International Association
for Cryptologic Research

This paper introduces PQConnect, a post-quantum end-to-end tunneling protocol that automatically protects all packets between clients that have installed PQConnect and servers that have installed and configured PQConnect.

Like VPNs, PQConnect does not require any changes to higher-level protocols and application software. PQConnect adds cryptographic protection to unencrypted applications, works in concert with existing pre-quantum applications to add post-quantum protection, and adds a second application-independent layer of defense to any applications that have begun to incorporate application-specific post-quantum protection.

Unlike VPNs, PQConnect automatically creates end-to-end tunnels to any number of servers using automatic peer discovery, with no need for the client administrator to configure per-server information. Each server carries out a client-independent configuration step to publish an announcement that the server's name accepts PQConnect connections. Any PQConnect client connecting to that name efficiently finds this announcement, automatically establishes a post-quantum point-to-point IP tunnel to the server, and routes traffic for that name through that tunnel.

The foundation of security in PQConnect is the server's long-term public key used to encrypt and authenticate all PQConnect packets. PQConnect makes a conservative choice of post-quantum KEM for this public key. PQConnect also uses a smaller post-quantum KEM for forward secrecy, and elliptic curves to ensure pre-quantum security even in case of security failures in KEM design or KEM software. Security of the handshake component of PQConnect has been symbolically proven using Tamarin.

We present an encrypted multi-map, a fundamental data structure underlying searchable encryption/structured encryption. Our protocol supports updates and is designed for applications demanding very strong data security. Not only it hides the information about queries and data, but also the query, access, and volume patterns. Our protocol utilizes a position-based ORAM and an encrypted dictionary. We provide two instantiations of the protocol, along with their operation-type-revealing variants, all using PathORAM but with different encrypted dictionary instantiations (AVL tree or BSkiplist). Their efficiency has been evaluated through both asymptotic and concrete complexity analysis, outperforming prior work while achieving the same level of strong security. We have implemented our instantiations and evaluated their performance on two real-world email databases (Enron and Lucene). We also discuss the strengths and limitations of our construction, including its resizability, and highlight that optimized solutions, even with heavy network utilization, may become practical as network speed improves.

Shadow is a family of lightweight block ciphers introduced by Guo, Li, and Liu in 2021, with Shadow-32 having a 32-bit block size and a 64-bit key, and Shadow-64 having a 64-bit block size and a 128-bit key. Both variants use a generalized Feistel network with four branches, incorporating the AND-Rotation-XOR operation similar to the Simon family for their bridging function. This paper reveals that the security claims of the Shadow family are not as strong as suggested. We present a key recovery attack that can retrieve the sequence of round keys used for encryption with only two known plaintext/ciphertext pairs, requiring time and memory complexity of $2^{43.23}$ encryptions and $2^{21.62}$ blocks of memory for Shadow-32, and complexity of $2^{81.32}$ encryptions and $2^{40.66}$ blocks of memory for Shadow-64. Notably, this attack is independent of the number of rounds and the bridging function employed. Furthermore, we critically evaluate one of the recent cryptanalysis on Shadow ciphers and identify significant flaws in the proposed key recovery attacks. In particular, we demonstrate that the distinguisher used in impossible differential attacks by Liu et al. is ineffective for key recovery, despite their higher claimed complexities compared to ours.

The Hermite Normal Form (HNF) of a matrix is an analogue of the echolon form over the integers. Any integer matrix can be transformed into its unique HNF. A common obstacle in computing the HNF is the extensive blow up of intermediate values. As first approach to this problem, we discuss the $Modulo Determinant Algorithm$. It keeps the entries bounded by $d$, the determinant of the lattice, and has a time complexity of $\mathcal{O}(n^3\log^2 d)$, where $n$ is the dimension of the matrix. Although this algorithm is very useful if the determinant is small, in the general case, the entries still become extremely large. Secondly, we study the $Linear Space Algorithm$. It has a time complexity of $\mathcal{O}(n^5\mathrm{polylog}(M, n))$, where $M$ denotes the largest absolute value of the input matrix. This is as fast as the best previously known algorithms, but in contrast, it assures space complexity linear in the input size, i.e. $\mathcal{O}(n^2\log M)$. As last algorithm to compute the HNF we analyze the $Heuristic Algorithm$, which is based on the first two algorithms. It achieves a much faster runtime in practice, yielding a heuristic runtime of $\mathcal{O}(n^4\mathrm{polylog}(M, n))$, while keeping the linear space complexity. Besides some performance speed ups, the $Linear Space Algorithm$ and $Heuristic Algorithm$ are precisely the algorithms implemented by SageMath.

The use of secure computation protocols within production software systems and applications is complicated by the fact that such protocols sometimes rely upon -- or are most compatible with -- unusual or restricted models of computation. We employ the features of a contemporary and widely used programming language to create an embedded domain-specific language for working with user-defined functions as binary matrices that operate on one-hot vectors. At least when working with small finite domains, this allows programmers to overcome the restrictions of more simple secure computation protocols that support only linear operations (such as addition and scalar multiplication) on private inputs. Notably, programmers are able to define their own input and output domains, to use all available host language features and libraries to define functions that operate on these domains, and to translate inputs, outputs, and functions between their usual host language representations and their one-hot vector or binary matrix forms. Furthermore, these features compose in a straightforward way with simple secure computation libraries available for the host language.

Traceable Receipt-free Encryption (TREnc) has recently been introduced as a verifiable public-key encryption primitive endowed with a unique security model. In a nutshell, TREnc allows randomizing ciphertexts in transit in order to remove any subliminal information up to a public trace that ensures the non-malleability of the underlying plaintext. A remarkable property of TREnc is the indistinguishability of the randomization of chosen ciphertexts against traceable chosen-ciphertext attacks (TCCA). The main application lies in voting systems by allowing voters to encrypt their votes, tracing whether a published ballot takes their choices into account, and preventing them from proving how they voted. While being a very promising primitive, the few existing TREnc mechanisms solely rely on discrete-logarithm related assumptions making them vulnerable to the well-known record-now/decrypt-later attack in the wait of quantum computers. We address this limitation by building the first TREnc whose privacy withstands the advent of quantum adversaries in the future. To design our construction, we first generalize the original TREnc primitive that is too restrictive to be easily compatible with built-in lattice-based semantically-secure encryption. Our more flexible model keeps all the ingredients generically implying receipt-free voting. Our instantiation relies on Ring Learning With Errors (RLWE) with pairing-based statistical zero-knowledge simulation sound proofs from Groth-Sahai, and further enjoys a public-coin common reference string removing the need of a trusted setup.

End-to-end encryption (E2EE) has become the gold standard for securing communications, bringing strong confidentiality and privacy guarantees to billions of users worldwide. However, the current push towards widespread integration of artificial intelligence (AI) models, including in E2EE systems, raises some serious security concerns.

This work performs a critical examination of the (in)compatibility of AI models and E2EE applications. We explore this on two fronts: (1) the integration of AI “assistants” within E2EE applications, and (2) the use of E2EE data for training AI models. We analyze the potential security implications of each, and identify conflicts with the security guarantees of E2EE. Then, we analyze legal implications of integrating AI models in E2EE applications, given how AI integration can undermine the confidentiality that E2EE promises. Finally, we offer a list of detailed recommendations based on our technical and legal analyses, including: technical design choices that must be prioritized to uphold E2EE security; how service providers must accurately represent E2EE security; and best practices for the default behavior of AI features and for requesting user consent. We hope this paper catalyzes an informed conversation on the tensions that arise between the brisk deployment of AI and the security offered by E2EE, and guides the responsible development of new AI features.

This document provides a definition of end-to-end encryption (E2EE). End-to-end encryption is an application of cryptographic mechanisms to provide security and privacy to communication between endpoints. Such communication can include messages, email, video, audio, and other forms of media. E2EE provides security and privacy through confidentiality, integrity, authenticity and forward secrecy for communication amongst people.

There are a variety of techniques for implementing read/write memory inside of zero-knowledge proofs and validating consistency of memory accesses. These techniques are generally implemented with the goal of implementing a RAM or ROM. In this paper, we present memory techniques for more specialized data structures: queues and stacks. We first demonstrate a technique for implementing queues in arithmetic circuits that requires 3 multiplication gates and 1 advice value per read and 2 multiplication gates per write. This is based on using Horner's Rule to evaluate 2 polynomials at random points and check that the values read from the queue are equal to the values written to the queue as vectors. Next, we present a stack scheme based on an optimized version of the RAM scheme of Yang and Heath that requires 5 multiplication gates and 4 advice values per read and 2 multiplication gates per write. This optimizes the RAM scheme by observing that reads and writes to a stack are already "paired" which avoids the need for inserting dummy operations for each access as in a stack. We also introduce a different notion of "multiplexing" or "operation privacy" that is better suited to the use case of stacks and queues. All of the techniques we provide are based on evaluating polynomials at random points and using randomly evaluated polynomials as universal hash functions to check set/vector equality.

To provide safe communication across an unprotected medium such as the internet, network protocols are being established. These protocols employ public key techniques to perform key exchange and authentication. Transport Layer Security (TLS) is a widely used network protocol that enables secure communication between a server and a client. TLS is employed in billions of transactions per second. Contemporary protocols depend on traditional methods that utilize the computational complexity of factorization or (elliptic curve) logarithm mathematics problems. The ongoing advancement in the processing power of classical computers requires an ongoing increase in the security level of the underlying cryptographic algorithms. This study focuses on the analysis of Curve448 and Edwards curve Ed448, renowned for their superior security features that offer a 224-bit level of security as part of the TLSv1.3 protocol. The exponential advancement of quantum computers, however, presents a substantial threat to secure network communication that depends on classical crypto schemes, irrespective of their degree of security. Quantum computers have the capability to resolve these challenges within a feasible timeframe. In order to successfully transition to Post-Quantum secure network protocols, it is imperative to concurrently deploy both classical and post-quantum algorithms. This is done to fulfill the requirements of both enterprises and governments, while also instilling more assurance in the reliability of the post-quantum systems. This paper presents a detailed hybrid implementation architecture of the TLSv1.3 network protocol. We showcase the first deployment of Curve448 and Crystals-Kyber for the purpose of key exchanging, and Ed448 and Crystals-Dilithium for verifying the authenticity of entities and for X.509 Public Key Infrastructure (PKI). We rely upon the widely used OpenSSL library and the specific wolfSSL library for embedded devices to provide our results for server and client applications.

Federated Learning (FL) enables collaborative model training while preserving data privacy by avoiding the sharing of raw data. However, in large-scale FL systems, efficient secure aggregation and dropout handling remain critical challenges. Existing state-of-the-art methods, such as those proposed by Liu et al. (UAI'22) and Li et al. (ASIACRYPT'23), suffer from prohibitive communication overhead, implementation complexity, and vulnerability to poisoning attacks. Alternative approaches that utilize partially connected graph structures (resembling client grouping) to reduce communication costs, such as Bell et al. (CCS'20) and ACORN (USENIX Sec'23), face the risk of adversarial manipulation during the graph construction process.

To address these issues, we propose ClusterGuard, a secure clustered aggregation scheme for federated learning. ClusterGuard leverages Verifiable Random Functions (VRF) to ensure fair and transparent cluster selection and employs a lightweight key-homomorphic masking mechanism, combined with efficient dropout handling, to achieve secure clustered aggregation. Furthermore, ClusterGuard incorporates a dual filtering mechanism based on cosine similarity and norm to effectively detect and mitigate poisoning attacks.

Extensive experiments on standard datasets demonstrate that ClusterGuard achieves over 2x efficiency improvement compared to advanced secure aggregation methods. Even with 20% of clients being malicious, the trained model maintains accuracy comparable to the original model, outperforming state-of-the-art robustness solutions. ClusterGuard provides a more efficient, secure, and robust solution for practical federated learning.

The RSA (Rivest-Shamir-Adleman) cryptosystem is a fundamental algorithm of public key cryptography and is widely used across various information domains. For an RSA modulus represented as $N = pq$, with its factorization remaining unknown, security vulnerabilities arise when attackers exploit the key equation $ed-k(p-1)(q-1)=1$. To enhance the security, Murru and Saettone introduced cubic Pell RSA --- a variant of RSA based on the cubic Pell equation, where the key equation becomes $ed-k(p^2+p+1)(q^2+q+1)=1$. In this paper, we further investigate the security implications surrounding the generalized key equation $eu-(p^2+p+1)(q^2+q+1)v=w$. We present a novel attack strategy aimed at recovering the prime factors $p$ and $q$ under specific conditions satisfied by $u$, $v$, and $w$. Our generalized attack employs lattice-based Coppersmith's techniques and extends several previous attack scenarios, thus deepening the understanding of mathematical cryptanalysis.

This paper investigates the Mersenne number-based $\mathsf{AJPS}$ cryptosystem, with a particular focus on its associated hard problem. Specifically, we aim to enhance the existing lattice-based attack on the Mersenne low Hamming ratio search problem. Unlike the previous approach of directly employing lattice reduction algorithm, we apply the lattice-based method to solving polynomial equations derived from the above problem. We extend the search range for vulnerabilities in weak keys and increase the success probability of key recovery attack. To validate the efficacy and accuracy of our proposed improvements, we conduct numerical computer experiments. These experiments serve as a concrete validation of the practicality and effectiveness of our improved attack.

Side-channel attacks exploit information leaked through non-primary channels, such as power consumption, electromagnetic emissions, or timing, to extract sensitive data from cryptographic devices. Over the past three decades, side-channel analysis has evolved into a mature research field with well-established methodologies for analyzing standard cryptographic algorithms like the Advanced Encryption Standard (AES). However, the integration of side-channel analysis with formal methods remains relatively unexplored. In this paper, we present a hybrid attack on AES that combines side-channel analysis with SAT. We model AES as a SAT problem and leverage hints of the input and output values of the S-boxes, extracted via profiled deep learning-based power analysis, to solve it. Experimental results on an ATXmega128D4 MCU implementation of AES-128 demonstrate that the SAT-assisted approach consistently recovers the full encryption key from a single trace, captured from devices different from those used for profiling, within one hour. In contrast, without SAT assistance, the success rate remains below 80% after 26 hours of key enumeration.

A universal thresholdizer (UT), constructed from a threshold fully homomorphic encryption by Boneh et. al , Crypto 2018, is a general framework for universally thresholdizing many cryptographic schemes. However, their framework is insufficient to construct strongly secure threshold schemes, such as threshold signatures and threshold public-key encryption, etc. In this paper, we strengthen the security definition for a universal thresholdizer and propose a scheme which satisfies our stronger security notion. Our UT scheme is an improvement of Boneh et. al ’s construction at the level of threshold fully homomorphic encryption using a key homomorphic pseudorandom function. We apply our strongly secure UT scheme to construct strongly secure threshold signatures and threshold public-key encryption.

This report covers our analysis (security, proofs, efficiency) of the Round-2 candidates to the Korean post-quantum competiton KpqC. Signature systems covered in the report are AIMer, HAETAE, MQ-Sign, and NCC-Sign; KEMs covered are NTRU+, Paloma, REDOG, and SMAUG-T.

Blind signatures are an important primitive for privacy-preserving technologies. To date, highly efficient pairing-free constructions rely on the random oracle model, and additionally, a strong assumption, such as interactive assumptions or the algebraic group model.

In contrast, for signatures we know many efficient constructions that rely on the random oracle model and standard assumptions. In this work, we develop techniques to close this gap. Compared to the most efficient pairing-free AGM-based blind signature by Crites et. al. (Crypto 2023), our construction has a relative overhead of only a factor $3\times$ and $2\times$ in terms of communication and signature size, and it is provable in the random oracle model under the DDH assumption. With one additional move and $\mathbb{Z}_p$ element, we also achieve one-more strong unforgeability.

Our construction is inspired by the recent works by Chairattana-Apirom, Tessaro, and Zhu (Crypto 2024) and Klooß, Reichle, and Wagner (Asiacrypt 2024), and we develop a tailored technique to circumvent the sources of inefficiency in their constructions. Concretely, we achieve signature and communication size of $192$ B and $608$ B, respectively.

We construct the first blind signature scheme that achieves all of the following properties simultaneously: - it is tightly secure under a standard (i.e., non-interactive, non-\(q\)-type) computational assumption, - it does not require pairings, - it does not rely on generic, non-black-box techniques (like generic NIZK proofs). The third property enables a reasonably efficient solution, and in fact signatures in our scheme comprise 10 group elements and 29 \(\mathbb{Z}_p\)-elements.

Our scheme starts from a pairing-based non-blind signature scheme (Abe et al., JoC 2023), and uses recent techniques of Chairattana-Apirom, Tessaro, and Zhu (CRYPTO 2024) to replace the pairings used in this scheme with non-interactive zero-knowledge proofs in the random oracle model. This conversion is not generic or straightforward (also because the mentioned previous works have converted only significantly simpler signature schemes), and we are required to improve upon and innovate existing techniques in several places.

As an interesting side note, and unlike previous works, our techniques only require a non-programmable random oracle, and our signature scheme achieves predicate blindness (which means that the user can prove statements about the signed message during the signing process).

We present a novel digital signature scheme grounded in non-commutative cryptography and implemented over a bilinear matrix group platform. At the core of our design is a unique equivocation function that obfuscates intermediate elements, effectively concealing outputs and minimizing observable information leakage. To the best of our knowledge, this is the first digital signature scheme to combine information-theoretic security with computational hardness, relying on a challenging instance of the Non-Abelian Hidden Subgroup Problem (NAHSP) and strengthened by practical guarantees. This dual-layered security approach ensures robustness against both classical and quantum adversaries while maintaining communication overheads competitive with RSA. Our work represents a significant advancement toward efficient, quantum-resilient digital signatures for real-world applications. This paper is an early pre-release intended to invite collaboration and feedback. The work is presented for research purposes only and is not intended for use in production systems.

A garbling scheme transforms a program (e.g., circuit) $C$ into a garbled program $\hat{C}$, along with a pair of short keys $(k_{i,0},k_{i,1})$ for each input bit $x_i$, such that $(C,\hat{C}, \{k_{i,x_i}\})$ can be used to recover the output $z = C(x)$ while revealing nothing else about the input $x$. This can be naturally generalized to partial garbling, where part of the input is public, and a computation $z = C(x, y)$ is decomposed into a public part $C_{\text{pub}}(x)$, depending only on the public input $x$, and a private part $z = C_{\text{priv}}(C_{\text{pub}}(x), y)$ that also involves a private input $y$.

A key challenge in garbling is to achieve succinctness, where the size of the garbled program may grow only with the security parameter and (possibly) the output length, but not with the size of $C$. Prior work achieved this strong notion of succinctness using heavy tools such as indistinguishability obfuscation (iO) or a combination of fully homomorphic encryption and attribute-based encryption.

In this work, we introduce new succinct garbling schemes based on variants of standard group-based assumptions. Our approach, being different from prior methods, offers a promising pathway towards practical succinct garbling. Specifically, we construct: - A succinct partial garbling scheme for general circuits, where the garbled circuit size scales linearly with the private computation $|C_{\text{priv}}|$ and is independent of the public computation $|C_{\text{pub}}|$. This implies fully succinct conditional disclosure of secrets (CDS) protocols for circuits. - Succinct (fully hiding) garbling schemes for simple types of programs, including truth tables, bounded-length branching programs (capturing decision trees and DFAs as special cases) and degree-2 polynomials, where the garbled program size is independent of the program size. This implies succinct private simultaneous messages (PSM) protocols for the same programs.

Our succinct partial garbling scheme can be based on a circular-security variant of the power-DDH assumption, which holds in the generic group model, or alternatively on the key-dependent message security of the Damgård-Jurik encryption. For bounded-depth circuits or the aforementioned simple programs, we avoid circular-security assumptions entirely.

At the heart of our technical approach is a new computational flavor of algebraic homomorphic MAC (aHMAC), for which we obtain group-based constructions building on techniques from the literature on homomorphic secret sharing. Beyond succinct garbling, we demonstrate the utility of aHMAC by constructing constrained pseudorandom functions (CPRFs) for general constraint circuits from group-based assumptions. Previous CPRF constructions were limited to $\mathsf{NC}^1$ circuits or alternatively relied on lattices or iO.