International Association
for Cryptologic Research

We introduce the use of machine learning in the cryptanalysis of code-based cryptography. Our focus is on distinguishing problems related to the security of NIST round-4 McEliece-like cryptosystems, particularly for Goppa codes used in ClassicMcEliece and Quasi-Cyclic Moderate Density Parity-Check (QC-MDPC) codes used in BIKE. We present DeepDistinguisher, a new algorithm for distinguishing structured codes from random linear codes that uses a transformer. The results show that the new distinguisher achieves a high level of accuracy in distinguishing Goppa codes, suggesting that their structure may be more recognizable by AI models. Our approach outperforms traditional attacks in distinguishing Goppa codes in certain settings and does generalize to larger code lengths without further training using a puncturing technique. We also present the first distinguishing results dedicated to MDPC and QC-MDPC codes.

In this paper, we study preimage resistance of the SHA-3 standard. We propose a squeeze meet-in-the-middle attack as a new preimage attack method for the sponge functions. This attack combines the squeeze attack and meet-in-the-middle attack, and is implemented by internal differentials. We analyze the inverse operation of the SHA-3 round function, and develop a new target internal differential algorithm as well as a linearization technique for the Sbox in the backward phase. In addition, we propose the concept of a value-difference distribution table (VDDT) to optimize the attack complexity. These techniques lead to faster preimage attacks on five (out of six) SHA-3 functions reduced to 4 rounds, and also bring preimage attacks on 5 rounds of four SHA-3 instances. The attack techniques are verified by performing practical preimage attack on a small variant of 4-round Keccak.

Presenting a novel use of encryption, not for hiding a secret, but for marking letters. Given a 2n letters plaintext, the transmitter encrypts the first n letters with key K1 to generate corresponding n cipherletters, and encrypts the second n letters with key K2 to generate n corresponding cipherletters. The transmitter sends the 2n cipherletters along with the keys, K1 and K2 The recipient (and any interceptor) will readily decrypt the 2n cipherletters to the original plaintext. This makes the above procedure equivalent to sending out the plaintext. So why bother? When decrypting the 2n cipherletters one will make a note of how the letters that were encrypted with K1 are mixed with the letters encrypted with K2 while keeping the original order of the letters encrypted with each key. There are 2^n possible mixings. Which means that the choice of mixing order can deliver a secret message, S, comprising n bits. So while on the surface a given plaintext is sent out from transmitter to recipient, this plaintext hides a secret. Imagine a text messaging platform that uses this protocol. An adversary will not know which plain innocent message harbors a secret message. This allows residents of cyberspace to communicate secrets without exposing the fact that they communicated a secret. Expect a big impact on the level of cyberspace privacy.

ChaCha is a widely deployed stream cipher and one of the most important symmetric primitives. Due to this practical importance, many cryptanalysis have been proposed. Until now, Probabilistic Neutral Bits (PNBs) have been the most successful. Given differential-linear distinguishers, PNBs are the technique for key recovery relying on an experimental backward correlation obtained through blackbox analysis. A careful theoretical analysis exploiting the round function design may find a better attack and improve our understanding, but the complicated nature of the ARX structure makes such analysis difficult. % We propose a theoretical methodology inspired by bit puncturing, which was recently proposed at Eurocrypt 2024. Our method has a theoretical foundation and is thus fundamentally different from PNBs, to which it is the first effective alternative. As a result, we significantly improved the attack complexity for 6, 7, and 7.5-round ChaCha. The 7-round attack is about $2^{40}$ times faster than the previous best. Furthermore, we propose the first 7.5-round attack with a non-negligible advantage over an exhaustive search.

This paper introduces a new one-more computational problem for lattice-based cryptography, which we refer to as the Algebraic One-More MISIS problem, or AOM-MISIS for short. It is a modification of the AOM-MLWE problem recently introduced by Espitau et al. (CRYPTO ’24) to prove security of new two-round threshold signatures.

Our first main result establishes that the hardness of AOM-MISIS is implied by the hardness of MSIS and MLWE (with suitable parameters), both of which are standard assumptions for efficient lattice-based cryptography. We prove this result via a new generalization of a technique by Tessaro and Zhu (EUROCRYPT ’23) used to prove hardness of a one-more problem for linear hash functions assuming their collision resistance, for which no clear lattice analogue was known. Since the hardness of AOM-MISIS implies the hardness of AOM-MLWE, our result resolves the main open question from the work of Espitau et al., who only provided a similar result for AOM-MLWE restricted to selective adversaries, a class which does not cover the use for threshold signatures.

Furthermore, we show that our novel formulation of AOM-MISIS offers a better interface to develop tighter security bounds for state-of-the-art two-round threshold signatures. We exemplify this by providing new proofs of security, assuming the hardness of MLWE and MSIS, for two threshold signatures, the one proposed in the same work by Espitau et al., as well as a recent construction by Chairattana-Apirom et al. (ASIACRYPT 2024). For the former scheme, we also show that it satisfies the strongest security notion (TS-UF-4) in the security hierarchy of Bellare et al. (CRYPTO ’22), as a result of independent interest.

This note discusses the problem of writing cryptographic implementations in software, free of timing-based side-channels, and many ways in which that endeavour can fail in practice. It is a pessimist view: it highlights why such failures are expected to become more common, and how constant-time coding is, or will soon become, infeasible in all generality.

In this paper, we propose a new type of non-interactive zero-knowledge (NIZK), called Fine-grained Verifier NIZK (FV-NIZK), which provides more flexible and more fine-grained verifiability of proofs than standard NIZK that supports public verifiability and designated-verifier NIZK (DV-NIZK) that supports private verifiability. FV-NIZK has two statistically (or computationally) equivalent verification approaches:

--- a master verification using the master secret key $msk$;

--- a fine-grained verification using a derived secret key $sk_d$, which is derived from $msk$ w.r.t. $d$ (which may stand for user identity, email address, vector, etc.).

We require unbounded simulation soundness (USS) of FV-NIZK to hold, even if an adversary obtains derived secret keys $sk_d$ with $d$ of its choices, and define proof pseudorandomness which stipulates the pseudorandomness of proofs for adversaries that are not given any secret key.

We present two instantiations of FV-NIZK for linear subspace languages, based on the matrix decisional Diffie-Hellman (MDDH) assumption. One of the FV-NIZK instantiations is pairing-free and achieves almost tight USS and proof pseudorandomness. We also adapt the two instantiations to support unbounded fine-grained secret key delegations.

We illustrate the usefulness of FV-NIZK by showing two applications and obtain the following pairing-free schemes:

--- the first almost tightly multi-challenge CCA (mCCA)-secure inner-product functional encryption (IPFE) scheme without pairings;

--- the first public-key encryption (PKE) scheme that reconciles the inherent contradictions between public verifiability and anonymity. We formalize such PKE as Fine-grained Verifiable PKE (FV-PKE), which derives a special key from the decryption secret key, such that for those who obtain the derived key, they can check the validity of ciphertexts but the anonymity is lost from their views (CCA-security still holds for them), while for others who do not get the derived key, they cannot do the validity check but the anonymity holds for them.

Our FV-PKE scheme achieves almost tight mCCA-security for adversaries who obtain the derived keys, and achieves almost tight ciphertext pseudorandomness (thus anonymity) for others who do not get any derived key.

Logic locking has surfaced as a notable safeguard against diverse hazards that pose a risk to the integrated circuit (IC) supply chain. Existing literature on logic locking largely encompasses the art of proposing new constructions, on the one hand, and unearthing weaknesses in such algorithms on the other. Somehow, in this race of make and break, the stress on automation of adopting such techniques on real-life circuits has been rather limited. For the first time, we present a generic end-to-end combinational logic locking CAD framework, MIDAS. This framework analyses circuit netlists and generates locked netlists. Due to its generic circuit analysis, it bridges the gap, integrates diverse logic locking techniques, and offers a scope of integration of potential future ones. MIDAS framework’s efficacy has been verified through its application on ISCAS’85 and ISCAS’99 benchmark circuits, locked using six different schemes such as EPIC, Anti-SAT, SFLL-HD, SFLL-fault, CAS-Lock, and LoPher. MIDAS minimizes the hardware overhead requirements of otherwise resource-intensive locking technique LoPher by extracting an influential portion of circuit to lock and utilizing a simple fitness function. We also assess the overhead increase for the aforementioned locking methods, thereby complicating the identification of influential nodes within the locked netlists. Finally, we evaluate MIDAS by selectively locking parts of a commercially-designed open-source RISC-V core.

Non-malleable zero-knowledge (NMZK), originally introduced in the seminal work of Dolev, Dwork, and Naor (STOC 91), is a fundamental concept for modeling the security of proof systems against man-in-the-middle attacks.

Recently, Kim, Liang, and Pandey (CRYPTO 2022) presented the first efficient constant-round NMZK argument system based solely on symmetric-key cryptography. Their construction relies on a non-black-box use of the involved cryptographic primitives and on multiple executions of Ligero (CCS 2017) that affect both the round complexity and the computational efficiency of their protocol. Their work left open the natural important challenge of achieving NMZK using the underlying primitives only in a black-box fashion (regardless of the number of rounds and actual efficiency).

In this paper, we solve the aforementioned open problem by presenting the first NMZK argument system based on the black-box use of cryptographic primitives. Our work is optimal in the use of primitives since we only need one-way functions, and asymptotically optimal in the number of rounds since we only require a constant number of rounds. Our argument system is non-malleable with respect to the strong "simulation-extractability" flavor of non-malleability.

Furthermore, we also show that our construction can be efficiently instantiated in Minicrypt, significantly improving upon the work of Kim et al., both in terms of round complexity and computational efficiency.

Recently, Jiang et al. (EUROCRYPT 2025) proposed a universal framework for constructing commitment schemes using group actions, and instantiated it with the Lattice Isomorphism Problem (LIP). This paper attempts to construct an instantiation based on module-LIP with this framework. More precisely, we first present a reduction from $\mathcal{O}_{\mathbb{L}}^2$-LIP to $\mathcal{O}_{\mathbb{L}}^2$-LAP. Then we develop a re-randomized algorithm based on the self-reduction framework of Module-LIP (Ducas et al. ASIACRYPT 2022), adapting it to the framework to construct commitment schemes.

Anonymous tokens with private metadata bit (ATPM) have received increased interest as a method for anonymous client authentication while also embedding trust signals that are only readable by the authority who holds the issuance secret key and nobody else. A drawback of all existing ATPM constructions is that they require client-issuer interaction during the issuance process. In this work, we build the first non-interactive anonymous tokens (NIAT) with private metadata bit, inspired by the recent work of Hanzlik (Eurocrypt '23) on non-interactive blind signatures. We discuss how the non-interaction property during the issuance process allows for more efficient issuance protocols that avoid the need for online signing. We construct an efficient NIAT scheme based on Structure-preserving Signatures on Equivalence Classes (SPS-EQ) and experimentally evaluate its performance. We also present an extension to our NIAT construction that allows the identification of clients who attempt to double-spend (i.e., present the same token twice).

Bootstrapping in approximate homomorphic encryption involves evaluating the modular reduction function. Traditional methods decompose the modular reduction function into three components: scaled cosine, double-angle formula, and inverse sine. While these approaches offer a strong trade-off between computational cost and level consumption, they lack flexibility in parameterization.

In this work, we propose a new method to decompose the modular reduction function with improved parameterization, generalizing prior trigonometric approaches. Numerical experiments demonstrate that our method achieves near-optimal approximation errors. Additionally, we introduce a technique that integrates the rescaling operation into matrix operations during bootstrapping, further reducing computational overhead.

The Research Training Group "Cybercrime and Forensic Computing" aims to systematically analyse research questions arising from the interaction between computer science and criminal law. The following research areas are particularly relevant to one of the PhD positions:

• Applied Cryptography
• Provable Security
• Privacy and Anonymity
• Modern Cryptographic Communication Protocols (TLS, Noise, MLS, Double Ratchet, ...)
• Building Blocks of Secure Messaging Protocols

More information about the project can be found at https://cybercrime.fau.de

Applicants should have an excellent academic record, hold an MSc or an equivalent university degree in computer science or related disciplines, and have the goal to finish a PhD degree within three years. For the particular position in applied cryptography, applicants should have a practical understanding of modern cryptographic protocols deployed in the real world and a background in provable security (e.g., game-based security definitions, reduction-based proofs, ...).

Closing date for applications:

Contact: Felix Freiling (felix.freiling@fau.de) for general questions and the application process, Paul Rösler (paul.roesler@fau.de) for questions about the position in the applied cryptography group.

More information: https://www.jobs.fau.de/jobs/7-phd-positions-m-f-d-salary-level-13-tv-l-in-computer-science-full-time-and-3-phd-position-m-f-d-salary-level-13-tv-l-in-law-part-time-75-91680455/

• Professor of Department of Mathematical and Computing Science https://educ.titech.ac.jp/is/eng/
• Research Fields: Theoretical Computer Science, Theory of Computational Complexity, Theory of Algorithms, Theory of Cryptography, Programming Theory, Software Verification Theory, Blockchain Technology, Theory and Practice of Cybersecurity, etc.

Closing date for applications:

Contact: Keisuke Tanaka (keisuke@comp.isct.ac.jp), School of Computing, Institute of Science Tokyo

More information: https://jrecin.jst.go.jp/seek/SeekJorDetail?id=D125021037&ln=1

Applications are invited for a Lecturer (Assistant Professor) position in Cyber Security & Privacy to join the School of Informatics at the University of Edinburgh. The post is based within the School’s Cyber Security, Privacy and Trust group, which is a broad group of researchers whose expertise ranges from cryptography and formal verification to human factors and social aspects.

This position is part of a broader collaborative research initiative on privacy-enhanced secure computations at scale, in partnership with Input-Output Global (IOG). Applications are invited from candidates with research expertise in all areas of Cyber Security & Privacy, with special consideration given to those specializing in blockchain and distributed ledgers, multi-party computation, post-quantum cryptography, and data privacy.

We are seeking current and future leaders in the field. The successful candidates will have (or be near to completing) a PhD, outstanding track record of research, evidence of growing reputation, a committed vision and research agenda, the enthusiasm and ability to undertake original research, to manage a research group, and to engage with teaching and academic supervision.

The School of Informatics at the University of Edinburgh is one of the largest in Europe, with more than 120 academic staff and a total of over 500 post-doctoral researchers, research students and support staff. Informatics at Edinburgh rated highest on Research Power in the most recent Research Excellence Framework. The School has strong links with industry, with dedicated business incubator space and well-established enterprise and business development programmes. The University of Edinburgh has recently established the Bayes Centre for Data Science and Artificial Intelligence, which provides a locus for fruitful multi-disciplinary work, including a range of companies collocated in it. The School holds a Silver Athena SWAN award in recognition of our commitment to advance the representation of women in science, mathematics, engineering and technology. We are also Stonewall Scotland Diversity Champions actively promoting LGBT equality.

Closing date for applications:

Contact: Prof. Aggelos Kiayias

More information: https://elxw.fa.em3.oraclecloud.com/hcmUI/CandidateExperience/en/sites/CX_1001/job/12182/?utm_medium=jobshare&utm_source=External+Job+Share

In this paper, we analyze the subtle issues of complexity estimates related to state-of-the-art cryptanalytic efforts on ChaCha. In this regard, we demonstrate that the currently best-known cryptanalytic result on $7$-round ChaCha with time $2^{189.7}$ and data $2^{102.63}$ [Xu et al., ToSC 2024] can be estimated as $2^{178.12}$ for time and $2^{101.09}$ for data complexity. We improve the best-known result for the $7.25$ round by obtaining an improved set of Probabilistic Neutral Bits and considering our revised estimation. Our result with time complexity $2^{212.43}$ and data complexity $2^{100.56}$ improves the result of Xu et al., where they could achieve time and data complexity $2^{223.9}$ and $2^{100.80}$, respectively. For both the $7$ and $7.25$ rounds, we can show an improvement of the order of $2^{11}$ in the time complexity. For $7.5$-round, we improve the result of Dey [IEEE-IT 2024], which reports the time and data complexity of $2^{255.24}$ and $2^{32.64}$, respectively. By applying the formula of the same paper and incorporating additional PNBs, we obtain improved time and data complexity of $2^{253.23}$ and $2^{34.47}$, respectively. Thus, this paper describes the currently best-known cryptanalytic results against reduced round ChaCha. Our results do not affect the security claims of the complete algorithm with 20 rounds. Also, we provide a rebuttal of the Work by Wang et al. \cite{wangeprint} and analyze their claim about the error in the ``Divide-and-Conquer'' Approach.

We explore advanced security notions for threshold signature schemes, focusing on Beyond UnForgeability Features (BUFF), introduced by Cremers et al. (S&P’21) in the non-threshold setting. The BUFF properties protect against attacks based on maliciously chosen keys, e.g., expropriating a message-signature pair under a new public key (called exclusive ownership). We first formalize these notions in the threshold setting and examine their relationships. Notably, unlike regular signature schemes, the hierarchy of variants of exclusive ownership notions only holds for threshold schemes if they are also robust. We then present a generic compiler that transforms any threshold signature scheme to satisfy exclusive ownership, and message-bound signature properties with minimal overhead. Furthermore, we modify the threshold BLS signature scheme to achieve these additional properties without increasing the signature size. Lastly, we identify specific structures in threshold signature schemes where BUFF properties can be naturally extended from the underlying standard signature scheme, and we analyze and prove the security properties in some of the existing threshold schemes.

The Message Layer Security (MLS) protocol has recently been standardized by the IETF. MLS is a scalable secure group messaging protocol expected to run more efficiently compared to the Signal protocol at scale, while offering a similar level of strong security. Even though MLS has undergone extensive examination by researchers, the majority of the works have focused on confidentiality.

In this work, we focus on the authenticity of the application messages exchanged in MLS. Currently, MLS authenticates every application message with an EdDSA signature and while manageable, the overhead is greatly amplified in the post-quantum setting as the NIST-recommended Dilithium signature results in a 40x increase in size. We view this as an invitation to explore new authentication modes that can be used instead. We start by taking a systematic view on how application messages are authenticated in MLS and categorize authenticity into four different security notions. We then propose several authentication modes, offering a range of different efficiency and security profiles. For instance, in one of our modes, COSMOS++, we replace signatures with one-time tokens and a MAC tag, offering roughly a 75x savings in the post-quantum communication overhead. While this comes at the cost of weakening security compared to the authentication mode used by MLS, the lower communication overhead seems to make it a worthwhile trade-off with security.

This note demonstrates that the blind signature scheme based on cryptographic group actions, as proposed in ePrint paper 2025/397, fails to ensure blindness. Specifically, we construct an adversary that achieves a $1/8$ advantage in the blindness experiment. The attack leverages selective abort techniques (also known as selective failure attacks), a well-known strategy in the MPC literature.

Secure Two-Party Computation (2PC) enables secure inference with cryptographic guarantees that protect the privacy of the model owner and client. However, it adds significant performance overhead. In this work, we make 2PC-based secure inference efficient while considering important deployment scenarios. We observe that the hitherto unconsidered latency of fetching keys from storage significantly impacts performance, as does network speed. We design a Linear Secret Sharing (LSS)-based system $LSS^M$ and a Function Secret Sharing (FSS)-based system $FSS^M$ for secure inference, optimized for small key size and communication, respectively. Notably, our highly-optimized and hardware-aware CPU-based $LSS^M$ outperforms prior GPU-based LSS systems by up to $50\times$. We then show that the best choice between $LSS^M$ and $FSS^M$ depends on the deployment scenario. In fact, under certain deployments, a combination of $LSS^M$ and $FSS^M$ can leverage heterogeneous processing across CPU and GPU. Such protocol-system co-design lets us outperform state-of-the-art secure inference systems by up to $21\times$ (geomean $3.25\times$).