International Association
for Cryptologic Research

Anticipating the advent of large quantum computers, NIST started a worldwide competition in 2016 aiming to define the next cryptographic standards. HQC is one of these post-quantum schemes still in contention, with four others already in the process of being standardized. In 2022, Guo et al. introduced a timing attack that exploited an inconsistency in HQC rejection sampling function to recover its secret key in 866,000 calls to an oracle. The authors of HQC updated its specification by applying an algorithm to sample vectors in constant time. A masked implementation of this function was then proposed for BIKE but it is not directly applicable to HQC. In this paper we propose a masked specification-compliant version of HQC vector sampling function which relies, to our knowledge, on the first masked implementation of the Barrett reduction.

We have proposed a novel FHE scheme that uniquely encodes the plaintext with noise in a way that prevents the increasing noise from overflowing and corrupting the plaintext. This allows users to perform computations on encrypted data smoothly. The scheme is constructed using the Chinese Remainder Theorem (CRT), supporting a predefined number of modular operations on encrypted plaintext without the need for bootstrapping. Although FHE recently became popular after Gentry's work and various developments have occurred in the last decade, the idea of "Fully Homomorphic Encryption (FHE)" scheme was first introduced in the 1970s by Rivest. The Chinese Remainder Theorem is one of the most suitable tools for developing a FHE Scheme because it forms a ring homomorphism \( Z_{p_1} \times Z_{p_2} \times \ldots \times Z_{p_k} \cong Z_{p_1 p_2 \ldots p_k} \). Various attempts have been made to develop a FHE using CRT, but most of them were unsuccessful, mainly due to the chosen plaintext attack (CPA). The proposed scheme overcomes the chosen plaintext attack. The scheme also adds random errors to the message during encryption. However, these errors are added in such a way that, when homomorphic operations are performed over encrypted data, the increasing values of errors never overwrite the values of the messages, as happens in LWE-based homomorphic schemes. Therefore, one can perform a predefined number of homomorphic operations (both addition and multiplication) without worrying about the increasing values of errors.

We address the black-box complexity of constructing pseudorandom functions (PRF) from pseudorandom generators (PRG). The celebrated GGM construction of Goldreich, Goldwasser, and Micali (Crypto 1984) provides such a construction, which (even when combined with Levin's domain-extension trick) has super-logarithmic depth. Despite many years and much effort, this remains essentially the best construction we have to date. On the negative side, one step is provided by the work of Miles and Viola (TCC 2011), which shows that a black-box construction which just calls the PRG once and outputs one of its output bits, cannot be a PRF.

In this work, we make significant further progress: we rule out black-box constructions of PRF from PRG that follow certain structural constraints, but may call the PRG adaptively polynomially many times. In particular, we define ``tree constructions" which generalize the GGM structure: they apply the PRG $G$ along a tree path, but allow for different choices of functions to compute the children of a node on the tree and to compute the next node on the computation path down the tree. We prove that a tree construction of logarithmic depth cannot be a PRF (while GGM is a tree construction of super-logarithmic depth). We also show several other results and discuss the special case of one-call constructions.

Our main results in fact rule out even weak PRF constructions with one output bit. We use the oracle separation methodology introduced by Gertner, Malkin, and Reingold (FOCS 2001), and show that for any candidate black-box construction $F^G$ from $G$, there exists an oracle relative to which $G$ is a PRG, but $F^G$ is not a PRF.

The multilinear extension of an $m$-variate function $f : \{0,1\}^m \to \mathbb{F}$, relative to a finite field $\mathbb{F}$, is the unique multilinear polynomial $\hat{f} : \mathbb{F}^m \to \mathbb{F}$ that agrees with $f$ on inputs in $\{0,1\}^m$. In this note we show how, given oracle access to $f : \{0,1\}^m \to \mathbb{F}$ and a point $z \in \mathbb{F}^m$, to compute $\hat{f}(z)$ using $O(2^m)$ field operations and only $O(m)$ space. This improves on a previous algorithm due to Vu et al. (SP, 2013), which similarly uses $O(2^m)$ field operations but requires $O(2^m)$ space. Furthermore, the number of field additions in our algorithm is about half of that in Vu et al.'s algorithm, whereas the number of multiplications is the same up to small additive terms.

We show that the scalar product protocol [IEEE Trans. Parallel Distrib. Syst. 2023, 1060-1066] is insecure against semi-honest server attack, not as claimed. Besides, its complexity increases exponentially with the number $n$, which cannot be put into practice.

In this paper we introduce new concept of tropical increasing matrices and then prove that two tropical increasing matrices are commute. Using this property, we modified Stickel’s protocol. This idea similar to [5] where modified Stickel’s protocol using commuting matrices (Linde De La Puente Matrices).

A Blind Signature Scheme (BSS) is a cryptographic primitive that enables a user to obtain a digital signature on a message from a signer without revealing the message itself. The standard security notion against malicious users for a BSS is One-More Unforgeability (OMUF). One of the earliest and most well-studied blind signature schemes is the Schnorr BSS, although recent results show it does not satisfy OMUF. On the other hand, the Schnorr BSS does satisfy the weaker notion of sequential OMUF --- which restricts adversaries to opening signing sessions one at a time --- in the Algebraic Group Model (AGM) + Random Oracle Model (ROM). In light of this result, a natural question arises: does the Schnorr BSS satisfy OMUF with regard to adversaries that open no more than a small number of signing sessions concurrently?

This paper serves as a first step towards characterizing the security of the Schnorr BSS in the limited concurrency setting. Specifically, we demonstrate that the Schnorr BSS satisfies OMUF when at most two signing sessions can be open concurrently (in the AGM+ROM). Our argument suggests that it is plausible that the Schnorr BSS satisfies OMUF for up to polylogarithmically many concurrent signing sessions.

With concerns about data privacy growing in a connected world, cryptography researchers have focused on fully homomorphic encryption (FHE) for promising machine learning as a service solutions. Recent advancements have lowered the computational cost by several orders of magnitude, but the latency of fully homomorphic neural networks remains a barrier to adoption. This work proposes using multi-exit neural networks (MENNs) to accelerate the FHE inference. MENNs are network architectures that provide several exit points along the depth of the network. This approach allows users to employ results from any exit and terminate the computation early, saving both time and power. First, this work weighs the latency, communication, accuracy, and computational resource benefits of running FHE-based MENN inference. Then, we present the TorMENNt attack that can exploit the user's early termination decision to launch a concrete side-channel on MENNs. We demonstrate that the TorMENNt attack can predict the private image classification output of an image set for both FHE and plaintext threat models. We discuss possible countermeasures to mitigate the attack and examine their effectiveness. Finally, we tie the privacy risks with a cost-benefit analysis to obtain a practical roadmap for FHE-based MENN adoption.

(Receiver) Anamorphic encryption, introduced by Persiano $ \textit{et al.}$ at Eurocrypt 2022, considers the question of achieving private communication in a world where secret decryption keys are under the control of a dictator. The challenge here is to be able to establish a secret communication channel to exchange covert (i.e. anamorphic) messages on top of some already deployed public key encryption scheme.

Over the last few years several works addressed this challenge by showing new constructions, refined notions and extensions. Most of these constructions, however, are either ad hoc, in the sense that they build upon specific properties of the underlying PKE, or impose severe restrictions on the size of the underlying anamorphic message space.

In this paper we consider the question of whether it is possible to have realizations of the primitive that are both generic and allow for large anamorphic message spaces. We give strong indications that, unfortunately, this is not the case.

Our first result shows that $ \textit{any black-box realization} $ of the primitive, i.e. any realization that accesses the underlying PKE only via oracle calls, $ \textit{must} $ have an anamorphic message space of size at most $poly(\lambda)$ ($\lambda$ security parameter).

Even worse, if one aims at stronger variants of the primitive (and, specifically, the notion of asymmetric anamorphic encryption, recently proposed by Catalano $ \textit{et al.} $) we show that such black-box realizations are plainly impossible, i.e. no matter how small the anamorphic message space is.

Finally, we show that our impossibility results are rather tight: indeed, by making more specific assumptions on the underlying PKE, it becomes possible to build generic AE where the anamorphic message space is of size $\Omega(2^\lambda)$.

In this work we prove lower bounds on the (communication) cost of maintaining a shared key among a dynamic group of users. Being "dynamic'' means one can add and remove users from the group. This captures important protocols like multicast encryption (ME) and continuous group-key agreement (CGKA), which is the primitive underlying many group messaging applications.

We prove our bounds in a combinatorial setting where the state of the protocol progresses in rounds. The state of the protocol in each round is captured by a set system, with each of its elements specifying a set of users who share a secret key. We show this combinatorial model implies bounds in symbolic models for ME and CGKA that capture, as building blocks, PRGs, PRFs, dual PRFs, secret sharing, and symmetric encryption in the setting of ME, and PRGs, PRFs, dual PRFs, secret sharing, public-key encryption, and key-updatable public-key encryption in the setting of CGKA. The models are related to the ones used by Micciancio and Panjwani (Eurocrypt'04) and Bienstock et al. (TCC'20) to analyze ME and CGKA, respectively.

We prove - using the Bollobás' Set Pairs Inequality - that the cost (number of uploaded ciphertexts) for replacing a set of $d$ users in a group of size $n$ is $\Omega(d\ln(n/d))$. Our lower bound is asymptotically tight and both improves on a bound of $\Omega(d)$ by Bienstock et al. (TCC'20), and generalizes a result by Micciancio and Panjwani (Eurocrypt'04), who proved a lower bound of $\Omega(\log(n))$ for $d=1$.

Transitioning from classically to quantum secure key agreement protocols may require to exchange fundamental components, for example, exchanging Diffie-Hellman-like key exchange with a key encapsulation mechanism (KEM). Accordingly, the corresponding security proof can no longer rely on the Diffie-Hellman assumption, thus invalidating the security guarantees. As a consequence, the security properties have to be re-proven under a KEM-based security notion. We initiate the study of the LDACS key agreement protocol (Edition 01.01.00 from 25.04.2023), which is soon-to-be-standardized by the International Civil Aviation Organization. The protocol's cipher suite features Diffie-Hellman as well as a KEM-based key agreement protocol to provide post-quantum security. While the former results in an instantiation of an ISO key agreement inheriting all security properties, the security achieved by the latter is ambiguous. We formalize the computational security using the systematic notions of de Saint Guilhem, Fischlin and Warinshi (CSF '20), and prove the exact security that the KEM-based variant achieves in this model; primarily entity authentication, key secrecy and key authentication. To further strengthen our ``pen-and-paper'' findings, we model the protocol and its security guarantees using Tamarin, providing an automated proof of the security against a Dolev-Yao attacker.

The collision-resistant hash function is an early cryptographic primitive that finds extensive use in various applications. Remarkably, the Merkle-Damgård and Merkle tree hash structures possess the collision-resistance preserving property, meaning the hash function remains collision-resistant when the underlying compression function is collision-resistant. This raises the intriguing question of whether reducing the number of underlying compression function calls with the collision-resistance preserving property is possible. In pursuit of addressing these inquiries, we prove that for an ℓn-to-sn-bit collision-resistance preserving hash function designed using r tn-to-n-bit compression function calls, we must have r ≥ ⌈(ℓ−s)/(t−1)⌉. Throughout the paper, all operations other than the compression function are assumed to be linear (which we call linear hash mode).

Event date: 21 November to 22 November 2024
Submission deadline: 18 September 2024
Notification: 30 October 2024

Event date: 16 December to 18 December 2024

Event date: 4 September 2024
Submission deadline: 10 July 2024
Notification: 19 July 2024

Event date: 14 December to 16 December 2024
Submission deadline: 15 August 2024
Notification: 10 November 2024

We are seeking an Associate Research Fellow to join our team through support from the Australian Research Council Linkage Project, focusing on "Cryptographic Group Actions". This research-only opportunity requires proficiency in cryptography research, particularly in post-quantum cryptography, group actions, and security proofs. The Institute of Cybersecurity and Cryptology is a premier research institute that conducts research in cybersecurity and cryptology. The institute was awarded the Excellence of Research Assessment with score 5 for cryptography research. Please apply online only (not via email). Selection criteria is available online via the link below.

Closing date for applications:

Contact: Prof Willy Susilo

More information: https://www.uow.edu.au/about/jobs/jobs-available/#en/sites/CX_1/job/4604/?utm_medium=jobshare

Key Responsibilities: • Design and implementation of secure web services for key and data distribution • Design and implementation of unit and integration tests for automated test execution in CI/CD pipelines, including quality aspects • Contributing to architectural and security concepts to ensure end-to-end protection of sensitive key material and data Your Profile: • University degree in computer science, software engineering, security, telematics, mathematics, or equivalent • Experience in software engineering, seasoned Java developer, 3+ years • Familiar with Java Spring Boot • Experience in (embedded) C development • Familiar with security and cryptography • Interested in implementing and testing reliable, high-secure, high-throughput services • Familiar with SQL and NoSQL databases (nice to have) • Familiar with containerization including orchestration (Kubernetes and Docker) (nice to have) • Familiar with Maven Build System and Jenkins Build Automation (nice to have) • Structured approach towards complex software challenges • Self-organized and team-oriented • Solid English communication skills (oral and written) • Open-minded and communicative Ready to create a smarter world? Join the future of Innovation. Join NXP. Apply online!

Closing date for applications:

Contact: Kerstin Krauss

More information: https://nxp.wd3.myworkdayjobs.com/careers/job/Gratkorn/Senior-Web-Service-Java-Software-Engineer-for-Trust-Provisioning--m-f-d-_R-10053960-1

TU Wien is Austria's largest institution of research and higher education in the fields of technology and natural sciences. With over 26,000 students and more than 4000 scientists, research, teaching, and learning dedicated to the advancement of science and technology have been conducted here for more than 200 years, guided by the motto "Technology for People". As a driver of innovation, TU Wien fosters close collaboration with business and industry and contributes to the prosperity of society.

At the Institute of Logic and Computation, in the Research Unit of Security and Privacy (in the upcoming Research Unit Privacy Enhanced Technologies) at TU Wien is offering two 40hours/week positions as university assistant (prae-doc) limited to expected 4 years. Expected start: September 2024

Tasks:
- Research in the area of privacy enhancing technologies, cryptocurrencies, and (applied) cryptography
- Teaching tasks (exercises and exams), student guidance
- Teaching in German and English is expected
- Assistance with thesis supervision
- Scientific publishing (journal and conference papers, dissertation) - Participation in scientific events
- Assistance with organizational and administrative tasks


Your profile: - Completion of a master or diploma curriculum in one of these fields: computer Science, math, or smilar fields
- Knowledge of privacy-enhancing technologies, such as cryptography, differential privacy, and related areas.
- Very good skills in German and English communication and writing Interest in academic research and teaching
- Advanced problem solving skills and scientific curiosity
- Team player with very good communication skills


We offer: A highly visible and connected international research group A broad range of opportunities in a thriving research area Hybrid working style with home office option A range of attractive social benefits (see Fringe-Benefit Catalogue of TU Wien) Internal and external training opportunities, various career options Central location of workplace as well as good accessibility (U1/U4 Karlsplatz)

Closing date for applications:

Contact: Univ.-Prof. Dr. Dominique Schröder dominique.schroeder@tuwien.ac.at

More information: https://jobs.tuwien.ac.at/Job/235902

We are looking for postdocs interested in working with us (Chris Brzuska and Russell W. F. Lai) on topics including but not limited to:

• Lattice-based cryptography, with special focus on the design, application, and analysis of non-standard lattice assumptions
• Succinct and/or zero-knowledge proof and argument systems
• Advanced (e.g. homomorphic, attribute-based, functional, laconic) encryption and (e.g. ring, group, threshold, blind) signature schemes
• Fine-grained cryptography (e.g. against bounded-space-time adversaries)
• Lower bounds and impossibility results

For questions about the topics, feel free to drop us an email to discuss.

For more details about the position, and for the instructions of how to apply, please refer to https://www.hiit.fi/ict-community-postdoctoral-researcher-positions/.

Closing date for applications:

Contact:

• For the position: Chris Brzuska, Russell W. F. Lai
• For the recruiting system: HIIT coordinator (see link above)

More information: https://www.hiit.fi/ict-community-postdoctoral-researcher-positions/