International Association
for Cryptologic Research

Mix networks (mixnets) offer robust anonymity even against adversaries monitoring all network links; however, they impose high latency on communications. To address this, recent research has explored strategic low-latency routing within mixnets. While these strategies appear to reduce latency, their impact on mixnet anonymity has not been carefully assessed, raising concerns about potential deanonymization of clients. Tackling this challenge, this paper first quantifies the anonymity loss associated with low-latency routing techniques in mixnets. Building on these insights, second, we introduce a novel low-latency routing method that maintains mixnet anonymity while achieving significant latency reductions compared to the state-of-the-art solution LARMix (NDSS, 2024). Our approach also ensures a more balanced load distribution among mixnet nodes. Moreover, under adversarial conditions where parts of the mixnet are compromised, our method does not confer significant advantages to the adversary, unlike LARMix. Thus, our proposal emerges as the optimal choice for low-latency routing in mixnets. Furthermore, we note that this version expands on both the analytical and experimental results of the previously published paper in NCA 2024, specifically through investigating the anonymity of Loopix-like mixnets.

Reverse firewalls (RFs), introduced by Mironov and Stephens Davidowitz at Eurocrypt 2015, provide a defence mechanism for cryptographic protocols against subversion attacks. In a subversion setting, an adversary compromises the machines of honest parties, enabling the leakage of their secrets through the protocol transcript. Previous research in this area has established robust guarantees, including resistance against data exfiltration for an RF. In this work, we present a new perspective focused on the implementation specifics of RFs. The inherently untrusted nature of RFs exposes their real-world implementations to the risk of Trojan insertion — an especially pressing issue in today’s outsourced supply chain ecosystem. We argue how Trojan-affected RF implementations can compromise their core exfiltration resistance property, leading to a complete breakdown of the RF’s security guarantees.

Building on this perspective, we propose an enhanced definition for ``Trojan-resilient Reverse Firewalls'' (Tr-RF), incorporating an additional Trojan resilience property. We then present concrete instantiations of Tr-RFs for Coin Tossing (CT) and Oblivious Transfer (OT) protocols, utilizing techniques from Private Circuit III (CCS'16) to convert legacy RFs into Tr-RFs. We also give simulation-based proofs to claim the enhanced security guarantees of our Tr-RF instantiations. Additionally, we offer concrete implementations of our Tr-RF based CT and OT protocols utilizing the Open-Portable Trusted Execution Environment (OP-TEE). Through OP-TEE, we practically realize assumptions made in Private Circuit III that are critical to ensuring Tr-RF security, bridging the gap between theoretical models and real-world applications. To the best of our knowledge, this provides the first practical implementation of reverse firewalls for any cryptographic functionality. Our work emphasizes the importance of evaluating protocol specifications within implementation-specific contexts.

A group signature scheme enables users of a group to anonymously sign messages on behalf of the group, while a designated authority can revoke anonymity when needed to ensure user accountability. Among group signature schemes, fully dynamic ones are particularly desirable as they allow new users to join and misbehaved existing users to be revoked without requiring system-wide updates. This paper introduces DGSP, a post-quantum fully dynamic group signature scheme that addresses key limitations of existing schemes like DGMT and SPHINX-in-the-Head (SITH). Leveraging the properties of ${\rm SPHINCS}^+$, DGSP achieves a superior balance between scalability and efficiency by (i) supporting up to $2^{60}$ users, (ii) requiring negligible setup time, and (iii) featuring efficient algorithms with short signatures of constant-size. Notably, while DGMT is limited to $2^{15}$ users, DGSP extends this to $2^{60}$ while keeping signatures compact—only 3.03 to 4.93 times larger than those of DGMT, yet just 0.021 to 0.004 times the size of SITH signatures. This makes DGSP a compelling solution for applications requiring both large-scale user support and signature efficiency in the post-quantum setting. Moreover, DGSP strengthens managerial accountability compared to DGMT by (i) enabling users to verify their certificates generated by the manager and (ii) ensuring public verifiability of the manager's signature attribution. Although SITH claims to support $2^{60}$ users, our analysis reveals that its opening algorithm is highly inefficient, making it impractical to handle such a large number of users. Our security analysis shows that DGSP achieves unforgeability, anonymity, and traceability in the standard model. We also provide a complete implementation of DGSP. Our experimental study exhibits that DGSP is superior over existing schemes in terms of efficiency and scalability.

It is a widely accepted standard practice to implement cryptographic software so that secret inputs do not influence the cycle count. Software following this paradigm is often referred to as "constant-time" software and typically involves following three rules: 1) never branch on a secret-dependent condition, 2) never access memory at a secret-dependent location, and 3) avoid variable-time arithmetic operations on secret data. The third rule requires knowledge about such variable-time arithmetic instructions, or vice versa, which operations are safe to use on secret inputs. For a long time, this knowledge was based on either documentation or microbenchmarks, but critically, there were never any guarantees for future microarchitectures. This changed with the introduction of the data-operand-independent-timing (DOIT) mode on Intel CPUs and, to some extent, the data-independent-timing (DIT) mode on Arm CPUs. Both Intel and Arm document a subset of their respective instruction sets that are intended to leak no information about their inputs through timing, even on future microarchitectures if the CPU is set to run in a dedicated DOIT (or DIT) mode. In this paper, we present a principled solution that leverages DOIT to enable cryptographic software that is future-proof constant-time, in the sense that it ensures that only instructions from the DOIT subset are used to operate on secret data, even during speculative execution after a mispredicted branch or function return location. For this solution, we build on top of existing security type systems in the Jasmin framework for high-assurance cryptography. We then use our solution to evaluate the extent to which existing cryptographic software built to be "constant-time" is already secure in this stricter paradigm implied by DOIT and what the performance impact is to move from constant-time to future-proof constant-time.

AES-GCM has been the status quo for efficient symmetric encryption for decades. As technology and cryptographic applications evolved over time, AES-GCM has posed some challenges to certain use-cases due to its default 96-bit nonce size, 128-bit block size, and lack of key commitment. Nonce-derived schemes are one way of addressing these challenges: Such schemes derive multiple keys from nonce values, then apply standard AES-GCM with the derived keys (and possibly another 96-bit nonce). The approach overcomes the nonce length and data limit issues since each derived key is only used to encrypt a few messages. By itself, the use of nonce-derived keys does not address key commitment, however. Some schemes chose to include a built-in key commitment mechanism, while others left it out of scope.

In this work, we explore efficient key commitment methods that can be added to any nonce-derived scheme in a black-box manner. Our focus is on options that use the underlying block cipher and no other primitive, are efficient, and only use standard primitives which are FIPS-approved. For concreteness we focus here specifically on adding key commitment to XAES-256-GCM, a nonce-scheme originally proposed by Filippo Valsorda, but these methods can be adapted to any other nonce-derived scheme. We propose an efficient CMAC-based key commitment solution, and prove its security in the ideal-cipher model. We argue that adding this solution yields a FIPS-compliant mode, quantify the data and message length limits of this mode and compare this combination to other nonce-derived modes. We also benchmark our key committing XAES-256-GCM performance.

Threshold public-key encryption securely distributes private key shares among multiple participants, requiring a minimum number of them to decrypt messages. We introduce a quantum-resistant threshold public-key encryption scheme based on the code-based Niederreiter cryptosystem that achieves security against chosen ciphertext attacks. A previous attempt was made recently by Takahashi, Hashimoto, and Ogata (published at DCC in 2023) but we show that it contains a critical security flaw that allow adversaries to exploit malformed ciphertexts to gain information about the secret key.

In this work, we formalize a generic conversion enhancing security of (classical) public-key encryption from one-wayness against passive attacks to indistinguishability against chosen-ciphertext attacks. The conversion uses a non-interactive zero-knowledge argument with strong security properties to ensure ciphertext well-formedness. We then provide an instantiation for Niederreiter encryption based on recent techniques introduced in the "MPC-in-the-head" paradigm. The publicly verifiable validity of ciphertexts makes this scheme suitable for threshold public-key encryption and prevents an attack similar to the one on Takahashi-Hashimoto-Ogata scheme. To improve the multi-party computation protocol for decryption (involving secure computations on polynomials), we introduce a field-switching technique that allows to significantly reduce the shared secret key size and computational overhead.

This work addresses the hintless single-server Private Information Retrieval (PIR) from the perspective of high-level protocol design and introduces PIRCOR and PIRCOR$^{*}$ that outperform the state-of-the-art PIRANA (Liu et. al., IEEE S&P 2024) and YPIR (Menon and Wu, USENIX Security 2024) in terms of the query size and the query generation time. In PIRCOR, we construct an efficient Rotation-based Expanded Binary Code (REBC) to expand $\alpha$ primary codewords into $\beta$ expanded codewords by the Rotation-Mutual-Multiplication operation. By leveraging the innovative REBC, PIRCOR reduces the query size for single-query PIR by a factor of $\mathcal{O}\left(N^{\frac{\delta-1}{\delta}}\right)$ compared to PIRANA, while also avoiding the $\mathcal{O}(N +\frac{|\mathrm{DB}|}{N})$ linear scaling inherent in YPIR ($N$, $\delta$ and $|\mathrm{DB}|$ are the (R)LWE secret dimension, the number of codewords with a Hamming weight of $1$ and the number of database elements). Based on PIRCOR, we further present PIRCOR$^{*}$ by additionally introducing the Rotation-self-Multiplication operation, which achieves a $\mathbf{50\%}$ reduction in rotation operations and a smaller query size when $\delta = 2$. Building upon PIRCOR and PIRCOR$^{*}$, we further propose their optimized variants, PIRCOR-op and PIRCOR$^{*}$-op, to further reduce the online response time. Similar to YPIR that leverage pre-processing, PIRCOR-op and PIRCOR$^{*}$-op allow all rotations and part of multiplications to be carried out in an offline stage before receiving the query. Additionally, we also design FHE-operator acceleration with leveled optimization and implementation optimization of ciphertext rotation. For 8 KB element retrieval in an 8 GB database, PIRCOR achieves a $\mathbf{10.7\times}$ query size reduction compared to PIRANA. When benchmarked against YPIR, the improvements are even more striking: PIRCOR reduces the query size by $\mathbf{26.8\times}$ and accelerates query generation by a staggering $\mathbf{6,080\times}$. Notably, the enhanced PIRCOR$^{*}$ achieves a $\mathbf{53.6\times}$ reduction in query size compared to YPIR, while improving query generation time by an impressive $\mathbf{12,160\times}$.

We show that the data aggregation scheme [IEEE TDSC, 2023, 20(3), 2011-2024] is flawed, because the signer only signs a part of data, not the whole data. An adversary can replace the unsigned component to cheat the verifier. To frustrate this attack, all components of the target data should be concatenated together and then be hashed and signed, so as to ensure that the signature verification can prove the whole message integrity.

Let us assume that one of two trusted parties (administrator) manages the information system (IS) and another one (user) is going to use the resources of this IS during the certain time interval. So they need establish secure user’s access password to the IS resources of this system via selected authenticated key exchange protocol. So they need to communicate via insecure communication channel and secretly con-struct a cryptographically strong session key that can serve for the establishment of secure passwords in the form of tuples in certain alphabet during the certain time interval. Nowadays selected protocol has to be postquantum secure. We propose the implementation of this scheme in terms of Symbolic Computa-tions. The key exchange protocol is one of the key exchange algorithms of Noncommutative Cryptography with the platform of multivariate transformation of the affine space over selected finite commutative ring. The session key is a multivariate map on the affine space. Platforms and multivariate maps are construct-ed in terms of Algebraic Graph Theory.

Proof-carrying data (PCD) is a powerful cryptographic primitive for computational integrity in a distributed setting. State-of-the-art constructions of PCD are based on accumulation schemes (and, closely related, folding schemes). We present WARP, the first accumulation scheme with linear prover time and logarithmic verifier time. Our scheme is hash-based (secure in the random oracle model), plausibly post-quantum secure, and supports unbounded accumulation depth. We achieve our result by constructing an interactive oracle reduction of proximity that works with any linear code over a sufficiently large field. We take a novel approach by constructing a straightline extractor that relies on erasure correction, rather than error-tolerant decoding like prior extractors. Along the way, we introduce a variant of straightline round-by-round knowledge soundness that is compatible with our extraction strategy.

Smart contracts are integral to decentralized systems like blockchains and enable the automation of processes through programmable conditions. However, their immutability, once deployed, poses challenges when addressing errors or bugs. Existing solutions, such as proxy contracts, facilitate upgrades while preserving application integrity. Yet, proxy contracts bring issues such as storage constraints and proxy selector clashes - along with complex inheritance management. This paper introduces a novel upgradeable smart contract framework with version control, named "decentraLized vErsion control and updAte manaGement in upgrAdeable smart coNtracts (LEAGAN)." LEAGAN is the first decentralized updatable smart contract framework that employs data separation with Incremental Hash (IH) and Revision Control System (RCS). It updates multiple contract versions without starting anew for each update, and reduces time complexity, and where RCS optimizes space utilization through differentiated version control. LEAGAN also introduces the first status contract in upgradeable smart contracts, and which reduces overhead while maintaining immutability. In Ethereum Virtual Machine (EVM) experiments, LEAGAN shows 40\% better space utilization, 30\% improved time complexity, and 25\% lower gas consumption compared to state-of-the-art models. It thus stands as a promising solution for enhancing blockchain system efficiency.

Range counting is the problem of preprocessing a set $P\subset R^d$ of $n$ points, such that given a query range $\gamma$ we can efficiently compute $|P\cap\gamma|$. In the more general range searching problem the goal is to compute $f(P\cap\gamma)$, for some function $f$.

It was already shown (Kushnir et al. PETS'24) how to efficiently answer a range searching query under FHE using a technique they called Copy-and-Recurse to traverse partition trees.

In the Range emptiness problem the goal is to compute only whether $P\cap\gamma =\emptyset$. This was shown (in plaintext) to be done more efficiently. Range emptiness is interesting on its own and also used as a building block in other algorithms.

In this paper we improve and extend the results of Kushnir et al. First, for range searching we reduce the overhead term to the optimal $O(n)$, so for example if the ranges are halfspaces in $R^d$ bounded by hyperplanes then range searching can be done with a circuit of size $O(t\cdot n^{1-1/d+\varepsilon}+n)$, where $t$ is the size of the sub-circuit that checks whether a point lies under a hyperplane.

Second, we introduce a variation of copy-and-recurse that we call leveled copy-and-recurse. With this variation we improve range searching in the 1-dimensional case as well as traversal of other trees (e.g., binary trees and B-trees). Third, we show how to answer range emptiness queries under FHE more efficiently than range counting.

We implemented our algorithms and show that our techniques for range emptiness yield a solution that is $\times 3.6$ faster than the previous results for a database of $2^{25}$ points.

Secure rate-distortion-perception (RDP) trade-offs arise in critical applications, such as semantic compression and privacy-preserving generative coding, where preserving perceptual quality while minimizing distortion is vital. This paper studies a framework for secure RDP over noiseless and noisy broadcast channels under strong secrecy constraints. We first characterize the exact secure RDP region for noiseless transmission channels. We then develop an inner bound on the secure RDP region for a memoryless broadcast channel with correlated noise components at the receivers' observations and prove its tightness under a more capable broadcast channel assumption. Our results demonstrate how optimized binning schemes simultaneously achieve high perceptual quality, low distortion, and strong secrecy, illuminating fundamental information-theoretic limits for next-generation trustworthy computation systems.

Quantum computers leverage qubits to solve certain computational problems significantly faster than classical computers. This capability poses a severe threat to traditional cryptographic algorithms, leading to the rise of post-quantum cryptography (PQC) designed to withstand quantum attacks. FALCON, a lattice-based signature algorithm, has been selected by the National Institute of Standards and Technology (NIST) as part of its post-quantum cryptography standardization process. However, due to the computational complexity of PQC, especially in cloud-based environments, throughput limitations during peak demand periods have become a bottleneck, particularly for FALCON. In this paper, we introduce GOLF (GPU-accelerated Optimization for Lattice-based FALCON), a novel GPU-based parallel acceleration framework for FALCON. GOLF includes algorithm porting to the GPU, compatibility modifications, multi-threaded parallelism with distinct data, single-thread optimization for single tasks, and specific enhancements to the Fast Fourier Transform (FFT) module within FALCON. Our approach achieves unprecedented performance in FALCON acceleration on GPUs. On the NVIDIA RTX 4090, GOLF reaches a signature generation throughput of 42.02 kops/s and a signature verification throughput of 10,311.04 kops/s. These results represent a 58.05$\times$ / 73.14$\times$ improvement over the reference FALCON implementation and a 7.17$\times$ / 3.79$\times$ improvement compared to the fastest known GPU implementation to date. GOLF demonstrates that GPU acceleration is not only feasible for post-quantum cryptography but also crucial for addressing throughput bottlenecks in real-world applications.

The Classic McEliece key encapsulation mechanism (KEM), a candidate in the fourth-round post-quantum cryptography (PQC) standardization process by the National Institute of Standards and Technology (NIST), stands out for its conservative design and robust security guarantees. Leveraging the code-based Niederreiter cryptosystem, Classic McEliece delivers high-performance encapsulation and decapsulation, making it well-suited for various applications. However, there has not been a systematic implementation of Classic McEliece on GPU platforms. This paper presents the first high-performance implementation of Classic McEliece on NVIDIA GPUs. Firstly, we present the first GPU-based implementation of Classic McEliece, utilizing a ``CPU-GPU'' heterogeneous approach and a kernel fusion strategy. We significantly reduce global memory accesses, optimizing memory access patterns. This results in encapsulation and decapsulation performance of 28,628,195 ops/s and 3,051,701 ops/s, respectively, for McEliece348864. Secondly, core operations like Additive Fast Fourier Transforms (AFFT), and Transpose AFFT (TAFFT) are optimized. We introduce the concept of the (T)AFFT stepping chain and propose two universal schemes: Memory Access Stepping Strategy (MASS) and Layer-Fused Memory Access Stepping Strategy (LFMASS), which achieve a speedup of 30.56% and 38.37%, respectively, compared to the native GPU-based McEliece6960119 implementation. Thirdly, extensive experiments on the NVIDIA RTX4090 show significant performance gains, achieving up to 344$\times$ higher encapsulation and 125$\times$ higher decapsulation compared to the official CPU-based AVX implementation, decisively outperforming existing ARM Cortex-M4 and FPGA implementations.

Event date: 1 June to 16 June 2025

Event date: 16 December to 20 December 2025
Submission deadline: 10 July 2025
Notification: 30 September 2025

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Submission deadline: 25 April 2025
Notification: 17 May 2025

Event date: 6 June to

This work introduces Zemlyanika, a post-quantum IND-CCA secure key encapsulation mechanism based on the Module-LWE problem. The high-level design of Zemlyanika follows a well-known approach where a passively secure public-key encryption scheme is transformed into an actively secure key encapsulation mechanism using the Fujisaki-Okamoto transform.

Our scheme features three main elements: a power-of-two modulus, explicit rejection, and revised requirements for decapsulation error probability.

The choice of a power-of-two modulus is atypical for Module-LWE based schemes due to the unavailability of Number Theoretic Transform (NTT). However, we argue that this option offers advantages that are often underestimated. We employ explicit rejection because it is more efficient than implicit rejection. Recent works show that both types of rejection are equally secure, so we do not reduce the security by this choice. Finally, we present compelling arguments that the probability of decapsulation failure may be higher than commonly accepted. This allows us to increase performance and security against attacks on the Module-LWE.