International Association
for Cryptologic Research

We introduce $\mathsf{ChalametPIR}$: a single-server Private Information Retrieval (PIR) scheme supporting fast, low-bandwidth keyword queries, with a conceptually very simple design. In particular, we develop a generic framework for converting PIR schemes for index queries over flat arrays (based on the Learning With Errors problem) into keyword PIR. This involves representing a key-value map using any probabilistic filter that permits reconstruction of elements from inclusion queries (e.g. Cuckoo filters). In particular, we make use of recently developed Binary Fuse filters to construct $\mathsf{ChalametPIR}$, with minimal efficiency blow-up compared with state-of-the-art index-based schemes (all costs bounded by a factor of \(\leq 1.08\)). Furthermore, we show that $\mathsf{ChalametPIR}$ achieves runtimes and financial costs that are factors of between \(6\times\)-\(11\times\) and \(3.75\times\)-\(11.4\times\) more efficient, respectively, than state-of-the-art keyword PIR approaches, for varying database configurations. Bandwidth costs are additionally reduced or remain competitive, depending on the configuration. Finally, we believe that our application of Binary Fuse filters in the cryptography setting may bring immediate independent value towards developing efficient variants of other related primitives that benefit from using such filters.

The paper presents a short survey of the History of Multivariate Cryptography together with the usage of old broken multivariate digital signatures in the new protocol based cryptosystems constructed in terms of Noncommutative Cryptography. The general schemes of New cryptosystems is a combinations of Eulerian maps and quadratic maps with their trapdoor accelerators, which are pieces of information such than the knowledge of them allow to compute the reimages in a polynomial time. These schemes are illustrated by historical examples of Imai – Matsumoto multivariate digital signatures schemes and Unbalanced Oil and Vinegar Cryptosystems.

Federated Learning (FL) is a data-minimization approach enabling collaborative model training across diverse clients with local data, avoiding direct data exchange. However, state-of-the-art FL solutions to identify fraudulent financial transactions exhibit a subset of the following limitations. They (1) lack a formal security definition and proof, (2) assume prior freezing of suspicious customers’ accounts by financial institutions (limiting the solutions’ adoption), (3) scale poorly, involving either $O(n^2)$ computationally expensive modular exponentiation (where $n$ is the total number of financial institutions) or highly inefficient fully homomorphic encryption, (4) assume the parties have already completed the identity alignment phase, hence excluding it from the implementation, performance evaluation, and security analysis, and (5) struggle to resist clients’ dropouts. This work introduces Starlit, a novel scalable privacy-preserving FL mechanism that overcomes these limitations. It has various applications, such as enhancing financial fraud detection, mitigating terrorism, and enhancing digital health. We implemented Starlit and conducted a thorough performance analysis using synthetic data from a key player in global financial transactions. The evaluation indicates Starlit’s scalability, efficiency, and accuracy.

Event date: 21 January to 19 July 2024
Submission deadline: 10 February 2024
Notification: 15 March 2024

Event date: 28 August to 30 August 2024
Submission deadline: 7 February 2024
Notification: 20 March 2024

Event date: 11 September to 13 September 2024
Submission deadline: 24 April 2024
Notification: 24 June 2024

In the current paper we investigate the possibility of designing secure two-party signature scheme with the same verification algorithm as in the Russian standardized scheme (GOST scheme). We solve this problem in two parts. The first part is a (fruitless) search for an appropriate scheme in the literature. It turned out that all existing schemes are insecure in the strong security models. The second part is a synthesis of new signature scheme and ends fruitfully. We synthesize a new two-party GOST signature scheme, additionally using the commitment scheme, guided by the features of the GOST signature scheme, as well as the known attacks on existing schemes. We prove that this scheme is secure in a bijective random oracle model in the case when one of the parties is malicious under the assumption that the classical GOST scheme is unforgeable in a bijective random oracle model and the commitment scheme is modelled as a random oracle.

PERK is a digital signature scheme submitted to the recent NIST Post-Quantum Cryptography Standardization Process for Additional Digital Signature Schemes. For NIST security level I, PERK features sizes ranging from 6kB to 8.5kB, encompassing both the signature and public key, depending on the parameter set. Given its inherent characteristics, PERK's signing and verification algorithms involve the computation of numerous large objects, resulting in substantial stack-memory consumption ranging from 300kB to 1.5MB for NIST security level I and from 1.1MB to 5.7MB for NIST security level V. In this paper, we present a memory-versus-performance trade-off strategy that significantly reduces PERK's memory consumption to a maximum of approximately 82kB for any security level, enabling PERK to be executed on resource-constrained devices. Additionally, we explore various optimizations tailored to the Cortex M4 and introduce the first implementation of PERK designed for this platform.

Quantstamp is looking for an applied cryptographer. Quantstamp often deals with a wide range of cryptographic problems, including reviewing implementations and tackling new theoretical problems using cryptography. For example, Quantstamp regularly receives requests to review code bases which either invoke or implement (custom) cryptography, as part of an audit.

Zero knowledge applications are becoming more common across all ecosystems, and this kind of math will also soon be the basis of several scaling solutions - in particular, zero knowledge rollups. These applications often use a zero-knowledge Succinct Non-interactive Argument of Knowledge (zk-SNARK) proof system, or a zero-knowledge Succinct Transparent Argument of Knowledge (zk-STARK) proof system.

Your work will involve doing research about various cryptographic protocols. Some protocols of interest will be those found in the code of our audit clients, others will be protocols of interest for future audits. In particular, in anticipation of audits of, or for, zero-knowledge rollup systems, you’ll be asked to understand various zero-knowledge proof systems. There are a lot of those - we don’t expect mastery in all of them, but mastery of at least one would be ideal.

Required

Mastery of at least one zk-SNARK/zk-STARK proof system, or a strong enough technical background to understand one (and this should have some direct connection to cryptography)

Ability to code and develop software. You should have experience with at least one major language, like Python, Java, or C; the exact language is not too important. You should be familiar with versioning software (specifically, GitHub), testing, and a familiarity with algorithms and data structures.

Ability to read and interpret academic papers

Ability to communicate ideas

Partial availability (2-6h) during EST work hours

Familiarity with existing ZK Rollup designs and multiple ZK proof systems

Knowledge of software development in Solidity, including testing and various development frameworks like Hardhat

Familiarity with blockchain ecosystems, particularly Ethereum

Familiarity with Circom

Closing date for applications:

Contact: See the link below for more information and to apply: https://jobs.ashbyhq.com/quantstamp/6ae4fc70-98bb-42e1-9f24-c40e7af441cc

More information: https://quantstamp.com/careers

The Faculty of Mathematics, Informatics and Mechanics of the University of Warsaw (MIM UW) invites applications for the positions of Assistant Professor (“adiunkt” in Polish) in Computer Science, starting on 1st October 2024 or 1st February 2025.

MIM UW is one of the leading computer science faculties in Europe. It is known for talented students (e.g., two wins and multiple top tens in the International Collegiate Programming Contest) and strong research teams, especially in algorithms, logic and automata, and computational biology. There is also a growing number of successful smaller groups in diverse areas including cryptography, distributed systems, game theory, and machine learning. There are seven ERC grants in computer science running at MIM UW at the moment.

In the current call, the following positions are offered:

1. S. Eilenberg Assistant Professor (reduced teaching load and increased salary),
2. Assistant Professor,
3. Assistant Professor in the teaching group (increased teaching, no research required).

Deadline for applications: 12th February, 2024.

Closing date for applications:

Contact: For further information about the procedure, requirements, conditions, etc., please contact Prof. Lukasz Kowalik (kowalik@mimuw.edu.pl) or Prof. Filip Murlak (fmurlak@mimuw.edu.pl).

More information: https://jobs.uw.edu.pl/en-gb/offer/WMIM_2024/field/ADIUNKT/

Homomorphic encryption (HE) is in the spotlight as a solution for privacy-related issues in various real-world scenarios. However, the limited types of operations supported by each HE scheme have been a major drawback in applications. While HE schemes based on learning-with-error (LWE) problem provide efficient lookup table (LUT) evaluation in terms of latency, they have downsides in arithmetic operations and low throughput compared to HE schemes based on ring LWE (RLWE) problem. The use of HE on circuits containing LUT has been partly limited if they contain arithmetic operations or their computational width is large.

In this paper, we propose homomorphic algorithms for batched queries on LUTs by using RLWE-based HE schemes. To look up encrypted LUTs of size $n$ on encrypted queries, our algorithms use $O(\log{n})$ homomorphic comparisons and $O(n)$ multiplications. For unencrypted LUTs, our algorithms use $O(\log{n})$ comparisons, $O(\sqrt{n})$ ciphertext multiplications, and $O(n)$ scalar multiplications.

We provide a proof-of-concept implementation based on CKKS scheme (Asiacrypt 2017). The amortized running time for an encrypted (Resp. unencrypted) LUT of size $512$ is $0.041$ (Resp. $0.025$) seconds. Our implementation reported roughly $2.4$-$6.0$x higher throughput than the current implementation of LWE-based schemes, with more flexibility on the structure of the LUTs.

Gröbner bases are nowadays central tools for solving various problems in commutative algebra and algebraic geometry. A typical use of Gröbner bases is the multivariate polynomial system solving, which enables us to construct algebraic attacks against post-quantum cryptographic protocols. Therefore, the determination of the complexity of computing Gröbner bases is very important both in theory and in practice: One of the most important cases is the case where input polynomials compose an (overdetermined) affine semi-regular sequence. The first part of this paper aims to present a survey on the Gröbner basis computation and its complexity. In the second part, we shall give an explicit formula on the (truncated) Hilbert-Poincaré series associated to the homogenization of an affine semi-regular sequence. Based on the formula, we also study (reduced) Gröbner bases of the ideals generated by an affine semi-regular sequence and its homogenization. Some of our results are considered to give mathematically rigorous proofs of the correctness of methods for computing Gröbner bases of the ideal generated by an affine semi-regular sequence.

The present article explains how to generalize the hash function SwiftEC (in an elementary quasi-unified way) to any elliptic curve $E$ over any finite field $\mathbb{F}_{\!q}$ of characteristic $> 3$. The new result apparently brings the theory of hash functions onto elliptic curves to its logical conclusion. To be more precise, this article provides compact formulas that define a hash function $\{0,1\}^* \to E(\mathbb{F}_{\!q})$ (deterministic and indifferentible from a random oracle) with the same working principle as SwiftEC. In particular, both of them equally compute only one square root in $\mathbb{F}_{\!q}$ (in addition to two cheap Legendre symbols). However, the new hash function is valid with much more liberal conditions than SwiftEC, namely when $3 \mid q-1$. Since in the opposite case $3 \mid q-2$ there are already indifferentiable constant-time hash functions to $E$ with the cost of one root in $\mathbb{F}_{\!q}$, this case is not processed in the article. If desired, its approach nonetheless allows to easily do that mutatis mutandis.

A tweakable wide blockcipher is a construction which behaves in the same way as a tweakable blockcipher, with the difference that the actual block size is flexible. Due to this feature, a tweakable wide blockcipher can be directly used as a strong encryption scheme that provides full diffusion when encrypting plaintexts to ciphertexts and vice versa. Furthermore, it can be the basis of authenticated encryption schemes fulfilling the strongest security notions. In this paper, we present two instantiations of the docked double decker tweakable wide blockcipher: $\mathit{ddd}\text{-}\mathit{AES}$ and $\mathit{bbb}\text{-}\mathit{ddd}\text{-}\mathit{AES}$. Both instances exclusively use similar building blocks as AES-GCM (AES and finite field multiplication), are designed for maximal parallelism, and hence, can make efficient use of existing hardware accelerators. Moreover, $\mathit{bbb}\text{-}\mathit{ddd}\text{-}\mathit{AES}$ builds upon a novel beyond birthday bound secure pseudorandom function, a tweakable variant of the XOR of permutations, facilitating in the need to include a tweak in the AES evaluations without sacrificing flexibility in docked double decker.

We continue the study of $t$-wise independence of substitution-permutation networks (SPNs) initiated by the recent work of Liu, Tessaro, and Vaikuntanathan (CRYPTO 2021). Our key technical result shows that when the S-boxes are randomly and independently chosen and kept secret, an $r$-round SPN with input length $n = b \cdot k$ is $2^{-\Theta(n)}$-close to $t$-wise independent within $r = O(\min\{k, \log t\})$ rounds for any $t$ almost as large as $2^{b/2}$. Here, $b$ is the input length of the S-box and we assume that the underlying mixing achieves maximum branch number. We also analyze the special case of AES parameters (with random S-boxes), and show it is $2^{-128}$-close to pairwise independent in $7$ rounds. Central to our result is the analysis of a random walk on what we call the layout graph, a combinatorial abstraction that captures equality and inequality constraints among multiple SPN evaluations. We use our technical result to show concrete security bounds for SPNs with actual block cipher parameters and small-input $S$-boxes. (This is in contrast to the large body of results on ideal-model analyses of SPNs.) For example, for the censored-AES block cipher, namely AES with most of the mixing layers removed, we show that 192 rounds suffice to attain $2^{-128}$-closeness to pairwise independence. The prior such result for AES (Liu, Tessaro and Vaikuntanathan, CRYPTO 2021) required more than 9000 rounds.

A major unresolved question in quantum cryptography is whether it is possible to obfuscate arbitrary quantum computation. Indeed, there is much yet to understand about the feasibility of quantum obfuscation even in the classical oracle model, where one is given for free the ability to obfuscate any classical circuit.

In this work, we develop a new array of techniques that we use to construct a quantum state obfuscator, a powerful notion formalized recently by Coladangelo and Gunn (arXiv:2311.07794) in their pursuit of better software copy-protection schemes. Quantum state obfuscation refers to the task of compiling a quantum program, consisting of a quantum circuit $C$ with a classical description and an auxiliary quantum state $\ket{\psi}$, into a functionally-equivalent obfuscated quantum program that hides as much as possible about $C$ and $\ket{\psi}$. We prove the security of our obfuscator when applied to any pseudo-deterministic quantum program, i.e. one that computes a (nearly) deterministic classical input / classical output functionality. Our security proof is with respect to an efficient classical oracle, which may be heuristically instantiated using quantum-secure indistinguishability obfuscation for classical circuits.

Our result improves upon the recent work of Bartusek, Kitagawa, Nishimaki and Yamakawa (STOC 2023) who also showed how to obfuscate pseudo-deterministic quantum circuits in the classical oracle model, but only ones with a completely classical description. Furthermore, our result answers a question of Coladangelo and Gunn, who provide a construction of quantum state indistinguishability obfuscation with respect to a quantum oracle, but leave the existence of a concrete real-world candidate as an open problem. Indeed, our quantum state obfuscator together with Coladangelo-Gunn gives the first candidate realization of a ``best-possible'' copy-protection scheme for all polynomial-time functionalities.

Our techniques deviate significantly from previous works on quantum obfuscation. We develop several novel technical tools which we expect to be broadly useful in quantum cryptography. These tools include a publicly-verifiable, linearly-homomorphic quantum authentication scheme with classically-decodable ZX measurements (which we build from coset states), and a method for compiling any quantum circuit into a "linear + measurement" ($\LM$) quantum program: an alternating sequence of CNOT operations and partial ZX measurements.

Federated Learning (FL) accomplishes collaborative model training without the need to share local training data. However, existing FL aggregation approaches suffer from inefficiency, privacy vulnerabilities, and neglect of poisoning attacks, severely impacting the overall performance and reliability of model training. In order to address these challenges, we propose SuperFL, an efficient two-server aggregation scheme that is both privacy preserving and secure against poisoning attacks. The two semi-honest servers $\mathcal{S}_0$ and $\mathcal{S}_1$ collaborate with each other, with a shuffle server $\mathcal{S}_0$ in charge of privacy-preserving random clustering, while an analysis server $\mathcal{S}_1$ responsible for robustness detection, identifying and filtering malicious model updates. Our scheme employs a novel combination of homomorphic encryption and proxy re-encryption to realize secure server-to-server collaboration. We also utilize a novel sparse matrix projection compression technique to enhance communication efficiency and significantly reduce communication overhead. To resist poisoning attacks, we introduce a dual-filter algorithm based on trusted root, combine dimensionality reduction and norm calculation to identify malicious model updates. Extensive experiments validate the efficiency and robustness of our scheme. SuperFL achieves impressive compression ratios, ranging from $5\text{-}40$x, under different models while maintaining comparable model accuracy as the baseline. Notably, our solution demonstrates a maximal model accuracy decrease of no more than $2\%$ and $6\%$ on the MNIST and CIFAR-10 datasets respectively, under specific compression ratios and the presence of malicious clients.

The security of lattice-based crytography (LWE, NTRU, and FHE) depends on the hardness of the shortest-vector problem (SVP). Sieving algorithms give the lowest asymptotic runtime to solve SVP, but depend on exponential memory. Memory access costs much more in reality than in the RAM model, so we consider a computational model where processors, memory, and meters of wire are in constant proportions to each other. While this adds substantial costs to route data during lattice sieving, we modify existing algorithms to amortize these costs and find that, asymptotically, a classical computer can achieve the previous RAM model cost of $2^{0.2925d+o(d)}$ to sieve a $d$-dimensional lattice for a computer existing in 3 or more spatial dimensions, and can reach $2^{0.3113d+o(d)}$ in 2 spatial dimensions, where "spatial dimensions" are the dimensions of the physical geometry in which the computer exists.

Under some assumptions about the constant terms of memory access, we estimate increases in bit security between $3$ to $29$ bits for different Kyber parameter sets and $4$ to $28$ bits for Dilithium.

Number theoretic transform (NTT) has been a very useful tool in computations for number theory, algebra and cryptography. Its performance affects some post-quantum cryptosystems. In this paper, we discuss the butterfly operation of NTT. This basic module of NTT requires heavy modular arithmetics. Montgomery reduction is commonly used in this setting. Recently several variants of Montgomery have been proposed for the purpose of speeding up NTT. We observe that the Chinese remainder theorem (CRT) can be involved in this type of algorithms in nature and transparent ways. In this paper, a framework of using CRT to model Montgomery type algorithms is described. The derivation of these algorithms as well as their correctness are all treated in the CRT framework. Under our approach, some problems of a modular reduction algorithm ((published in IACR Transactions on Cryptographic Hardware and Embedded Systems, doi:10.46586/tches.v2022.i4.614-636 ) are identified, and a counterexample is generated to show that the algorithm is incorrect.

In recent years, the number of third-party services that can access highly-sensitive data has increased steadily, e.g., in the financial sector, in eGovernment applications, or in high-assurance identity services. Protocols that enable this access must provide strong security guarantees.

A prominent and widely employed protocol for this purpose is the OpenID Foundation's FAPI protocol. The FAPI protocol is already in widespread use, e.g., as part of the UK's Open Banking standards and Brazil's Open Banking Initiative as well as outside of the financial sector, for instance, as part of the Australian government's Consumer Data Rights standards.

Based on lessons learned from FAPI 1.0, the OpenID Foundation has developed a completely new protocol, called FAPI 2.0. The specifications of FAPI 2.0 include a concrete set of security goals and attacker models under which the protocol aims to be secure.

Following an invitation from the OpenID Foundation's FAPI Working Group (FAPI WG), we have accompanied the standardization process of the FAPI 2.0 protocol by an in-depth formal security analysis. In this paper, we report on our analysis and findings.

Our analysis incorporates the first formal model of the FAPI 2.0 protocol and is based on a detailed model of the web infrastructure, the Web Infrastructure Model, originally proposed by Fett, Küsters, and Schmitz. Our analysis has uncovered several types of attacks on the protocol, violating the aforementioned security goals set by the FAPI WG. We subsequently have worked with the FAPI WG to fix the protocol, resulting in several changes to the specifications. After adapting our model to the changed specifications, we have proved the security properties to hold under the strong attacker model defined by the FAPI WG.