International Association
for Cryptologic Research

This paper focuses on equivalences between Generalised Feistel Networks (GFN) of type-II. We introduce a new definition of equivalence which captures the concept that two GFNs are identical up to re-labelling of the inputs/outputs, and give a procedure to test this equivalence relation. Such two GFNs are therefore cryptographically equivalent for several classes of attacks. It induces a reduction of the space of possible GFNs: the set of the $(k!)^2$ possible even-odd GFNs with $2k$ branches can be partitioned into $k!$ different classes.

This result can be very useful when looking for an optimal GFN regarding specific computationally intensive properties, such as the minimal number of active S-boxes in a differential trail. We also show that in several previous papers, many GFN candidates are redundant as they belong to only a few classes. Because of this reduction of candidates, we are also able to suggest better permutations than the one of WARP: they reach 64 active S-boxes in one round less and still have the same diffusion round that WARP. Finally, we also point out a new family of permutations with good diffusion properties.

In some linear key recovery attacks, the function which determines the value of the linear approximation from the plaintext, ciphertext and key is replaced by a similar map in order to improve the time or memory complexity at the cost of a data complexity increase. We propose a general framework for key recovery map substitution, and introduce Walsh spectrum puncturing, which consists of removing carefully-chosen coefficients from the Walsh spectrum of this map. The capabilities of this technique are illustrated by describing improved attacks on reduced-round Serpent (including the first 12-round attack on the 192-bit key variant), GIFT-128 and NOEKEON, as well as the full DES.

Learning with Errors (LWE) is a hard math problem underlying post-quantum cryptography (PQC) systems for key exchange and digital signatures, recently standardized by NIST. Prior work [Wenger et al., 2022; Li et al., 2023a;b] proposed new machine learning (ML)-based attacks on LWE problems with small, sparse secrets, but these attacks require millions of LWE samples to train on and take days to recover secrets. We propose three key methods—better pre-processing, angular embeddings and model pre-training—to improve these attacks, speeding up preprocessing by 25× and improving model sample efficiency by 10×. We demonstrate for the first time that pre-training improves and reduces the cost of ML attacks on LWE. Our architecture improvements enable scaling to larger-dimension LWE problems: this work is the first instance of ML attacks recovering sparse binary secrets in dimension n = 1024, the smallest dimension used in practice for homomorphic encryption applications of LWE where sparse binary secrets are proposed.

Speculative out-of-order execution is a strategy of masking execution latency by allowing younger instructions to execute before older instructions. While originally considered to be innocuous, speculative out-of-order execution was brought into the spotlight with the 2018 publication of the Spectre and Meltdown attacks. These attacks demonstrated that microarchitectural side channels can leak sensitive data accessed by speculatively executed instructions that are not part of the normal program execution. Since then, a significant effort has been vested in investigating how microarchitectural side channels can leak data from speculatively executed instructions and how to control this leakage. However, much less is known about how speculative out-of-order execution affects microarchitectural side-channel attacks.

In this paper, we investigate how speculative out-of-order execution affects the Evict+Time cache attack. Evict+Time is based on the observation that cache misses are slower than cache hits, hence by measuring the execution time of code, an attacker can determine if a cache miss occurred during the execution. We demonstrate that, due to limited resources for tracking out-of-order execution, under certain conditions an attacker can gain more fine-grained information and determine whether a cache miss occurred in part of the executed code.

Based on the observation, we design the Evict+Spec+Time attack, a variant of Evict+Time that can learn not only whether a cache miss occurred, but also in which part of the victim code it occurred. We demonstrate that Evict+Spec+Time is an order of magnitude more efficient than Evict+Time when attacking a T-table-based implementation of AES. We further show an Evict+Spec+Time attack on an S-box-based implementation of AES, recovering the key with as little as 14389 decryptions. To the best of our knowledge, ours is the first successful Evict+Time attack on such a victim.

Biscuit is a recent multivariate signature scheme based on the MPC-in-the-Head paradigm. It has been submitted to the NIST competition for additional signature schemes. Signatures are derived from a zero-knowledge proof of knowledge of the solution of a structured polynomial system. This extra structure enables efficient proofs and compact signatures. This short note demonstrates that it also makes these polynomial systems easier to solve than random ones. As a consequence, the original parameters of Biscuit failed to meet the required security levels and had to be upgraded.

Fault injection attacks are a serious concern for cryptographic hardware. Adversaries may extract sensitive information from the faulty output that is produced by a cryptographic circuit after actively disturbing its computation. Alternatively, the information whether an output would have been faulty, even if it is withheld from being released, may be exploited. The former class of attacks, which requires the collection of faulty outputs, such as Differential Fault Analysis (DFA), then either exploits some knowledge about the position of the injected fault or about its value. The latter class of attacks, which can be applied without ever obtaining faulty outputs, such as Statistical Ineffective Fault Attacks (SIFA), then either exploits a dependency between the effectiveness of the fault injection and the value to be faulted (e.g., an LSB stuck-at-0 only affecting odd numbers), denoted as SIFA-1, or a conditional propagation of a faulted value based on a sensitive intermediate (e.g., multiplication of a faulted value by 0 prevents propagation), denoted as SIFA-2. The aptitude of additive masking schemes, which were designed to prevent side-channel analysis, to also thwart fault attacks is typically assumed to be limited. Common fault models, such as toggle/bit-flip, stuck-at-0 or stuck-at-1 survive the recombination of Boolean shares well enough for generic attacks to succeed. More precisely, injecting a fault into one or multiple Boolean shares often results in the same, or at least a predictable, error appearing in the sensitive variable after recombination. In this work, we show that additive masking in prime-order fields breaks such relationships, causing frequently exploited biases to decrease exponentially in the number of shares. As a result, prime masking offers surprisingly strong protection against generic statistical attacks, which require a dependency between the effectiveness of an injected fault and the secret variable that is manipulated, such as SIFA-1. Operation-dependent statistical attacks, such as SIFA-2 and Fault Template Attacks (FTA), may still be performed against certain prime-field structures, even if they are masked with many shares. Yet, we analyze the corresponding cases and are able to provide specific guidelines on how to avoid vulnerabilities either at the cipher design or implementation level by making informed decisions about the primes, non-linear mappings and masked gadgets used. Since prime-field masking appears to be one of the rare instances of affordable countermeasures that naturally provide sound protection against sidechannel analysis and certain fault injection attacks, we believe there is a strong incentive for developing new ciphers to leverage these advantages.

This work introduces several algorithms related to the computation of orientations in endomorphism rings of supersingular elliptic curves. This problem boils down to representing integers by ternary quadratic forms, and it is at the heart of several results regarding the security of oriented-curves in isogeny-based cryptography. Our main contribution is to show that there exists efficient algorithms that can solve this problem for quadratic orders of discriminant $n$ up to $O(p^{4/3})$. Our approach improves upon previous results by increasing this bound from $O(p)$ to $O(p^{4/3})$ and removing some heuristics. We introduce several variants of our new algorithm and provide a careful analysis of their asymptotic running time (without heuristic when it is possible). The best proven asymptotic complexity of one of our variant is $O(n^{3/4}/p)$ in average. The best heuristic variant has a complexity of $O(p^{1/3})$ for big enough $n$. We then introduce several results regarding the computation of ideals between oriented orders. The first application of this is a simplification of the known reduction from vectorization to computing the endomorphism ring, removing the assumption on the factorization of the discriminant. As a second application, we relate the problem of computing fixed-degree isogenies between supersingular curves to the problem of computing orientations in endomorphism rings, and we show that for a large range of degree $d$, our new algorithms improve on the state-of-the-art, and in important special cases, the range of degree $d$ for which there exist a polynomial-time algorithm is increased. In the most special case we consider, when both curves are oriented by a small degree endomorphism, we show heuristically that our techniques allow the computation of isogenies of any degree, assuming they exist.

A Proof of Exponentiation (PoE) allows a prover to efficiently convince a verifier that $y=x^e$ in some group of unknown order. PoEs are the basis for practical constructions of Verifiable Delay Functions (VDFs), which, in turn, are important for various higher-level protocols in distributed computing. In applications such as distributed consensus, many PoEs are generated regularly, motivating protocols for secure aggregation of batches of statements into a few statements to improve the efficiency for both parties. Rotem (TCC 2021) recently presented two such generic batch PoEs.

In this work, we introduce two batch PoEs that outperform both proposals of Rotem and we evaluate their practicality. First, we show that the two batch PoEs of Rotem can be combined to improve the overall efficiency by at least a factor of two. Second, we revisit the work of Bellare, Garay, and Rabin (EUROCRYPT 1998) on batch verification of digital signatures and show that, under the low order assumption, their bucket test can be securely adapted to the setting of groups of unknown order. The resulting batch PoE quickly outperforms the state of the art in the expected number of group multiplications with the growing number of instances, and it decreases the cost of batching by an order of magnitude already for hundreds of thousands of instances. Importantly, it is the first batch PoE that significantly decreases both the proof size and complexity of verification. Our experimental evaluations show that even a non-optimized implementation achieves such improvements, which would match the demands of real-life systems requiring large-scale PoE processing.

Finally, even though our proof techniques are conceptually similar to Rotem, we give an improved analysis of the application of the low order assumption towards secure batching of PoE instances, resulting in a tight reduction, which is important when setting the security parameter in practice.

We give an alternative derivation of (N,N)-isogenies between fast Kummer surfaces which complements existing works based on the theory of theta functions. We use this framework to produce explicit formulae for the case of N = 3, and show that the resulting algorithms are more efficient than all prior (3,3)-isogeny algorithms.

The notion of collaborative zk-SNARK is introduced by Ozdemir and Boneh (USENIX 2022), which allows multiple parties to jointly create a zk-SNARK proof over distributed secrets (also known as the witness). This approach ensures the privacy of the witness, as no corrupted servers involved in the proof generation can learn anything about the honest servers' witness. Later, Garg et al. continued the study, focusing on how to achieve faster proof generation (USENIX 2023). However, their approach requires a powerful server that is responsible for the most resource-intensive computations and communications during the proof generation. This requirement results in a scalability bottleneck, making their protocols unable to handle large-scale circuits.

In this work, we address this issue by lifting a zk-SNARK called Libra (Crypto 2019) to a collaborative zk-SNARK and achieve a fully distributed proof generation, where all servers take roughly the same portion of the total workload. Further, our protocol can be adapted to be secure against a malicious adversary by incorporating some verification mechanisms. With 128 consumer machines and a 4Gbps network, we successfully generate a proof for a data-parallel circuit containing $2^{23}$ gates in merely 2.5 seconds and take only 0.5 GB memory for each server. This represents a $19\times$ speed-up, compared to a local Libra prover. Our benchmark further indicates an impressive 877$\times$ improvement in running time and a 992$\times$ enhancement in communication compared to the implementation in previous work. Furthermore, our protocol is capable of handling larger circuits, making it scalable in practice.

To enable parallel processing, the Directed Acyclic Graph (DAG) structure is introduced to the design of asynchronous Byzantine Fault Tolerant (BFT) consensus protocols, known as DAG-based BFT. Existing DAG-based BFT protocols operate in successive waves, with each wave containing three or four Reliable Broadcast (RBC) rounds to broadcast data, resulting in high latency due to the three communication steps required in each RBC. For instance, Tusk, a state-of-the-art DAG-based BFT protocol, has a good-case latency of 7 communication steps and an expected worst latency of 21 communication steps.

To reduce latency, we propose GradedDAG, a new DAG-based BFT consensus protocol based on our adapted RBC called Graded RBC (GRBC) and the Consistent Broadcast (CBC), with each wave consisting of only one GRBC round and one CBC round. Through GRBC, a replica can deliver data with a grade of 1 or 2, and a non-faulty replica delivering the data with grade 2 can ensure that more than 2/3 of replicas have delivered the same data. Meanwhile, through CBC, data delivered by different non-faulty replicas must be identical. In each wave, a block in the GRBC round will be elected as the leader. If a leader block has been delivered with grade 2, it and all its ancestor blocks can be committed. GradedDAG offers a good-case latency of 4 communication steps and an expected worst latency of 7.5 communication steps, significantly lower than the state-of-theart. Experimental results demonstrate GradedDAG’s feasibility and efficiency.

We implement a secure platform for statistical analysis over multiple organizations and multiple datasets. We provide a suite of protocols for different variants of JOIN and GROUP-BY operations. JOIN allows combining data from multiple datasets based on a common column. GROUP-BY allows aggregating rows that have the same values in a column or a set of columns, and then apply some aggregation summary on the rows (such as sum, count, median, etc.). Both operations are fundamental tools for relational databases. One example use case of our platform is in data marketing in which an analyst would join purchase histories and membership information, and then obtain statistics, such as "Which products were bought by people earning this much per annum?"

Both JOIN and GROUP-BY involve many variants, and we design protocols for several common procedures. In particular, we propose a novel group-by-median protocol that has not been known so far. Our protocols rely on sorting protocols, and work in the honest majority setting and against malicious adversaries. To the best of our knowledge, this is the first implementation of JOIN and GROUP-BY protocols secure against a malicious adversary.

Adaptor signature is a novel cryptographic primitive which ties together the signature and the leakage of a secret value. It has become an important tool for solving the scalability and interoperability problems in the blockchain. Aumayr et al. (Asiacrypt 2021) recently provide the formalization of the adaptor signature and present a provably secure ECDSA-based adaptor signature, which requires zero-knowledge proof in the pre-signing phase to ensure the signer works correctly. However, the number of zero-knowledge proofs is linear with the number of participants.

In this paper, we propose efficient ECDSA-based adaptor signature schemes and give security proofs based on ECDSA. In our schemes, the zero-knowledge proofs in the pre-signing phase can be generated in a batch and offline. Meanwhile, the online pre-signing algorithm is similar to the ECDSA signing algorithm and can enjoy the same efficiency as ECDSA. In particular, considering specific verification scenarios, such as (batched) atomic swaps, our schemes can reduce the number of zero-knowledge proofs in the pre-signing phase to one, independent of the number of participants. Last, we conduct an experimental evaluation, demonstrating that the performance of our ECDSA-based adaptor signature reduces online pre-signing time by about 60% compared with the state-of-the-art ECDSA-based adaptor signature.

Garbled Circuit (GC) techniques usually work with Boolean circuits. Despite intense interest, efficient arithmetic generalizations of GC were only known from heavy assumptions, such as LWE.

We construct arithmetic garbled circuits from circular correlation robust hashes, the assumption underlying the celebrated Free XOR garbling technique. Let $\lambda$ denote a computational security parameter, and consider the integers $\mathbb{Z}_m$ for any $m \geq 2$. Let $\ell = \lceil \log_2 m \rceil$ be the bit length of $\mathbb{Z}_m$ values. We garble arithmetic circuits over $\mathbb{Z}_m$ where the garbling of each gate has size $O(\ell \cdot \lambda)$ bits. Constrast this with Boolean-circuit-based arithmetic, requiring $O(\ell^2\cdot \lambda)$ bits via the schoolbook multiplication algorithm, or $O(\ell^{1.585}\cdot \lambda)$ bits via Karatsuba's algorithm.

Our arithmetic gates are compatible with Boolean operations and with Garbled RAM, allowing to garble complex programs of arithmetic values.

Event date: 14 March to 15 March 2024
Submission deadline: 18 February 2024
Notification: 26 February 2024

Event date: 14 August to 16 August 2024
Submission deadline: 10 March 2024
Notification: 31 May 2024

Our modern society relies upon numerous electronic devices that use encryption to communicate and operate securely. However, even strong cryptography can break when the device hardware is attacked. Thus, the University of Amsterdam is looking for a strong MSc graduate that is interested in Side-Channel Analysis, Hardware Security and Cryptographic Implementations.

What are you going to do?

Conduct high-quality research in the field of Side-Channel Analysis and Cryptographic Engineering, resulting in academic publications in peer-reviewed international conferences

Contribute to the open-source teaching material in our Bachelor and Master courses (in English)

Meet regularly with your academic supervisor and the international team to discuss and analyse the technical details of your ongoing research

Perform research-oriented internships in the industry

Your experience and profile:

Completed or soon-to-be-completed MSc in Computer Security, Computer Science, Mathematics, Computer Engineering, Electrical Engineering or related discipline

Strong interest in learning hardware security, applied cryptography and side-channel analysis, through regular tutoring by the academic supervisor

Background in Machine Learning, Signal Processing and/or background in Cryptography, Computer Security

Fluency in oral and written English, good presentation skills

Apply using the link:

https://vacatures.uva.nl/UvA/job/PhD-in-Side-Channel-Analysis/786914702/

Closing date for applications:

Contact: Kostas Papagiannopoulos, k.papagiannopoulos@uva.nl, kostaspap88@gmail.com

More information: https://vacatures.uva.nl/UvA/job/PhD-in-Side-Channel-Analysis/786914702/

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 open PhD/Postdoc positions in the following areas:

• Privacy-enhancing technologies in cyber-physical systems.
• AI-based network attack detection and simulation
• AI-enabled penetration testing

The available positions are 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 positions are filled.

Closing date for applications:

Contact: Ivan Pryvalov (ivan.pryvalov@b-tu.de)

We are looking for two motivated PhD students in the area of cryptography. The candidates will work at the Institute of Cybersecurity and Cryptology at the University of Wollongong. The research topic is public-key cryptography, especially about post-quantum cryptography and privacy-preserving cryptography. The positions are fully funded and will start at 1st July 2024 or thereafter. The period of the position is 3 years. The applicants should have a solid background in Computer Science or Mathematics (or relevant fields). Furthermore, it is preferable that the applicants have some research experience in the field of (public-key) cryptography. If interested, please send your CV, transcripts and a short paragraph about your research background and research interest to contact.

Closing date for applications:

Contact: Dr. Zuoxia Yu (given name_first name at uow.edu.au)

The Security and Privacy Research Unit at TU Wien is offering a fully funded PhD position in Formal Methods for Security and Privacy under the supervision of Univ.-Prof. Dr. Matteo Maffei.
Your profile:

• Master degree in computer science or equivalent (degree completion by employment start)
• Excellent English, communication, and teamwork skills
• Background in formal methods (e.g., automated reasoning, type systems, or proof assistants) or cryptography
• Experience in research is a plus

We offer:

• A job in an internationally renowned group, which regularly publishes in top security venues, and consists of an international, diverse, and inclusive team with expertise in formal methods, cryptography, security, privacy, and blockchains
• Diverse research topics in formal methods for security and privacy, with a specific focus on cryptographic protocols and blockchains
• An international English-speaking environment (German not required)
• A competitive salary
• Flexible hours

Applications are to be performed online (https://tools.spycode.at/recruiting/call/5). The application material should include:

• Motivation letter
• Bachelor/Master’s transcripts
• Publication list (if available)
• Curriculum vitae
• Contact information for two referees

We strongly encourage applications from underrepresented groups. Applications are welcome until the position is filled. The applications will be evaluated in a bi-weekly fashion, and applicants will be contacted only in case they are selected for an interview.

Closing date for applications:

Contact: Univ.-Prof. Dr. Matteo Maffei

More information: https://tools.spycode.at/recruiting/call/5