International Association
for Cryptologic Research

A 2-year post-doctoral research associate position is available at the Cyber Security & Privacy Research Group, Department of Computer Science, University of York, UK, to work on a research project at the intersection of cyber security and AI.
The ideal candidate will have expertise in security and privacy and familiarity with AI concepts. Experience in cryptographic design of privacy-enhancing technologies including zero-knowledge proofs, secure multi-party computation, or differential privacy is particularly welcome.
You will be working with a multi-disciplinary team spanning 7 universities across the UK and several industrial project partners (see project website: https://phawm.org).

Closing date for applications:

Contact: Dr. Siamak Shahandashti (siamak.shahandashti@york.ac.uk) for informal enquiries.
Applications need to be formally made by 1/10/2024 at https://jobs.york.ac.uk/vacancy/research-associate-566847.html, where further information about the position can also be found.

More information: https://jobs.york.ac.uk/vacancy/research-associate-566847.html

The area of modern zero-knowledge proof systems has seen a significant rise in popularity over the last couple of years, with new techniques and optimized constructions emerging on a regular basis.

As the field matures, the aspect of implementation attacks becomes more relevant, however side-channel attacks on zero-knowledge proof systems have seen surprisingly little treatment so far. In this paper we give an overview of potential attack vectors and show that some of the underlying finite field libraries, and implementations of heavily used components like hash functions, are vulnerable w.r.t. cache attacks on CPUs.

On the positive side, we demonstrate that the computational overhead to protect against these attacks is relatively small.

The see-in-the-middle (SITM) attack combines differential cryptanalysis and the ability to observe differential patterns in the side-channel leakage traces to reveal the secret key of SPN-based ciphers. While SITM presents a fresh perspective to side-channel analysis and allows attacks on deeper cipher rounds, there are practical difficulties that come with this method. First, one must realize a visual inspection of millions of power traces. Second, there is a strong requirement to reduce noise to a minimum, achieved by averaging over 1000 traces in the original work, to see the patterns. Third, the presence of a jitter-based countermeasure greatly affects pattern identification, making the visual inspection infeasible. In this paper we aim to tackle these difficulties by using a machine learning approach denoted as DL-SITM (deep learning SITM). The fundamental idea of our approach is that, while a collision obscured by noise is imperceptible in a manual inspection, a powerful deep learning model can identify it, even when a jitter-based countermeasure is in place. As we show with a practical experiment, the proposed DL-SITM approach can distinguish the two valid differentials from over 4M differential traces with only six false positives. Extrapolating from the parameters of this experiment, we get a rough estimate of $2^{43}$ key candidates for the post-processing step of our attack, which places it easily in the practical range. At the same time, we show that even with a jitter countermeasure shifting the execution by $\pm15$ samples, the testing f1-score stays at a relatively high (0.974).

We introduce $\mathsf{pKt}$ complexity, a new notion of time-bounded Kolmogorov complexity that can be seen as a probabilistic analogue of Levin's $\mathsf{Kt}$ complexity. Using $\mathsf{pKt}$ complexity, we upgrade two recent frameworks that characterize one-way functions ($\mathsf{OWF}$) via symmetry of information and meta-complexity, respectively. Among other contributions, we establish the following results:

- $\mathsf{OWF}$ can be based on the worst-case assumption that $\mathsf{BPEXP}$ is not contained infinitely often in $\mathsf{P}/\mathsf{poly}$ if the failure of symmetry of information for $\mathsf{pKt}$ in the $\textit{worst-case}$ implies its failure on $\textit{average}$. - $\mathsf{OWF}$ exist if and only if the average-case easiness of approximating $\mathsf{pKt}$ with $\textit{two-sided}$ error implies its (mild) average-case easiness with $\textit{one-sided}$ error.

Previously, in a celebrated result, Liu and Pass (CRYPTO 2021 and CACM 2023) proved that one can base (infinitely-often) $\mathsf{OWF}$ on the assumption that $\mathsf{EXP} \nsubseteq \mathsf{BPP}$ if and only if there is a reduction from computing $\mathsf{Kt}$ on average with $\textit{zero}$ error to computing $\mathsf{Kt}$ on average with $\textit{two-sided}$ error. In contrast, our second result shows that closing the gap between two-sided error and one-sided error average-case algorithms for approximating $\mathsf{pKt}$ is both necessary and sufficient to $\textit{unconditionally}$ establish the existence of $\mathsf{OWF}$.

Functional Encryption (FE) is a cryptographic technique established to guarantee data privacy while allowing the retrieval of specific results from the data. While traditional decryption methods rely on a secret key disclosing all the data, FE introduces a more subtle approach. The key generation algorithm generates function-specific decryption keys that can be adaptively provided based on policies. Adaptive access control is a good feature for privacy-preserving techniques. Generic schemes have been designed to run basic functions, such as linear regression. However, they often provide a narrow set of outputs, resulting in a lack of thorough analysis. The bottom line is that despite significant research, FE still requires appropriate constructions to unleash its full potential in securely analyzing data and providing more insights. In this article, we introduce SPADE, a novel FE scheme that features multiple users and offers fine-grained access control through partial decryption of the ciphertexts. Unlike existing FE schemes, our construction also supports qualitative data, such as genomics, expanding the applications of privacy-preserving analysis to enable a comprehensive study of the data. SPADE is a significant advancement that balances privacy and data analysis with clear implications in healthcare and finance. To verify its applicability, we conducted extensive experiments on datasets used in sleep medicine (hypnogram data) and DNA analysis (genomic records).

In recent years, cybersecurity has also become relevant for Operational Technology (OT). Critical systems like industrial automation systems or transportation systems are faced with new threats, and therefore require the implementation of thorough security measures. Regulations further mandate the deployment and regular verification of these security measures. However, OT systems differ from well-known systems of classic Information Technology (IT), such as mission times spanning decades, infrequent updates only during on-site maintenance, or diverse devices with varying support for security measures. The growing field of crypto-agility examines approaches to integrate security measures in an agile and flexible way, making updates easier and, therefore, encouraging a more frequent deployment of them. This paper contributes to this research field in the context of secure communication in two ways. We first examine the current state of crypto-agility by providing an overview of existing measures for OT systems. Then, we propose a new architecture concept with different deployment approaches to integrate security measures in a crypto-agile way. Based on a security library with a generic interface and a flexible proxy application, our architecture is capable of securing both new OT systems and existing ones via retrofit.

The field of distributed certification is concerned with certifying properties of distributed networks, where the communication topology of the network is represented as an arbitrary graph; each node of the graph is a separate processor, with its own internal state. To certify that the network satisfies a given property, a prover assigns each node of the network a certificate, and the nodes then communicate with one another and decide whether to accept or reject. We require soundness and completeness: the property holds if and only if there exists an assignment of certificates to the nodes that causes all nodes to accept. Our goal is to minimize the length of the certificates, as well as the communication between the nodes of the network. Distributed certification has been extensively studied in the distributed computing community, but it has so far only been studied in the information- theoretic setting, where the prover and the network nodes are computationally unbounded. In this work we introduce and study computationally bounded distributed certification: we define locally verifiable distributed $\mathsf{SNARG}$s ($\mathsf{LVD}$-$\mathsf{SNARG}$s), which are an analog of SNARGs for distributed networks, and are able to circumvent known hardness results for information-theoretic distributed certification by requiring both the prover and the verifier to be computationally efficient (namely, PPT algorithms). We give two $\mathsf{LVD}$-$\mathsf{SNARG}$ constructions: the first allows us to succinctly certify any network property in $\mathsf{P}$, using a global prover that can see the entire network; the second construction gives an efficient distributed prover, which succinctly certifies the execution of any efficient distributed algorithm. Our constructions rely on non-interactive batch arguments for $\mathsf{NP}$ ($\mathsf{BARG}$s) and on $\mathsf{RAM}$ $\mathsf{SNARG}$s, which have recently been shown to be constructible from standard cryptographic assumptions.

Password-protected key retrieval (PPKR) enables users to store and retrieve high-entropy keys from a server securely. The process is bootstrapped from a human-memorizable password only, addressing the challenge of how end-users can manage cryptographic key material. The core security requirement is protection against a corrupt server, which should not be able to learn the key or offline- attack it through the password protection. PPKR is deployed at a large scale with the WhatsApp Backup Protocol (WBP), allowing users to access their encrypted messaging history when switching to a new device. Davies et al. (Crypto’23) formally analyzed the WBP, proving that it satisfies most of the desired security. The WBP uses the OPAQUE protocol for password-based key exchange as a building block and relies on the server using a hardware security module (HSM) for most of its protection. In fact, the security analysis assumes that the HSM is incorruptible – rendering most of the heavy cryptography in the WBP obsolete. In this work, we explore how provably secure and efficient PPKR can be built that either relies strongly on an HSM – but then takes full advantage of that – or requires less trust assumption for the price of more advanced cryptography. To this end, we expand the definitional work by Davies et al. to allow the analysis of PPKR with fine-grained HSM corruption, such as leakage of user records or attestation keys. For each scenario, we aim to give minimal PPKR solutions. For the strongest corruption setting, namely a fully corrupted HSM, we propose a protocol with a simpler design and better efficiency than the WBP. We also fix an attack related to client authentication that was identified by Davies et al.

In this article, we derive the weight distribution of linear codes stemming from a subclass of (vectorial) $p$-ary plateaued functions (for a prime $p$), which includes all the explicitly known examples of weakly and non-weakly regular plateaued functions. This construction of linear codes is referred in the literature as the first generic construction. First, we partition the class of $p$-ary plateaued functions into three classes $\mathscr{C}_1, \mathscr{C}_2,$ and $\mathscr{C}_3$, according to the behavior of their dual function $f^*$. Using these classes, we refine the results presented in a series of articles \cite{Mesnager2017, MesOzSi,Pelen2020, RodPasZhaWei, WeiWangFu}. Namely, we derive the full weight distributions of codes stemming from all $s$-plateaued functions for $n+s$ odd (parametrized by the weight of the dual $wt(f^*)$), whereas for $n+s$ even, the weight distributions are derived from the class of $s$-plateaued functions in $\mathscr{C}_1$ parametrized using two parameters (including $wt(f^*)$ and a related parameter $Z_0$). Additionally, we provide more results on the different weight distributions of codes stemming from functions in subclasses of the three different classes. The exact derivation of such distributions is achieved by using some well-known equations over finite fields to count certain dual preimages. In order to improve the dimension of these codes, we then study the vectorial case, thus providing the weight distributions of a few codes associated to known vectorial plateaued functions and obtaining codes with parameters $[p^n-1,2n, p^n-p^{n-1} - {p}^{(n+s-2)/2}(p-1)]$. For the first time, we provide the full weight distributions of codes from (a subclass of) vectorial $p$-ary plateaued functions. This class includes all known explicit examples in the literature. The obtained codes are minimal and self-orthogonal virtually in all cases.

An ongoing research challenge in symmetric cryptography is to design an authenticated encryption (AE) with a commitment to the secret key or preferably to the entire context. One way to achieve this is to use a transform on an existing AE scheme, if possible with no output length expansion. At EUROCRYPT'22, Bellare and Hoang proposed the HtE transform, which lifts key-commitment to context-commitment. In the same year at ESORICS'22, Chan and Rogaway proposed the CTX transform, which works on any AE scheme where the tag is not required for decryption. However, for AE schemes which are not key-committing to begin with and which use the tag for decryption, no such transform exists till date. The latter category encompasses all AE schemes based on the design paradigms SIV, MAC-then-Encrypt, and Encode-then-Encipher. In this work, we propose PACT, a transform to convert any AE scheme into a context-committing one without any output length expansion. In addition, PACT preserves both nonce-respecting and nonce-misuse security of the legacy AE scheme. However, this is not the case with all the existing transforms. To demonstrate this, we show that a combination of CTY and SC (proposed by Bellare and Hoang, CRYPTO'24) doesn't preserve the nonce-misuse security of the legacy AE scheme. PACT requires only one call to a collision-resistant unkeyed hash function and one call to a block cipher. Finally, we propose a lighter transform comPACT, which converts a nonce-respecting AE scheme into a context-committing one.

AES implementation has been vastly analysed against side-channel attacks in the last two decades particularly targeting resource-constrained microcontrollers. Still, less research has been conducted on AES implementations on advanced hardware platforms. In this study, we examine the resilience of AES on an ARM Cortex A72 processor within the Raspberry Pi 4B model. Unlike their microcontroller counterparts, these platforms operate within the complex ecosystem of an operating system (OS), resulting in EM traces characterized by low signal-to-noise ratios and jitter. We discuss the inefficacy of traditional CPA attacks in the presence of noise, misalignment, and jitter (in trace and trigger signals). The interrupts and daemons cause these effects, resulting in context switch overheads leading to increased variability in execution times. Additionally, there are no fixed methods or set rules for pre-processing; the approach varies depending on the target device. Our experiments show that CPA is ineffective against masked and unmasked AES implementations on ARM Cortex A72. Therefore, we resort to deep learning-based side-channel analysis (DL-SCA) techniques, that do not require extensive data pre-processing and can effectively work with EM traces that have low signal-to-noise ratios. Using DL-SCA we could recover the AES secret key. Our experiments underscore the formidable challenge posed by breaking AES on ARM Cortex processors compared to conventional microcontroller-based implementations. Importantly, our findings extend beyond previous studies, marking the first successful attack on ARM Cortex A72 and demonstrating the efficacy of DL-SCA even when pre-processing techniques are varied and not standardized.

Secure elements are small microcontrollers whose main purpose is to generate/store secrets and then execute cryptographic operations. They undergo the highest level of security evaluations that exists (Common Criteria) and are often considered inviolable, even in the worst-case attack scenarios. Hence, complex secure systems build their security upon them.

FIDO hardware tokens are strong authentication factors to sign in to applications (any web service supporting FIDO); they often embed a secure element and the FIDO protocol uses Elliptic Curve Digital Signature Algorithm (ECDSA for short) as its core cryptographic primitive. YubiKey 5 Series are certainly the most widespread FIDO hardware tokens, their secure element is an Infineon SLE78.

This document shows how – finding a JavaCard open platform (the Feitian A22) based on a similar Infineon SLE78 – we understood the Infineon ECDSA implementation, found a side-channel vulnerability and designed a practical side-channel attack. The attack is then demonstrated on a YubiKey 5Ci. Finally, we show that the vulnerability extends to the more recent Infineon Optiga Trust M and Infineon Optiga TPM security microcontrollers.

Our work unearths a side-channel vulnerability in the cryptographic library of Infineon Technologies, one of the biggest secure element manufacturers. This vulnerability – that went unnoticed for 14 years and about 80 highest-level Common Criteria certification evaluations – is due to a non constant-time modular inversion.

The attack requires physical access to the secure element (few local electromagnetic side-channel acquisitions, i.e. few minutes, are enough) in order to extract the ECDSA secret key. In the case of the FIDO protocol, this allows to create a clone of the FIDO device.

All YubiKey 5 Series (with firmware version below 5.7) are impacted by the attack and in fact all Infineon security microcontrollers (including TPMs) that run the Infineon cryptographic library (as far as we know, any existing version) are vulnerable to the attack. These security microcontrollers are present in a vast variety of secure systems – often relying on ECDSA – like electronic passports and crypto-currency hardware wallets but also smart cars or homes. However, we did not check (yet) that the EUCLEAK attack applies to any of these products.

Bootstrapping stands as a fundamental component of fully homomorphic encryption (FHE) schemes, facilitating an infinite number of operations by recovering the ciphertext modulus. This work is aimed at significantly reducing the consumption of modulus in bootstrapping, thereby enhancing the efficiency of FHE performance, specifically for the Cheon--Kim--Kim--Song (CKKS) scheme proposed by Cheon et al. Building on the EvalRound bootstrapping method proposed by Kim et al., which includes the steps of ModRaise, CoeffToSlot, EvalRound and SlotToCoeff, we introduce $\textrm{EvalRound}^{+}$ bootstrapping. This bootstrapping inherits the advantage of EvalRound bootstrapping in CoeffToSlot and resolves its disadvantage in SlotToCoeff. Furthermore, we conduct a set of rigorous and comprehensive analyses to precisely determine the optimal choices of the parameters. The implementation of $\textrm{EvalRound}^{+}$ bootstrapping, along with optimal choices, has achieved a reduction in modulus consumption by over $40\%$ for CoeffToSlot and SlotToCoeff. Additionally, it has increased the number of levels for general multiplication by 2-4 in the most widely used bootstrapping parameter sets.

Blind signatures have garnered significant attention in recent years, with several efficient constructions in the random oracle model relying on well-understood assumptions. However, this progress does not apply to pairing-free cyclic groups: fully secure constructions over cyclic groups rely on pairings, remain inefficient, or depend on the algebraic group model or strong interactive assumptions. To address this gap, Chairattana-Apirom, Tessaro, and Zhu (CTZ, Crypto 2024) proposed a new scheme based on the CDH assumption. Unfortunately, their construction results in large signatures and high communication complexity.

In this work, we propose a new blind signature construction in the random oracle model that significantly improves upon the CTZ scheme. Compared to CTZ, our scheme reduces communication complexity by a factor of more than 10 and decreases the signature size by a factor of more than 45, achieving a compact signature size of only 224 Bytes. The security of our scheme is based on the DDH assumption over pairing-free cyclic groups, and we show how to generalize it to the partially blind setting.

In this paper, we study the security definitions of various threshold symmetric primitives. Namely, we analyze the security definitions for threshold pseudorandom functions, threshold message authentication codes and threshold symmetric encryption. In each case, we strengthen the existing security definition, and we present a scheme that satisfies our stronger notion of security. In particular, we propose indifferentiability definition and IND-CCA2 definition for a threshold pseudorandom function and a threshold symmetric encryption scheme, respectively. Moreover, we show that these definitions are achievable. Notably, we propose the first IND-CCA2 secure threshold symmetric encryption scheme.

Fully homomorphic encryption schemes are methods to perform compu- tations over encrypted data. Since its introduction by Gentry, there has been a plethora of research optimizing the originally inefficient cryptosystems. Over time, different families have emerged. On the one hand, schemes such as BGV, BFV, or CKKS excel at performing coefficient-wise addition or multiplication over vectors of encrypted data. In contrast, accumulator-based schemes such as FHEW and TFHE provide efficient methods to evaluate boolean circuits and means to efficiently compute functions over small plaintext space of up to 4-5 bits in size. In this paper, we focus on the second family. At a high level, accumulator-based schemes encode the range of a function f in the coefficients of a polynomial, which is then encrypted in a homomorphic accumulator. Given an input ciphertext, the coefficients of the encrypted polynomial are homomorphically rotated, such that there is a correspondence between the constant term of the result and the message contained in the ciphertext. In the end, it is possible to derive a ciphertext of the constant term encrypted with regard to the same encryption scheme as the input ciphertext. To summarize, by appropriately encoding the function f on the accumulated polynomial, we can compute f on the plaintext of the input ciphertext, where the output ciphertext has its noise magnitude independent of the input ciphertext. However, by default, it is necessary to impose restrictions on the type of functions we evaluate or drastically limit the plaintext space that can be correctly processed. Otherwise, the procedure’s output will be incorrect and hard to predict. In this work, we describe two novel algorithms that have no such restrictions. Furthermore, we derive an algorithm that enables a user to evaluate an arbitrary amount of functions at a computational cost that differs only by a constant amount compared to a single function. Our methods lead to an evaluation that is between 15 and 31% faster than previous works while also being conceptually simpler.

In this article, we propose a generic hybrid encryption scheme providing entity authentication. The scheme is based on lossy trapdoor functions relying on the hardness of the Learning With Errors problem. The construction can be used on a number of different security requirements with minimal reconfiguration. It ensures entity authentication and ciphertext integrity while providing security against adaptive chosen ciphertext attacks in the standard model. As a desired characteristic of schemes providing entity authentication, we prove the strong unforgeability under chosen message attack for the construction. In addition, the scheme is post-quantum secure based on the hardness of the underlying assumption.

In 2022, Wang et al. proposed the multivariate signature scheme SNOVA as a UOV variant over the non-commutative ring of $\ell \times \ell $ matrices over $\mathbb{F}_q$. This scheme has small public key and signature size and is a first round candidate of NIST PQC additional digital signature project. Recently, Ikematsu and Akiyama, and Li and Ding show that the core matrices of SNOVA with $v$ vinegar-variables and $o$ oil-variables are regarded as the representation matrices of UOV with $\ell v$ vinegar-variables and $\ell o$ oil-variables over $\mathbb{F}_q$, and thus we can apply existing key recovery attacks as a plain UOV. In this paper, we propose a method that reduces SNOVA to smaller UOV with $v$ vinegar-variables and $o$ oil-variables over $\mathbb{F}_{q^\ell }$. As a result, we show that the previous first round parameter sets at $\ell = 2$ do not meet the NIST PQC security levels. We also confirm that the present parameter sets are secure from existing key recovery attacks with our approach.

To be competitive with other signature schemes, the MLWE instance $\bf (A,t)$ on which Dilithium is based is compressed: the least significant bits of $\bf t$, which are denoted $\textbf{t}_0$, are considered part of the secret key. Knowing $\bf t_0$ does not provide any information about the other data in the secret key, but it does allow the construction of much more efficient side-channel attacks. Yet to the best of our knowledge, there is no kown way to recover $\bf t_0$ from Dilithium signatures. In this work, we show that each Dilithium signature leaks information on $\bf t_0$, then we construct an attack that retrieves the vector $\bf t_0$ from Dilithium signatures. Experimentally, for Dilithium-2, $4\,000\,000$ signatures and $2$ hours are sufficient to recover $\textbf{t}_0$ on a desktop computer.

Achieving malicious security with high efficiency in dishonest-majority secure multiparty computation is a formidable challenge. The milestone works SPDZ and TinyOT have spawn a large family of protocols in this direction. For boolean circuits, state-of-the-art works (Cascudo et. al, TCC 2020 and Escudero et. al, CRYPTO 2022) have proposed schemes based on reverse multiplication-friendly embedding (RMFE) to reduce the amortized cost. However, these protocols are theoretically described and analyzed, resulting in a significant gap between theory and concrete efficiency.

Our work addresses existing gaps by refining and correcting several issues identified in prior research, leading to the first practically efficient realization of RMFE. We introduce an array of protocol enhancements, including RMFE-based quintuples and (extended) double-authenticated bits, aimed at improving the efficiency of maliciously secure boolean and mixed circuits. The culmination of these efforts is embodied in Coral, a comprehensive framework developed atop the MP-SPDZ library. Through rigorous evaluation across multiple benchmarks, Coral demonstrates a remarkable efficiency gain, outperforming the foremost theoretical approach by Escudero et al. (which incorporates our RMFE foundation albeit lacks our protocol enhancements) by a factor of 16-30×, and surpassing the leading practical implementation for Frederiksen et al. (ASIACRYPT 2015) by 4-7×.