International Association
for Cryptologic Research

Ongoing advances in quantum computing represent a growing risk to modern cryptography (potentially threatening both asymmetric and symmetric encryption protocols), thereby challenging the foundations of digital security. In response, global cybersecurity communities, led by standardization bodies such as NIST and ETSI, launched initiatives to establish migration pathways toward post-quantum cryptography (PQC). However, the migration of legacy systems to quantum-safe cryptography presents many challenges that have not yet been addressed due to their limited cryptographic agility, outdated infrastructure, and regulatory constraints. These legacy environments, even though they rely on aging technologies and constrained hardware, are still vital to major sectors (such as finance, energy, healthcare, and government). This paper explores some obstacles to the implementation of PQC in these environments, such as hard-coded cryptographic functions, outdated programming languages, hardware limitations, vendor lock-in, interoperability constraints, and certification issues. This shows that, in contrast to contemporary systems, legacy systems cannot be readily modified or easily re-engineered. A critical review of existing standards and academic publications revealed key limitations: their focus on algorithms specifications, the abstract guidance provided without operational depth, the lack of empirical validations, and the insufficient risk modeling and attention to legacy constraints. These gaps prevent effective planning and secure execution of the PQC migration in legacy systems. Consequently, this position paper argues that existing deliverables remain insufficient to address the specific challenges of PQC migration in legacy systems. It proposes the elaboration of a field-informed risk-based framework for PQC Migration in Legacy Systems to guide this transition. This proposed framework combines three interdepedent layers: a diagnostic characterization of legacy system constraints, a qualitative risk assessment grounded in those constraints, and a quantitative evaluation of migration options through an ROI-based analysis to support decision-making. Unlike existing approaches that treat legacy as generic labels, this framework begins by exploring what makes each system legacy in its specific context before applying the risk model. Its development is informed by an empirical survey conducted among large organizations across critical sectors, ensuring relevance beyond theoretical assumptions. Future work will focus on elaborating the framework through applied research, tool development, and real-world case studies in collaboration with financial institutions and critical infrastructure operators. In addition, continued engagement with cyber authorities and standardization bodies will help us ensure alignment with emerging regulations.

Many blockchain-based applications can be seen as instances of fair exchange of two signatures. Adaptor signatures (AS) and, more concretely, their extractability property, are commonly combined with blockchain-based economic incentives to achieve fairness in the exchange of two signatures in the blockchain. Certain blockchain applications require unique signatures (e.g., BLS), but it is formally impossible to build AS from unique signatures. Other applications need blind signatures, however, we found a tension between extractability and blindness. To address these limitations, we observe that fair exchange protocols based on AS only require extractability for one of the two exchanged signatures. This observation allows the other AS to be replaced with a primitive that provides similar security guarantees without inheriting the limitations of AS with respect to unique and blind signatures. A natural candidate is verifiably encrypted signatures (VES), introduced by Boneh et al. (Eurocrypt'03). However, this primitive predates blockchain systems and relies on a trusted party, the adjudicator.

Our first contribution is to eliminate the need for an adjudicator by shifting trust to the blockchain and redefining the VES security model accordingly. We introduce two new security notions and prove that our notions imply existing guarantees. We revisit classical VES constructions by Boneh et al. (Eurocrypt'03) for unique signatures and by Hanser et al. (ESORICS'15) for probabilistic signatures, and show that they satisfy our new definitions. Furthermore, we compare our new notions with AS, and conclude that our revised VES is equivalent in terms of security to AS without extractability. Our second contribution extends VES to support blind and non-interactive blind signatures, introducing a new primitive: Verifiably Encrypted Blind Signatures (VEBS). We present a novel construction for non-interactive blind signatures and prove its security. We implement our construction and demonstrate its practical efficiency: encryption requires 3 ms, verification 6 ms, and decryption 13 ms, with a communication cost of 912 bytes. Finally, we discuss how VES/VEBS apply to diverse use cases, including anonymous credentials, contingent payments, atomic swaps, intermediated payments, coin mixing, and applications involving blind signatures.

In this work, we introduce the concept of Obliviously Checkable Secret-Carrying Puzzles (OxSP) and build proof-friendly Garbled Circuits (GCs) to enable their practical implementation. OxSPs allow one to publicly pose puzzles and verify purported solutions received in response, keeping the desired parts of the puzzles and the responses hidden.

We show how OxSPs can be based on Garbled Circuits (GCs). However, this requires ZK-SNARK proofs of correctness of garbling. We note that combining existing GC and ZK-SNARK constructions results in very large computational costs for the OxSP solvers. Our main technical contribution is to design a new proof-friendly GC construction which cuts down the cost of generating a proof of correct garbling to almost a third, without resorting to non-standard cryptographic assumptions.

Beyond its use in OxSP, we expect our proof-friendly GCs to be of significant independent interest, as a tool for auditable secure 2-party computation.

BBS+ signatures are widely adopted in privacy-preserving systems such as anonymous credentials and Direct Anonymous Attestation (DAA). To strengthen key security and eliminate single points of failure, threshold variants of BBS+ signatures have become increasingly important. However, existing constructions suffer from notable inefficiencies: some entail excessive communication overhead (e.g., DKL+23, S&P 2023), while others impose substantial computational costs and require additional interaction rounds (e.g., WMC24, NDSS 2024).

In this work, we present a novel and efficient three-round threshold BBS+ signature scheme from the Castagnos–Laguillaumie (CL) cryptosystem. Our construction achieves best communication–computation trade-offs than previous works. Specifically, compared to the four-round WMC24 scheme, our protocol reduces communication by $77.4\%$ and demonstrates faster computation, with benchmarks indicating speedups of $10.6$--$16.6\times$ in single-threading and $3.3$--$5.4\times$ in multi-threading. Against the three-round protocol DKL+23, our scheme exhibits an asymptotic slowdown factor of $4\times$, but enhances communication by two orders of magnitude.

We further extend our techniques to threshold BBS signatures, Dodis-Yampolskiy verifiable random functions (DY VRFs), and multiplication protocols (DNP25 and LLZ+25, CCS'25). This yields: (1) a three-round threshold protocol for the original BBS scheme; (2) two-round threshold protocols for both DY VRFs (focusing on its oblivious variant) and the AGM-secure BBS variant; and (3) one fewer group element in broadcasts for the multiplication protocol with reduced ZKP costs via simplified relations.

The integral-resistance property provides strong arguments against integral distinguishers. Recently, Zeng and Tian proposed a new method to show this property for AES. In this paper, we provide a generalized framework and tool called intres to extend and apply this method to other ciphers with complex linear layers. We derive properties that a cipher must fulfill in order for the method to be applicable. Furthermore, we introduce a degree propagation model which helps us determine the valid key masks for the integral-resistance matrix. The degree model can also be used to upper-bound the algebraic degree of cipher constructions. This allows us to provide tighter upper bounds for the degree of Rijndael-256. We propose algorithmic improvements to substantially decrease the runtime of the offline phase with the intres framework. As a result, we are able to show the integral-resistance property for 7 rounds of PRINCE and 6 rounds of Beanie. Finally, we develop a heuristic MILP-based approach to search for lower-latency alternatives to the MixColumns matrices of PRINCE while maintaining integral resistance. After showing that using this new matrix we still achieve 7-round integral resistance, we validate our method with SAT-based trail counting. While using a MixColumns matrix only optimized for integral resistance might affect security against other types of attacks, we believe these lower-latency matrices have their place in constructions similar to ZIP-ciphers, where integral resistance is particularly critical.

MPC-in-the-Head protocols enable post-quantum digital signatures based solely on symmetric primitives, with PICNIC being a prominent example built on the LowMC block cipher. While existing analyses assume exact Boolean circuit semantics, recent advances in neural representations suggest that piecewise-linear implementations may introduce activation boundary leakage. In this work, we investigate whether such leakage can be exploited in the context of LowMC and MPC-in-the-Head transcripts. We propose a perturbation-based probing methodology that models neural leakage and reduces round-key recovery to independent binary hypothesis tests via majority voting. Exploiting the linear structure of the LowMC key schedule, we demonstrate that recovery of the first-round key enables efficient reconstruction of the master key with linear complexity. Experimental results confirm successful recovery of 128-, 192-, and 256-bit keys under the proposed model, highlighting a new dimension in symmetric cryptanalysis and the need to consider learning-based leakage in future designs.

Passwords remain the default mechanism for user authentication despite well known weaknesses such as offline dictionary attacks and pervasive password reuse. Single Password Authentication (SPA) solutions mitigate these risks by securely protecting high entropy authentication secrets under a single human memorable password and storing them across untrusted storage provider(s). However, existing SPA schemes leave important practical gaps: storage providers cannot verify password knowledge, enabling preemption and overwrite attacks on stored shares, and current designs do not securely and efficiently support secret or password updates. We present $\mathsf{UpSPA}$, an efficient, secure, and updatable threshold SPA that closes these gaps without necessitating login-server-side changes. $\mathsf{UpSPA}$ introduces a high entropy, storage-provider-specific identifier secret to prevent preemption, enables secret updates via implicit authentication, and supports password updates via explicit authentication using a password protected signing key. We prove security in the ideal-real paradigm, including resistance to offline dictionary attacks under standard static threshold corruption assumptions. Our evaluation shows that $\mathsf{UpSPA}$ incurs low overhead on commodity hardware and remains competitive across threshold settings relative to prior SPA work that does not support password updates.

Puncturable attribute-based encryption ($\mathsf{PABE}$) not only supports fine-grained access control over encrypted data, but also enables users to revoke the decryption capability for specific messages by puncturing tags, thereby achieving fine-grained forward security. It finds wide applications in scenarios such as sharing government classified documents and personal health records. However, existing $\mathsf{PABE}$ schemes only support tag-by-tag puncturing, where each puncturing operation is done through key delegation, which causes the key size to grow with the number of punctured tags. This inefficiency makes $\mathsf{PABE}$ impractical in scenarios that require frequent puncturing or mass revocations. To address this limitation, it is crucial to support batch puncturing of tags, i.e., the decryption capability for messages associated with multiple tags can be revoked simultaneously via a single puncture.

In this work, we construct a ciphertext-policy attribute-based encryption ($\mathsf{CPABE}$) scheme for circuits with batch-puncturing. Notably, the size of the punctured key in our scheme is independent of the number of punctured tags, as well as the size and depth of the circuits. This is achieved by leveraging the evasive learning with errors ($\mathsf{LWE}$) and tensor $\mathsf{LWE}$ assumptions. In addition, we observe that puncturable $\mathsf{CPABE}$ can be re-stated by dual-policy $\mathsf{ABE}$ ($\mathsf{DPABE}$) with key delegation, and generalize batch-puncturing $\mathsf{CPABE}$ to provide the first lattice-based construction of $\mathsf{DPABE}$ for circuits. Moreover, inspired by the observation of Agrawal and Yamada (Eurocrypt '20), we introduce the puncturing property into optimal broadcast encryption ($\mathsf{BE}$), capturing a new primitive called puncturable $\mathsf{BE}$, which allows the receiver to securely erase sensitive messages without communicating with the authority.

We give a simple counterexample which shows that, for Reed--Solomon codes over multiplicative subgroups of prime fields, proximity gaps do not hold near capacity, at least not as conjectured by Ben-Sasson, et al., in BCIKS20. For relative distance $\theta = 1-\rho-\eta$, where $\rho$ is the rate of the code, and positive $\eta = \Theta_\rho(1/\log n)$, where $n$ is the length of the code, we construct an affine line that is not entirely $\theta$-close to the code but still contains $2^{\Omega_\rho(1/\eta)}$ such points. The same construction gives a slightly stronger list-decoding lower bound. The proof uses a new additive-combinatorics lemma on sums of roots of unity.

We present Panther, a family of six robust hybrid key encapsulation mechanism (KEM) combiners that pair FrodoKEM (unstructured LWE) with ML-KEM (module-LWE, FIPS 203) so that IND-CCA2 security holds whenever either assumption is hard. The family includes five hardened variants of the textbook combiners—parallel HKDF, SHAKE256 splitkey, sequential chaining, XOR, and nested—each made to satisfy a uniform robustness predicate (transcript binding, domain separation, implicit rejection, length normalisation, ∨-security), together with a novel structural-splicing construction Panther-SS that interleaves the constituent ciphertexts and binds the cut-positions via a structural tag. Every combiner admits a systematic Market-Theoretic Security Framework proof in which each bidding round is documented by its purpose, the scheme component it replaces, and its complexity cost; the framework extends cleanly to correctness, unbounded session security, QROM security, and quantitative side-channel resistance. We complement the theory with extensive benchmarks on liboqs-backed reference implementations, including a head-to-head comparison of Panther combiners against the keyencapsulation candidates that appeared in NIST PQC Rounds 1–4 (Kyber/ML-KEM, FrodoKEM, NTRU, SABER, NTRU Prime, Classic McEliece, BIKE, HQC). The experiments cover keygen/encaps/decaps latency, throughput, memory footprint, ciphertext and key sizes, scaling with query count, CPU-cycle counts, security-vs-performance Pareto analysis, and an attack-vsdefence matrix against published side-channel attacks on both constituents. The results confirm that hybrid robustness is essentially free over the slower constituent, that Panther-SS uniquely achieves full robustness with combiner-only overhead below half a percent of total latency, and that the Panther family sits on the Pareto frontier of post-quantum KEM candidates.

In this paper, we explore the use of Montgomery multiplication with a multi-limb redundant representation of integers, in particular in combination with signed reduction factors. We develop techniques that are particularly suited to software platforms on which carry propagation is expensive, in particular RISC-V CPUs which lack hardware support for carries. We also show how to perform a whole-primitive range analysis that demonstrates that overflows are not possible, thus allowing liberal use of unreduced limb-wise additions and subtractions, which are small and fast. The implementation and analysis techniques are illustrated in a codegolfing exercise, to produce size-optimized implementations of ECDSA signature verification over NIST curve P-256; use of a virtual CPU with a custom instruction set with byte-size encoding ("bytecode") allows the production of an implementation as small as 848 bytes on x86 CPUs (in 64-bit mode); RISC-V (984 bytes), Armv8-A (1136 bytes) and portable C implementations (about 2200 to 2800 bytes) are also provided. In the process, an AI is utterly discomfited.

The plausible threat of a Cryptographically Relevant Quantum Computer (CRQC) has rightly stimulated a move away from traditional methods of asymmetric cryptography to new post-quantum secure equivalents. Digital signature is the cryptographic primitive that authenticates an internet server’s identity by signing each certificate in an X.509 certificate chain. A suggested response to the CRQC threat is to deploy a hybrid classical/post-quantum digital signature, combining a traditional tried-and-tested scheme with a post-quantum alternative, where certificates are signed using both methods. Here we propose a fused signature scheme that adopts the same approach, but introduces minimal friction into existing TLS architectures

The transition to post-quantum cryptography (PQC) is constrained by the limited cryptanalytic history of individual PQC algorithms. Hybrid constructions, which combine several primitives so that breaking the hybrid requires breaking each component, address this concern directly. This paper presents Cobra, a hybrid Key Encapsulation Mechanism (KEM) that integrates FrodoKEM (unstructured LWE), ML-KEM (FIPS 203 module-LWE), HQC (code-based), and a Dummy KEM for agility, and analyses all 15 mathematically distinct composition methods spanning parallel, cascading, multi-stage, and nested topologies. We prove that every Cobra method achieves IND-CCA2 security within the MarketTheoretic Security Framework (MTSF), which subsumes and strictly extends both Universal Composability and the Random Oracle Model. An explicit 10-round bidding-round chain per method yields post-quantum ask prices of approximately 2−127 at NIST Level 1 together with composability under arbitrary TLS 1.3 embeddings, per-session CNF auditing, and unbounded-session security via pinging. Although all fifteen methods are security-equivalent, encapsulation latency varies by 3.2× (1.2–3.8 ms) and Theorem 7.1 reduces deployment selection to a Pareto-optimal set of five archetypes. Three real-world TLS 1.3 case studies (financial, healthcare, government) confirm the prediction, with infrastructure overhead clustering at 15–22% across sectors.

Event date: 26 May to 29 May 2026
Submission deadline: 25 April 2026
Notification: 30 April 2026

Event date: 1 June to 2 June 2026

Side-channel attacks exploiting Plaintext-Checking Oracles (PCOs) instantiated thanks to the leakage of the re-encryption step taking place during decapsulation are a well-known weakness of ML-KEM. An already wide literature investigated how to efficiently exploit such oracles, leading to easy (full) key recoveries. Somewhat surprisingly, the investigation of how to best instantiate PCOs against ML-KEM's most leaking operations is less investigated, in particular when it comes to quantitative evaluations against concrete masked implementations. In this paper, we first remedy this lack by systematically instantiating PCOs against three open source masked software implementations of the Keccak function used in ML-KEM, based on different masking techniques and programming styles. We evaluate the accuracy of PCOs for increasing number of shares using state-of-the-art profiled attacks against ARM Cortex-M4 implementations, and succeed obtaining high accuracy for up to 7 shares by leveraging the leakage of approximately 50 ML-KEM executions only. Doing so, we confirm the ``computing more implies leaking more'' adage and conclude that enforcing high security levels on such platforms will not be affordable. Next, we consolidate recent solutions for exploiting PCOs. For this purpose, we start by introducing a simple, concrete and re-usable model for PCOs targeting masked implementations of Keccak. We follow by clarifying that approaches based on hard decisions are suboptimal compared to soft (probabilistic) ones. We finally open a study of how to best exploit the adversary's computational power in a security evaluation. We show that (even naive) lattice based attacks are a promising approach for this purpose, leaving the design of a generic estimator that could efficiently leverage physical (side-channel) information as an interesting research direction.

Side-channel attacks are an important class of security exploits, in which an attacker gains access to confidential data by observing information inadvertently leaked by a system. Writing constant-time code is a common defense against time-based and microarchitectural side-channel attacks.

Many approaches have been proposed to automatically verify that a program is constant-time. Sound methods can detect all information leaks but, to efficiently analyze large programs, most of them rely on overapproximation which can yield false alarms (i.e., reports of non-existent information leaks). Each finding produced by such analyses therefore requires manual inspection. Additionally, most existing approaches do not perform binary-level analysis and thus miss vulnerabilities introduced by compilation.

In this paper, we present a novel sound analysis for detecting information leaks under the constant-time threat model. Compared with existing work, our technique improves taint analysis by systematically tracking precision loss to determine whether a detected information leak could be caused by overapproximation. Findings for which no precision loss is detected are reported as confirmed; as long as they do not arise from dead code, confirmed findings are guaranteed to be true and thus do not require significant manual inspection. Only findings with detected precision loss need classical human verification. Our analysis operates directly on binary executables. We instantiate our technique within the abstract interpretation framework and provide a proof of correctness.

We implemented our approach in a prototype tool, SCOUT-CT, and evaluated it on a benchmark of constant-time and non-constant-time programs, including real-world cryptographic implementations. Our results show that SCOUT-CT is effective: our tool detected all 98 timing leaks in the benchmark and automatically classified 97 as confirmed findings that do not require significant manual inspection.

This paper presents a comprehensive analysis of modular addition from a cryptanalytic perspective, focusing on both linear and differential cryptanalysis techniques. We examine the probability distribution of carry bits in modular addition operations and demonstrate how these probabilities affect linear approximations. The paper provides detailed algorithms for constructing Linear Approximation Tables (LAT) and Difference Distribution Tables (DDT) for modular addition operations, along with theoretical proofs and practical examples. Our analysis reveals that the probability of carry bits approaches 1/2 as the bit position increases, which significantly impacts the effectiveness of linear cryptanalysis. Furthermore, we demonstrate how to extend DDTs for larger bit sizes by leveraging smaller tables and carry bit relationships. The findings have direct implications for the cryptanalysis of ARX ciphers.

Masking is a representative side-channel countermeasure that provides provable security. Among masking schemes, Boolean masking (BM) is widely adopted due to its simple sharing structure, while inner product masking (IPM) and code-based masking (CM) have been studied as alternatives that achieve a higher security order with the same number of shares---a property known as security order amplification in the bit-probing model. Recent work by Gaspoz and Dhooghe (TCHES 2025) proposed an IPM multiplication gadget and CM gadgets with provable bit-level security; however, the overhead of CM gadgets for linear operations, the overhead of IPM multiplication, and the lack of a complete provably secure implementation exploiting IPM security order amplification remain open challenges. In this paper, we address all three challenges. First, we propose BM-to-IPM and IPM-to-BM conversion gadgets satisfying bit $t$-MIMO-SNI in the bit-probing model, enabling composable and provably secure interoperation between the two masking domains. Second, we optimize the TCHES 2025 IPM multiplication gadget via Row Packing and Reduction in Rows, reducing the fresh random bit requirement from $\frac{1}{2}t(n^2-1)k^2(k+1)$ to $tk(n-1)(kn+W)$ bits with a proportional reduction in XOR gates, while maintaining bit $t$-SNI security. Third, we present a hybrid IPM-BM framework in which multiplications are performed in IPM with fewer shares and all Boolean linear operations are handled share-wise in BM at no additional randomness cost, and show that this hybrid approach requires significantly fewer gates and random bits than a pure CM approach. As a concrete instantiation, we implement a second-order masked AES-128 with a 2-share IPM / 3-share BM hybrid architecture, prove that the implementation satisfies bit 2-PINI, and evaluate its practical side-channel security via first- and second-order TVLA on an ARM Cortex-M4 with up to one million traces. To the best of our knowledge, this is the first end-to-end cryptographic implementation that provably preserves IPM's security order amplification in the bit-probing model.

Session is a decentralised secure (anonymous) messenger that combines onion routing with the Oxen Proof-of-Stake blockchain to provide metadata-private communication. Our study presents the first comprehensive analysis of Session's messaging protocol and its integration with the Oxen blockchain. In analysing Session and the underlying Oxen blockchain, we uncovered seven vulnerabilities.

Most notably we discovered flaws in the Oxen consensus protocol which could allow network takeover in a realistic setting, thereby undermining the integrity guarantees on which Session's anonymity layer depends. We also discovered serious vulnerabilities in Version 1 of Session's group chat protocol. We conducted extensive simulations to analyse the impact of these vulnerabilities and provide recommendations to reinforce both the Oxen protocol and the Session client to mitigate these attacks.