represent it as an arithmetic circuit (while maintaining full functionality and security). In order to achieve our results in a memory-efficient way, we take advantage of the structure of the computation and plonky2's ability to efficiently prove its own verification circuit to implement a recursion-based IVC scheme.

An Oblivious Pseudo-Random Function (OPRF) is a two-party protocol for jointly evaluating a Pseudo-Random Function (PRF), where a user has an input x and a server has an input k. At the end of the protocol, the user learns the evaluation of the PRF using key k at the value x, while the server learns nothing about the user's input or output.

OPRFs are a prime tool for building secure authentication and key exchange from passwords, private set intersection, private information retrieval, and many other privacy-preserving systems. While classical OPRFs run as fast as a TLS Handshake, current *quantum-safe* OPRF candidates are still practically inefficient.

In this paper, we propose a framework for constructing OPRFs from post-quantum multi-party computation. The framework captures a family of so-called "2Hash PRFs", which sandwich a function evaluation in between two hashes. The core of our framework is a compiler that yields an OPRF from a secure evaluation of any function that is key-collision resistant and one-more unpredictable. We instantiate this compiler by providing such functions built from Legendre symbols, and from AES encryption. We then give a case-tailored protocol for securely evaluating our Legendre-based function, built from oblivious transfer (OT) and zero-knowledge proofs (ZKP). Instantiated with lattice-based OT and ZKPs, we obtain a quantum-safe OPRF that completes in 0.57 seconds, with less than 1MB of communication.

$n$-out-of-$n$ distributed signatures are a special type of threshold $t$-out-of-$n$ signatures. They are created by a group of $n$ signers, each holding a share of the secret key, in a collaborative way. This kind of signatures has been studied intensively in recent years, motivated by different applications such as reducing the risk of compromising secret keys in cryptocurrencies. Towards maintaining security in the presence of quantum adversaries, Damgård et al. (J Cryptol 35(2), 2022) proposed lattice-based constructions of $n$-out-of-$n$ distributed signatures and multi-signatures following the Fiat-Shamir with aborts paradigm (ASIACRYPT 2009). Due to the inherent issue of aborts, the protocols either require to increase their parameters by a factor of $n$, or they suffer from a large number of restarts that grows with $n$. This has a significant impact on their efficiency, even if $n$ is small. Moreover, the protocols use trapdoor homomorphic commitments as a further cryptographic building block, making their deployment in practice not as easy as standard lattice-based Fiat-Shamir signatures. In this work, we present a new construction of $n$-out-of-$n$ distributed signatures. It is designed specifically for applications with small number of signers. Our construction follows the Fiat-Shamir with aborts paradigm, but solves the problem of large number of restarts without increasing the parameters by a factor of $n$ and utilizing any further cryptographic primitive. To demonstrate the practicality of our protocol, we provide a software implementation and concrete parameters aiming at 128 bits of security. Furthermore, we select concrete parameters for the construction by Damgård et al. and for the most recent lattice-based multi-signature scheme by Chen (CRYPTO 2023), and show that our approach provides a significant improvement in terms of all efficiency metrics. Our results also show that the multi-signature schemes by Damgård et al. and Chen as well as a multi-signature variant of our protocol produce signatures that are not smaller than a naive multi-signature derived from the concatenation of multiple standard signatures.

In this study, we investigate the newly developed low energy lightweight block cipher (LELBC), specifically designed for resource-constrained Internet of Things (IoT) devices in smart agriculture. The designers conducted a preliminary differential cryptanalysis of LELBC through mixed-integer linear programming (MILP). This paper further delves into LELBC’s differential characteristics in both single and related-key frameworks using MILP, identifying a nine-round differential characteristic with a probability of $2^{-60}$ in a single-key framework and a 12-round differential characteristic with a probability of $2^{-60}$ in a related-key framework.

The Fully Homomorphic Encryption research group at CEA-LIST, a French public government funded research organisation, is one of the largest research groups that dedicatedly works on homomorphic encryption. Potential use cases include Privacy preserving Machine learning and Deep learning, to name a few. However, different schemes are suited for different use-cases, and thus there is a greater need of efficient switching between schemes.

Thus we are looking for a highly motivated PhD candidate with a string background in applied cryptography including FHE/MPC.

The candidate must meet the following requirements

Education: Hold an M.Sc. degree (or equivalent) with excellent grades in Information security or Computer science.

Experience and Knowledge: Strong background in (applied) cryptography with a particular focus on cryptographic protocols/FHE/MPC. Good software development/programming skills.

Personality and Working Practice: Self-motivated and enthusiastic, independent, reliable, creative, and able to work in an international team with diverse background.

Language Fluent English, knowledge of French not required The successful candidate will:

become a part of the team of 10 full time researchers (one of largest teams in the world dedicated to FHE) and advance research on FHE.

develop new protocols and techniques in FHE,

publish and present your results in top-tier journals and conferences.

The position is based at the CEA-LIST Nano-Innov campus in Palaiseau, France (30 mins from central Paris), fully funded for three years, no teaching duties, annual leaves, and the usual benefits.

Closing date for applications:

Contact: Olive Chakraborty (olive.chakraborty@cea.fr )

Contact us with your CV and Cover letter for more details on the subject.

Event date: 20 November to 21 November 2024
Submission deadline: 20 June 2024
Notification: 10 September 2024

Event date: 2 December to 7 December 2024

Transport Layer Security ( TLS ) is foundational for safeguarding client-server communication. However, it does not extend integrity guarantees to third-party verification of data authenticity. If a client wants to present data obtained from a server, it cannot convince any other party that the data has not been tampered with.

TLS oracles ensure data authenticity beyond the client-server TLS connection, such that clients can obtain data from a server and ensure provenance to any third party, without server-side modifications. Generally, a TLS oracle involves a third party, the verifier, in a TLS session to verify that the data obtained by the client is accurate. Existing protocols for TLS oracles are communication-heavy, as they rely on interactive protocols. We present ORIGO, a TLS oracle with constant communication. Similar to prior work, ORIGO introduces a third party in a TLS session, and provides a protocol to ensure the authenticity of data transmitted in a TLS session, without forfeiting its confidentiality. Compared to prior work, we rely on intricate details specific to TLS 1.3, which allow us to prove correct key derivation, authentication and encryption within a Zero Knowledge Proof (ZKP). This, combined with optimizations for TLS 1.3, leads to an efficient protocol with constant communication in the online phase. Our work reduces online communication by $375 \times$ and online runtime by up to $4.6 \times$, compared to prior work.

With the ongoing advances in machine learning (ML), cybersecurity solutions and security primitives are becoming increasingly vulnerable to successful attacks. Strong physically unclonable functions (PUFs) are a potential solution for providing high resistance to such attacks. In this paper, we propose a generalized attack model that leverages multiple chips jointly to minimize the cloning error. Our analysis shows that the entropy rate over different chips is a relevant measure to the new attack model as well as the multi-bit strong PUF classes. We explain the sources of randomness that affect unpredictability and its possible measures using models of state-of-the-art strong PUFs. Moreover, we utilize min-max entropy estimators to measure the unpredictability of multi-bit strong PUF classes for the first time in the PUF community. Finally, we provide experimental results for a multi-bit strong PUF class, the hybrid Boolean network PUF, showing its high unpredictability and resistance to ML attacks.

Structure-preserving signatures (SPS) have emerged as an important cryptographic building block, as their compatibility with the Groth-Sahai (GS) NIZK framework allows to construct protocols under standard assumptions with reasonable efficiency. Over the last years there has been a significant interest in the design of threshold signature schemes. However, only very recently Crites et al. (ASIACRYPT 2023) have introduced threshold SPS (TSPS) along with a fully non-interactive construction. While this is an important step, their work comes with several limitations. With respect to the construction, they require the use of random oracles, interactive complexity assumptions and are restricted to so called indexed Diffie-Hellman message spaces. Latter limits the use of their construction as a drop-in replacement for SPS. When it comes to security, they only support static corruptions and do not allow partial signature queries for the forgery. In this paper, we ask whether it is possible to construct TSPS without such restrictions. We start from an SPS from Kiltz, Pan and Wee (CRYPTO 2015) which has an interesting structure, but thresholdizing it requires some modifications. Interestingly, we can prove it secure in the strongest model (TS-UF-1) for fully non-interactive threshold signatures (Bellare et al., CRYPTO 2022) and even under fully adaptive corruptions. Surprisingly, we can show the latter under a standard assumption without requiring any idealized model. All known constructions of efficient threshold signatures in the discrete logarithm setting require interactive assumptions and idealized models. Concretely, our scheme in type III bilinear groups under the SXDH assumption has signatures consisting of 7 group elements. Compared to the TSPS from Crites et al. (2 group elements), this comes at the cost of efficiency. However, our scheme is secure under standard assumptions, achieves strong and adaptive security guarantees and supports general message spaces, i.e., represents a drop-in replacement for many SPS applications. Given these features, the increase in the size of the signature seems acceptable even for practical applications.

We introduce a private cryptocurrency design based on the original e-cash protocol. Our proposal allows for private payments on existing blockchain systems. In our design, the issuance of the private cash is transparent and is associated with a blockchain transfer to provide stronger security.

Sparse binary LWE secrets are under consideration for standardization for Homomorphic Encryption and its applications to private computation. Known attacks on sparse binary LWE secrets include the sparse dual attack and the hybrid sparse dual-meet in the middle attack, which requires significant memory. In this paper, we provide a new statistical attack with low memory requirement. The attack relies on some initial parallelized lattice reduction. The key observation is that, after lattice reduction is applied to the rows of a q-ary-like embedded random matrix A, the entries with high variance are concentrated in the early columns of the extracted matrix. This allows us to separate out the “hard part” of the LWE secret. We can first solve the sub-problem of finding the “cruel” bits of the secret in the early columns, and then find the remaining “cool” bits in linear time. We use statistical techniques to distinguish distributions to identify both the cruel and the cool bits of the secret. We provide concrete attack timings for recovering secrets in dimensions n = 256, 512, and 768. For the lattice reduction stage, we leverage recent improvements in lattice reduction (flatter) applied in parallel. We also apply our new attack in the RLWE setting for 2-power cyclotomic rings, showing that these RLWE instances are much more vulnerable to this attack than LWE.

In the rapidly evolving fields of encryption and blockchain technologies, the efficiency and security of cryptographic schemes significantly impact performance. This paper introduces a comprehensive framework for continuous benchmarking in one of the most popular cryptography Rust libraries, fastcrypto. What makes our analysis unique is the realization that automated benchmarking is not just a performance monitor and optimization tool, but it can be used for cryptanalysis and innovation discovery as well. Surprisingly, benchmarks can uncover spectacular security flaws and inconsistencies in various cryptographic implementations and standards, while at the same time they can identify unique opportunities for innovation not previously known to science, such as providing a) hints for novel algorithms, b) indications for mix-and-match library functions that result in world record speeds, and c) evidences of biased or untested real world algorithm comparisons in the literature.

Our approach transcends traditional benchmarking methods by identifying inconsistencies in multi-threaded code, which previously resulted in unfair comparisons. We demonstrate the effectiveness of our methodology in identifying the fastest algorithms for specific cryptographic operations like signing, while revealing hidden performance characteristics and security flaws. The process of continuous benchmarking allowed fastcrypto to break many crypto-operations speed records in the Rust language ecosystem.

A notable discovery in our research is the identification of vulnerabilities and unfair speed claims due to missing padding checks in high-performance Base64 encoding libraries. We also uncover insights into algorithmic implementations such as multi-scalar elliptic curve multiplications, which exhibit different performance gains when applied in different schemes and libraries. This was not evident in conventional benchmarking practices. Further, our analysis highlights bottlenecks in cryptographic algorithms where pre-computed tables can be strategically applied, accounting for L1 and L2 CPU cache limitations.

Our benchmarking framework also reveals that certain algorithmic implementations incur additional overheads due to serialization processes, necessitating a refined `apples to apples' comparison approach. We identified unique performance patterns in some schemes, where efficiency scales with input size, aiding blockchain technologies in optimal parameter selection and data compression.

Crucially, continuous benchmarking serves as a tool for ongoing audit and security assurance. Variations in performance can signal potential security issues during upgrades, such as cleptography, hardware manipulation or supply chain attacks. This was evidenced by critical private key leakage vulnerabilities we found in one of the most popular EdDSA Rust libraries. By providing a dynamic and thorough benchmarking approach, our framework empowers stakeholders to make informed decisions, enhance security measures, and optimize cryptographic operations in an ever-changing digital landscape.

We formally define the Lattice Isomorphism Problem for module lattices (module-LIP) in a number field $K$. This is a generalization of the problem defined by Ducas, Postlethwaite, Pulles, and van Woerden (Asiacrypt 2022), taking into account the arithmetic and algebraic specificity of module lattices from their representation using pseudo-bases. We also provide the corresponding set of algorithmic and theoretical tools for the future study of this problem in a module setting. Our main contribution is an algorithm solving module-LIP for modules of rank $2$ in $K^2$, when $K$ is a totally real number field. Our algorithm exploits the connection between this problem, relative norm equations and the decomposition of algebraic integers as sums of two squares. For a large class of modules (including $\mathcal{O}_K^2$), and a large class of totally real number fields (including the maximal real subfield of cyclotomic fields) it runs in classical polynomial time in the degree of the field and the residue at 1 of the Dedekind zeta function of the field (under reasonable number theoretic assumptions). We provide a proof-of-concept code running over the maximal real subfield of some cyclotomic fields.

Soft Analytical Side Channel Attacks (SASCA) are a powerful family of Side Channel Attacks (SCA) that allow to recover secret values with only a small number of traces. Their effectiveness lies in the Belief Propagation (BP) algorithm, which enables efficient computation of the marginal distributions of intermediate values. Post-quantum schemes such as Kyber, and more recently, Hamming Quasi-Cyclic (HQC), have been targets of SASCA. Previous SASCA on HQC focused on Reed-Solomon (RS) codes and successfully retrieved the shared key with a high success rate for high noise levels using a single trace. In this work, we present new SASCA on HQC where both the shared key and the secret key are targeted. Unlike the previous SASCA, we take a closer look at the Reed-Muller (RM) code. The advantage of this choice, is that the RM decoder is applied before the RS decoder. This is what makes it possible to attack the two keys. We build a factor graph of the Fast Hadamard Transform (FHT) function from the HQC reference implementation of April 2023. The information recovered from BP allows us to retrieve the shared key with a single trace. In addition to the previous SASCA targeting HQC, we also manage to recover the secret key with two chosen ciphertext attacks. One of them require a single trace and is successful until high noise levels.

The threshold implementation technique has been proposed in 2006 by Nikova et al. as a countermeasure to mitigate cryptographic side-channel attacks on hardware implementations when the effect of glitches is taken into account. This technique is based on Boolean sharing (also called masking) and it was developed for securing symmetric ciphers such as AES. In 2023, Piccione et al. proposed a general construction of threshold implementations that is universal for S-boxes that are bijective vectorial Boolean function (functions from a binary vector space $\mathbb{F}_{2}^n$ into itself). In this paper, we further generalize the construction and we propose a general theory of threshold implementations for any type of S-boxes. We investigate the case of functions (also not necessarily bijective) that are defined between two finite Abelian groups and we use the definition of threshold implementation given by Dhooghe et al. in 2019 with additive sharing. To show that this generalized notion is as useful as the one for Boolean sharing, we prove that many classical results still hold. An important tool in this theory is the notion of functional degree introduced by Aichinger and Moosbauer in 2021 which generalizes the algebraic degree of a vectorial Boolean function. We show that if $F$ has functional degree (at most) $d$ and the cardinality of the domain is divisible by the cardinality of the codomain, then $F$ admits a threshold implementation $\mathcal{F}$ with $s\geq d+2$ shares in input and $d+2$ shares in output. Moreover, we provide a complete overview on which are the available tools for studying the functional degree and how to represent those functions using a Integer-Valued (IV) polynomial representation. Then we apply our theory for the following applications: defining the inner product masking in our setting, providing a threshold implementation of any multiplication map, and computing the functional degree and the IV polynomial representations of the conversion maps between $\mathbb{F}_p^n$ and $\mathbb{Z}_{p^n}$.

Side-channel analysis has become a cornerstone of modern hardware security evaluation for cryptographic accelerators. Recently, these techniques are also being applied in fields such as AI and Machine Learning to investigate possible threats. Security evaluations are reliant on standard test setups including commercial and open-source evaluation boards such as, SASEBO/SAKURA and ChipWhisperer. However, with shrinking design footprints and overlapping tasks on the same platforms, the quality of the side channel information as well as the speed of data capture can significantly influence security assessment.

In this work, we designed EFFLUX-F2, a hardware security evaluation board to improve the quality and speed of side-channel information capture. We also designed a measurement setup to benchmark the signal differences between target boards. Multiple experimental evaluations like noise analysis, CPA and TVLA performed on EFFLUX-F2 and competing evaluation boards showcase the significant superiority of our design in all aspects.

This note reveals a vulnerability of MuSig and BN multi-signatures when used with delayed message selection. Despite the fact that both schemes can be correctly implemented with preprocessing of the first two signing rounds before the message to sign is selected, we show that they are insecure (i.e. not existentially unforgeable against chosen message attacks) when the message selection is deferred to the third signing round and when parallel signing sessions are permitted. The attack, which uses the algorithm by Benhamouda et al. to solve the ROS problem, is practical and runs in polynomial time.

We define a (small) augmentation to the FROST threshold signature scheme that additionally allows for re-randomizable public and secret keys. We build upon the notion of re-randomizable keys in the literature, but tailor this capability when the signing key is secret-shared among a set of mutually trusted parties. We do not make any changes to the plain FROST protocol, but instead define additional algorithms to allow for randomization of the threshold public key and participant’s individual public and secret key shares.

We show the security of this re-randomized extension to FROST with respect to the algebraic one-more discrete logarithm (AOMDL) problem in the random oracle model, the same security assumptions underlying plain FROST.