International Association
for Cryptologic Research

Substitution-permutation networks (SPNs) are the most popular construction method for block ciphers and sponge permutations. Basically, we can divide SPNs into two groups by considering their diffusion layer profiles: bit-wise versus byte-wise (or nibble-wise) diffusion. In this paper, we suitably combine both approaches and present a more efficient and flexible construction that achieves a combination of small S-boxes, linear layers consisting only of XOR operations and bit permutations, and a small number of rounds. As concrete results, we present the family SEFA of several lightweight and flexible block ciphers, AEADs, and cryptographic hash functions, including:

* SEFA-128/256: A TFHE-friendly block cipher with 128-bit block, 256-bit key, using 4-bit S-boxes, through 16 rounds.

* SEFA-512/256: A wide-block AEAD suitable for encrypting 256-bit blocks with 256-bit keys, using 8-bit S-boxes, through 16 and 10 rounds.

* SEFA-1536: A hash function using sponge construction with a 1536-bit state permutation including 8-bit S-boxes and 12 rounds.

We analyze the security from several attack perspectives. Also, we present hardware implementation results, along with comprehensive performance benchmarks, to demonstrate performance and flexibility.

There are two kinds of cryptographic group actions: restricted and unrestricted. While unrestricted actions like (qt-)PEGASIS are needed for more advanced constructions, restricted ones like dCTIDH are sufficient for instantiating a NIKE and usually much more efficient.

In this work, we propose CORAL, a significantly faster algorithm to evaluate the same action as (qt-)PEGASIS, but in a restricted fashion; CORAL only computes two-dimensional $2$-isogenies to evaluate the action and outperforms both recent unrestricted (KLaPoTi, (qt-)PEGASIS) and (restricted) CSIDH-based approaches (SQALE, dCTIDH). In essence, CORAL trades off unrestrictedness for efficiency.

Our unoptimised C implementation evaluates a group-action in 240 ms with a 2032-bit prime. When used to construct a non-interactive key exchange, CORAL yields an actively secure post-quantum NIKE with compact public keys (e.g. 256 bytes for 2032-bit primes).

ML-DSA is a lattice-based signature scheme that has recently been standardized by NIST as FIPS 204. Among the many subroutines used by ML-DSA are the high bits (Hb) and low bits (Lb) functions, which, as their name suggest, return only the higher bits or the lower bits of some integer. Recently, Seck and Roux-Langlois (IACR CiC 2025) conjectured that the high bits of a sum of two integers t and r can be expressed as the sum of the high bits of each integer, plus an error term that can take at most seven different values. In this paper, we study the properties of the functions Hb and Lb and we prove that the conjecture of Seck and Roux-Langlois holds. In addition, we provide a complete characterization of the error term. As an application, we explain how these properties can be used to design lattice-based signature schemes with advanced features.

Rotating leader Byzantine fault-tolerant (BFT) protocols, also known as protocols in the leader-speaks-once (LSO) model, have become a topic of interest with the rise of blockchains. Many recent efforts have been made to lower the latency and simplify the design. However, existing works often sacrifice expected latency (latency when failures might occur) to enjoy higher good-case latency.

In this work, we present a generic approach to building rotating leader BFT protocols that are both simple and fast. We present two variants: HumBird-1, a synchronous BFT under the $f

The selfish-mining literature has progressively lowered the hashrate threshold at which a rational Bitcoin miner strictly profits by deviating from the honest protocol, from Eyal and Sirer's 25% down to single digits under richer attacker models. Two such models remain unreconciled: Gervais et al. (CCS 2016) parameterize network-layer adversaries through an eclipse fraction omega and an honest-only stale rate o, but assume constant block rewards; WeRLman (IEEE S&P 2023) models fee-rich rewards through a deep-RL solver, but assumes a network-clean attacker. An adversary that exploits both surfaces has not been analyzed.

We present the first joint analysis. We formulate a Markov decision process that integrates Gervais's network-layer channels with WeRLman's whale-transaction tracking, solve it at a 5.7 x 10^8 state truncation, and anchor the construction by reduction to each prior model at the appropriate parameter limit.

Our threat model targets a state-level adversary with per-block routing influence over a meaningful fraction of honest hash power, not an attacker who eclipses individual nodes. Under this model, the joint threshold collapses far below either single-channel result. At L = 3, the F=10 baseline (no network channels) crosses at alpha = 13.2%; a 10% eclipse fraction alone drops it to 2.6%, a 10% stale rate alone to 4.8%, and both channels together to 1.4%. At L = 5 the joint configuration drops further to 0.9%, an at least 15x collapse. An analytical lemma accounts for the interaction: the honest-chain growth probability factors as (1 - alpha - omega)(1 - o), isolating a cross-term omega * o that is a property of the joint transition kernel itself. The cross-term is structural, not a training artifact, and its presence implies that mitigations targeting only one channel leave the joint contribution structurally present whenever the other channel is non-zero.

The security of decentralized asset custody and blockchain consensus increasingly relies on threshold signatures to eliminate single points of failure. In these high-stakes environments, static security models are an insufficient theoretical abstraction. Real-world blockchain deployments demand adaptive security to ensure protocols remain secure against adversaries who reactively compromise participants based on observed network traffic and protocol state.

While the NIST standardization effort and Bitcoin’s Taproot (BIP340) upgrade have accelerated interest in adaptively secure threshold Schnorr signatures, a threshold system is only as secure as its setup phase. In decentralized settings, this necessitates a Distributed Key Generation (DKG) protocol to eliminate reliance on a trusted dealer. However, a critical gap remains: current DKG protocols are incompatible with the specific key structures required by promising schemes (EUROCRYPT'26). Furthermore, a result from CRYPTO'25 demonstrated that unique key commitments necessitate a non-falsifiable assumption for adaptive security. To avoid non-standard assumptions, DKG protocols must be key-share hiding, a property existing DKGs fail to provide.

We close this gap by proposing two novel DKG protocols that support identifiable abort and tolerate a dishonest majority and prove them secure under new ideal functionalities in the UC framework. Our first protocol achieves optimal round complexity via a single broadcast round at the cost of allowing adversarial bias, while our second protocol eliminates the bias in two rounds. In addition, the protocols provide a proactive key refresh mechanism that allows long-lived decentralized networks to refresh internal key material periodically. This enables the system to recover from transient node compromises without the operational burden of rotating the underlying public key. Our evaluation demonstrates that these constructions are practically efficient, incurring minimal communication and computation overhead for modern high-throughput architectures.

We prove that row-wise interleaving does not impose a linear loss on two coding-theoretic soundness properties used in recent IOP/SNARK analyses: generator mutual correlated agreement and curve decodability.

For generator-MCA, let $G:\Omega\to\mathbb{F}_q^\ell$ be a coefficient generator over a finite seed set and let $C$ be an $\mathbb{F}_q$-additive code. For every interleaving width $s$ and distance parameter $\delta$, we show $$ \varepsilon_G(C,\delta) \le \varepsilon_G(C^{\equiv s},\delta) \le \left(1+\frac1q+\cdots+\frac1{q^{s-1}}\right)\varepsilon_G(C,\delta). $$ Moreover, if $|\Omega|\le q$, then the transfer is exact: $$ \varepsilon_G(C^{\equiv s},\delta)=\varepsilon_G(C,\delta). $$ In particular, affine-line MCA is invariant under row-wise interleaving. This answers the known interleaving-loss question and removes the linear interleaving factor from the affine-line MCA bound. It also implies that polynomial-generator MCA bounds transfer to interleaved codes without an additional interleaving-width factor.

We further establish interleaving stability for curve decodability. We introduce a marked formulation, prove its equivalence to the standard definition for $\mathbb{F}_q$-additive codes and $1\le b\le a\le q$, and use it to transfer curve decodability to row-wise interleavings. If $C$ is $(\ell,\delta,a,b)$-curve-decodable and $\binom{a}{b}\le q$, then $C^{\equiv s}$ is also $(\ell,\delta,a,b)$-curve-decodable for every $s$. We also give a field-size-weighted variant that transfers larger base-code witness parameters to smaller interleaved-code witness parameters.

We study the Lattice Isomorphism Problem (LIP) for both indefinite and definite quadratic forms, with applications to the signature schemes DEFI and HAWK. By combining arithmetic and algorithmic techniques, we obtain efficient attacks on DEFIv2, an efficient digital signature scheme based on isotropic quadratic forms. Our approach to the Decision/Distinguishing-LIP draws on the arithmetic theory of quadratic forms, with particular emphasis on indefinite forms of dimension at least~$3$. We show that such forms arise naturally in the analysis of DEFI and prove that, under suitable assumptions, the genus, spinor genus, and equivalence class coincide. This structural collapse leads to a classical polynomial-time algorithm for the Decision/Distinguishing-LIP instances obtained from DEFI. In addition, we present a quantum polynomial-time algorithm for recovering the secret key of DEFIv2 and demonstrate practical signature forgeries within minutes using the authors' public challenge instances. Finally, we evaluate t

Ring signatures provide authentication over messages, whilst providing anonymity amongst a set of signer-defined public keys. They see active use in cryptocurrencies, e-voting and concurrent signature domains. However, post-quantum constructions typically rely on lattices, specifically utilising the Learning with Errors (LWE) and Short-Integer-Solution (SIS) problems, which cause inefficiencies when compared with classical constructions.

One promising route to circumvent the inherent challenges of these underlying assumptions is the Lattice Isomorphism Problem (LIP), which underpins the HAWK signature scheme by Ducas et. al, currently a second round candidate in the NIST standardisation project Post Quantum Cryptography: Additional Digital Signature Schemes. It offers significant performance improvements over standard lattice assumptions due to its improved decoding, however, the only known construction of a ring signature from LIP has been shown to not satisfy linkability or correctness.

In this paper we propose RingSLIP, a secure linkable ring signature based on LIP, utilising the HAWK signature. The resulting ring signature is logarithmic in the number of ring members, and concretely has size 46KB when targeting 128 bits of security for 4096 ring members, which is competitive with other lattice-based schemes. Furthermore, we observe that our construction also benefits from online/offline computation, resulting in a signature with online signing and verification only requiring $8.54 \times 10^4$ and $1.48 \times 10^5$ CPU cycles respectively, compared to $1.35 \times 10^{11}$ without these optimisations.

We introduce BlindReview, an anonymous and end-to-end verifiable peer review system that cryptographically guarantees both privacy and auditability throughout the reviewing process. We formally define these security properties and provide rigorous proofs that BlindReview satisfies them. We also present an implementation demonstrating our protocol’s practicality. This work serves as a foundation for verifiable and privacy-preserving peer review, offering a concrete solution to enhance transparency and reduce bias in the academic peer review process.

Atomic swaps are a fundamental primitive for the trustless exchange of digital assets across blockchains: they guarantee that either both parties receive the agreed assets or neither party transfers. While this all-or-nothing guarantee is powerful, it also imposes an inherent determinism that rules out exchanges whose intended outcome is probabilistic. As a result, existing atomic swaps cannot realize trustless exchanges in which one party pays for a fixed chance of receiving a larger asset or reward, as in lotteries, randomized allocation mechanisms, and probabilistic cross-chain trades.

We introduce probabilistic swaps, a new cryptographic primitive that extends atomic swaps to the probabilistic setting. In a probabilistic swap, one party's transfer is executed with a fixed, publicly specified probability embedded in the protocol and cannot be biased by either party. This yields a trustless mechanism for randomized exchange with verifiable odds and no trusted intermediary.

Our construction combines adaptor signatures with oblivious pseudorandom functions (OPRFs) to realize the desired probabilistic outcome while ensuring that neither party can predict or bias it in advance. Along the way, we introduce a new mechanism for the atomic exchange of OPRF evaluations for payments, which may be of independent interest. A key feature of our approach is that it preserves the minimal on-chain footprint of modern atomic-swap protocols. The protocol relies only on standard Bitcoin scripts, such as digital signatures and timelocks, and is deployable on any blockchain that already supports atomic swaps. Consequently, probabilistic swaps are indistinguishable from ordinary on-chain transactions, which helps preserve privacy and fungibility. We provide formal security foundations and demonstrate practicality through a probabilistic swap between the Bitcoin and Litecoin testnets, as well as in the Lightning Network.

Dwork and Naor (FOCS 2000) showed a generic transformation to construct a ZAP (a two-round public-coin witness-indistinguishable proof) from any non-interactive zero-knowledge (NIZK) proof with statistical soundness in the common random string model. In recent years, a number of works have shown how to construct NIZK arguments in the common random string model from a broad range of assumptions including decisional Diffie-Hellman (DDH), learning with errors (LWE), or combinations of multiple assumptions. While a number of previous works have developed specialized tools to build ZAPs using these same assumptions (through a non-trivial adaptation of the underlying NIZK), a natural question is whether we can generically obtain a ZAP from these NIZK arguments à la Dwork-Naor.

In this work, we introduce the notion of a sometimes-constricting generator and show how to use it to generically upgrade any computational (resp., statistical) NIZK argument in the common random string model into a computational (resp., statistical) ZAP argument. We then show how to build sometimes-constricting generators from either the DDH assumption (over pairing-free groups) or the LWE assumption. Our transformation immediately allows us to recover constructions of ZAPs from assumptions like DDH or LWE, as well as enables new constructions from different combinations of cryptographic assumptions with properties that were not previously attainable. More broadly, our compiler provides a general mechanism to convert any future NIZK construction in the common random string model into a ZAP.

Pairing-based cryptography relies heavily on the efficiency of bilinear pairings, the computation of which is dominated by the final exponentiation step. This paper describes an efficient cubing operation in the cyclotomic subgroup of $\mathbb{F}_{q^6}$ for $q\equiv1\mod{6}$. As an application, we use existing results for computing Frobenius maps to optimize the cost of the optimal Ate pairing final exponentiation over the SG54 curve. Furthermore, we introduce a novel decomposition for the hard part of the final exponentiation for this curve. Additionally, we apply established methods for cyclotomic cubing to accelerate the final exponentiation for the BLS15 and BLS27 curves. Compared to previous works, our approach achieves efficiency gains of $24\%$ for SG54 and $22\%$ for the BLS15 and BLS27 curves.

We present a machine-checked security proof for the NTOR key exchange protocol, which is used to establish connections in the Tor onion routing system. It was previously studied by Goldberg et al. (DCC 2013), but within a slightly non-standard model that did not explicitly capture forward secrecy. Our proof is fully formalized in EasyCrypt, adding to the still small set of cryptographic protocols verified in the computational model. A key contribution is a systematic treatment of halting reductions involving failure events expressed as global properties of the execution. In the course of this work, we also contributed improvements to the EasyCrypt framework itself. We prove NTOR secure in a new model of unilaterally authenticated key exchange that captures forward secrecy, and is intentionally close to established bilaterally AKE models (such as eCK). By examining more carefully how identities and public keys are used in key exchange proto- cols, we obtain simpler formal arguments and introduce several variants of our UAKE security model, connected by general reductions that, in the case of NTOR, are also realized in EasyCrypt. This allows us to carry out the main proof in a simpler setting and then derive the desired security guarantee for NTOR via these reductions.

In 1997, Lo proved that if one of the parties is malicious, it is not possible to achieve unconditional security in quantum bit-commitment (Phy. Rev. Lett, 1997) and hence in two-party quantum computation (Phy. Rev. A, 1997). However, restricting the computational power of the adversary, via assumptions like the existence of one-wayness, it has been shown that secure quantum bit-commitment is indeed possible by Dumais et al. (EUROCRYPT 2000). In their paper, they showed that with the assumption of a one-way permutation that is secure against any polynomial-time quantum adversary, one can achieve a secure quantum bit-commitment scheme. Although security is guaranteed, complete fairness remains an issue in quantum two-party computation for the real-world framework.

In this paper, we show for the first time that there are some functions for which secure two-party quantum computation with complete fairness is achievable. This seems in sharp contrast with the impossibility result of Ben-Or et al. (FOCS, 2006). In Ben-Or et al.'s work, they have considered a malicious quantum adversary with unconditional computational power and used a broadcast channel. We have used the hybrid model idea of Gordon et al. (STOC 2008) using a non-simultaneous channel and the idea of composition of multiparty computations by Ran Canetti (Journal of Cryptology, 2000), to achieve complete fairness in the quantum domain, under the assumption of computationally bounded adversary. The functions we study are of two types (similar to Gordon et al.): one is any function without an embedded XOR, and the other is a particular function containing an embedded XOR.

At first, we design secure two-party computation protocols in a hybrid model, using a trusted third party. Then we prove how these protocols achieve complete fairness in the hybrid model. Finally, we construct a secure two-party communication protocol and show how we can use this to remove the trusted third party of the hybrid model, and achieve both security and complete fairness in the real-world model.

We introduce new techniques for verifiable reporting of abusive messages in private messaging platforms. Our techniques are compatible with both metadata-hiding messaging systems, where the platform does not know who speaks to whom, and with third-party moderation, where the platform is not involved in the process of verifying or judging reported content.

While prior work in this space considers the question of how a moderator would verify reports, prior works do not address the question of how a moderator and platform would collaborate to enforce moderation decisions. In a setting where the platform does not wish to be involved in or responsible for enforcing moderation decisions, or in federated settings where it is not clear who would be responsible for enforcement, this presents an additional challenge.

Our work solves this problem with a lightweight credentialing and revocation mechanism that does not involve the platform in moderation enforcement at all. In order to support this added functionality, we build on Asymmetric Message Franking (Crypto '19) and improve performance over the original scheme, reducing moderator computation and communication costs to verify reports by $6\times$ and $7\times$, respectively.

Succinct non-interactive arguments (SNARGs) for NP allow an efficient prover to convince a verifier that an NP statement is true with a proof that is much shorter than the original NP witness. Gentry and Wichs (STOC ’11) showed that adaptive soundness of such SNARGs cannot be proven via a black-box reduction from any falsifiable assumption. However, recent works by Waters, Wu and Zhandry (STOC ’24, CRYPTO ’24, CRYPTO ’25) circumvent this negative result by relying on subexponential hardness assumptions and having a long common reference string (CRS) that is longer than the statement size.

In this work, we study unique SNARGs where each statement has at most one accepting proof. The above constructions of adaptively sound SNARGs are not unique and crucially rely on the existence of multiple valid proofs in their security analysis. We explore to what extent this is inherent as follows: - On the negative side, we give a strengthened Gentry-Wichs style black-box separation for the case of perfectly complete and unique SNARGs for NP with adaptive security. Our black-box separation extends even to reductions that rely on subexponentially hard falsifiable assumptions and to SNARGs that have an arbitrarily long CRS. - On the positive side, we construct perfectly unique and adaptively secure SNARGs for NP with a long CRS based on subexponentially hard one-way functions and indistinguishability obfuscation. We do so by relaxing perfect completeness and allowing for a negligible completeness error. This is the first unique SNARG with a proof of adaptive security from falsifiable assumptions, even restricted to P

Pseudorandom Generators (PRGs) based on Threshold-XOR predicates with large locality and high stretches have recently gained traction, since they lend themselves as shallow weak Pseudorandom Functions (PRFs) to fast multiparty computation protocols. In this work, we present novel fast attacks on such PRGs that achieve substantial advantages. Concretely, we break the security levels of most parameters collected by Boura, Couteau, Perrin and Rotella (ToSC'25), as well as those proposed by Fu, Li, Lyu and Liu (EC'26). On the asymptotic side, we prove that our attacks achieve an advantage of $n^{-n/a}$ where $n$ denotes the seed length and $a$ the locality of threshold predicates. As a consequence, when $a \in \Theta(n)$, we get poly-time attacks with noticeable advantage. These results close current gaps on the theoretical study of THR-XOR based Goldreich PRGs.

Additionally, we demonstrate how the attack advantage can be amplified for PRGs with large output lengths (which are common in the context of weak PRFs). Finally, we discuss alternative predicates for Goldreich PRGs that resist all currently known attacks.

Fair exchange protocols on blockchain enable atomic exchange of digital goods and cryptocurrency between untrusted parties. Two prominent protocols, Zero-Knowledge Contingent Payment (ZKCP) and FairSwap, guarantee fairness: either both parties receive the expected items (digital goods and cryptocurrency) or neither does. However, both protocols lack accountability: when an exchange terminates abnormally, the protocol cannot identify which party caused the failure. This lack of accountability undermines the applicability of fair exchange protocols in decentralized settings. To fill this gap, we identify the common accountability issue in both protocols and revise them by adding signature exchange and chaining. We formally define the revised protocols in the universal composability framework and provide a proof sketch, and measure the computational overhead of the revised protocols in our implementation and experiment.

Analyzing time-series databases in a privacy-preserving manner has gained significant attention, especially when the data contains sensitive personal information such as medical records or spatio-temporal data such as trajectories. Motivated by scenarios where a user must show whether an anomaly (or similarity) is detected in a time series containing sensitive data, we propose a toolkit for proving these properties on (committed) private time series. We leverage Matrix Profile (MP), a state-of-the-art data-mining structure, to detect subsequence anomalies and similarities in time series, in contrast to many works that only detect anomalies and similarities on complete time series. As recent findings have shown, the aggregated data used by MP (such as subsequence distances or MP values) leak critical information about the time series. It is therefore crucial to consider a strong adversary model where all information other than the presence or absence of anomalies/similarities remains protected. To guarantee this, we propose a combination of commitment and zero-knowledge proof systems that ensure both the validity of the proven result and the (unconditional) protection of the time series. The proposed schemes maintain reasonable execution times, even for large real-time time series.