International Association
for Cryptologic Research

This paper introduces a new method for training decision trees and random forests using CKKS homomorphic encryption (HE) in cloud environments, enhancing data privacy from multiple sources. The innovative Homomorphic Binary Decision Tree (HBDT) method utilizes a modified Gini Impurity index (MGI) for node splitting in encrypted data scenarios. Notably, the proposed training approach operates in a single cloud security domain without the need for decryption, addressing key challenges in privacy-preserving machine learning. We also propose an efficient method for inference utilizing only addition for path evaluation even when both models and inputs are encrypted, achieving O(1) multiplicative depth. Experiments demonstrate that this method surpasses the previous study by Akavia et al.'s by at least 3.7 times in the speed of inference. The study also expands to privacy-preserving random forests, with GPU acceleration ensuring feasibly efficient performance in both training and inference.

Determining the complexity of computing Gröbner bases is an important problem both in theory and in practice, and for that the solving degree plays a key role. In this paper, we study the solving degrees of affine semi-regular sequences and their homogenized sequences. Some of our results are considered to give mathematically rigorous proofs of the correctness of methods for computing Gröbner bases of the ideal generated by an affine semi-regular sequence. This paper is a sequel of the authors’ previous work and gives additional results on the solving degrees and important behaviors of Gröbner basis computation.

Recent improvements to garbled circuits are mainly focused on reducing their size. The state-of-the-art construction of Rosulek and Roy (Crypto 2021) requires $1.5\kappa$ bits for garbling AND gates in the free-XOR setting. This is below the previously proven lower bound $2\kappa$ in the linear garbling model of Zahur, Rosulek, and Evans (Eurocrypt 2015).

Recently, Ashur, Hazay, and Satish (eprint 2024/389) proposed a scheme that requires $4/3\kappa + O(1)$ bits for garbling AND gates. Precisely they extended the idea of slicing introduced by Rosulek and Roy to garble 3-input gates of the form $g(u,v,w) := u(v+w)$. By setting $w = 0$, it can be used to garble AND gates with the improved communication costs.

However, in this paper, we observe that the scheme proposed by Ashur, Hazy, and Satish leaks information on the permute bits, thereby allowing the evaluator to reveal information on the private inputs. To be precise, we show that in their garbling scheme, the evaluator can compute the bits $\alpha$ and $\beta + \gamma$, where $\alpha$, $\beta$, and $\gamma$ are the private permute bits of the input labels $A$, $B$, and $C$, respectively.

Fischlin's transform (CRYPTO 2005) is an alternative to the Fiat-Shamir transform that enables straight-line extraction when proving knowledge. In this work we focus on the problem of using the Fischlin transform to construct UC-secure zero-knowledge from Sigma protocols, since UC security -- that guarantees security under general concurrent composition -- requires straight-line (non-rewinding) simulators. We provide a slightly simplified transform that is much easier to understand, and present algorithmic and implementation optimizations that significantly improve the running time. It appears that the main obstacles to the use of Fischlin in practice is its computational cost and implementation complexity (with multiple parameters that need to be chosen). We provide clear guidelines and a simple methodology for choosing parameters, and show that with our optimizations the running-time is far lower than expected. For just one example, on a 2023 MacBook, the cost of proving the knowledge of discrete log with Fischlin is only 0.41ms (on a single core). We also extend the transform so that it can be applied to batch proofs, and show how this can be much more efficient than individually proving each statement. As a contribution of independent interest, we present a new algorithm for polynomial evaluation on any series of sequential points that does not require roots of unity. We hope that this paper will both encourage and help practitioners implement the Fischlin transform where relevant.

Biometric authentication eliminates the need for users to remember secrets and serves as a convenient mechanism for user authentication. Traditional implementations of biometric-based authentication store sensitive user biometry on the server and the server becomes an attractive target of attack and a source of large-scale unintended disclosure of biometric data. To mitigate the problem, we can resort to privacy-preserving computation and store only protected biometrics on the server. While a variety of secure computation techniques is available, our analysis of privacy-preserving biometric computation and biometric authentication constructions revealed that available solutions fall short of addressing the challenges of privacy-preserving biometric authentication. Thus, in this work we put forward new constructions to address the challenges.

Our solutions employ a helper server and use strong threat models, where a client is always assumed to be malicious, while the helper server can be semi-honest or malicious. We also determined that standard secure multi-party computation security definitions are insufficient to properly demonstrate security in the two-phase (enrollment and authentication) entity authentication application. We thus extend the model and formally show security in the multi-phase setting, where information can flow from one phase to another and the set of participants can change between the phases. We implement our constructions and show that they exhibit practical performance for authentication in real time.

The sumcheck protocol is an interactive protocol for verifying the sum of a low-degree polynomial over a hypercube. This protocol is widely used in practice, where an efficient implementation of the (honest) prover algorithm is paramount. Prior work contributes highly-efficient prover algorithms for the notable special case of multilinear polynomials (and related settings): [CTY11] uses logarithmic space but runs in superlinear time; in contrast, [VSBW13] runs in linear time but uses linear space. In this short note, we present a family of prover algorithms for the multilinear sumcheck protocol that offer new time-space tradeoffs. In particular, we recover the aforementioned algorithms as special cases. Moreover, we provide an efficient implementation of the new algorithms, and our experiments show that the asymptotics translate into new concrete efficiency tradeoffs.

In "Keeping up with the KEMs" Cremers et al. introduced various binding models for KEMs. The authors show that ML-KEM is LEAK-BIND-K-CT and LEAK-BIND-K-PK, i.e. binding the ciphertext and the public key in the case of an adversary having access, but not being able to manipulate the key material. They further conjecture that ML-KEM also has MAL-BIND-K-PK, but not MAL-BIND-K-CT, the binding of public key or ciphertext to the shared secret in the case of an attacker with the ability to manipulate the key material. This short paper demonstrates that ML-KEM does neither have MALBIND-K-CT nor MAL-BIND-K-PK, due to the attacker being able to produce mal-formed private keys, giving concrete examples for both. We also suggest mitigations, and sketch a proof for binding both ciphertext and public key when the attacker is not able to manipulate the private key as liberally.

In their paper, Wei et al. proposed a lightweight protocol for conditional privacy-preserving authentication in VANET. The protocol aims to achieve ultra-low transmission delay and efficient system secret key (SSK) updating. Their protocol uses a signature scheme with message recovery to authenticate messages. This scheme provides security against adaptively chosen message attacks. However, our analysis reveals a critical vulnerability in the scheme. It is susceptible to replay attacks, meaning a malicious vehicle can replay a message multiple times at different timestamps. This action undermines the overall effectiveness of conditional privacy. We suggest possible solutions to address these vulnerabilities and enhance the security of VANET communication.

In this paper, we propose a novel isogeny-based public key encryption (PKE) scheme named LIT-SiGamal. This is based on a LIT diagram and SiGamal. SiGamal is an isogeny-based PKE scheme that uses a commutative diagram with an auxiliary point. LIT-SiGamal uses a LIT diagram which is a commutative diagram consisting of large-degree horizontal isogenies and relatively small-degree vertical isogenies, while the original SiGamal uses a CSIDH diagram.

A strength of LIT-SiGamal is efficient encryption and decryption. QFESTA is an isogeny-based PKE scheme proposed by Nakagawa and Onuki, which is a relatively efficient scheme in isogeny-based PKE schemes. In our experimentation with our proof-of-concept implementation, the computational time of the encryption of LIT-SiGamal is as efficient as that of QFESTA, and that of the decryption of LIT-SiGamal is about $5$x faster than that of QFESTA.

We describe a small tweak to Cuckoo filters that allows securing them under insertions using the techniques from Filić et al. (ACM CCS 2022), without the need for an outer PRF call.

Given that the tropical Stickel protocol and its variants are all vulnerable to the generalized Kotov-Ushakov attack, we suggest employing the max-min semiring and, more generally, max-$T$ semiring where the multiplication is based on a $T-$norm, as a framework to implement the Stickel protocol. While the Stickel protocol over max-min semiring or max-$T$ semiring remains susceptible to a form of Kotov-Ushakov attack, we demonstrate that it exhibits significantly increased resistance against this attack when compared to the tropical (max-plus) implementation.

Cryptographic agility, or crypto-agility, is a design feature that enables agile updates to new cryptographic algorithms and standards without the need to modify or replace the surrounding infrastructure. This paper examines the prerequisites for crypto-agility and proposes its desired design feature. More specifically, we investigate the design characteristics of widely deployed cybersecurity paradigms, i.e., zero trust, and apply its design feature to crypto-agility, achieving greater visibility and automation in cryptographic management.

Biextensions associated to line bundles on abelian varieties allows to reinterpret the usual Weil, Tate, Ate, optimal Ate, \ldots, pairings as monodromy pairings. We introduce a cubical arithmetic, derived from the canonical cubical torsor structure of these line bundles, to obtain an efficient arithmetic of these biextensions.

This unifies and extends Miller's standard algorithm to compute pairings along with other algorithms like elliptic nets and theta functions, and allows to adapt these algorithms to pairings on any model of abelian varieties with a polarisation $\Phi_D$, as long as we have an explicit theorem of the square for $D$.

In particular, we give explicit formulas for the arithmetic of the biextension (and cubical torsor structure) associated to the divisor $D=2(0_E)$ on an elliptic curve. We derive very efficient pairing formulas on elliptic curves and Kummer lines. Notably for generic pairings on Montgomery curves, our cubical biextension ladder algorithm to compute pairings costs only $15M$ by bits, which as far as I know is faster than any pairing doubling formula in the literature.

Searchable symmetric encryption (SSE) schemes provide users with the ability to perform keyword searches on encrypted databases without the need for decryption. While this functionality is advantageous, it introduces the potential for inadvertent information disclosure, thereby exposing SSE systems to various types of attacks. In this work, we introduce a new inference attack aimed at enhancing the query recovery accuracy of RefScore (presented at USENIX 2021). The proposed approach capitalizes on both similar data knowledge and an additional volume leakage as auxiliary information, facilitating the extraction of keyword matches from leaked data. Empirical evaluations conducted on multiple real-world datasets demonstrate a notable enhancement in query recovery accuracy, up to 19.5%. We also analyze the performance of the proposed attack in the presence of diverse countermeasures.

Searchable symmetric encryption has been vulnerable to inference attacks that rely on uniqueness in leakage patterns. However, many keywords in datasets lack distinctive leakage patterns, limiting the effectiveness of such attacks. The file injection attacks, initially proposed by Cash et al. (CCS 2015), have shown impressive performance with 100% accuracy and no prior knowledge requirement. Nevertheless, this attack fails to recover queries with underlying keywords not present in the injected files. To address these limitations, our research introduces a novel attack strategy called LEAP-Hierarchical Fusion Attack (LHFA) that combines the strengths of both file injection attacks and inference attacks. Before initiating keyword injection, we introduce a new approach for inert/active keyword selection. In the phase of selecting injected keywords, we focus on keywords without unique leakage patterns and recover them, leveraging their presence for document recovery. Our goal is to achieve an amplified effect in query recovery. We demonstrate a minimum query recovery rate of 1.3 queries per injected keyword with a 10% data leakage of a real-life dataset, and initiate further research to overcome challenges associated with non-distinctive keywords.

Zero-Knowledge Proof (ZKP) technology marks a revolutionary advancement in the field of cryptography, enabling the verification of certain information ownership without revealing any specific details. This technology, with its paradoxical yet powerful characteristics, provides a solid foundation for a wide range of applications, especially in enhancing the privacy and security of blockchain technology and other cryptographic systems. As ZKP technology increasingly becomes a part of the blockchain infrastructure, its importance for security and integrity becomes more pronounced. However, the complexity of ZKP implementation and the rapid iteration of the technology introduce various vulnerabilities, challenging the privacy and security it aims to offer.

This study bases on the integrity, soundness, and zero-knowledge properties of ZKP to meticulously classify existing vulnerabilities and deeply explores multiple categories of vulnerabilities, including integrity issues, soundness problems, information leakage, and non-standardized cryptographic implementations. Furthermore, we propose a set of defense strategies that include a rigorous security audit process and a robust distributed network security ecosystem. This audit strategy employs a divide-and-conquer approach, segmenting the project into different levels, from the application layer to the platform-nature infrastructure layer, using threat modeling, linear code checking, and internal cross-review, among other means, aimed at comprehensively identifying vulnerabilities in ZKP circuits, revealing design flaws in ZKP applications, and accurately identifying inaccuracies in the integration process of ZKP primitives.

Event date: 9 December to 13 December 2024
Submission deadline: 28 May 2024
Notification: 25 August 2024

Event date: 1 July to 5 July 2024

Quantum computers have the potential to solve hard problems that are nearly impossible to solve by classical computers, this has sparked a surge of research to apply quantum technology and algorithm against the cryptographic systems to evaluate for its quantum resistance. In the process of selecting post-quantum standards, NIST categorizes security levels based on the complexity that quantum computers would require to crack AES encryption (levels 1, 3, and 5) and SHA-2 or SHA-3 (levels 2 and 4).

In assessing the security strength of cryptographic algorithms against quantum threats, accurate predictions of quantum resources are crucial. Following the work of Jaques et al. in Eurocrypt 2020, NIST estimated security levels 1, 3, and 5, corresponding to quantum circuit size for finding the key for AES-128, AES-192, and AES-256, respectively. This work has been recently followed-up by Huang et al. (Asiacrypt'22) and Liu et al. (Asiacrypt'23). However, for levels 2 and 4, which relate to the collision finding for the SHA-2 and SHA-3 hash functions, quantum attack complexities are probably not well-studied.

In this paper, we present novel techniques for optimizing the quantum circuit implementations for SHA-2 and SHA-3 algorithms in all the categories specified by NIST. After that, we evaluate the quantum circuits of target cryptographic hash functions for quantum collision search. Finally, we define the optimal quantum attack complexity for levels 2 and 4, and comment on the security strength of the extended level. We present new concepts to optimize the quantum circuits at the component level and the architecture level.

As the National Institute of Standards and Technology (NIST) concludes its post-quantum cryptography (PQC) competition, the winning algorithm, Dilithium, enters the deployment phase in 2024. This phase underscores the importance of conducting thorough practical security evaluations. Our study offers an in-depth side-channel analysis of Dilithium, showcasing the ability to recover the complete private key, ${s}_1$, within ten minutes using just two signatures and achieving a 60 success rate with a single signature. We focus on analyzing the polynomial addition in Dilithium, $z=y+{cs}_1$, by breaking down the attack into two main phases: the recovery of $y$ and ${cs}_1$ through side-channel attacks, followed by the resolution of a system of error-prone equations related to ${cs}_1$. Employing Linear Regression-based profiled attacks enables the successful recovery of the full $y$ value with a 40% success rate without the necessity for initial filtering. The extraction of ${cs}_1$ is further improved using a CNN model, which boasts an average success rate of 75%. A significant innovation of our research is the development of a constrained optimization-based residual analysis technique. This method efficiently recovers ${s}_1$ from a large set of error-containing equations concerning ${cs}_1$, proving effective even when only 10% of the equations are accurate. We conduct a practical attack on the Dilithium2 implementation on an STM32F4 platform, demonstrating that typically two signatures are sufficient for complete private key recovery, with a single signature sufficing in optimal conditions. Using a general-purpose PC, the full private key can be reconstructed in ten minutes.