International Association
for Cryptologic Research

Existing PVSS schemes suffer from at least $O(n)$ online complexity due to the need to individually encrypt and prove/ verify each of the $n$ shares. In this work, we present a generic framework for constructing PVSS schemes with $O(1)$ complexity for share distribution and (the expected to be repeated numerous times) public verification. Our key insight lies in establishing a novel connection between PVSS and CCA2-Secure threshold encryption (CCATE), which enables public verifiability enforced by Non-Interactive Zero-Knowledge (NIZK) proofs. We show that a CCATE scheme can be generically transformed into a secure PVSS scheme, eliminating the $O(n)$ bottleneck per on-line operations. We instantiate the framework by presenting two CCATE constructions: 1) A pairing-free scheme based on a committee-based Distributed Key Generation (DKG) protocol and Threshold ElGamal encryption. 2) A silent setup scheme leveraging a non-interactive distributed key generation, relying on Power-of-Tau ceremony. Furthermore, we introduce solutions for dynamic membership updates in both DKG constructions, demonstrating their practicality and adaptability for real-world applications. The scheme is based on an off-line setup stage (before a specific value to share is given) where the $O(n)$ complexity is dealt with. Although our schemes incur higher setup costs, they drastically reduce the complexity of the critical distribution and verification stages to constant time. This trade-off marks a significant advancement in the scalability of PVSS-based systems, especially in the context of blockchain modern transactions. Conceptually, the work points out how variants of the notion of Threshold Encryption can potentially serve as a ``compression mechanism'' for information sharing schemes.

Germany is currently rolling out an opt-out, nation-scale database of the medical records of the majority of its population, with low-income people being disproportionally represented among its users. While there has been considerable criticism of the system coming from civil society, independent academic analysis of the system by the cryptography and information security community has been largely absent. In this paper, we aim to raise awareness of the system’s existence and, based on the system’s public specifications, highlight several concerning cryptographic engineering decisions. Our core observations is that the system’s most sensitive long-term user keys are derived by a rudimentary, home-grown centralized key escrow mechanism. This mechanism relies on a per-use salt and only 256 bit of entropy, shared globally across millions of users. Furthermore, the system’s specification mandates only level 3 compliance with the obsolete FIPS 140-2 security standard, which requires “hard, opaque potting”, but lacks active tamper sensing. As a result, the system remains vulnerable to attacks by nation states and other well-funded adversaries.

Security Meshes are patterns of sensing traces covering an area that are used in Hardware Security Modules (HSMs) and other systems to detect attempts to physically intrude into the device's protective shell. State-of-the-art solutions manufacture meshes in bespoke processes from carefully chosen materials, which is expensive and makes replication challenging. Additionally, state-of-the-art monitoring circuits sacrifice either monitoring precision or cost efficiency. In this paper, we present an embeddable security mesh monitoring circuit constructed from low-cost, standard components that utilizes Time Domain Reflectometry (TDR) to create a unique fingerprint of a mesh. Our approach is both low-cost and precise, and enables the use of inexpensive standard Printed Circuit Boards (PCBs) as security mesh material. We demonstrate a working prototype of our TDR circuit costing less than 10 € in components that achieves both time resolution and rise time better than 200 ps—a 25 × improvement over previous work. We demonstrate a simple classifier that detects several types of advanced attacks such as probing using an oscilloscope probe or micro-soldering attacks with no false negatives.

In this paper, we analyze the clash between privacy-oriented cryptocurrencies and emerging legal frameworks for combating financial crime, focusing in particular on the recent European Union regulations. We analyze Monero, a leading "privacy coin" and a major point of concern for law enforcement, and study the scope of due diligence that must be exercised under the new law with regard to Monero trading platforms and how it translates to the technical capabilities of the Monero protocol. We both recognize flaws in the legislation and identify technical pitfalls threatening either the effective compliance of, say, Monero exchanges or the anonymity endeavour of Monero itself. Of independent interest is that we turn to anamorphic cryptography (marking one of the first practical applications of the concept) and leverage it to build a hidden transaction layer embedded in the Monero blockchain that obfuscates illegal money flow and circumvents transaction-level attempts at enforcing the EU law.

Over the past decades, extensive research has been conducted on lightweight cryptographic primitives. The linear layer plays an important role in their security.

In this paper, we propose a family of linear layers consisting of XORs and rotations, which is called multiple rows mixers (MRM). It is a family designed for LS-type ciphers, but mixing elements from several rows. We investigate the impact of the linear layers on the 3-round trail weight of permutations and explore the properties of the inverse of the linear layers with a low XOR count. We employ a generic and extensible approach to determine the parameters of MRM. This approach can automatically generate linear layers that meet the requirements of a given branch number.

By applying these design principles and methods, we derive a linear layer that has a dimension of 5 × 64, a differential branch number of 12, a linear branch number of 5 and a computational cost of 2.6 XOR operations per bit. MRM is not limited to fixed dimension and can be extended to other dimensions. In addition, we present a concrete instantiation of a 320-bit permutation using a more efficient instance of MRM, named Hsilu. Its non-linear layer employs the χ operating on columns.

Compared with the permutations of Gaston and NIST lightweight standard Ascon, the round function of Hsilu requires fewer XOR operations. Hsilu exhibits competitive security and performance with Ascon and Gaston. We demonstrate that the best-found 3-round differential and linear trails of Hsilu have much higher weights than those of Ascon. Hsilu outperforms Gaston and Ascon in terms of both software and hardware performance.

Private Simultaneous Messages (PSM) and Conditional Disclosure of Secrets (CDS) are two fundamental primitives in information-theoretic cryptography. In a PSM protocol, each party sends a single message to a referee, who can then compute a function $f$ of their private inputs but learns nothing else. A CDS protocol follows the same model as PSM, but the goal is to disclose a secret shared among all parties to the referee if and only if the function $f$ evaluates to $1$. Minimizing the communication complexity of these primitives is a central question in this area. Since the best known constructions for general functions require high communication complexity, recent studies have attempted to obtain more efficient constructions by focusing on symmetric functions, whose outputs are invariant under permutations of inputs. However, the extent to which exploiting symmetry can improve efficiency has remained unclear. In this work, we show upper and lower bounds that relate the optimal communication complexities of PSM and CDS for symmetric functions to those for general functions. When the number of parties is larger than the input domain size, our upper bound for PSM improves the best known communication complexity for symmetric functions. Furthermore, our upper bound for CDS demonstrates for the first time that symmetry can be exploited to reduce communication in CDS protocols. In contrast, when the number of parties is constant, our lower bounds show that focusing on symmetric functions yields only a constant-factor improvement. We also derive an analogous implication for PSM protocols under a plausible conjecture. In addition, our new constructions for symmetric functions lead to improvements over the state-of-the-art results in related models such as ad hoc PSM and secret sharing.

We present a simple IND-CCA lattice-based threshold KEM. At a high level, our design is based on the BCHK transform (Canetti et al., EUROCRYPT 2004), which we adapt to the lattice setting by combining it with the FO transform (Fujisaki and Okamoto, PKC 1999) in order to achieve decryption consistency.

As for the BCHK transform, our construction requires a threshold identity-based encryption (TIBE) scheme with suitable properties. We build such an IBE by combining the ABB IBE (Agrawal, Boneh, Boyen, EUROCRYPT 2010) with recent advances in lattice threshold cryptography, such as the threshold-friendly signature Plover (Esgin et al., EUROCRYPT 2024) and a variant of the Threshold Raccoon scheme (Katsumata et al., CRYPTO 2024).

The security proof of our scheme relies on a new assumption which we call the Coset-Hint-MLWE assumption, and which is a natural generalisation of the Hint-MLWE assumption (Kim et al., CRYPTO 2023). We prove the hardness of Coset-Hint-MLWE under standard assumptions. We believe this new assumption may be of independent interest.

Unlike prior works on IND-CCA lattice-based threshold KEMs, our construction only relies on simple algorithmic tools and does not use heavy machinery such as multi-party computation or threshold fully homomorphic encryption.

Encrypted matrix multiplication (MM) is a fundamental primitive in privacy-preserving machine learning and encrypted data search, but it remains a significant performance bottleneck. Recently, Bae et al.~(Crypto’24) and Park~(Eurocrypt’25) introduced novel algorithms for ciphertext–plaintext (CPMM) and ciphertext–ciphertext (CCMM) matrix multiplications. These algorithms reduce encrypted MM operations to plaintext matrix multiplications (PPMM), enabling implementation through highly optimized BLAS libraries. While these reduction-based methods offer significant improvements, their applicability is limited to scenarios where the matrix dimension $d$ is comparable to the ring dimension $N$ in RLWE-based CKKS schemes. As a result, they fall short for matrix multiplications involving small or medium-sized matrices.

We extend the reduction-based CPMM/CCMM into small-sized matrix operations by batching instances. We use the Slots-in-Coefficient (SinC) encoding where a ring element is represented by a polynomial with coefficients each of which is the Discrete Fourier Transform of matrix entries at the same position. This encoding enables reductions of encrypted batch MM algorithms to a small number of batch PPMMs, which can be efficiently accelerated by BLAS libraries. Our batch encrypted MM flexibly accommodates diverse matrix dimensions and batch sizes independent of the ring dimension $N$, thereby extending its applicability to practical real-world settings.

For two $d \times d$ matrices with $N/d$ batches, our batch CPMM and CCMM algorithms achieve complexity $O(d^2N)$, improving upon Bae et al. at $O(dN^2)$ and Jiang et al~(CCS’18) at $O(d^2 N\log (N))$. We further extend our techniques to rectangular matrices, achieving $O(dN^2)$ for multiplying a $d \times N$ and an $N \times N$ matrix, improving previous $O(N^3)$ methods. A proof-of-concept implementation validates these improvements: multiplying 128 batches of $64 \times 64$ matrices takes $0.20$s (CPMM) and $0.91$s (CCMM), yielding $205\times$ and $64\times$ speedups over previous methods. For a $64 \times 2048$ by $2048 \times 2048$ multiplication, our CCMM completes in $7.8$s, achieving a $28\times$ speedup compared to Park's algorithm.

The NIST Post-Quantum Cryptography (PQC) standardization has entered its fourth round, underscoring the critical importance of addressing side-channel attacks (SCA), a dominant threat in real-world cryptographic implementations, especially on embedded devices. This paper presents a novel chosen-ciphertext side-channel attack against CRYSTALS-Kyber (standardized as ML-KEM) implementations with Fisher-Yates shuffled polynomial reduction. We propose an efficient and fault-tolerant key recovery algorithm that, by crafting malicious ciphertexts, induces changes in the Hamming weight distribution of an intermediate polynomial's coefficients (the output of the shuffled polynomial reduction during decapsulation), enabling recovery of secret key coefficients from these changes. To ensure robustness, we propose an error-correction strategy that leverages the Hamming weight classifier's behavior to constrain and shrink the correction search space, maintaining effectiveness even with less accurate classifiers or in low-SNR environments. A Multi-Layer Perceptron (MLP) is employed for Hamming weight classification from side-channel traces, achieving 97.11\% accuracy. We combine statistical analysis with explainable deep learning for precise trace segmentation during pre-processing. Experimental results demonstrate full key recovery with only an average of $10 + 354 \times 3$ ciphertext queries and a success rate of 97.98\%, reducing the adversarial effort by 95.36\% compared to contemporary bit-flip techniques. Although shuffling aims to disrupt temporal correlations, our results show that statistical features persist and leak through shuffled implementations. This work reveals enduring SCA risks in shuffled implementations and informs a broader reassessment of PQC side-channel resilience.

Aggregate signatures allow compressing multiple single-signer signatures into a single short aggregate signature. This primitive has attracted new attention due to applications in blockchains and cryptocurrencies. In multisig addresses, which is one of such applications, aggregate signatures reduce the sizes of transactions from multisig addresses. Security of aggregate signatures under adaptive corruptions of signing keys is important, since one of the motivations of multisig addresses was a countermeasure against signing key exposures. We propose the first aggregate signature scheme tightly secure under adaptive corruptions using pairings. An aggregate signature includes two source group elements of bilinear groups plus a bit vector whose length is equal to the number of single-signer signatures being aggregated. To construct a scheme, we employ a technique from quasi-adaptive non-interactive zero-knowledge arguments. Our construction can be seen as modularization and tightness improvement of Libert et al.'s threshold signature scheme supporting signature aggregation (Theoretical Computer Science 645) in a non-threshold setting.

Side-channel analysis (SCA) exploits physical leakages such as power consumption or electromagnetic emissions to extract secret information from cryptographic implementations. Leakage model is the critical link between observable physical emanations (e.g., power consumption) and internal cryptographic states. When a leakage model fails to match the device’s underlying physical leakage, even powerful attacks like Correlation Power Analysis (CPA) are rendered ineffective. This fundamental challenge limits the success of traditional SCA, especially against noisy or masked targets. In this work, we introduce the Neural Leakage Model (NLM), a novel neural network architecture trained to learn and accurately characterize complex physical leakage to enhance CPA. Moving beyond NLM's superior attack performance, we directly address the critical ``black box'' problem in deep learning-based side-channel analysis (DLSCA) by proposing a novel methodology to transform the trained NLM into a mathematically equivalent, closed-form polynomial expression. This provides interpretability, offering evaluators transparent insight into the precise leakage model the network has learned. We validate NLM’s performance across four diverse datasets spanning three platforms, from low-end microcontrollers to high-end multi-core ARM systems and dedicated FPGAs. Notably, NLM successfully recovers the secret key from the highly challenging and noisy CHES2025 dataset, representing the first known successful CPA attack on this benchmark. Furthermore, NLM demonstrates superior or comparable performance when extended to higher-order CPA against masking countermeasures. Our findings establish NLM as a powerful, generalizable, and interpretable approach to modern side-channel analysis.

Very recently, Crites et al. (CRYPTO 2025) gave a proof for the full adaptive security of FROST (Komlo and Goldberg, SAC 2020), the state-of-the-art two-round threshold Schnorr signature scheme, which is currently used in real-world applications and is covered by an RFC standard. Their security proof, however, relies on the computational hardness of a new search problem they call “low-dimensional vector representation” (LDVR). In fact, the authors show that hardness of LDVR is necessary for adaptive security of a large class of threshold Schnorr signatures to hold, including FROST and its two-round variants. Given that LDVR is a new assumption and its hardness has not been seriously scrutinized, it remains an open problem whether a two-round threshold Schnorr signature with full adaptive security can be constructed based on more well-established assumptions.

In this paper, we resolve this open problem by presenting ms-FROST. Our scheme is partially non-interactive and supports any t - 1 < n adaptive corruptions, where n is the number of signers and t is the signing threshold. Its security relies on the algebraic one-more discrete logarithm (AOMDL) assumption, the algebraic group model (AGM), and the random oracle model (ROM). Further, it achieves the strongest security notion (TS-UF-4) in the security hierarchy of Bellare et al. (CRYPTO 2022). To justify our use of the algebraic group model, we show an impossibility result: We rule out any black-box algebraic security reduction in the ROM from AOMDL to the adaptive TS-UF-0 security of ms-FROST.