International Association
for Cryptologic Research

Kyber was selected by NIST as a Post-Quantum Cryptography Key Encapsulation Mechanism standard. This means that the industry now needs to transition and adopt these new standards. One of the most demanding operations in Kyber is the modular arithmetic, making it a suitable target for optimization. This work offers a novel modular reduction design with the lowest area on Xilinx FPGA platforms. This novel design, through K-reduction and LUT-based reduction, utilizes 49 LUTs and 1 DSP as opposed to Xing and Li’s 2021 CHES design requiring 90 LUTs and 1 DSP for one modular multiplication. Our design is the smallest modular multiplier reported as of today.

Applications are invited for the MS and PhD positions at the Department of Computer Science and Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan. The successful candidate will work under the guidance of Dr. Arijit Karati on the diverse topics in applied cryptology and network security.

Candidates for applied cryptography domain must comprehend formal security analysis, secure coding, and effective security integration in the application domains.

Candidates for ML/AI domain, must comprehend seach/optimization algorithms, classification, regression and other essential aspects.

Responsibilities: Apart from academic work, student must involve in several activities in a group or individually, such as (not limited to):

Design and implementation of safety protocol.

Assesment of the security and performance metric.

Research meeting with the supervisor.

Requirements: (02 MS and 02 PhD positions)
Apart from the university's basic admission policies (https://cse.nsysu.edu.tw/?Lang=en), students are desired to have following key requirements:

Strong motivation on cryptography or AI security.

Knowledge of modern technology.

Knowledge of basic mathematics.

Knowledge of at least two programming languages, such as Python/Java/C/C++.

Master's thesis must match respective research fields. (for Phd positions)

Scholarship:

Under the university policy.

Project funding (based on availability for MS/Ph.D. students).

What students can expect:

Cooperation from the supervisor and labmates.

The rich culture in research and related activities.

Flexibility in communication, e.g., English.

What the supervisor can expect:
Apart from academic and research works, students are expected to have

Good moral character.

Hardworking and dedication.

Deadline for online application: September 30, 2024

Closing date for applications:

Contact: Arijit Karati (arijit.karati@mail.cse.nsysu.edu.tw)

More information: https://www.canseclab.com/

Homomorphic encryption is a cryptographic technique that enables arithmetic operations to be performed on encrypted data. However, word-wise fully homomorphic encryption schemes, such as BGV, BFV, and CKKS schemes, only support addition and multiplication operations on ciphertexts. This limitation makes it challenging to perform non-linear operations directly on the encrypted data. To address this issue, prior research has proposed efficient approximation techniques that utilize iterative methods, such as functional composition, to identify optimal polynomials. These approximations are designed to have a low multiplicative depth and a reduced number of multiplications, as these criteria directly impact the performance of the approximated operations.

In this paper, we propose a novel method, named as adaptive successive over-relaxation (aSOR), to further optimize the approximations used in homomorphic encryption schemes. Our experimental results show that the aSOR method can significantly reduce the computational effort required for these approximations, achieving a reduction of 2–9 times compared to state-of-the-art methodologies. We demonstrate the effectiveness of the aSOR method by applying it to a range of operations, including sign, comparison, ReLU, square root, reciprocal of m-th root, and division. Our findings suggest that the aSOR method can greatly improve the efficiency of homomorphic encryption for performing non-linear operations.

Digital signatures are fundamental building blocks in various protocols to provide integrity and authenticity. The development of the quantum computing has raised concerns about the security guarantees afforded by classical signature schemes. CRYSTALS-Dilithium is an efficient post-quantum digital signature scheme based on lattice cryptography and has been selected as the primary algorithm for standardization by the National Institute of Standards and Technology. In this work, we present a high-throughput GPU implementation of Dilithium. For individual operations, we employ a range of computational and memory optimizations to overcome sequential constraints, reduce memory usage and IO latency, address bank conflicts, and mitigate pipeline stalls. This results in high and balanced compute throughput and memory throughput for each operation. In terms of concurrent task processing, we leverage task-level batching to fully utilize parallelism and implement a memory pool mechanism for rapid memory access. We propose a dynamic task scheduling mechanism to improve multiprocessor occupancy and significantly reduce execution time. Furthermore, we apply asynchronous computing and launch multiple streams to hide data transfer latencies and maximize the computing capabilities of both CPU and GPU. Across all three security levels, our GPU implementation achieves over 160× speedups for signing and over 80× speedups for verification on both commercial and server-grade GPUs. This achieves microsecond-level amortized execution times for each task, offering a high-throughput and quantum-resistant solution suitable for a wide array of applications in real systems.

In this work, we consider the setting where one or more users with low computational resources would lie to outsource the task of proof generation for SNARKs to one external entity, named Prover. We study the scenario in which Provers have access to all statements and witnesses to be proven beforehand. We take a different approach to proof aggregation and design a new protocol that reduces simultaneously proving time and communication complexity, without going through recursive proof composition. Our two main contributions: We first design FLIP, a communication efficient folding scheme where we apply the Inner Pairing Product Argument to fold R1CS instances of the same language into a single relaxed R1CS instance. Then, any proof system for relaxed R1CS language can be applied to prove the final instance. As a second contribution, we build a novel variation of Groth16 with the same communication complexity for relaxed R1CS and two extra pairings for verification, with an adapted trusted setup. Compared to SnarkPack - a prior solution addressing scaling for multiple Groth16 proofs - our scheme improves in prover complexity by orders of magnitude, if we consider the total cost to generated the SNARK proofs one by one and the aggregation effort. An immediate application of our solution is Filecoin, a decentralized storage network based on incentives that generates more than 6 million SNARKs for large circuits of 100 million constraints per day.

In this paper, we present an improved attack on the stream cipher Salsa20. Our improvements are based on two technical contributions. First, we make use of a distribution of a linear combination of several random variables that are derived from different differentials and explain how to exploit this in order to improve the attack complexity. Secondly, we study and exploit how to choose the actual value for so-called probabilistic neutral bits optimally. Because of the limited influence of these key bits on the computation, in the usual attack approach, these are fixed to a constant value, often zero for simplicity. As we will show, despite the fact that their influence is limited, the constant can be chosen in significantly better ways, and intriguingly, zero is the worst choice. Using this, we propose the first-ever attack on 7.5-round of $128$-bit key version of Salsa20. Also, we provide improvements in the attack against the 8-round of $256$-bit key version of Salsa20 and the 7-round of $128$-bit key version of Salsa20.

Data availability sampling allows clients to verify availability of data on a peer-to-peer network provided by an untrusted source. This is achieved without downloading the full data by sampling random positions of the encoded data.

The long-term vision of the Ethereum community includes a comprehensive data availability protocol using polynomial commitments and tensor codes. As the next step towards this vision, an intermediate solution called PeerDAS is about to integrated, to bridge the way to the full protocol. With PeerDAS soon becoming an integral part of Ethereum's consensus layer, understanding its security guarantees is essential.

This document aims to describe the cryptography used in PeerDAS in a manner accessible to the cryptographic community, encouraging innovation and improvements, and to explicitly state the security guarantees of PeerDAS.

Self-Sovereign Identity (SSI) empowers individuals and organizations with full control over their data. Decentralized identifiers (DIDs) are at its center, where a DID contains a collection of public keys associated with an entity, and further information to enable entities to engage via secure and private messaging across different platforms. A crucial stepping stone is DIDComm, a cryptographic communication layer that is in production with version 2. Due to its widespread and active deployment, a formal study of DIDComm is highly overdue.

We present the first formal analysis of DIDComm’s cryptography, and formalize its goal of (sender-) anonymity and authenticity. We follow a composable approach to capture its security over a generic network, formulating the goal of DIDComm as a strong ideal communication resource. We prove that the proposed encryption modes reach the expected level of privacy and authenticity, but leak beyond the leakage induced by an underlying network (captured by a parameterizable resource).

We further use our formalism to propose enhancements and prove their security: first, we present an optimized algorithm that achieves simultaneously anonymity and authenticity, conforming to the DIDComm message format, and which outperforms the current DIDComm proposal in both ciphertext size and computation time by almost a factor of 2. Second, we present a novel DIDComm mode that fulfills the notion of anonymity preservation, in that it does never leak more than the leakage induced by the network it is executed over. We finally show how to merge this new mode into our improved algorithm, obtaining an efficient all-in-one mode for full anonymity and authenticity.

In the post-quantum migration of TLS 1.3, an ephemeral Diffie-Hellman must be replaced with a post-quantum key encapsulation mechanism (KEM). At EUROCRYPT 2022, Huguenin-Dumittan and Vaudenay [EC:HugVau22] demonstrated that KEMs with standard CPA security are sufficient for the security of the TLS1.3 handshake. However, their result is only proven in the random oracle model (ROM), and as the authors comment, their reduction is very much non-tight and not sufficient to guarantee security in practice due to the $O(q^6)$-loss, where $q$ is the number of adversary’s queries to random oracles. Moreover, in order to analyze the post-quantum security of TLS 1.3 handshake with a KEM, it is necessary to consider the security in the quantum ROM (QROM). Therefore, they leave the tightness improvement of their ROM proof and the QROM proof of such a result as an interesting open question.

In this paper, we resolve this problem. We improve the ROM proof in [EC:HugVau22] from an $O(q^6)$-loss to an $O(q)$-loss with standard CPA-secure KEMs which can be directly obtained from the underlying public-key encryption (PKE) scheme in CRYSTALS-Kyber. Moreover, we show that if the KEMs are constructed from rigid deterministic public-key encryption (PKE) schemes such as the ones in Classic McElieceand NTRU, this $O(q)$-loss can be further improved to an $O(1)$-loss. Hence, our reductions are sufficient to guarantee security in practice. According to our results, a CPA-secure KEM (which is more concise and efficient than the currently used CCA/1CCA-secure KEM) can be directly employed to construct a post-quantum TLS 1.3. Furthermore, we lift our ROM result into QROM and first prove that the CPA-secure KEMs are also sufficient for the post-quantum TLS 1.3 handshake. In particular, the techniques introduced to improve reduction tightness in this paper may be of independent interest.

The impossible differential (ID) attack is one of the most important cryptanalytic techniques for block ciphers. There are two phases to finding an ID attack: searching for the distinguisher and building a key recovery upon it. Previous works only focused on automated distinguisher discovery, leaving key recovery as a manual post-processing task, which may lead to a suboptimal final complexity. At EUROCRYPT~2023, Hadipour et al. introduced a unified constraint programming (CP) approach based on satisfiability for finding optimal complete ID attacks in strongly aligned ciphers. While this approach was extended to weakly-aligned designs like PRESENT at ToSC~2024, its application to ARX and AndRX ciphers remained as future work. Moreover, this method only exploited ID distinguishers with direct contradictions at the junction of two deterministic transitions. In contrast, some ID distinguishers, particularly for ARX and AndRX designs, may not be detectable by checking only the existence of direct contradictions.

This paper fills these gaps by extending Hadipour et al.'s method to handle indirect contradictions and adapting it for ARX and AndRX designs. We also present a similar method for identifying zero-correlation (ZC) distinguishers. Moreover, we extend our new model for finding ID distinguishers to a unified optimization problem that includes both the distinguisher and the key recovery for AndRX designs. Our method improves ID attacks and introduces new distinguishers for several ciphers, such as SIMON, SPECK, Simeck, ChaCha, Chaskey, LEA, and SipHash. For example, we achieve a one-round improvement in the ID attacks against SIMON-64-96, SIMON-64-128, SIMON-128-128, SIMON-128-256 and a two-round improvement in the ID attacks against SIMON-128-192. These results significantly contribute to our understanding of the effectiveness of automated tools in the cryptanalysis of different design paradigms.

Sieving using near-neighbor search techniques is a well-known method in lattice-based cryptanalysis, yielding the current best runtime for the shortest vector problem in both the classical [BDGL16] and quantum [BCSS23] setting. Recently, sieving has also become an important tool in code-based cryptanalysis. Specifically, using a sieving subroutine, [GJN23, DEEK24] presented a variant of the information-set decoding (ISD) framework, which is commonly used for attacking cryptographically relevant instances of the decoding problem. The resulting sieving-based ISD framework yields complexities close to the best-performing classical algorithms for the decoding problem such as [BJMM12, BM18]. It is therefore natural to ask how well quantum versions perform. In this work, we introduce the first quantum algorithms for code sieving by designing quantum variants of the aforementioned sieving subroutine. In particular, using quantum-walk techniques, we provide a speed-up over the best known classical algorithm from [DEEK24] and over a variant using Grover's algorithm [Gro96]. Our quantum-walk algorithm exploits the structure of the underlying search problem by adding a layer of locality-sensitive filtering, inspired by the quantum-walk algorithm for lattice sieving from [CL21]. We complement our asymptotic analysis of the quantum algorithms with numerical results, and observe that our quantum speed-ups for code sieving behave similarly as those observed in lattice sieving. In addition, we show that a natural quantum analog of the sieving-based ISD framework does not provide any speed-up over the first presented quantum ISD algorithm [Ber10]. Our analysis highlights that the framework should be adapted in order to outperform the state-of-the-art of quantum ISD algorithms [KT17, Kir18].

Cryptocurrencies have gained high popularity in recent years, with over 9000 of them, including major ones such as Bitcoin and Ether. Each cryptocurrency is implemented on one blockchain or over several such networks. Recently, various technologies known as blockchain interoperability have been developed to connect these different blockchains and create an interconnected blockchain ecosystem. This paper aims to provide insights on the blockchain ecosystem and the connection between blockchains that we refer to as the interoperability graph. Our approach is based on the analysis of the correlation between cryptocurrencies implemented over the different blockchains. We examine over 4800 cryptocurrencies implemented on 76 blockchains and their daily prices over a year. This experimental study has potential implications for decentralized finance (DeFi), including portfolio investment strategies and risk management.

Due to the ubiquitous requirements and performance leap in the past decade, it has become feasible to execute garbling and secure computations in settings sensitive to side-channel attacks, including smartphones, IoTs and dedicated hardwares, and the possibilities have been demonstrated by recent works. To maintain security in the presence of a moderate amount of leaked information about internal secrets, we investigate {\it leakage-resilient garbling}. We augment the classical privacy, obliviousness and authenticity notions with leakages of the garbling function, and define their leakage-resilience analogues. We examine popular garbling schemes and unveil additional side-channel weaknesses due to wire label reuse and XOR leakages. We then incorporate the idea of label refreshing into the GLNP garbling scheme of Gueron et al. and propose a variant GLNPLR that provably satisfies our leakage-resilience definitions. Performance comparison indicates that GLNPLR is 60X (using AES-NI) or 5X (without AES-NI) faster than the HalfGates garbling with second order side-channel masking, for garbling AES circuit when the bandwidth is 2Gbps.

The Paillier cryptosystem is renowned for its applications in electronic voting, threshold ECDSA, multi-party computation, and more, largely due to its additive homomorphism. In these applications, range proofs for the Paillier cryptosystem are crucial for maintaining security, because of the mismatch between the message space in the Paillier system and the operation space in application scenarios.

In this paper, we present novel range proofs for the Paillier cryptosystem, specifically aimed at optimizing those for both Paillier plaintext and affine operation. We interpret encryptions and affine operations as commitments over integers, as opposed to solely over $\mathbb{Z}_{N}$. Consequently, we propose direct range proof for the updated cryptosystem, thereby eliminating the need for auxiliary integer commitments as required by the current state-of-the-art. Our work yields significant improvements: In the range proof for Paillier plaintext, our approach reduces communication overheads by approximately $60\%$, and computational overheads by $30\%$ and $10\%$ for the prover and verifier, respectively. In the range proof for Paillier affine operation, our method reduces the bandwidth by $70\%$, and computational overheads by $50\%$ and $30\%$ for the prover and verifier, respectively. Furthermore, we demonstrate that our techniques can be utilized to improve the performance of threshold ECDSA and the DCR-based instantiation of the Naor-Yung CCA2 paradigm.

We provide a novel perspective on a long-standing challenge to the integrity of votes cast without the supervision of a voting booth: "improper influence,'' which we define as any combination of vote buying and voter coercion. In comparison with previous proposals, our system is the first in the literature to protect against a strong adversary who learns all of the voter's keys---we call this property "extreme coercion resistance.'' When keys are stolen, each voter, or their trusted agents (which we call "hedgehogs''), may "nullify'' (effectively cancel) their vote in a way that is unstoppable and irrevocable, and such that the nullification action is forever unattributable to that voter or their hedgehog(s). We demonstrate the security of our VoteXX system in the universal composability model.

As in many other coercion-resistant systems, voters are authorized to vote with public-private keys. Each voter registers their public keys with the Election Authority (EA) in a way that convinces the EA that the voter has memorized a passphrase that corresponds to their private keys. As a consequence, if an adversary obtains a voter's keys, the voter also retains a copy. Voters concerned about adversaries stealing their private keys can themselves, or by delegating to one or more untrusted hedgehog(s), monitor the bulletin board for malicious ballots cast with their keys, and can act to nullify these ballots in a privacy-preserving manner with zero-knowledge proofs.

In comparison with previous proposals, our system offers some protection against even the strongest adversary who learns all keys. Other coercion-resistant protocols either do not address these attacks, place strong limitations on adversarial abilities, or rely on fully trusted parties to assist voters with their keys.

When integer and rational arithmetics are performed using modular arithmetics over $\mathbb{Z}/q\mathbb{Z}$, overflows naturally occur due to the mismatch between the infinite cardinality of $\mathbb{Z}$ or $\mathbb{Q}$ and the finite cardinality of $\mathbb{Z}/q\mathbb{Z}$. Since $\mathbb{Z}/q\mathbb{Z}$ is also the (sub) message space for many secure computation designs, secure computations of integer and rational arithmetics using these schemes must also consider the overflow problem.

Previous works [CLPX, CT-RSA'18] and [HDRdS, ACNS'23] perform integer and rational arithmetics using the CLPX homomorphic encryption scheme, where overflows are avoided by restricting supported circuits. This introduces an additional constraint beyond the noise budget limitation. In our work, we discuss the possibilities of tolerating overflows. Firstly, we explain that when input messages and the final result are well-bounded, intermediate values can go arbitrarily large without affecting output correctness. This kind of overflow is called pseudo-overflow and does not need to be avoided. Secondly, we note that for prime-power modulus $q=p^r$, overflow errors are small in the $p$-adic norm. Therefore, we apply the $p$-adic encoding technique in [HDRdS, ACNS'23] to the BGV/BFV homomorphic encryption scheme with plaintext modulus $p^r$. Compared to [CLPX, CT-RSA'18] and [HDRdS, ACNS'23], our method supports circuits that are up to $2 \times$ deeper under the same ciphertext parameters, at the cost of an output error bounded by $p^{-r}$ in the $p$-adic norm.

Attribute-based encryption (ABE) is a powerful primitive that has found applications in important real-world settings requiring access control. Compared to traditional public-key encryption, ABE has established itself as a considerably more complex primitive that is additionally less efficient to implement. It is therefore paramount that the we can simplify the design of ABE schemes that are efficient, provide strong security guarantees, minimize the complexity in their descriptions and support all practical features that are desirable for common real-world settings. One of such practical features that is currently still difficult to achieve is multi-authority support. Motivated by NIST's ongoing standardization efforts around multi-authority schemes, we put a specific focus on simplifying the support of multiple authorities in the design of schemes.

To this end, we present ISABELLA, a framework for constructing pairing-based ABE with advanced functionalities under strong security guarantees. At a high level, our approach builds on various works that systematically and generically construct ABE schemes by reducing the effort of proving security to a simpler yet powerful ''core'' called pair encodings. To support the amount of adaptivity required by multi-authority ABE, we devise a new approach to designing schemes from pair encodings, while still being able to benefit from the advantages that pair encodings provide. As a direct result of our framework, we obtain various improvements for existing (multi-authority) schemes as well as new schemes.

A linear error-correcting code exhibits proximity gaps if each affine line of words either consists entirely of words which are close to the code or else contains almost no such words. In this short note, we prove that for each linear code which exhibits proximity gaps within the unique decoding radius, that code's interleaved code also does. Combining our result with an argument suggested to us by Angeris, Evans and Roh ('24), we extend those authors' sharpening of the tensor-based proximity gap of Diamond and Posen (Commun. Cryptol. '24) up to the unique decoding radius, at least in the Reed–Solomon setting.

This short note describes an update to the sca25519 library, an ECC implementation computing the X25519 key-exchange protocol on the Arm Cortex-M4 microcontroller. The sca25519 software came with extensive mitigations against various side-channel and fault attacks and was, to our best knowledge, the first to claim affordable protection against multiple classes of attacks that are motivated by distinct real-world application scenarios.

This library is protected against various passive and active side-channel threats. However, both classes of attacks were considered separately, i.e., combining the attacks is considered out-of-scope because to successfully execute such a combined attack, the adversary would need to be very powerful (e.g., a very well-equipped security laboratory). Protection against such powerful adversaries is considered infeasible without using dedicated protected hardware with which Arm Cortex-M4 is not equipped.

However, there exists a particular class of easy and cheap active attacks: they are called tearing, and they are well known in the smartcard context. In this paper, we extend the scope of the library to also consider a combination of tearing and side-channel attacks. In this note, we show how we can mitigate such a combination by performing a small code update. The update does not affect the efficiency of the library.

Privacy set intersection (PSI) and private information retrieval (PIR) are important areas of research in privacy protection technology. One of the key tools for both is the oblivious pseudorandom function (OPRF). Currently, existing oblivious pseudorandom functions either focus solely on efficiency without considering quantum attacks, or are too complex, resulting in low efficiency. The aim of this paper is to achieve a balance: to ensure that the oblivious pseudorandom function can withstand quantum attacks while simplifying its structure as much as possible. This paper constructs an efficient oblivious pseudorandom function based on the ideal lattice hardness assumption and the oblivious transfer (OT) technique by Chase and Miao (CRYPTO 2020), and also constructs PSI and PIR.