International Association
for Cryptologic Research

Despite improvements to authentication mechanisms, account compromise remains frequent and users need a trustworthy way to determine what devices have accessed their accounts. Doing so, however, is in tension with privacy goals on the modern web, which mandate that web services not learn static device identifiers. Recent work aims to address this tension via client-side encrypted access logging (CSAL), but their approach does not allow retrieving all log entries and users may miss information about adversarial accesses.

We present Trace, a new CSAL system that achieves complete logging while preserving privacy. Trace records verifiable evidence of each authentication in an encrypted log stored by an independent logging service, ensuring that only the user can inspect it. The web service remains unaware of the logging, preserving backward compatibility with existing authentication infrastructures. Unlike prior approaches, Trace simultaneously achieves verifiable device attribution, backward compatibility, and formally-analyzed security against malicious adversaries. Our prototype implementation reaches over 10 K authentications per second on a single core, suggesting it can scale efficiently for large services.

In 2022, Castryck and Decru introduced an attack that broke several isogeny-based schemes, including SIKE, which had advanced to the final round of the NIST Post-Quantum Cryptography Standardization Competition. Despite this attack, research on isogeny-based cryptography has continued, primarily due to the compact key sizes offered by these schemes compared to other post-quantum approaches. There are now many isogeny-based schemes that are resistant to the Castryck-Decru attack. These schemes typically involve advanced mathematical structures that may require significant time and effort to study.

In this paper, we provide a structured survey of isogeny-based signature schemes that are resistant to the Castryck-Decru attack, aiming to facilitate an understanding of the current landscape and the most practically relevant schemes in this area. We categorize these signature schemes into two main classes: those based on the CSIDH group action and the SQIsign family. For each class, we discuss their fundamental design principles, security assumptions, and specific constructions. We also compare their performance and compactness. Additionally, we describe one representative scheme from each class that is particularly relevant in practice due to its efficiency or compactness. In conclusion, we compare the performance of the schemes discussed in this work with other post-quantum signature schemes.

Post-quantum cryptography focuses on research of cryptographic primitives, including public key encryption and signatures, that can resist the attacks mounted by an adversary with an access to a quantum computer. An alternative is to employ quantum cryptography to protect communication links by employing principles of quantum physics to protect security of the key exchange. Recently, a group key establishment protocol that combines these approaches in a secure way was presented by Steinwandt and Gonzales Vasco. We have successfully imple- mented and employed this protocol in a prototype application. In this article we describe the overall architecture and specific details of the implementation that can be of interest for scientific community. We conclude with a discussion of specific challenges, options and open problems that can accompany similar imple- mentation task.

Security evaluation of masking in low-noise regimes remains poorly understood: increasing the masking order does not automatically translate into higher concrete resistance once many correlated intermediates are processed by a full implementation. A common approach is to reduce noisy side-channel leakage to the random probing model (RPM), but existing reductions can be too loose to yield meaningful leakage rates in practice, and current RPM analyses often rely on costly simulations or numerically propagated bounds.

This work develops analytic and algorithmic tools for estimating and upper-bounding RPM security of masked gadgets and their compositions. First, for noisy Hamming-weight leakage over $\mathbb{F}_{2^u}$ we compute concrete RPM leakage-rate parameters for a tighter $\mathbb{F}_2$-linear reduction based on binary inner products, providing a tangible link between SNR and probing rate. Second, for $\mathbb{F}_q$-linear circuits we leverage a vector-space representation to characterize RPM leakage as an erasure event, yielding a direct connection to local metrics such as advantage and implying global simulability for refreshed, block-separated executions. Third, we improve Monte Carlo estimation of rare leakage events using importance sampling, enabling evaluation in low-rate/high-order regimes that are infeasible with naive sampling. Finally, we revisit the leakage-diagram technique and derive explicit bounds for refresh gadgets, and we apply the same viewpoint to composition through \emph{bridges}, showing that SNI—while sufficient for threshold probing model (TPM)—does not capture the RPM phenomenon governing refresh boundaries.

We implement our methods in \textsf{LAPSE}, a tool that compiles gadget descriptions into linear-algebraic representations and supports exact computation as well as Monte Carlo/importance-sampling estimation of RPM security parameters.

The problem of computing an isogeny of large prime degree from a supersingular elliptic curve of unknown endomorphism ring is assumed to be hard both for classical as well as quantum computers. In this work, we first build a two-round identification protocol whose security reduces to this problem. The challenge consists of a random large prime $q$ and the prover simply replies with an efficient representation of an isogeny of degree $q$ from its public key. Using the hash-and-sign paradigm, we then derive a signature scheme with a very simple and flexible signing procedure and prove its security in the standard model. The most efficient variant of our signature schemes features a signing which is $1.4\times$ to $1.6\times$ faster than the most recent implementaion of SQIsign, whereas verification ranges from $1.2\times$ slower to $1.01\times$ faster depending on the security level. The sizes of public key and signature are comparable to existing schemes.

SMAUG-T and HAETAE, designated as target algorithms for national standardization via the Korean Post-Quantum Cryptography (KpqC) competition, run efficiently on general-purpose platforms. On ARM Cortex-M4 class microcontrollers, however, peak stack usage becomes a key constraint: while SMAUG-T can be executed on typical Cortex-M4 boards, the baseline HAETAE implementation exceeds the available SRAM (e.g., 91{,}176\,B stack for signing), motivating dedicated memory optimization. To address this problem, we propose a suite of memory optimization techniques for SMAUG-T and HAETAE that enable their practical operation within the strict memory budget of the Cortex-M4. Experimental results demonstrate that, compared to the KpqClean\_ver2 baseline, peak stack usage was reduced by 73--83~\% for SMAUG-T5 (e.g., 24{,}300\,B$\rightarrow$4{,}240\,B in decapsulation) and by about 90~\% for HAETAE5 (e.g., 91{,}176\,B$\rightarrow$8{,}092\,B in signing). Furthermore, a branchless constant-time design was applied throughout to ensure that the optimized implementations remain robust against side-channel threats such as timing attacks. This work provides a practical methodology for deploying KpqC lattice-based cryptography in memory-constrained embedded environments.

Private Set Intersection (PSI) allows two mutually distrusting parties to compute the intersection of their private sets without revealing any additional information. Fuzzy PSI, an approximate variant of PSI, allows the receiver to learn points of the sender that are ``close" to its points. More formally, the receiver learns all $y$ in the sender's set that satisfy $dist(x,y)< \delta$ for some element $x$ in the receiver's set and threshold parameter $\delta$. Recently, there has been significant progress on Fuzzy PSI, as it allows us to realize several important applications such as password matching, facial recognition, and contact tracing in a privacy-preserving manner. However, existing Fuzzy PSI constructions make strong assumptions on the input sets, such as receiver set disjointedness or projected disjointedness. In this work, we analyze those strong assumptions from a practical viewpoint and observe a gap between theory and practice, i.e., real-world data sets do not abide to those assumptions. To bridge the gap, we first define a new relaxed and weaker assumption based on the low density of sets, demonstrate the assumption to be practical, and build a compiler that converts constructions under the strong assumption to those under the new, practical assumption. At the core of our transformation is a novel idea involving higher-dimensional lifting and coloring. Combining our transformation with current Fuzzy PSI protocols under the strong assumption yields efficient and practical Fuzzy PSI protocols. We also concretely analyze the run-time and overhead of our transformed protocols for parameters for illustrative applications, such as password matching.

The transition to post-quantum cryptography (PQC) significantly increases the computational cost of TLS~1.3 handshakes. In particular, hybrid handshakes incur even greater overhead, as they require performing both classical and PQC algorithms for key exchange and authentication. This paper systematically analyzes the performance of hybrid PQC TLS~1.3 handshakes using a POSIX thread pool-based parallel execution model. We evaluate a total of 135 combinations comprising 3 classical KEMs, 3 ML-KEM variants, 3 classical DSAs, and 5 PQC DSAs. Sequential execution times range from 429.2 to 1,907.0~$\mu$s, while parallel execution times range from 356.7 to 1,380.8~$\mu$s, achieving speedups of 1.08$\times$ to 1.40$\times$ across all combinations. The highest speedup is observed in P-384-based configurations, where the overlap between classical and PQC operations is most pronounced. Furthermore, we recommend both throughput-oriented combinations based on FN-DSA and currently standardized ML-DSA combinations for each NIST security level. These results provide practical design guidance for mitigating performance degradation in hybrid PQC TLS deployments.

We specify OCH, the first authenticated encryption with associated data scheme built to provide 128-bit multi-user AE security, 128-bit context commitment security, and 256-bit nonces with optional nonce privacy. It therefore addresses pressing limitations of currently widely-deployed schemes. We construct and formally analyze the security of OCH in a modular fashion, with transforms that are of broader applicability. On Intel Raptor Lake CPUs, OCH using the Areion permutation family has a peak encryption speed of 0.62 cycles per byte (cpb), not far off from AES128-GCM (0.38cpb) and outperforming both ChaCha20/Poly1305 (1.63cpb) and TurboSHAKE128-Wrap (3.52cpb).

Private set intersection (PSI) has become extremely practical, in large part due to the fact that modern protocols rely almost exclusively on cheap, symmetric-key cryptography. The same cannot be said for the variant of PSI called updatable PSI (UPSI; Badrinarayanan et al., PoPETS 2022), where parties’ input sets evolve over time, and the cost of re-computing the intersection depends only on the changes to their sets. In existing UPSI protocols, the number of public-key operations scales with the number of items.

In this work, we introduce the first UPSI protocol that largely avoids public-key operations. In fact, our protocol uses mostly the same protocol tools/techniques that have been so successful in making (plain) PSI truly practical. By leveraging symmetric-key primitives, our implementation achieves orders-of-magnitude improvements over prior work.

Additionally, we observe that existing UPSI security proofs do not consider an adversary who can choose protocol inputs adaptively (i.e., choose which items to add to the set the current epoch based on the adversary’s view in previous epochs). We observe that several existing UPSI protocols are trivially broken by such adaptive input selection (even with semi-honest corruption). Several variants of our protocol are secure in the presence of adaptively chosen inputs.

Along the way, we also introduce a new and cleaner abstraction for a common idiom of using an oblivious key-value store (OKVS; Garimella et al., Crypto 2021) to represent a set of items. Our new abstraction, called affine set encoding, may be of independent interest.

Recently, Stateful Private Information Retrieval (PIR) has emerged as a promising new paradigm of PIR. Despite significant recent progress, state-of-the-art single-server schemes in this paradigm still suffer from practical inefficiencies in communication, computation, and/or client storage. In this work, we construct a new single-server stateful PIR scheme called HarmonyPIR that achieves efficient communication, computation, and client storage. From a technical standpoint, we build on the recent work of Wang and Ren (EUROCRYPT 25) and propose a new hint organization that uses only a single random permutation. The random permutation can be instantiated using either AES or the recently standardized FF1 Format-Preserving Encryption, yielding two variants of HarmonyPIR. Our new scheme achieves up to two orders of magnitude better amortized computation and up to five times better amortized communication than state-of-the-art schemes.

Post-quantum assumptions may not rely on the difficulty of finding secret subgroups as many classical schemes did. Instead, several assumptions make use of more general group actions, with the belief that quantum algorithms are not helpful in this less structured setting. Famously, some isogeny constructions use the action of an ideal class group on elliptic curves, but equivalence problems in error-correcting codes and lattices also exhibit such structures.

Previous works hence presented anonymity-preserving constructions in a generic group action framework; however, they were not general enough to encompass the group action underlying the Lattice Isomorphism Problem (LIP), for which the acting group is infinite (in fact, not even compact) and non-commutative.

We bridge this gap by, from zero-knowledge proofs of OR statements, building generic blind signature and strong designated-verifier signature with non-delegability constructions from standard assumptions corresponding to a generalised group action inverse problem.

Traceable secret sharing complements traditional schemes by enabling the identification of parties who sell their shares. Recently, two independent works extended traceable secret sharing to general access structures.

Goyal, Jain, and Partap [EC'26] introduced a model in which a reconstruction box is augmented with a label $I \subseteq [n]$ and is only required to distinguish between two secrets when queried with the shares of parties in $I$. Based on how this label relates to the corrupted set $J$ that built the box, they defined two notions of traceability. If $I \cap J = \emptyset$, the model is called $\emptyset$-strong traceability, for which they presented a construction based on indistinguishability obfuscation (iO). Otherwise, their model is calledstrong traceability, for which they proved an impossibility result. Farràs and Guiot [EC'26] proposed a different model, which we call hiding traceability, where the reconstruction box has no label and the access structure is hidden from the parties.

In this work, we improve traceable secret sharing for general access structures in three directions. First, we present a fully information-theoretic scheme for the $\emptyset$-strong traceability model, eliminating the need for strong cryptographic assumptions. This resolves an open question posed by Goyal, Jain, and Partap, who asked what are the minimal assumptions needed in the $\emptyset$-strong traceability model.

Second, motivated by the impossibility of strong traceability, we introduce a relaxed notion calledhidden mildly strong traceability. This model is relevant in practice and bridges the strong and hidden models. For this setting, we present an information-theoretic scheme for general access structures.

Finally, we consider the more general model of stateful traceability, where reconstruction boxes may keep state across queries, and we prove an impossibility result for this setting.

End-to-end cloud storage solutions are deployed at large scale, yet recent works have demonstrated severe attacks against their confidentiality and integrity. Motivated by this, a first formal treatment of secure cloud storage was given at CRYPTO 2024 by Backendal, Davis, Günther, Haller and Paterson (BDGHP). They define syntax and security notions, capturing client-to-client security of cloud storage schemes with respect to a password distribution. They also give an efficient construction using the Two-Hash Diffie-Hellman (2HDH) OPRF and standard cryptographic building blocks, which they prove secure under selective corruptions in the random oracle model. However, several aspects of practical security guarantees remain open. We extend and refine the work of BDGHP along multiple dimensions, advancing the analysis of secure cloud storage schemes. First, we prove that their construction can be proven secure against adaptive corruptions (with a slight modification), circumventing technical challenges posed by file sharing. Second, we modularize the scheme further by introducing an abstraction for the authentication procedure. This allows us to identify the concrete role of 2HDH and alternative instantiations. Third, we introduce a weaker model that captures adversaries who can arbitrarily control the network, except during registration. This allows us to prove concrete guarantees about online password guessing attacks, whereas the stronger model inherently allows for offline guessing. Finally, we formalize and prove explicit authentication, relying on the security of our new authentication abstraction and the MAC scheme, where the latter was previously not used in the security analysis.

This paper presents the first round-optimal threshold blind signature without random oracles. Our construction achieves security in the algebraic group model (AGM) for asymmetric pairing groups, and tolerates adaptive corruption of up to $t-1$ signers, where $t$ is the threshold. We improve upon the recent threshold blind signatures of Lehmann, Nazarian and Özbay (EUROCRYPT 2025) and Jarecki and Nazarian (ASIACRYPT 2025) in two ways: we eliminate both the reliance on random oracles and the need for $q$-type assumptions in the AGM. As a core building block, we introduce a new pairing-based round-optimal blind signature without random oracles, based on the $2$-DL assumption in the AGM. Both blind signature schemes achieve communication and computation costs only twice that of the celebrated blind BLS signature.

The processing of ML-KEM (formerly CRYSTALS-Kyber), a key encapsulation mechanism with post-quantum security, is performed by multiplication, addition, and subtraction of polynomials whose coefficients lie in the finite field ${\mathbb F}_{3329}$. To reduce the number of such operations, it is common to use the Number Theoretic Transform (NTT). This paper focuses on arithmetic over ${\mathbb F}_{3329}$ and proposes the use of a logarithmic representation with respect to a primitive element $\alpha$ of ${\mathbb F}_{3329}^*$ for implementing multiplication, addition, and subtraction over ${\mathbb F}_{3329}$. In this representation, multiplication in ${\mathbb F}_{3329}^*$ can be reduced to addition in $\mathbb{Z}_{3328}$. Furthermore, addition and subtraction in ${\mathbb F}_{3329}^*$ can be computed in the logarithmic domain by using Zech's logarithm. However, special treatment is required when $0 \in {\mathbb F}_{3329}$ is involved in the operations. This paper proposes a new implementation method of the logarithmic representation for arithmetic over ${\mathbb F}_{3329}$, including the handling of such exceptional cases.

We revisit the three-round threshold Schnorr signature scheme Sparkle of Crites, Komlo, and Maller (CRYPTO 2023), as well as its variant Sparkle+. While Sparkle+ was accompanied by a claim of full adaptive security, subsequent work identified a gap in the analysis. Moreover, the original—and simpler and more efficient—Sparkle scheme has so far lacked even a proof of static security.

We resolve this state of affairs by giving the first proof of static security for Sparkle and then, as our main result, a tight proof of full adaptive security in the pure random oracle model, i.e. without relying on the algebraic group model. The core obstacle is that, in the fully adaptive setting for Sparkle, rewinding arguments fundamentally break down. To address this, our proof is based on a new Vandermonde circular discrete-logarithm (VCDL) assumption, an interactive strengthening of the circular discrete-logarithm assumption of Cho et al. (CRYPTO 2025), originally introduced to prove tight security of basic Schnorr signatures. In particular, circular-style assumptions eliminate the need for rewinding. Beyond tightness, our analysis highlights circular-style assumptions as a general approach to achieving security in settings—such as full adaptive security—where rewinding is inherently problematic.

We justify VCDL by reducing it to the low-dimensional vector representation (LDVR) problem of Crites et al. (CRYPTO 2025) in the elliptic-curve generic group model; conversely, VCDL implies LDVR in the standard model. Finally, we generalize VCDL (and similarly LDVR) by abstracting away the specific choice of Vandermonde vectors. As an application, we identify a different assumption within this framework that yields a tight proof of adaptive multi-user security for the basic Schnorr signature scheme, a result of independent interest.

It has been a very long standing open question whether the CFS signature scheme whose security is basically that of a McEliece scheme based on very high rate binary Goppa codes could be attacked or not. There was a first cryptanalytic result by Faugère et al in 2011 consisting in finding a distinguisher for the binary Goppa codes used in this scheme showing that these codes can be distinguished in polynomial time from a random binary linear code. However despite numerous cryptanalytic attempts and even if the original distinguisher has been significantly improved, no attack on the McEliece scheme based on binary Goppa codes has been found so far except for very peculiar Goppa codes of degree $2$. We show here that the Pfaffian modeling used in the distinguishing attack of Couvreur, Mora and Tillich of Asiacrypt 2023 can actually be used together with a shortening trick and looking for squares in the corresponding ideal to find a polynomial attack on the CFS scheme based on very high rate binary Goppa codes.This breaks this 25 years old signature scheme. We demonstrate the effectiveness of this approach by recovering the key of TII McEliece challenges with a claimed key security of up to 210 bits.

Range queries can filter, aggregate, and retrieve database entries that lie in a specified multi-dimensional rectangle. Private range queries allow a client to query a server's public database while keeping the client's multi-dimensional rectangle hidden.

We construct RangeR, a constant-round private range query scheme that supports any associative aggregation function (e.g., SUM, MAX, TOP-K) and works with any number of servers. In the single-server setting, RangeR is orders of magnitude faster and uses 50%-90% less communication than HADES (VLDB 2025), a prior single-server private range query scheme that only supports linear aggregation functions.

We describe how RangeR can be used to implement a privacy-preserving map application that can return the highest-rated restaurants near a user. Using data from $\mathtt{OpenStreetMaps}$, we estimate that a user can find the highest-rated restaurants within one kilometer of their location within $2$ seconds, while revealing only that the user is somewhere in the USA.

The distributed nature of federated learning systems makes them vulnerable to backdoor attacks in which malicious clients manipulate local training data using trigger-dependent behaviors to cause targeted misclassification. Although homomorphic encryption preserves the privacy of model updates during aggregation, it limits the application of conventional defenses that require access to plaintext updates. Moreover, distinguishing poisoned models from benign variations becomes more challenging under non-independent and identically distributed (non-IID) data distributions.To address this challenge, we introduce a defense strategy that operates at inference time by identifying abnormal internal activation patterns within the aggregated global model, rather than filtering encrypted individual updates during training. The proposed approach analyzes neurons that exhibit low activation on clean inputs, referred to as "dormant" neurons, but become disproportionately active in the presence of trigger patterns. By constructing a statistical activation baseline using a small clean dataset, we derive class-specific thresholds that serve as decision boundaries to detect and reject suspicious predictions. Since the proposed method relies on global model behavior at inference time instead of inspecting individual client updates, it does not introduce additional training overhead and remains robust under non-IID data settings. Our approach maintains a strong balance between privacy, security, and accuracy by defending against backdoor attacks without requiring access to client updates. Experimental results demonstrate that even with a 99% attack success rate and 90% main-task accuracy, the proposed defense method successfully detects 100% poisoned images.