International Association
for Cryptologic Research

Servan-Schreiber et al. (S&P 2023) presented a new notion called private access control lists (PACL) for function secret sharing (FSS), where the FSS evaluators can ensure that the FSS dealer is authorized to share the given function. Their construction relies on costly non-interactive secret-shared proofs and is not secure in post-quantum setting. We give a construction of PACL from publicly verifiable secret sharing (PVSS) under short integer solution (SIS). Our construction adapts the Gentry et al’s scheme (Eurocrypt 2022) for post-quantum setting based on learning with error assumption (LWE). The implementation of our PACL with different files showed that it is feasible even at different sizes, and should remain so even with large secret vectors. This construction has many applications for access control by applying FSS. We show how to apply the proposed PACL construction to secure data retrieval. We also present a scheme for secure data retrieval with access control, which might be of independent interest.

Private Stream Aggregation (PSA) allows clients to send encryptions of their private values to an aggregator that is then able to learn the sum of these values but nothing else. It has since found many applications in practice, e.g. for smart metering or federated learning. In 2018, Becker et al. proposed the first lattice-based PSA scheme LaPS (NDSS 2018), with putative post-quantum security, which has subsequently been patented. In this paper, we describe two attacks on LaPS that break the claimed aggregator obliviousness security notion, where the second attack even allows to recover the secret keys of the clients, given enough encryptions. Moreover, we review the PSA literature for other occurrences of the responsible flawed proof steps. By explicitly tracking down and discussing these flaws, we clarify and hope to contribute to the literature on PSA schemes, in order to prevent further insecure schemes in practice. Finally, we point out that a Real-or-Random variant of the security notion that is often used as a substitute to make proofs easier, is not well-defined and potentially weaker than the standard PSA security notion. We propose a well defined variant and show that it is equivalent to the standard security notion of PSA under mild assumptions.

We show that the authentication key agreement scheme [IEEE Trans. Smart Grid, 2023, 14(5), 3816-3827] is flawed due to its inconsistent computations. We also show that the scheme fails to keep anonymity, not as claimed.

We investigate shift-invariant vectorial Boolean functions on $n$ bits that are lifted from Boolean functions on $k$ bits, for $k\leq n$. We consider vectorial functions that are not necessarily permutations, but are, in some sense, almost bijective. In this context, we define an almost lifting as a Boolean function for which there is an upper bound on the number of collisions of its lifted functions that does not depend on $n$. We show that if a Boolean function with diameter $k$ is an almost lifting, then the maximum number of collisions of its lifted functions is $2^{k-1}$ for any $n$. Moreover, we search for functions in the class of almost liftings that have good cryptographic properties and for which the non-bijectivity does not cause major security weaknesses. These functions generalize the well-known map $\chi$ used in the Keccak hash function.

Let K be a commutative ring. We refer to a connected bipartite graph G = G_n(K) with partition sets P = K^n (points) and L = K^n (lines) as an affine graph over K of dimension dim(G) = n if the neighbourhood of each vertex is isomorphic to K. We refer to G as an algebraic affine graph over K if the incidence between a point (x_1, x_2, . . . , x_n) and line [y_1, y_2, . . . , y_n] is defined via a system of polynomial equations of the kind f_i = 0 where f_i ∈ K[x_1, x_2, . . . , x_n, y_1, y_2, . . . , y_n]. We say that an affine algebraic graph is a Jordan-Gauss graph over K if the incidences between points and lines are given by a quadratic system of polynomial equations, and the neighbourhood of each vertex is given as a solution set of the system of linear equations in row-echelon form. For each integral domain K we consider the known explicit construction of the family of Jordan-Gauss graphs A(n, K), n = 2, 3, . . . with cycle indicator ≥ 2n + 2. Additionally several constructions of families of edge intransitive Jordan-Gauss graphs over K of increasing girth with well defined projective limit will be presented. This projective limit is a forest defined by the system of algebraic equations. In the case K = F_q, q ≥ 3 we present results of computer experiments for the evaluation of girth, cycle indicator, diameter and the second largest eigenvalue of the constructed graphs, and we formulate several conjectures on their properties. One of the conjectures is that the girth of A(n, F_q) is 2[(n+ 5)/2]. We discuss briefly some applications of Jordan-Gauss graphs of large girth to Graph Theory, Algebraic Geometry and the theory of LDPC codes; and we consider ideas to use groups related to these graphs in Noncommutative Cryptography and Stream Ciphers Design.

Auditing throughout a fiscal year is integral to organizations with transactional activity. Organizations transact with each other and record the details for all their economical activities so that a regulatory committee can verify the lawfulness and legitimacy of their activity. However, it is computationally infeasible for the committee to perform all necessary checks for each organization. To overcome this, auditors assist in this process: organizations give access to all their internal data to their auditors, who then produce reports regarding the consistency of the organization's data, alerting the committee to any inconsistencies. Despite this, numerous issues that result in fines annually revolve around such inconsistencies in bookkeeping across organizations. Notably, committees wishing to verify the correctness of auditor-provided reports need to redo all their calculations; a process which is computationally proportional to the number of organizations. In fact, it becomes prohibitive when considering real-world settings with thousands of organizations. In this work, we propose two protocols, CLOSC and CLOLC, whose goals are to enable auditors and a committee to verify the consistency of transactions across different ledgers. Both protocols ensure that for every transaction recorded in an organization's ledger, there exists a dual one in the ledger of another organization while safeguarding against other potential attacks. Importantly, we minimize the information leakage to auditors and other organizations and guarantee three crucial security and privacy properties that we propose: (i) transaction amount privacy, (ii) organization-auditor unlinkability, and (iii) transacting organizations unlinkability. At the core of our protocols lies a two-tier ledger architecture alongside a suite of cryptographic tools. To demonstrate the practicality and scalability of our designs, we provide extensive performance evaluation for both CLOSC and CLOLC. Our numbers are promising, i.e., all computation and verification times lie in the range of seconds, even for millions of transactions, while the on-chain storage costs for an auditing epoch are encouraging i.e. in the range of GB for millions of transactions and thousands of organizations.

Handling congestion in blockchain systems is a fundamental problem given that the security and decentralization objectives of such systems lead to designs that compromise on (horizontal) scalability (what sometimes is referred to as the ``blockchain trilemma''). Motivated by this, we focus on the question whether it is possible to design a transaction inclusion policy for block producers that facilitates fee and delay predictability while being incentive compatible at the same time.

Reconciling these three properties is seemingly paradoxical given that the dominant approach to transaction processing is based on first-price auctions (e.g., as in Bitcoin) or dynamic adjustment of the minimum admissible fee (e.g. as in Ethereum EIP-1559) something that breaks fee predictability. At the same time, in fixed fee mechanisms (e.g., as in Cardano), fees are trivially predictable but are subject to relatively inexpensive bribing or denial of service attacks where transactions may be delayed indefinitely by a well funded attacker, hence breaking delay predictability.

In this work, we set out to address this problem by putting forward blockchain space tokenization (BST), namely a new capability of a blockchain system to tokenize its capacity for transactions and allocate it to interested users who are willing to pay ahead of time for the ability to post transactions regularly for a period of time. We analyze our system in the face of worst-case transaction-processing attacks by introducing a security game played between the mempool mechanism and an adversary. Leveraging this framework, we prove that BST offers predictable and asymptotically optimal delays, predictable fees, and is incentive compatible, thus answering the question posed in the affirmative.

Zero-knowledge succinct non-interactive argument of knowledge (zk-SNARK) is a kind of proof system that enables a prover to convince a verifier that an NP statement is true efficiently. In the last decade, various studies made a lot of progress in constructing more efficient and secure zk-SNARKs. Our research focuses on designated-verifier zk-SNARKs, where only the verifier knowing some secret verification state can be convinced by the proof. A natural idea of getting a designated-verifier zk-SNARK is encrypting a publicly-verifiable zk-SNARK's proof via public-key encryption. This is also the core idea behind the well-known transformation proposed by Bitansky et al. in TCC 2013 to obtain designated-verifier zk-SNARKs. However, the transformation only applies to zk-SNARKs which requires the complicated trusted setup phase and sticks on storage-expensive common reference strings. The loss of the secret verification state also makes the proof immediately lose the designated-verifier property.

To address these issues, we first define "strong designated-verifier" considering the case where the adversary has access to the secret verification state, then propose a construction of strong designated-verifier zk-SNARKs. The construction inspired by designated verifier signatures based on two-party ring signatures does not use encryption and can be applied on any public-verifiable zk-SNARKs to yield a designated-verifiable variant. We introduce our construction under the circuit satisfiability problem and implement it in Circom, then test it on different zk-SNARKs, showing the validity of our construction.

Bottleneck complexity is an efficiency measure of secure multiparty computation (MPC) protocols introduced to achieve load-balancing in large-scale networks, which is defined as the maximum communication complexity required by any one player within the protocol execution. Towards the goal of achieving low bottleneck complexity, prior works proposed MPC protocols for computing symmetric functions in the correlated randomness model, where players are given input-independent correlated randomness in advance. However, the previous protocols with polylogarithmic bottleneck complexity in the number of players $n$ require a large amount of correlated randomness that is linear in $n$, which limits the per-party efficiency as receiving and storing correlated randomness are the bottleneck for efficiency. In this work, we present for the first time MPC protocols for symmetric functions such that bottleneck complexity and the amount of correlated randomness are both polylogarithmic in $n$, assuming collusion of size at most $n-o(n)$ players. Furthermore, one of our protocols is even computationally efficient in that each player performs only $\mathrm{polylog}(n)$ arithmetic operations while the computational complexity of the previous protocols is $O(n)$. Technically, our efficiency improvements come from novel protocols based on ramp secret sharing to realize basic functionalities with low bottleneck complexity, which we believe may be of interest beyond their applications to secure computation of symmetric functions.

Deduplication is a vital preprocessing step that enhances machine learning model performance and saves training time and energy. However, enhancing federated learning through deduplication poses challenges, especially regarding scalability and potential privacy violations if deduplication involves sharing all clients’ data. In this paper, we address the problem of deduplication in a federated setup by introducing a pioneering protocol, Efficient Privacy-Preserving Multi-Party Deduplication (EP-MPD). It efficiently removes duplicates from multiple clients’ datasets without compromising data privacy. EP-MPD is constructed in a modular fashion, utilizing two novel variants of the Private Set Intersection protocol. Our extensive experiments demonstrate the significant benefits of deduplication in federated learning of large language models. For instance, we observe up to 19.61% improvement in perplexity and up to 27.95% reduction in running time. EP-MPD effectively balances privacy and performance in federated learning, making it a valuable solution for large-scale applications.

Isogeny-based schemes often come with special requirements on the field of definition of the involved elliptic curves. For instance, the efficiency of SQIsign, a promising candidate in the NIST signature standardisation process, requires a large power of two and a large smooth integer $T$ to divide $p^2-1$ for its prime parameter $p$.

We present two new methods that combine previous techniques for finding suitable primes: sieve-and-boost and XGCD-and-boost. We use these methods to find primes for the NIST submission of SQIsign. Furthermore, we show that our methods are flexible and can be adapted to find suitable parameters for other isogeny-based schemes such as AprèsSQI or POKE. For all three schemes, the parameters we present offer the best performance among all parameters proposed in the literature.

In this work, we introduce enhanced high-order masking techniques tailored for Dilithium, the post-quantum signature scheme recently standardized by NIST. We improve the masked generation of the masking vector $\vec{y}$, based on a fast Boolean-to-arithmetic conversion modulo $q$. We also describe an optimized gadget for the high-order masked rejection sampling, with a complexity independent from the size of the modulus $q$. We prove the security of our gadgets in the classical ISW $t$-probing model. Finally, we detail our open-source C implementation of these gadgets integrated into a fully masked Dilithium implementation, and provide an efficiency comparison with previous works.

In this short note, we introduce a specific class of rank two lattices over CM fields endowed with additional symmetries, which are involved in the decomposition of algebraic integers in Hermitian squares. As an application, we show an elementary reduction from the module-LIP problem in rank 2 over a CM or totally real number field to the finding of a square basis in such lattices.

In this article we present a non-uniform reduction from rank-2 module-LIP over Complex Multiplication fields, to a variant of the Principal Ideal Problem, in some fitting quaternion algebra. This reduction is classical deterministic polynomial-time in the size of the inputs. The quaternion algebra in which we need to solve the variant of the principal ideal problem depends on the parameters of the module-LIP problem, but not on the problem’s instance. Our reduction requires the knowledge of some special elements of this quaternion algebras, which is why it is non-uniform.

In some particular cases, these elements can be computed in polynomial time, making the reduction uniform. This is in particular the case for the Hawk signature scheme: we show that breaking Hawk is no harder than solving a variant of the principal ideal problem in a fixed quaternion algebra (and this reduction is uniform).

Event date: 18 October 2024
Submission deadline: 22 July 2024
Notification: 26 August 2024

The research group for Cryptographic Protocols located at the University of Luxembourg and the KASTEL Security Research Labs (Germany) is looking for a PhD student working on cryptographic primitives and protocols enabling privacy, accountability, and transparency. A background in provable security (e.g., successfully attended courses or a master’s thesis on the subject) is expected.

The candidate will be based at the University of Luxembourg but also profit from regular visits at and joint research projects with the KASTEL Security Research Labs at KIT, Germany. The candidate’s research will be dealing with privacy-enhancing cryptographic building blocks and protocols for important application scenarios and result in both theoretical contributions (protocol designs, security models and proofs, etc.) and their efficient implementation. Privacy-preserving payments and data analytics, misuse-resistant lawful interception, and anonymous communication are research topics of particular interest to us.

If you are interested in joining our group, please send an email including your CV, transcripts, and two references to andy.rupp@uni.lu. As the position should be filled as soon as possible, your application will be considered promptly.

Closing date for applications:

Contact: Andy Rupp (andy.rupp@uni.lu)

More information: https://www.uni.lu/fstm-en/research-groups/cryptographic-protocols/

Are you fascinated by the theoretical underpinnings of security that allow for protecting privacy in an ever more interconnected world? Are you willing to take on the challenge of upgrading cryptography to deal with the threat posed by quantum computation? Do you enjoy working in a team of young and motivated researchers? The TCS Group from the Informatics Institute of the University of Amsterdam is seeking a PhD student to carry out cutting-edge research in theoretical computer science, with an expected focus on code-based cryptography.

Closing date for applications:

Contact: Nicolas Resch

More information: https://vacatures.uva.nl/UvA/job/PhD-Position-in-Code-Based-Cryptography/777741202/

We are seeking two highly motivated and talented students to join our research group to pursue a Ph.D in the field of cryptography at School of Computer Science, University of Sydney. The student will work on cutting-edge research in topics such as

security and fairness in multi-party computation

distributed payment protocols

privacy-preserving blockchains and cryptocurrencies

Security and game-theoretic aspects in blockchains

and other topics in cryptographic theory and applications.

Qualifications:

An UG or Master’s degree in CS, Mathematics, Electrical Engineering, or a related field, with one year of research experience (For eg., research-based thesis).

Strong background in theoretical computer science, number theory, probability.

Proficiency in English, both written and spoken

Good communication and teamwork skills.

Optional skills:

Excellent programming skills and experience with cryptographic libraries is a plus.

Benefits:

Competitive salary and benefits package.

Work in a dynamic and international research environment.

Support for international collaborations and travel.

About University of Sydney:

The University of Sydney is one of the world's leading universities, known for its outstanding research and teaching excellence (ranked 18 in the world - QS rankings 2025 ). Our vibrant campus is located in the heart of Sydney (one of the top livable cities of the world), offering an exceptional environment for both academic and personal growth and the perfect work-life balance. The School of Computer Science is among the top ranked in the world ( ranked 22 in the world for CS - US news and world report 2024-25 ) constantly expanding year-on-year with strong faculty and students.

Application Process: Interested candidates should contact via email with

a detailed CV, including list of publications (if any)

Transcript, degree certificate

Contact of two references

Closing date for applications:

Contact: Sri AravindaKrishnan Thyagarajan aravind.thyagarajan@sydney.edu.au

More information: https://www.sydney.edu.au/courses/courses/pr/doctor-of-philosophy-engineering.html

We are looking for a bright, ambitious, and motivated PhD student to join the cryptography group in the Cybersecurity Engineering Section at DTU Compute in the Copenhagen region of Denmark. The 3-year PhD position will preferably start on 1 November 2024 (or according to mutual agreement). The goal of the PhD project is to improve the state of threshold post-quantum cryptography. You will join the growing cryptography team at DTU and be able to work with researchers in- and outside of the Copenhagen region and Denmark.

Responsibilities and qualifications
Your main task will be to design new threshold cryptographic algorithms with post-quantum security.
You will investigate distributed alternatives to existing post-quantum algorithms such as Dilithium, Falcon and Picnic, and the long-term security of threshold cryptography, in particular with respect to proactive and post-quantum security. To succeed in this research effort, you will gain familiarity with:

• post-quantum cryptographic primitives such as signatures or OPRFs
• threshold cryptographic techniques such as secret sharing and multiparty computation
• cryptographic foundations of post-quantum cryptography such as lattices, MPC-in-the-head, FHE and similar tools

In addition to the research project, you will conduct a limited amount of small-class teaching during your PhD period.

As formal qualification, you must have a two-year master's degree (120 ECTS points) or a similar degree with an academic level equivalent to a two-year master's degree. Furthermore, to ensure a smooth start into the project, it is preferable that you have previous experience with either threshold or post-quantum cryptography.

Salary and appointment terms
The appointment will be based on the collective agreement with the Danish Confederation of Professional Associations. The allowance will be agreed upon with the relevant union. The period of employment is 3 years. The position is a full-time position and the starting date is 1 November 2024 (or according to mutual agreement).

Closing date for applications:

Contact: Carsten Baum (cabau@dtu.dk)

More information: https://efzu.fa.em2.oraclecloud.com/hcmUI/CandidateExperience/da/sites/CX_1/job/2872/

We are looking for a person to extend our team as postdoc in the Horizon Europe Next Generation Internet pilot NGI TALER. Your task will be to carry out foundational research in the context of the payment system GNU Taler. More precisely, you will be tasked with proving the security of post-quantum replacements for the cryptography used to secure GNU Taler. The position is initially 1 year with funding for a 1-year extension available.

GNU Taler is a privacy-preserving payment system. Customers can stay anonymous, but merchants cannot hide their income through payments with GNU Taler. This helps to avoid tax evasion and money laundering while providing users with a privacy-preserving way of electronic payment. As part of a Next Generation Internet pilot, the cryptography used in GNU Taler will be future-proofed by developing post-quantum secure variants of the involved protocols. Your task will be to prove these new protocols secure against quantum adversaries, closely collaborating with the team that develops the protocols.

If you have a PhD in cryptography or a related area, please apply online via the TU/e website.

Closing date for applications:

Contact: Andreas Hülsing a.t.huelsing [put at here] tue.nl and Kathrin Hövelmanns k.hovelmanns [put at here] tue.nl

More information: https://jobs.tue.nl/en/vacancy/postdoc-in-postquantum-cryptography-1094802.html