International Association
for Cryptologic Research

AES has cemented its position as the primary symmetric-key primitive for a wide range of cryptographic applications, which motivates the analysis on the concrete security of AES's instantiations in practice, for instance, the collision resistance of AES-based hashing, the key commitment security of AES-based authenticated encryption schemes, and the one-wayness of AES-based one-way functions in ZK and MPC protocols. In this work, we introduce single-color initial structures into meet-in-the-middle (MITM) attacks, a systematic technique to identify attack trails that enable efficient neutral word generation and low-memory attacks. As a result, we have attained: (1) the first classical one-block collision attack on 7-round AES-MMO/MP, marking the first advancement in attack rounds for more than a decade and matching the attack rounds in the quantum setting; (2) the first one-block collision attack on 4-round AES-128-DM, which bridges the gap in Taiyama et al.'s claim at Asiacrypt 2024 from an MITM perspective; (3) the first improvement in single known plaintext key recovery attack on 5-round AES-128 in over a decade; (4) comprehensive results on the security margin of Rijndael-192 and Rijndael-256 in multiple instantiations. These breakthroughs deepen our understanding of AES-like structure, and contribute as a scrutiny of the security of AES-based instantiations.

We show that the data classification scheme [IEEE Trans. Sustain. Comput., 2023, 8(4), 652-669)] failed to check the compatibility of encoding algorithm and homomorphic encryption algorithm. Some calculations should be revised to ensure all operands are first encoded using the same scaling factors. The canonical embedding map depending on the natural projection should be explicitly arranged so as to construct an efficient decoding algorithm.

Multi-party private set union (mPSU) allows multiple parties to compute the union of their private input sets without revealing any additional information. Existing efficient mPSU protocols can be categorized into symmetric key encryption (SKE)-based and public key encryption (PKE)-based approaches. However, neither type of mPSU protocol scales efficiently to a large number of parties, as they fail to fully utilize available computational resources, leaving participants idle during various stages of the protocol execution.

This work examines the limitation of existing protocols and proposes a unified framework for designing efficient mPSU protocols. We then introduce an efficient Parallel mPSU for Large-Scale Entities (PULSE) that enables parallel computation, allowing all parties/entities to perform computations without idle time, leading to significant efficiency improvements, particularly as the number of parties increases. Our protocol is based on PKE and secure even when up to $n-1$ semi-honest parties are corrupted. We implemented PULSE and compared it to state-of-the-art mPSU protocols under different settings, showing a speedup of $1.91$ to $3.57\times$ for $n=8$ parties for various set sizes.

Secure multi-party computation (MPC) enables parties to compute a function over private inputs while maintaining confidentiality. Although MPC has advanced significantly and attracts a growing industry interest, open-source implementations are still at an early stage, with no production-ready code and a poor understanding of their actual security guarantees. In this work, we study the real-world security of modern MPC implementations, focusing on the SPDZ protocol (Damgård et al., CRYPTO 2012, ESORICS 2013), which provides security against malicious adversaries when all-but-one of the participants may be corrupted. We identify a novel type of MAC key leakage in the MAC check protocol of SPDZ, which can be exploited in concurrent, multi-threaded settings, compromising output integrity and, in some cases, input privacy. In our analysis of three SPDZ implementations (MP-SPDZ, SCALE-MAMBA, and FRESCO), two are vulnerable to this attack, while we also uncover further issues and vulnerabilities with all implementations. We propose mitigation strategies and some recommendations for researchers, developers and users, which we hope can bring more awareness to these issues and avoid them reoccurring in future.

An identity-based ring signature (IRS) is an attractive cryptographic primitive having numerous applications including e-cash and e-voting, that enables a user to authenticate sensitive documents on behalf of a ring of user identities chosen by the signer and provides anonymity and unforgeability. We introduce the first quantum-tokenized identity-based ring signature (QTIRS) scheme qtIRS and its variant D-qtIRS with signature delegation assuming the existence of obfuscated membership-checking circuits and a computationally sound non-interactive zero-knowledge (NIZK) proof system. We emphasize that our schemes are dynamic where a user can join or leave the system without disturbing others, allow unrestricted ring size with signature size independent of the ring size and provide security in the standard model. We enhance our security framework by minimizing restrictions for the adversary and allowing a broader domain to forge and introduce the notion of strong existential unforgeability against adaptive-chosen-message-and-identity attack (sEU-ACMI) and strong anonymity (sAnon). Our scheme qtIRS achieves sEU-ACMI and sAnon security while our scheme D-qtIRS attains strong existential unforgeability against adaptive-chosen-message-and-identity attack with signature delegation (sEU-ACMID) and sAnon. Most interestingly, our D-qtIRS features signature delegation and leased key revocation in the IRS setting, enabling a lessor to grant a limited signing authority to a lessee and revoke the delegated authority whenever needed. Additionally, we have shown the existence of obfuscated membership-checking circuits which is of independent interest.

We put forth a new paradigm for practical secure multiparty computation (MPC) in the preprocessing model, where a feasible one-time setup can enable a lifetime of efficient online secure computations. Our protocols match the security guarantees and low costs of the cheapest category of MPC solutions, namely 3-party protocols (3PC) secure against a single malicious party, with the qualitative advantages that one party communicates data sublinear in the circuit size, and can go offline after its initial messages. This "2+1"-party structure can alternatively be instantiated between 2 parties with the aid of a (possibly untrusted) dealer. Within such existing protocols, we provide comparable online performance while improving the storage and offline dealer-to-party communication requirements by more than 3 orders of magnitude.

At the technical level, we build on a novel combination of the Fully Linear Interactive Oracle Proof (FLIOP)-based protocol design of Boyle et al. (CRYPTO 2021) and pseudorandom correlation generators. We provide an extensive assortment of algorithmic and implementation-level optimizations, design efficient distributed proofs of well-formedness of complex FLIOP correlations, and make them circuit-independent. We implement and benchmark our end-to-end system against the state of the art in the $(2+1)$ regime, a dealer-aided variant of SPDZ for Boolean circuits.

We additionally extend our techniques to the $(n+1)$ party setting, where a dealer aids general dishonest-majority MPC, and provide a variant of the protocol which further achieves security with identifiable abort.

In this work, we present an efficient secure multi-party computation MPC protocol that provides strong security guarantees in settings with a dishonest majority of participants who may behave arbitrarily. Unlike the popular MPC implementation known as SPDZ [Crypto ’12], which only ensures security with abort, our protocol achieves both complete identifiability and robustness. With complete identifiability, honest parties can detect and unanimously agree on the identity of any malicious party. Robustness allows the protocol to continue with the computation without requiring a restart, even when malicious behavior is detected. Additionally, our approach addresses the performance limitations observed in the protocol by Cunningham et al. [ICITS ’17], which, while achieving complete identifiability, is hindered by the costly exponentiation operations required by the choice of commitment scheme.

Our protocol is based on the approach by Rivinius et al. [S&P ’22], utilizing lattice-based commitment for better efficiency. We achieves robustness with the help of a semi-honest trusted third party. We benchmark our robust protocol, showing the efficient recovery from parties’ malicious behavior.

Finally, we benchmark our protocol on a ML-as-a-service scenario, wherein clients off-load the desired computation to the servers, and verify the computation result. We benchmark on linear ML inference, running on various datasets. While our efficiency is slightly lower compared to SPDZ’s, we offer stronger security properties that provide distinct advantages.

Authenticated encryption with associated data (AEAD) schemes are responsible for securing increasingly critical digital infrastructures, worldwide. Unfortunately, current widely deployed schemes suffer from various limitations that make them difficult to use securely in practice. For example, schemes like AES-GCM limit the amount of data that can be encrypted with a single key, therefore limiting its secure scaling to modern workloads. At the same time, practitioners may not be able to move away from the use of AES-GCM due to mature and widely deployed implementations, legacy constraints, and compliance. In this paper, we provide approaches to improve the secure scaling of AEAD schemes via what we call derived-nonce, derived-key (DNDK) transforms. At a high level, such transforms use a root key to derive a nonce and key for use with an underlying scheme. The challenge is doing so in a way that introduces as little overhead as possible, and relying on a small number of assumptions on the used primitives. We provide some general results about secure scaling transforms and a concrete design for AES-GCM that is called DNDK-GCM. It requires as little as three additional AES calls to enable use of the same key to encrypt up to $2^{64}$ bytes of data, even when using random nonces. We also provide a detailed performance analysis. DNDK-GCM is now a draft IETF standard, and is already deployed at the cloud scale by companies including Meta.

The CKKS fully homomorphic encryption scheme enables efficient homomorphic operations in terms of throughput, but its bootstrapping algorithm incurs a significant latency. In this work, we introduce SHIP, a novel bootstrapping algorithm for CKKS ciphertexts. SHIP enjoys a very shallow homomorphic multiplicative depth compared to state-of-the-art CKKS bootstrapping algorithms. Bootstrapping depth directly impacts the required Ring-LWE modulus, and hence the Ring- LWE degree. The massive depth saving allows us to report the first bootstrapping of CKKS ciphertexts for full-dimensional cleartext vectors in ring degree N=2^13, without resorting to an expensive scheme switching to DM/CGGI. SHIP also enjoys great parallelizability, with minimal communication between threads. The combined ring size reduction and high parallelizability lead to very low latency. In ring degree N=2^13, our experimental implementation runs in 215ms on a 32-core CPU for real-valued cleartext vectors. This is 2.5x lower than the smallest latency we could observe with the HEaaN library (using 48 cores). For binary cleartext vectors, the latency is lowered to 174ms, which is 2.2x lower than Bae et al [Eurocrypt’24] (with 32 cores).

The power of adaptivity in algorithms has been intensively studied in diverse areas of theoretical computer science. In this paper, we obtain a number of sharp lower bound results which show that adaptivity provides a significant extra power in cryptanalytic time-space tradeoffs with (possibly unlimited) preprocessing time.

Most notably, we consider the discrete logarithm (DLOG) problem in a generic group of $N$ elements. The classical `baby-step giant-step' algorithm for the problem has time complexity $T=O(\sqrt{N})$, uses $O(\sqrt{N})$ bits of space (up to logarithmic factors in $N$) and achieves constant success probability.

We examine a generalized setting where an algorithm obtains an advice string of $S$ bits and is allowed to make $T$ arbitrary non-adaptive queries that depend on the advice string (but not on the challenge group element for which the DLOG needs to be computed). We show that in this setting, the $T=O(\sqrt{N})$ online time complexity of the baby-step giant-step algorithm cannot be improved, unless the advice string is more than $\Omega(\sqrt{N})$ bits long. This lies in stark contrast with the classical adaptive Pollard's rho algorithm for DLOG, which can exploit preprocessing to obtain the tradeoff curve $ST^2=O(N)$. We obtain similar sharp lower bounds for the problem of breaking the Even-Mansour cryptosystem in symmetric-key cryptography and for several other problems. To obtain our results, we present a new model that allows analyzing non-adaptive preprocessing algorithms for a wide array of search and decision problems in a unified way. Since previous proof techniques inherently cannot distinguish between adaptive and non-adaptive algorithms for the problems in our model, they cannot be used to obtain our results. Consequently, we rely on information-theoretic tools for handling distributions and functions over the space $S_N$ of permutations of $N$ elements. Specifically, we use a variant of Shearer's lemma for this setting, due to Barthe, Cordero-Erausquin, Ledoux, and Maurey (2011), and a variant of the concentration inequality of Gavinsky, Lovett, Saks and Srinivasan (2015) for read-$k$ families of functions, that we derive from it. This seems to be the first time a variant of Shearer's lemma for permutations is used in an algorithmic context, and it is expected to be useful in other lower bound arguments.

The dream of achieving data privacy during external computations has become increasingly concrete in recent years. Indeed, since the early days of Fully Homomorphic Encryption (FHE) more than a decade ago, new cryptosystems and techniques have constantly optimized the efficiency of computation on encrypted data. However, one of the main disadvantages of FHE, namely its significant ciphertext expansion factor, remains at the center of the efficiency bottleneck of FHE schemes. To tackle the issue of slow uplink FHE data transmission, we use transciphering. With transciphering, the client naturally encrypts its data under a symmetric scheme and sends them to the server with (once and for all) an FHE encryption of the symmetric scheme’s key. With its larger computing power, the server then evaluates the symmetric scheme’s decryption algorithm within the homomorphic domain to obtain homomorphic ciphertexts that allow it to perform the requested calculations. Since the first use of this method a bit more than ten years ago, papers on the homomorphic evaluation of AES have been numerous. And as the AES execution is the application chosen by NIST in the FHE part of its recent call for proposals on threshold encryption, the stakes of such work go up another level. But what about other standardized block ciphers? Is the AES the more efficient option? In this work, we leverage on two methods which have successfully been applied to the homomorphic evaluation of AES to study several state-of-the-art symmetric block ciphers (namely CLEFIA, PRESENT, PRINCE, SIMON, SKINNY). That is to say, we implement a representative set of symmetric block ciphers using TFHE. These implementations allow us to compare the efficiency of this set of symmetric schemes and to categorize them. We highlight the characteristics of block ciphers that are fast to execute in the homomorphic domain and those that are particularly costly. Finally, this classification of operation types enables us to sketch out what the ideal block cipher for transciphering homomorphic data in integer mode might look like.

Event date: 8 October to 9 October 2025
Submission deadline: 1 July 2025
Notification: 23 August 2025

The Augot-Finiasz system is a public-key encryption (PKE) scheme based on Reed-Solomon codes and was later followed by analogous versions in the rank metric. Although these schemes were eventually broken, their fundamental idea remains exciting. Notably, these schemes are significantly different from the McEliece system as there is no need to hide the code and, as such, promise much better parameters. Further, they admit a simple description where both the public key and ciphertext are just corrupted codewords of a public code. An interesting question is whether the general idea can be made to work, i.e., resist all known attacks, by using other code classes. This paper shows how to generalize the Augot-Finiasz system to other code families. We reduce the correctness and security of this framework to simple assertions about the code class with which it is instantiated. Specifically, its correctness is equivalent to the existence of an efficient error-erasure decoder, and its security reduces to an easily understood hardness assumption, called "supercode decoding", close to the syndrome decoding problem.

In the $k$-Orthogonal Vectors ($k$-OV) problem we are given $k$ sets, each containing $n$ binary vectors of dimension $d=n^{o(1)}$, and our goal is to pick one vector from each set so that at each coordinate at least one vector has a zero. It is a central problem in fine-grained complexity, conjectured to require $n^{k-o(1)}$ time in the worst case.

We propose a way to plant a solution among vectors with i.i.d. $p$-biased entries, for appropriately chosen $p$, so that the planted solution is the unique one. Our conjecture is that the resulting $k$-OV instances still require time $n^{k-o(1)}$ to solve, on average.

Our planted distribution has the property that any subset of strictly less than $k$ vectors has the same marginal distribution as in the model distribution, consisting of i.i.d. $p$-biased random vectors. We use this property to give average-case search-to-decision reductions for $k$-OV.

For very long messages, the reliable broadcast protocol with the best communication complexity to date is the Minicast protocol of Locher & Shoup [2024]. To reliably broadcast a message $m$ to $n$ parties, Minicast has communication complexity $\sim 1.5 |m| n$, when $|m|$ is large. However, the round complexity of Minicast is 4, which is worse than the 3 rounds of the classical protocol of Bracha. We give a new reliable broadcast protocol whose communication complexity is essentially the same as that of Minicast, but whose round complexity is just 3. Like Minicast, our new protocol does not rely on any cryptography other than hash functions. We also give a new 2-round protocol that relies on signatures. For large $|m|$, its communication complexity is also $\sim 1.5 |m| n$, unless the sender provably misbehaves, in which case its communication complexity may degrade to at most $\sim 2 |m| n$.

One-way functions (OWFs) form the foundation of modern cryptography, yet their unconditional existence remains a major open question. In this work, we study this question by exploring its relation to lossy reductions, i.e., reductions $R$ for which it holds that $I(X;R(X)) \ll n$ for all distributions $X$ over inputs of size $n$. Our main result is that either OWFs exist or any lossy reduction for any promise problem $\Pi$ runs in time $2^{\Omega(\log\tau_\Pi / \log\log n)}$, where $\tau_\Pi(n)$ is the infimum of the runtime of all (worst-case) solvers of $\Pi$ on instances of size $n$. More precisely, by having a reduction with a better runtime, for an arbitrary promise problem $\Pi$, and by using a non-uniform advice, we construct (a family of) OWFs. In fact, our result requires a milder condition, that $R$ is lossy for sparse uniform distributions (which we call mild-lossiness). It also extends to $f$-reductions as long as $f$ is a non-constant permutation-invariant Boolean function, which includes $\text{And-, Or-, Maj-, Parity-}$, $\text{Mod}_k$-, and $\text{Threshold}_k$-reductions.

Additionally, we show that worst-case to average-case Karp reductions and randomized encodings are special cases of mild-lossy reductions and improve the runtime above as $2^{\Omega(\log \tau_\Pi)}$ when these mappings are considered. Restricting to weak fine-grained OWFs, this runtime can be further improved as $\Omega(\tau_\Pi)$. Intuitively, the latter asserts that if weak fine-grained OWFs do not exist then any instance randomization of any $\Pi$ has the same runtime (up to a constant factor) as the best worst-case solver of $\Pi$.

Taking $\Pi$ as $k\text{Sat}$, our results provide sufficient conditions under which (fine-grained) OWFs exist assuming the Exponential Time Hypothesis (ETH). Conversely, if (fine-grained) OWFs do not exist, we obtain impossibilities on instance compressions (Harnik and Naor, FOCS 2006) and instance randomizations of $k\text{Sat}$ under the ETH. Moreover, the analysis can be adapted to studying such properties of any $\text{NP}$-complete problem.

Finally, we partially extend these findings to the quantum setting; the existence of a pure quantum mildly-lossy reduction for $\Pi$ within the runtime $2^{o(\log\tau_\Pi / \log\log n)}$ implies the existence of one-way state generators, where $\tau_\Pi$ is defined with respect to quantum solvers.

Fully Homomorphic Encryption (FHE) enables arbitrary and unlimited computations directly on encrypted data. Notably, the TFHE scheme allows users to encrypt bits or small numbers (4-6 bits) and compute any univariate function using programmable bootstrapping (PBS), while simultaneously refreshing the ciphertext noise. Since both linear and non-linear functions can be evaluated using PBS, it is possible to compute arbitrary functions and circuits of unlimited depth without any accuracy loss. Nevertheless, a major limitation of TFHE, compared to other FHE schemes, is that it operates on a single ciphertext at a time, and the underlying message size remains small. For larger messages with longer bit sizes, the execution overhead of PBS grows exponentially with the number of message bits. A recent approach, called Without-padding PBS (WoPBS), enables computation of much larger lookup tables (10-28 bits), with the execution cost scaling linearly with the number of message bits. The significant encoding mismatch between the PBS and WoPBS, however, complicates the use of both approaches within the same circuit execution.

In this work, we introduce novel switching algorithms that enable ciphertexts to be converted back and forth between the PBS and WoPBS contexts without impacting the input noise. Moreover, we introduce a new method to bootstrap ciphertexts within the WoPBS context, allowing for unlimited XOR operations at negligible cost. To enhance runtime, we further introduce optimized parameters for both contexts. We validate our techniques through the homomorphic evaluation of AES encryption and decryption, demonstrating transciphering applications that outperform related works.

This whitepaper introduces Clementine, a secure, collateral-efficient, trust-minimized, and scalable Bitcoin bridge based on BitVM2 that enables withdrawals from rollups or other side systems to Bitcoin. Clementine proposes a new Bitcoin light client that remains secure against adversaries controlling less than 50% of Bitcoin’s hash rate, assuming at least one honest Watchtower in a permissioned set. The protocol is collateral-efficient, reusing locked funds over time and reducing unnecessary dust outputs through the strategic use of 0-value outputs, and scalable, enabling a single challenge per Operator to slash multiple misbehaviors. This increases throughput and reduces on-chain load without compromising security. Clementine enables trust-minimized and efficient peg-outs from Citrea to Bitcoin, making zk-rollups on Bitcoin practical and unlocking new paths for native scalability and interoperability.

We are seeking to recruit a researcher for an interdisciplinary project on notions of "explainability" in the context of side channel evaluations (considering technical approaches used in both FIPS style as well as CC style evaluations).

The project will run up to three years. It will require a mix of technical skills (we wish to propose and evaluate novel approaches to gather evidence for/against security of implementations given access to side channels) as well as an interest in developing social science research methodologies (we plan to engage with evaluation labs but also vendors to research useful notions of "explainable leakage").

The project will be co-supervised by Prof. Elisabeth Oswald and Prof. Katharina Kinder-Kurlanda; both situated in the interdisciplinary Digital Age Research Centre at the University of Klagenfurt (Austria).

We seek applicants with a mathematical/technical background. For applicants wishing to pursue a PhD, we expect that they have done a MSc/Bsc thesis on side channels/faults with a practical focus. For applicants who already possess a PhD, we expect a strong track record in applied cryptography with some publications in the area of side channels/faults in top venues.

The post holder will be expected to work in Klagenfurt (Austria), and to be able to do short term visits to evaluation labs/vendors throughout Europe.

In order to apply, please send a short CV, including your scientific outputs (e.g. papers, talks, seminars, open source artefacts, etc.), as a single pdf file to [email protected]. If you have questions, or wish to discuss informally, please contact Elisabeth Oswald.

We will review applications as they arrive and invite potentially suitable candidates for an online interview as soon as possible, with the intention to fill the post once a suitable candidate has been identified.

Closing date for applications:

Contact: Elisabeth Oswald (Elisabeth.Oswald AT aau.at)

The Young Investigator Group “COSYS - Control Systems and Cyber Security Lab” at the Chair of IT Security at the Brandenburg University of Technology Cottbus-Senftenberg has an open PhD/Postdoc position in the following areas:

AI-based Network Attack Detection and Simulation.

AI-enabled Penetration Testing.

Privacy-Enhancing Technologies in Cyber-Physical Systems.

The available position is funded as 100% TV-L E13 tariff in Germany and limited until 31.07.2026, with possibility for extension. Candidates must hold a Master’s degree (PhD degree for Postdocs) or equivalent in Computer Science or related disciplines, or be close to completing it. If you are interested, please send your CV, transcript of records from your Master studies, and an electronic version of your Master's thesis (if possible), as a single pdf file. Applications will be reviewed until the position is filled.

Closing date for applications:

Contact: Ivan Pryvalov ([email protected])