For any prime number $p$, we provide two classes of linear codes with few weights over a $p$-ary alphabet. These codes are based on a well-known generic construction (the defining-set method), stemming on a class of monomials and a class of trinomials over finite fields. The considered monomials are Dembowski-Ostrom monomials $x^{p^{\alpha}+1}$, for a suitable choice of the exponent $\alpha$, so that, when $p>2$ and $n\not\equiv 0 \pmod{4}$, these monomials are planar. We study the properties of such monomials in detail for each integer $n$ greater than two and any prime number $p$. In particular, we show that they are $t$-to-one, where the parameter $t$ depends on the field $\mathbb{F}_{p^n}$ and it takes the values $1, 2$ or $p+1$. Moreover, we give a simple proof of the fact that the functions are $\delta$-uniform with $\delta \in \{1,4,p\}$. This result describes the differential behaviour of these monomials for any $p$ and $n$. For the second class of functions, we consider an affine equivalent trinomial to $x^{p^{\alpha}+1}$, namely, $x^{p^{\alpha}+1}+\lambda x^{p^{\alpha}}+\lambda^{p^{\alpha}}x$ for $\lambda\in \mathbb{F}_{p^n}^*$. We prove that these trinomials satisfy certain regularity properties, which are useful for the specification of linear codes with three or four weights that are different than the monomial construction. These families of codes contain projective codes and optimal codes (with respect to the Griesmer bound). Remarkably, they contain infinite families of self-orthogonal and minimal $p$-ary linear codes for every prime number $p$. Our findings highlight the utility of studying affine equivalent functions, which is often overlooked in this context.

A $t$-out-of-$n$ threshold decryption system assigns key shares to $n$ parties so that any $t$ of them can decrypt a well-formed ciphertext. Existing threshold decryption systems are not secure when these parties are rational actors: an adversary can offer to pay the parties for their key shares. The problem is that a quorum of $t$ parties, working together, can sell the adversary a decryption key that reveals nothing about the identity of the traitor parties. This provides a risk-free profit for the parties since there is no accountability for their misbehavior --- the information they sell to the adversary reveals nothing about their identity. This behavior can result in a complete break in many applications of threshold decryption, such as encrypted mempools, private voting, and sealed-bid auctions. In this work we show how to add accountability to threshold decryption systems to deter this type of risk-free misbehavior. Suppose a quorum of $t$ or more parties construct a decoder algorithm $D(\cdot)$ that takes as input a ciphertext and outputs the corresponding plaintext or $\bot$. They sell $D$ to the adversary. Our threshold decryption systems are equipped with a tracing algorithm that can trace $D$ to members of the quorum that created it. The tracing algorithm is only given blackbox access to $D$ and will identify some members of the misbehaving quorum. The parties can then be held accountable, which may discourage them from selling the decoder $D$ in the first place. Our starting point is standard (non-threshold) traitor tracing, where $n$ parties each holds a secret key. Every party can decrypt a well-formed ciphertext on its own. However, if a subset of parties ${\cal J} \subseteq [n]$ collude to create a pirate decoder $D(\cdot)$ that can decrypt well-formed ciphertexts, then it is possible to trace $D$ to at least one member of ${\cal J}$ using only blackbox access to the decoder $D$. Traitor tracing received much attention over the years and multiple schemes have been developed. In this work we develop the theory of traitor tracing for threshold decryption, where now only a subset ${\cal J} \subseteq [n]$ of $t$ or more parties can collude to create a pirate decoder $D(\cdot)$. This problem has recently become quite important due to the real-world deployment of threshold decryption in encrypted mempools, as we explain in the paper. While there are several non-threshold traitor tracing schemes that we can leverage, adapting these constructions to the threshold decryption settings requires new cryptographic techniques. We present a number of constructions for traitor tracing for threshold decryption, and note that much work remains to explore the large design space.

We present a deterministic synchronous protocol for binary Byzantine Agreement against a corrupt minority with adaptive $O(n\cdot f)$ communication complexity, where $f$ is the exact number of corruptions. Our protocol improves the previous best-known deterministic Byzantine Agreement protocol developed by Momose and Ren (DISC 2021), whose communication complexity is quadratic, independent of the exact number of corruptions. Our approach combines two distinct primitives that we introduce and implement with $O(n\cdot f)$ communication, Reliable Voting, and Weak Byzantine Agreement. In Reliable Voting, all honest parties agree on the same value only if all honest parties start with that value, but there is no agreement guarantee in the general case. In Weak Byzantine Agreement, we achieve agreement, but validity requires that the inputs to the protocol satisfy certain properties. Our Weak Byzantine Agreement protocol is an adaptation of the recent Cohen et al. protocol (OPODIS 2022), in which we identify and address various issues.

Active fault injection is a credible threat to real-world digital systems computing on sensitive data. Arguing about security in the presence of faults is non-trivial, and state-of-the-art criteria are overly conservative and lack the ability of fine-grained comparison. However, comparing two alternative implementations for their security is required to find a satisfying compromise between security and performance. In addition, the comparison of alternative fault scenarios can help optimize the implementation of effective countermeasures. In this work, we use quantitative information flow analysis to establish a vulnerability metric for hardware circuits under fault injection that measures the severity of an attack in terms of information leakage. Potential use cases range from comparing implementations with respect to their vulnerability to specific fault scenarios to optimizing countermeasures. We automate the computation of our metric by integrating it into a state-of-the-art evaluation tool for physical attacks and provide new insights into the security under an active fault attacker.

We propose a new method to encode the problems of optimizing S-box implementations into SAT problems. By considering the inputs and outputs of gates as Boolean functions, the fundamental idea of our method is representing the relationships between these inputs and outputs according to their algebraic normal forms. Based on this method, we present several encoding schemes for optimizing S-box implementations according to various criteria, such as multiplicative complexity, bitslice gate complexity, gate complexity, and circuit depth complexity. The experimental results of these optimization problems show that, compared to the encoding method proposed in FSE 2016, which represents these relationships between Boolean functions by their truth tables, our new encoding method can significantly reduce accelerate the subsequent solving process by 2-100 times for the majority of instances. To further improve the solving efficiency, we propose several strategies to eliminate the redundancy of the derived equation system and break the symmetry of the solution space. We apply our method in the optimizations of the S-boxes used in Ascon, ICEPOLE, PRIMATEs, Keccak/Ketje/Keyak, Joltik/Piccolo, LAC, Minalpher, Prøst, and RECTANGLE. We achieve some new improved implementations and narrow the range of the optimal values for different optimization criteria of these S-boxes.

There has been a recent interest in proposing quantum protocols whose security relies on weaker computational assumptions than their classical counterparts. Importantly to our work, it has been recently shown that public-key encryption (PKE) from one-way functions (OWF) is possible if we consider quantum public keys. Notice that we do not expect classical PKE from OWF given the impossibility results of Impagliazzo and Rudich (STOC'89). However, the distribution of quantum public keys is a challenging task. Therefore, the main question that motivates our work is if quantum PKE from OWF is possible if we have classical public keys. Such protocols are impossible if ciphertexts are also classical, given the impossibility result of Austrin et al. (CRYPTO'22) of quantum enhanced key-agreement (KA) with classical communication. In this paper, we focus on black-box separation for PKE with classical public key and quantum ciphertext from OWF under the polynomial compatibility conjecture, first introduced in Austrin et al.. More precisely, we show the separation when the decryption algorithm of the PKE does not query the OWF. We prove our result by extending the techniques of Austrin et al. and we show an attack for KA in an extended classical communication model where the last message in the protocol can be a quantum state.

This paper presents MQ on my Mind (MQOM), a digital signature scheme based on the difficulty of solving multivariate systems of quadratic equations (MQ problem). MQOM has been submitted to the NIST call for additional post-quantum signature schemes. MQOM relies on the MPC-in-the-Head (MPCitH) paradigm to build a zero-knowledge proof of knowledge (ZK-PoK) for MQ which is then turned into a signature scheme through the Fiat-Shamir heuristic. The underlying MQ problem is non-structured in the sense that the system of quadratic equations defining an instance is drawn uniformly at random. This is one of the hardest and most studied problems from multivariate cryptography which hence constitutes a conservative choice to build candidate post-quantum cryptosystems. For the efficient application of the MPCitH paradigm, we design a specific MPC protocol to verify the solution of an MQ instance. Compared to other multivariate signature schemes based on non-structured MQ instances, MQOM achieves the shortest signatures (6.3-7.8 KB) while keeping very short public keys (few dozen of bytes). Other multivariate signature schemes are based on structured MQ problems (less conservative) which either have large public keys (e.g. UOV) or use recently proposed variants of these MQ problems (e.g. MAYO).

The LowMC family of SPN block cipher proposed by Albrecht et al. was designed specifically for MPC-/FHE-/ZKP-friendly use cases. It is especially used as the underlying block cipher of PICNIC, one of the alternate third-round candidate digital signature algorithms for NIST post-quantum cryptography standardization. The security of PICNIC is highly related to the difficulty of recovering the secret key of LowMC from a given plaintext/ciphertext pair, which raises new challenges for security evaluation under extremely low data complexity. In this paper, we improve the attacks on LowMC under low data complexity, i.e. 1 or 2 chosen plaintext/ciphertext pairs. For the difference enumeration attack with 2 chosen plaintexts, we propose new algebraic methods to better exploit the nonlinear relation inside the introduced variables based on the attack framework proposed by Liu et al. at ASIACRYPT 2022. With this technique, we significantly extend the number of attack rounds for LowMC with partial nonlinear layers and improve the success probability from around 0.5 to over 0.9. The security margin of some instances can be reduced to only 3/4 rounds. For the key-recovery attack using a single plaintext, we adopt a different linearization strategy to reduce the huge memory consumption caused by the polynomial methods for solving multivariate equation systems. The memory complexity reduces drastically for all 5-/6-round LowMC instances with full nonlinear layers at the sacrifice of a small factor of time complexity. For 5-round LowMC instances with a block size of 129, the memory complexity decreases from $2^{86.46}$ bits to $2^{48.18}$ bits while the time complexity even slightly reduces. Our results indicate that the security for different instances of LowMC under extremely low data complexity still needs further exploration.

The adoption of Distributed Ledger Technology (DLT) for critical financial infrastructures like Central Bank Digital Currencies (CBDCs) is hindered by a significant performance gap. Permissioned blockchains such as Hyperledger Fabric, which are conceptually suitable and have become popular platform used in many production deployments today, are nevertheless limited by architectural bottlenecks. Their monolithic peer design and consensus mechanisms prevent them from achieving the required scale for such demanding applications. This paper presents Fabric-X, a fundamental re-architecture of Hyperledger Fabric that addresses these challenges end-to-end. We have open-sourced Fabric-X as part of the Linux Foundation Decentralized Trust at \url{github.com/hyperledger/fabric-x}. We decompose the monolithic peer into independently scalable microservices for endorsement, validation, and committing. To maximize parallelism, we introduce a transaction dependency graph that enables the safe, concurrent validation of transactions across multiple blocks. Complementing the peer redesign, we introduce Arma, a novel sharded Byzantine Fault Tolerant (BFT) ordering service that dramatically increases throughput by ordering compact transaction digests rather than full transaction payloads. We implemented and benchmarked this framework with a UTXO-based CBDC application. Our evaluation demonstrates a peak throughput exceeding 100,000 transactions per second (TPS)—a multiple-orders-of-magnitude improvement over the standard implementation. This work proves that permissioned DLTs can be engineered for national-scale payment systems, providing a resilient and highly performant foundation for practical CBDC deployments and the integration of advanced, computationally intensive features.

Although we have known about fully homomorphic encryption (FHE) from circular security assumptions for over a decade [Gentry, STOC ’09; Brakerski–Vaikuntanathan, FOCS ’11], there is still a significant gap in understanding related homomorphic primitives supporting all *unrestricted* polynomial-size computations. One prominent example is attribute-based encryption (ABE). The state-of-the-art constructions, relying on the hardness of learning with errors (LWE) [Gorbunov–Vaikuntanathan–Wee, STOC ’13; Boneh et al., Eurocrypt ’14], only accommodate circuits up to a *predetermined* depth, akin to leveled homomorphic encryption. In addition, their components (master public key, secret keys, and ciphertexts) have sizes polynomial in the maximum circuit depth. Even in the simpler setting where a single key is published (or a single circuit is involved), the depth dependency persists, showing up in constructions of 1-key ABE and related primitives, including laconic function evaluation (LFE), 1-key functional encryption (FE), and reusable garbling schemes. So far, the only approach of eliminating depth dependency relies on indistinguishability obfuscation. An interesting question that has remained open for over a decade is whether the circular security assumptions enabling FHE can similarly benefit ABE. In this work, we introduce new lattice-based techniques to overcome the depth-dependency limitations. - Relying on a circular security assumption, we construct LFE, 1-key FE, 1-key ABE, and reusable garbling schemes capable of evaluating circuits of unbounded depth and size. - Based on the *evasive circular* LWE assumption, a stronger variant of the recently proposed *evasive* LWE assumption [Wee, Eurocrypt ’22; Tsabary, Crypto ’22], we construct full-fledged ABE and predicate encryption (PE) schemes for circuits of unbounded depth and size. Our LFE, 1-key FE, and reusable garbling schemes achieve almost optimal succinctness (up to polynomial factors in the security parameter). Their ciphertexts and input encodings have sizes linear in the input length, while function digest, secret keys, and garbled circuits have constant sizes independent of circuit parameters (for Boolean outputs). In fact, this gives the first constant-size garbled circuits without relying on indistinguishability obfuscation. Our ABE and PE schemes offer short components, with master public key and ciphertext sizes linear in the attribute length and secret key being constant-size.

Public key encryption with keyword search (PEKS), formalized by Boneh et al. [EUROCRYPT' 04], enables secure searching for specific keywords in the ciphertext. Nevertheless, in certain scenarios, varying user tiers are granted disparate data searching privileges, and administrators need to restrict the searchability of ciphertexts to select users exclusively. To address this concern, Jiang et al. [ACISP' 16] devised a variant of PEKS, namely public key encryption with authorized keyword search (PEAKS), wherein solely authorized users possess the ability to conduct targeted keyword searches. Nonetheless, it is vulnerable to resist quantum computing attacks. As a result, research focusing on authorizing users to search for keywords while achieving quantum security is far-reaching. In this work, we present a novel construction, namely lattice-based PEAKS (L-PEAKS), which is the first mechanism to permit the authority to authorize users to search different keyword sets while ensuring quantum-safe properties. Specifically, the keyword is encrypted with a public key, and each authorized user needs to obtain a search privilege from an authority. The authority distributes an authorized token to a user within a time period and the user will generate a trapdoor for any authorized keywords. Technically, we utilize several lattice sampling and basis extension algorithms to fight against attacks from quantum adversaries. Moreover, we leverage identity-based encryption (IBE) to alleviate the bottleneck of public key management. Furthermore, we conduct parameter analysis, security reduction, and theoretical complexity comparison of our scheme and perform comprehensive evaluations of a commodity machine for completeness. Our L-PEAKS satisfies IND-sID-CKA and T-EUF security and is efficient in terms of space and computation complexity compared to other existing primitives.

Parallel repetition refers to a set of valuable techniques used to reduce soundness error of probabilistic proofs while saving on certain efficiency measures. Parallel repetition has been studied for interactive proofs (IPs) and multi-prover interactive proofs (MIPs). In this paper we initiate the study of parallel repetition for probabilistically checkable proofs (PCPs). We show that, perhaps surprisingly, parallel repetition of a PCP can increase soundness error, in fact bringing the soundness error to one as the number of repetitions tends to infinity. This "failure" of parallel repetition is common: we find that it occurs for a wide class of natural PCPs for NP-complete languages. We explain this unexpected phenomenon by providing a characterization result: the parallel repetition of a PCP brings the soundness error to zero if and only if a certain "MIP projection" of the PCP has soundness error strictly less than one. We show that our characterization is tight via a suitable example. Moreover, for those cases where parallel repetition of a PCP does bring the soundness error to zero, the aforementioned connection to MIPs offers preliminary results on the rate of decay of the soundness error. Finally, we propose a simple variant of parallel repetition, called consistent parallel repetition (CPR), which has the same randomness complexity and query complexity as the plain variant of parallel repetition. We show that CPR brings the soundness error to zero for every PCP (with non-trivial soundness error). In fact, we show that CPR decreases the soundness error at an exponential rate in the repetition parameter.

In this paper, we revisit the problem of erasing sensitive data from memory and registers when returning from a cryptographic routine. While the problem and related attacker model are fairly easy to phrase, it turns out to be surprisingly hard to guarantee security in this model when implementing cryptography in common languages such as C/C++ or Rust. We revisit the issues surrounding zeroization and then present a principled solution in the sense that it guarantees that sensitive data is erased and it clearly defines when this happens. We implement our solution as an extension to the formally verified Jasmin compiler and extend the correctness proof of the compiler to cover zeroization. We show that the approach seamlessly integrates with state-of-the-art protections against microarchitectural attacks by integrating zeroization into Libjade, a cryptographic library written in Jasmin with systematic protections against timing and Spectre-v1 attacks. We present benchmarks showing that, in many cases, the overhead of zeroization is barely measurable and stays below 2% except for highly optimized symmetric crypto routines on short inputs.

Boolean Searchable Symmetric Encryption (BSSE) enables users to perform retrieval operations on the encrypted data while sup- porting complex query capabilities. This paper focuses on addressing the storage overhead and privacy concerns associated with existing BSSE schemes. While Patel et al. (ASIACRYPT’21) and Bag et al. (PETS’23) introduced BSSE schemes that conceal the number of single keyword re- sults, both of them suffer from quadratic storage overhead and neglect the privacy of search and access patterns. Consequently, an open ques- tion arises: Can we design a storage-efficient Boolean query scheme that effectively suppresses leakage, covering not only the volume pattern for singleton keywords, but also search and access patterns? In light of the limitations of existing schemes in terms of storage over- head and privacy protection, this work presents a novel solution called SESAME. It realizes efficient storage and privacy preserving based on Bloom filter and functional encryption. Moreover, we propose an en- hanced version, SESAME+, which offers improved search performance. By rigorous security analysis on the leakage functions of our schemes, we provide a formal security proof. Finally, we implement our schemes and demonstrate that SESAME+ achieves superior search efficiency and reduced storage overhead.

We demonstrate that a passive network attacker can opportunistically obtain private RSA host keys from an SSH server that experiences a naturally arising fault during signature computation. In prior work, this was not believed to be possible for the SSH protocol because the signature included information like the shared Diffie-Hellman secret that would not be available to a passive network observer. We show that for the signature parameters commonly in use for SSH, there is an efficient lattice attack to recover the private key in case of a signature fault. We provide a security analysis of the SSH, IKEv1, and IKEv2 protocols in this scenario, and use our attack to discover hundreds of compromised keys in the wild from several independently vulnerable implementations.

Zero-knowledge proofs for NP statements are an essential tool for building various cryptographic primitives and have been extensively studied in recent years. In a seminal result from Goldreich, Micali and Wigderson (JACM'91), zero-knowledge proofs for NP statements can be built from any one-way function, but this construction leads very inefficient proofs. To yield practical constructions, one often uses the additional structure provided by homomorphic commitments. In this paper, we introduce a relaxed notion of homomorphic commitments, called malleable commitments, which requires less structure to be instantiated. We provide a malleable commitment construction from the ElGamal-type isogeny-based group action (Eurocrypt’22). We show how malleable commitments with a group structure in the malleability can be used to build zero-knowledge proofs for NP statements, improving on the naive construction from one-way functions. We consider three representations: arithmetic circuits, rank-1 constraint systems and branching programs. This work gives the first attempt at constructing a post-quantum generic proof system from isogeny assumptions (the group action DDH problem). Though the resulting proof systems are linear in the circuit size, they possess interesting features such as non-interactivity, statistical zero-knowledge, and online-extractability.

In 2012, Ding, Xie and Lin designed a key exchange protocol based on Ring-LWE problem, called the DXL key exchange protocol, which can be seen as an extended version of the Diffie-Hellman key exchange. In this protocol, Ding et al. achieved key exchange between the communicating parties according to the associativity of matrix multiplications, that is, $(x^T\cdot A)\cdot y = x^T\cdot (A\cdot y)$, where $x,y$ are column vectors and $A$ is a square matrix. However, the DXL key exchange protocol cannot resist key reuse attacks. At ESORICS 2022, Qin et al. proposed a method that an adversary can recover the reused private key after forging the public keys for several times. Nevertheless, Qin et al.'s method leads to a lot of redundant operations. In this paper, we improve Qin et al.'s method to a more general case and propose an effective approach to combine signal leakage attacks with depth first search. Compared with state-of-the-art result appeared at ESORICS 2022, the number of reused private key have been decreased from 29 to 10. In other words, if the number of reuses exceeds 10, the private key will be restored. Moreover, we validate the effectiveness of the results through experiments.

The Boolean map $\chi_n \colon \mathbb{F}_2^n \to \mathbb{F}_2^n,\ x \mapsto y$ defined by $y_i = x_i + (x_{i+1}+1)x_{i+2}$ (where $i\in \mathbb{Z}/n\mathbb{Z}$) is used in various permutations that are part of cryptographic schemes, e.g., Keccak-f (the SHA-3-permutation), ASCON (the winner of the NIST Lightweight competition), Xoodoo, Rasta and Subterranean (2.0). In this paper, we study various algebraic properties of this map. We consider $\chi_n$ (through vectorial isomorphism) as a univariate polynomial. We show that it is a power function if and only if $n=1,3$. We furthermore compute bounds on the sparsity and degree of these univariate polynomials, and the number of different univariate representations. Secondly, we compute the number of monomials of given degree in the inverse of $\chi_n$ (if it exists). This number coincides with binomial coefficients. Lastly, we consider $\chi_n$ as a polynomial map, to study whether the same rule ($y_i = x_i + (x_{i+1}+1)x_{i+2}$) gives a bijection on field extensions of $\mathbb{F}_2$. We show that this is not the case for extensions whose degree is divisible by two or three. Based on these results, we conjecture that this rule does not give a bijection on any extension field of $\mathbb{F}_2$.

Several key exchange protocols based on tropical circulant matrices were proposed in the last two years. In this paper, we show that protocols offered by M. Durcheva [M. I. Durcheva. TrES: Tropical Encryption Scheme Based on Double Key Exchange. In: Eur. J. Inf. Tech. Comp. Sci. 2.4 (2022), pp. 11–17], by B. Amutha and R. Perumal [B. Amutha and R. Perumal. Public key exchange protocols based on tropical lower circulant and anti-circulant matrices. In: AIMS Math. 8.7 (2023), pp. 17307–17334.], and by H. Huang, C. Li, and L. Deng [H. Huang, C. Li, and L. Deng. Public-Key Cryptography Based on Tropical Circular Matrices. In: Appl. Sci. 12.15 (2022), p. 7401] are insecure.

Private Set Intersection Cardinality(PSI-CA) is a type of secure two-party computation. It enables two parties, each holding a private set, to jointly compute the cardinality of their intersection without revealing any other private information about their respective sets. In this paper, we manage to break two PSI-CA protocols by recovering the specific intersection items in polynomial time. Among them, the PSI-CA protocol proposed by De Cristofaro et al. in 2012 is the most popular PSI-CA protocol based on the Google Scholar search results and it is still deemed one of the most efficient PSI-CA protocols. In this paper, we also propose several solutions to these protocols' security problems.

This works introduces Basefold, a new $\textit{field-agnostic}$ Polynomial Commitment Scheme (PCS) for multilinear polynomials that has $O(\log^{2}(n))$ verifier costs and $O(n \log n)$ prover time. An important application of a multilinear PCS is constructing Succinct Non-interactive Arguments (SNARKs) from multilinear polynomial interactive oracle proofs (PIOPs). Furthermore, field-agnosticism is a major boon to SNARK efficiency in applications that require (or benefit from) a certain field choice. Our inspiration for Basefold is the Fast Reed-Solomon Interactive-Oracle Proof of Proximity (FRI IOPP), which leverages two properties of Reed-Solomon (RS) codes defined over "FFT-friendly'' fields: $O(n \log n)$ encoding time, and a second property that we call foldability. We first introduce a generalization of the FRI IOPP that works over any foldable linear code in linear time. Second, we construct a new family of linear codes which we call $\textit{random foldable codes}$, that are a special type of punctured Reed-Muller codes, and prove tight bounds on their minimum distance. Unlike RS codes, our new codes are foldable and have $O(n \log n)$ encoding time over ${any}$ sufficiently large field. Finally, we construct a new multilinear PCS by carefully interleaving our IOPP with the classical sumcheck protocol, which also gives a new multilinear PCS from FRI. Basefold is 2-3 times faster than prior multilinear PCS constructions from FRI when defined over the same finite field. More significantly, using Hyperplonk (Eurocrypt, 2022) as a multilinear PIOP backend for apples-to-apples comparison, we show that Basefold results in a SNARK that has better concrete efficiency across a range of field choices than with any prior multilinear PCS in the literature. Hyperplonk with Basefold has a proof size that is more than $10$ times smaller than Hyperplonk with Brakedown and its verifier is over $30$ times faster for circuits with more than $2^{20}$ gates. Compared to FRI, Hyperplonk with Basefold retains efficiency over any sufficiently large field. For illustration, with Basefold we can prove ECDSA signature verification over the secp256k1 curve more than $20$ times faster than Hyperplonk with FRI and the verifier is also twice as fast. Proofs of signature verification have many useful applications, including offloading blockchain transactions and enabling anonymous credentials over the web.

Security proofs of symmetric-key primitives typically consider an idealized world with access to a (uniformly) random function. The starting point of our work is the observation that such an ideal world can lead to underestimating the actual security of certain primitives. As a demonstrating example, $\mathsf{XoP2}$, which relies on two independent random permutations, has been proven to exhibit superior concrete security compared to $\mathsf{XoP}$, which employs a single permutation with domain separation. But the main reason for this is an artifact of the idealized model used in the proof, in particular, that (in the random-function-ideal world) $\mathsf{XoP}$ might hit a trivially bad event (outputting $\mathbf{0}$), which does not occur in the real/domain-separated world. Motivated by this, we put forth the analysis of such primitives in an updated ideal world, which we call the {\it fine-tuned} setting, where the above artifact is eliminated. We provide fine-tuned (and enhanced) security analyses for $\mathsf{XoP}$ and $\mathsf{XoP}$-based MACs: $\mathsf{nEHtM}$ and $\mathsf{DbHtS}$, and demonstrate how the transition to the fine-tuned setting can also yield better privacy/authentication tradeoff in authenticated encryption via the generic composition of encryption and MAC. Our analyses demonstrate that the security of $\mathsf{XoP}$-based and $\mathsf{XoP2}$-based constructions are, in fact, far more similar than what was previously proven. Our security proofs rely on a fine-tuned and extended version of Mirror theory for both lower and upper bounds, which yields more versatile and improved security proofs. Of independent interest, this extension allows us to prove the multi-user MAC security of $\mathsf{nEHtM}$ in the nonce-misuse model, while the previous analysis only applied to the multi-user PRF security in the nonce-respecting model. As a side note, we also point out (and fix) a flaw in the original analysis of Chen et al. [CRYPTO'23].

When outsourcing a database to an untrusted remote server, one might want to verify the integrity of contents while accessing it. To solve this, Blum et al. [FOCS `91] propose the notion of memory checking. Memory checking allows a user to run a RAM program on a remote server, with the ability to verify integrity of the storage with small local storage. In this work, we define and initiate the formal study of memory checking for Parallel RAMs (PRAMs). The parallel RAM model is very expressive and captures many modern architectures such as multi-core architectures and cloud clusters. When multiple clients run a PRAM algorithm on a shared remote server, it is possible that there are concurrency issues that cause inconsistencies. Therefore, integrity verification is even more desirable property in this setting. Assuming only the existence of one-way functions, we construct an online memory checker (one that reports faults as soon as they occur) for PRAMs with $O(\log N)$ simulation overhead in both work and depth. In addition, we construct an offline memory checker (one that reports faults only after a long sequence of operations) with amortized $O(1)$ simulation overhead in both work and depth. Our constructions match the best known simulation overhead of the memory checkers in the standard single-user RAM setting. As an application of our parallel memory checking constructions, we additionally construct the first maliciously secure oblivious parallel RAM (OPRAM) with polylogarithmic overhead.

Non-interactive zero-knowledge (NIZK) proof systems are a cornerstone of modern cryptography, but their security has received little attention in the quantum settings. Motivated by improving our understanding of this fundamental primitive against quantum adversaries, we propose a new definition of security against quantum adversary. Specifically, we define the notion of quantum simulation soundness (SS-NIZK), that allows the adversary to access the simulator in superposition. We show a separation between post-quantum and quantum security of SS-NIZK, and prove that Sahai’s construction for SS-NIZK (in the CRS model) can be made quantumly-simulation-sound. As an immediate application of our new notion, we prove the security of the Naor-Yung paradigm in the quantum settings, with respect to a strong quantum IND-CCA security notion. This provides the quantum analogue of the classical dual key approach to prove the security of encryption schemes. Along the way, we introduce a new notion of quantum-query advantage functions, which may be used as a general framework to show classical/quantum separation for other cryptographic primitives, and it may be of independent interest.

Integral, impossible-differential (ID), and zero-correlation (ZC) attacks are three of the most important attacks on block ciphers. However, manually finding these attacks can be a daunting task, which is why automated methods are becoming increasingly important. Most automatic tools regarding integral, ZC, and ID attacks have focused only on finding distinguishers rather than complete attacks. At EUROCRYPT 2023, Hadipour et al. proposed a generic and efficient constraint programming (CP) model based on satisfiability for finding ID, ZC, and integral distinguishers. This new model can be extended to a unified CP model for finding full key recovery attacks. However, it has limitations, including determining the contradiction location beforehand and a cell-wise model unsuitable for weakly aligned ciphers like Ascon and PRESENT. They also deferred developing a CP model for the partial-sum technique in key recovery as future work. In this paper, we enhance Hadipour et al.'s method in several ways. First, we remove the limitation of determining the contradiction location in advance. Second, we show how to extend the distinguisher model to a bit-wise model, considering the internal structure of S-boxes and keeping the model based on satisfiability. Third, we introduce a CP model for the partial-sum technique for the first time. To show the usefulness and versatility of our approach, we apply it to various designs, from strongly aligned ones like ForkSKINNY and QARMAv2 to weakly aligned ones such as Ascon and PRESENT, yielding significantly improved results. To mention a few of our results, we improve the integral distinguisher of QARMAv2-128 (resp. QARMAv2-64) by 7 (resp. 5) rounds, and the integral distinguisher of ForkSKINNY by 1 round, only thanks to our cell-wise distinguishe modelings. By using our new bit-wise modeling, our tool can find a group of $2^{155}$ 5-round ID and ZC distinguishers for Ascon in only one run, taking a few minutes on a regular laptop. The new CP model for the partial-sum technique enhances integral attacks on all SKINNY variants, notably improving the best attack on SKINNY-$n$-$n$ in the single-key setting by 1 round. We also enhance ID attacks on ForkSKINNY and provide the first analysis of this cipher in a limited reduced-round setting. Our methods are generic and applicable to other block ciphers.

Protocols for secure multi-party computation (MPC) supporting mixed-mode computation have found a lot of applications in recent years due to their flexibility in representing the function to be evaluated. However, existing mixed-mode MPC protocols are only practical for a small number of parties: they are either tailored to the case of two/three parties, or scale poorly for a large number of parties. In this paper, we design and implement a new system for highly efficient and scalable mixed-mode MPC tolerating an arbitrary number of semi-honest corruptions. Our protocols allow secret data to be represented in Encrypted, Boolean, Arithmetic, or Yao form, and support efficient conversions between these representations. 1. We design a multi-party table-lookup protocol, where both the index and the table can be kept private. The protocol is scalable even with hundreds of parties. 2. Using the above protocol, we design efficient conversions between additive arithmetic secret sharings and Boolean secret sharings for a large number of parties. For 32 parties, our conversion protocols require 1184× to 8141× less communication compared to the state- of-the-art protocols MOTION and MP-SPDZ; this leads to up to 1275× improvement in running time under 1 Gbps network. The improvements are even larger with more parties. 3. We also use new protocols to design an efficient multi-party distributed garbling protocol. The protocol could achieve asymptotically constant communication per party. Our implementation will be made public.

In this paper, we introduce Oblivious Homomorphic Encryption (OHE) which provably separates the computation spaces of multiple clients of a fully homomorphic encryption (FHE) service while keeping the evaluator blind about whom a result belongs. We justify the importance of this strict isolation property of OHE by showing an attack on a recently proposed key-private cryptocurrency scheme. Our two OHE constructions are based on a puncturing function where the evaluator can effectively mask ciphertexts from rogue and potentially colluding clients. In the first construction OHE1, we show that this can be im- plemented via an FHE scheme (with key privacy and weak wrong-key decryption properties) plus an anonymous commitment scheme. The second construction OHE2, for flexibility of primitive choice, achieves this via a combination of a standard FHE scheme, an encryption scheme with key privacy and weak wrong-key decryption, and an anonymous commitment scheme. OHE can be used to provide provable anonymity to cloud applications, single server implementations of anonymous messaging as well as account-based cryptocurrencies.

The idea of balancing the side-channel leakage in software was proposed more than a decade ago. Just like with other hiding-based countermeasures, the goal is not to hide the leakage completely but to significantly increase the effort required for the attack. Previous approaches focused on two directions: either balancing the Hamming weight of the processed data or deriving the code by using stochastic leakage profiling. In this brief, we build upon these results by proposing a novel approach that combines the two directions. We provide the theory behind our encoding scheme backed by experimental results on a 32-bit ARM Cortex-M4 microcontroller. Our results show that such a combination gives better side-channel resistance properties than each of the two methods separately.

This paper presents full round distinguishing and key recovery attacks on lightweight block cipher SAND-2 with 64-bit block size and 128-bit key size, which appears to be a mixture of the AND-Rotation-XOR (AND-RX) based ciphers SAND and ANT. However, the security arguments against linear and some other attacks are not fully provided. In this paper, we find that the combination of a SAND-like nibble-based round function and ANT-like bit-based permutations will cause dependencies and lead to iterative linear and differential trails with high probabilities. By exploiting these, full round distinguishing attacks on SAND-2 work with $2^{46}$ queries for linear and $2^{58.60}$ queries for differential in the single-key setting. Then, full round key recovery attacks are also mounted, which work with the time complexity $2^{48.23}$ for linear and $2^{64.10}$ for differential. It should be noted that the dependency observed in this paper only works for SAND-2 and will not threaten SAND and ANT. From the point of designers, our attacks show the risk of mixing the parts of different designs, even though each of them is well-studied to be secure.

We show that the Yang et al.'s key agreement scheme [Future Gener. Comput. Syst., 145, 415-428 (2023)] is flawed. (1) There are some inconsistent computations, which should be corrected. (2) The planned route of a target vehicle is almost exposed. The scheme neglects the basic requirement for bit-wise XOR, and tries to encrypt the route by the operator. The negligence results in some trivial equalities. (3) The scheme is insecure against impersonation attack launched by the next roadside unit.

Many secure computation schemes and protocols (such as numerous variants of secure multi-party computation and homomorphic encryption) have favorable performance characteristics when they are used to evaluate addition and scalar multiplication operations on private values that can be represented as ring elements. A purely algebraic argument (with no references to any specific protocol or scheme) can be used to show that the ability to perform these operations is sufficient to implement any univariate map that operates on private values when that map's domain is finite. Such implementations of univariate maps can be composed in sequence any number of times. Other forms of composition for such implementations can be realized by using multiplication operations involving ring elements, but it is possible that these can be substituted with scalar multiplication operations within certain secure computation workflows.

Motivated by applications in anonymous reputation systems and blockchain governance, we initiate the study of predicate aggregate signatures (PAS), which is a new primitive that enables users to sign multiple messages, and these individual signatures can be aggregated by a combiner, preserving the anonymity of the signers. The resulting PAS discloses only a brief description of signers for each message and provides assurance that both the signers and their description satisfy the specified public predicate. We formally define PAS and give a construction framework to yield a logarithmic size signature, and further reduce the verification time also to logarithmic. We also give several instantiations for several concrete predicates that may be of independent interest. To showcase its power, we also demonstrate its applications to multiple settings including multi-signatures, aggregate signatures, threshold sig- natures, (threshold) ring signatures, attribute-based signatures, etc, and advance the state of the art in all of them.

In the design of an identity-based encryption (IBE) scheme, the primary security assumptions center around quadratic residues, bilinear mappings, and lattices. Among these approaches, one of the most intriguing is introduced by Clifford Cocks and is based on quadratic residues. However, this scheme has a significant drawback: a large ciphertext to plaintext ratio. A different approach is taken by Zhao et al., who design an IBE still based on quadratic residues, but with an encryption process reminiscent of the Goldwasser-Micali cryptosystem. In the following pages, we will introduce an elementary method to accelerate Cocks' encryption process and adapt a space-efficient encryption technique for both Cocks' and Zhao et al.'s cryptosystems.

In Crypto 94, Chor, Fiat, and Naor first introduced the traitor tracing (TT) systems, which aim at helping content distributors identify pirates. Since its introduction, many traitor tracing schemes have been proposed. However, we observe until now almost all the traitor tracing systems using probabilistic public key (and secret key) encryption as the the content distribution algorithm, they do not consider this basic fact: the malicious encrypter can plant some trapdoor in the randomness of the ciphertexts and later he can use this trapdoor or the delegation of the trapdoor to construct decoding pirates, He can sell them to the black market and get his own benefits. At first sight, this new attack model is too strong to capture the real attack scenarios. But we think it is valuable at least for the following two reasons: (1) Note in many modern content distribution systems, there are at least existing three different roles: { the content provider, the content distributer and the content consumer. In this framework, the encrypter is not necessarily the content provider (or content owner). It can be a malicious employee in the content provider corporation, it can also be the malicious content distributer or its malicious employee}. In all these cases, the encrypter has its own benefits and has the potential intention to plant some trapdoor in the randomness for generating ciphertexts. (2) Also note in the related work, there is a conclusion that traitor tracing and differential privacy can have directly influence on each other, while differential privacy (DP) is at the heart of constructing modern privacy preserving systems. But if we consider this new insider attacker (the encrypter), at least some part arguments on the relationship between traitor tracing and differential privacy need more consideration. Therefore in this paper we carefully describe this new insider attacker and investigate thoroughly on its effect. Our main research results are the following: (1) We show that many existing public key traitor tracing systems with probabilistic encryption algorithm are failing to work correctly when facing this malicious encrypter.They are including the BSW, BW, GKSW, LCZ and BZ traitor tracing systems. Furthermore, we conclude that most of the existing traitor tracing systems using probabilistic encryption algorithm can not resist this attack. (2) When considering the insider attacker (the encrypter), if the traitor tracing schemes using probabilistic encryption algorithms, the conclusion on tight relationship between traitor tracing and differential privacy may need more consideration. (3) By employing the technique of hash function, we show how to design TT+ system which can resist this type of attack based on the existing traitor tracing system. Compared with the old traitor tracing system, our new proposal does not add much overhead and thus is practical too.

In this paper, we analyze the Espresso cipher from a related key chosen IV perspective. More precisely, we explain how one can obtain Key-IV pairs such that Espresso's keystreams either have certain identical bits or are shifted versions of each other. For the first case, we show how to obtain such pairs after $2^{32}$ iterations, while for the second case, we present an algorithm that produces such pairs in $2^{28}$ iterations. Moreover, we show that by making a minor change in the padding used during the initialization phase, it can lead to a more secure version of the cipher. Specifically, changing the padding increases the complexity of our second attack from $2^{28}$ to $2^{34}$. Finally, we show how related IVs can accelerate brute force attacks, resulting in a faster key recovery. Although our work does not have any immediate implications for breaking the Espresso cipher, these observations are relevant in the related-key chosen IV scenario.

We present a new method for doing multi-party private set intersection against a malicious adversary, which reduces the total communication cost to $ O(nl\kappa) $. Additionally, our method can also be used to build a multi-party Circuit-PSI without payload. Our protocol is based on Vector-OLE(VOLE) and oblivious key-value store(OKVS). To meet the requirements of the protocol, we first promote the definition of VOLE to a multi-party version. After that, we use the new primitive to construct our protocol and prove that it can tolerate all-but-two malicious corruptions. Our protocol follows the idea of [RS21], where each party encodes the respective set as a vector, uses VOLE to encrypt the vector, and finally construct an OPRF to get the result. When it comes to multi-party situation, we have to encrypt several vectors at one time. As a result, the VOLE used in [RS21] and follow-up papers is not enough, that brings our idea of an multi-party VOLE.

In this paper, inspired by the work of Beyne and Rijmen at CRYPTO 2022, we explore the accurate probability of $d$-differential in the fixed-key model. The theoretical foundations of our method are based on a special matrix $-$ quasi-$d$-differential transition matrix, which is a natural extension of the quasidifferential transition matrix. The role of quasi-$d$-differential transition matrices in polytopic cryptananlysis is analogous to that of correlation matrices in linear cryptanalysis. Therefore, the fixed-key probability of a $d$-differential can be exactly expressed as the sum of the correlations of its quasi-$d$-differential trails. Then we revisit the boomerang attack from a perspective of 3-differential. Different from previous works, the probability of a boomerang distinguisher can be exactly expressed as the sum of the correlations of its quasi-$3$-differential trails without any assumptions in our work. In order to illustrate our theory, we apply it to the lightweight block cipher GIFT. It is interesting to find the probability of every optimal 3-differential characteristic of an existing 2-round boomerang is zero, which can be seen as an evidence that the security of block ciphers adopting half-round key XOR might be overestimated previously to some extent in differential-like attacks.

GLS254 is an elliptic curve defined over a finite field of characteristic 2; it contains a 253-bit prime order subgroup, and supports an endomorphism that can be efficiently computed and helps speed up some typical operations such as multiplication of a curve element by a scalar. That curve offers on x86 and ARMv8 platforms the best known performance for elliptic curves at the 128-bit security level. In this paper we present a number of new results related to GLS254: - We describe new efficient and complete point doubling formulas (2M+4S) applicable to all ordinary binary curves. - We apply the previously described (x,s) coordinates to GLS254, enhanced with the new doubling formulas. We obtain formulas that are not only fast, but also complete, and thus allow generic constant-time usage in arbitrary cryptographic protocols. - Our strictly constant-time implementation multiplies a point by a scalar in 31615 cycles on an x86 Coffee Lake, and 77435 cycles on an ARM Cortex-A55, improving previous records by 13% and 11.7% on these two platforms, respectively. - We take advantage of the completeness of the formulas to define some extra operations, such as canonical encoding with (x, s) compression, constant-time hash-to-curve, and signatures. Our Schnorr signatures have size only 48 bytes, and offer good performance: signature generation in 18374 cycles, and verification in 27376 cycles, on x86; this is about four times faster than the best reported Ed25519 implementations on the same platform. - The very fast implementations leverage the carryless multiplication opcodes offered by the target platforms. We also investigate performance on CPUs that do not offer such an operation, namely a 64-bit RISC-V CPU (SiFive-U74 core) and a 32-bit ARM Cortex-M4 microcontroller. While the achieved performance is substantially poorer, it is not catastrophic; on both platforms, GLS254 signatures are only about 2x to 2.5x slower than Ed25519.

The Crossbred algorithm is one of the algorithms for solving a system of polynomial equations, proposed by Joux and Vitse in 2017. It has been implemented in Fukuoka MQ challenge, which is related to the security of multivariate crytography, and holds several records. A framework for estimating the complexity has already been provided by Chen et al. in 2017. However, it is generally unknown which parameters are actually available. This paper investigates how to select available parameters for the Crossbred algorithm. As a result, we provide formulae that give an available parameter set and estimate the complexity of the Crossbred algorithm.

One of the founding results of lattice based cryptography is a quantum reduction from the Short Integer Solution problem to the Learning with Errors problem introduced by Regev. It has recently been pointed out by Chen, Liu and Zhandry that this reduction can be made more powerful by replacing the learning with errors problem with a quantum equivalent, where the errors are given in quantum superposition. In the context of codes, this can be adapted to a reduction from finding short codewords to a quantum decoding problem for random linear codes. We therefore consider in this paper the quantum decoding problem, where we are given a superposition of noisy versions of a codeword and we want to recover the corresponding codeword. When we measure the superposition, we get back the usual classical decoding problem for which the best known algorithms are in the constant rate and error-rate regime exponential in the codelength. However, we will show here that when the noise rate is small enough, then the quantum decoding problem can be solved in quantum polynomial time. Moreover, we also show that the problem can in principle be solved quantumly (albeit not efficiently) for noise rates for which the associated classical decoding problem cannot be solved at all for information theoretic reasons. We then revisit Regev's reduction in the context of codes. We show that using our algorithms for the quantum decoding problem in Regev's reduction matches the best known quantum algorithms for the short codeword problem. This shows in some sense the tightness of Regev's reduction when considering the quantum decoding problem and also paves the way for new quantum algorithms for the short codeword problem.

In this paper, we introduce the subfield bilinear collision problem and use it to construct an identification protocol and a signature scheme. This construction is based on the MPC-in-the-head paradigm and uses the Fiat-Shamir transformation to obtain a signature.

Computing the distance between two non-normalized vectors $\mathbfit{x}$ and $\mathbfit{y}$, represented by $\Delta(\mathbfit{x},\mathbfit{y})$ and comparing it to a predefined public threshold $\tau$ is an essential functionality used in privacy-sensitive applications such as biometric authentication, identification, machine learning algorithms ({\em e.g.,} linear regression, k-nearest neighbors, etc.), and typo-tolerant password-based authentication. Tackling a widely used distance metric, {\sc Nomadic} studies the privacy-preserving evaluation of cosine similarity in a two-party (2PC) distributed setting. We illustrate this setting in a scenario where a client uses biometrics to authenticate to a service provider, outsourcing the distance calculation to two computing servers. In this setting, we propose two novel 2PC protocols to evaluate the normalising cosine similarity between non-normalised two vectors followed by comparison to a public threshold, one in the semi-honest and one in the malicious setting. Our protocols combine additive secret sharing with function secret sharing, saving one communication round by employing a new building block to compute the composition of a function $f$ yielding a binary result with a subsequent binary gate. Overall, our protocols outperform all prior works, requiring only two communication rounds under a strong threat model that also deals with malicious inputs via normalisation. We evaluate our protocols in the setting of biometric authentication using voice, and the obtained results reveal a notable efficiency improvement compared to existing state-of-the-art works.

MAYO is a popular high-calorie condiment as well as an auspicious candidate in the ongoing NIST competition for additional post-quantum signature schemes achieving competitive signature and public key sizes. In this work, we present high-speed implementations of MAYO using the AVX2 and Armv7E-M instruction sets targeting recent x86 platforms and the Arm Cortex-M4. Moreover, the main contribution of our work is showing that MAYO can be even faster when switching from a bitsliced representation of keys to a nibble-sliced representation. While the bitsliced representation was primarily motivated by faster arithmetic on microcontrollers, we show that it is not necessary for achieving high performance on Cortex-M4. On Cortex-M4, we instead propose to implement the large matrix multiplications of MAYO using the Method of the Four Russians (M4R), which allows us to achieve better performance than when using the bitsliced approach. This results in up to 21% faster signing. For AVX2, the change in representation allows us to implement the arithmetic much faster using shuffle instructions. Signing takes up to 3.2x fewer cycles and key generation and verification enjoy similar speedups. This shows that MAYO is competitive with lattice-based signature schemes on x86 CPUs, and a factor of 2-6 slower than lattice-based signature schemes on Cortex-M4 (which can still be considered competitive).

We prove that two variants of the Fujisaki-Okamoto (FO) transformations are selective opening secure (SO) against chosen-ciphertext attacks in the quantum random oracle model (QROM), assuming that the underlying public-key encryption scheme is one-way secure against chosen-plaintext attacks (OW-CPA). The two variants we consider are $\mathsf{FO}^{\not{\bot}}$ (Hofheinz, Hövelmanns, and Kiltz, TCC 2017) and $\mathsf{U}^{\not{\bot}}_\mathsf{m}$ (Jiang et al., CRYPTO 2018). This is the first correct proof in the QROM. The previous work of Sato and Shikata (IMACC 2019) showed the SO security of $\mathsf{FO}^{\not{\bot}}$ in the QROM. However, we identify a subtle gap in their work. To close this gap, we propose a new framework that allows us to adaptively reprogram a QRO with respect to multiple queries that are computationally hard to predict. This is a property that can be easily achieved by the classical ROM, but is very hard to achieve in the QROM. Hence, our framework brings the QROM closer to the classical ROM. Under our new framework, we construct the first tightly SO secure PKE in the QROM using lossy encryption. Our final application is proving $\mathsf{FO}^{\not{\bot}}$ and $\mathsf{U}^{\not{\bot}}_\mathsf{m}$ are bi-selective opening (Bi-SO) secure in the QROM. This is a stronger SO security notion, where an adversary can additionally corrupt some users' secret keys.

Deep learning-based profiling side-channel analysis has gained widespread adoption in academia and industry due to its ability to uncover secrets protected by countermeasures. However, to exploit this capability, an adversary must have access to a clone of the targeted device to obtain profiling measurements and know secret information to label these measurements. Non-profiling attacks avoid these constraints by not relying on secret information for labeled data. Instead, they attempt all key guesses and select the most successful one. Deep learning approaches form the foundation of several non-profiling attacks, but these methods often suffer from high computational complexity and limited performance in practical applications. This work explores the performance of multi-output regression (MOR) models in side-channel analysis. We start with the recently proposed multi-output regression (MOR) approach for non-profiling side-channel analysis. Then, we significantly improve its performance by updating the 1) loss function, 2) distinguisher, and 3) employing a novel concept of validation set to reduce overfitting. We denote our approach as MORE - Multi-Output Regression Enhanced, which emphasizes significantly better attack performance than MOR. Our results demonstrate that combining the MORE methodology, ensembles, and data augmentation presents a potent strategy for enhancing non-profiling side-channel attack performance and improving the reliability of distinguishing key candidates.

Weightwise degree-d functions are Boolean functions that take the values of a function of degree at most d on each set of fixed Hamming weight. The class of weightwise affine functions encompasses both the symmetric functions and the Hidden Weight Bit Function (HWBF). The good cryptographic properties of the HWBF, except for the nonlinearity, motivates to investigate a larger class with functions that share the good properties and have a better nonlinearity. Additionally, the homomorphic friendliness of symmetric functions exhibited in the context of hybrid homomorphic encryption and the recent results on homomorphic evaluation of Boolean functions make this class of functions appealing for efficient privacy-preserving protocols. In this article we realize the first study on weightwise degree-d functions, focusing on weightwise affine and weightwise quadratic functions. We show some properties on these new classes of functions, in particular on the subclass of cyclic weightwise functions. We provide balanced constructions and prove nonlinearity lower bounds for all cyclic weightwise affine functions and for a family of weightwise quadratic functions. We complement our work with experimental results, they show that other cyclic weightwise linear functions than the HWBF have better cryptographic parameters, and considering weightwise quadratic functions allows to reach higher algebraic immunity and substantially better nonlinearity.

The need for energy optimizations in modern systems forces CPU vendors to provide Dynamic Voltage Frequency Scaling (DVFS) interfaces that allow software to control the voltage and frequency of CPU cores. In recent years, the accessibility of such DVFS interfaces to adversaries has amounted to a plethora of fault attack vectors. In response, the current countermeasures involve either restricting access to DVFS interfaces or including additional compiler-based checks that let the DVFS fault occur but prevent an adversary from weaponizing it. However, such countermeasures are overly restrictive because (1) they prevent benign, non-SGX processes from utilizing DVFS, and (2) rely upon a less practical threat model than what is acceptable for Intel SGX. In this work, we hence put forth a new countermeasure perspective. We reason that all DVFS fault attacks are helped by system design decisions that allow an adversary to search through the entire space of frequency/voltage pairs which lead to DVFS faults on the victim system. Using this observation, we classify such frequency/voltage pairs causing DVFS faults as unsafe system states. We then develop a kernel module level countermeasure (in non-SGX execution context) that polls core frequency/voltage pairs to detect when the system is in an unsafe state, and force it back into a safe state. Our countermeasure completely prevents DVFS faults on three Intel generation CPUs: Sky Lake, Kaby Lake R, and Comet Lake, while allowing accessibility of DVFS features to benign non-SGX executions (something which prior works fail to achieve). Additionally, we also put forth the notion of maximal safe state, allowing our countermeasure to be implemented both as microcode (on the micro-architecture level) and as model-specific register (on the hardware level), as opposed to prior countermeasures which can not be implemented at the hardware level. Finally, we evaluate the overhead of our kernel module's execution on SPEC2017, observing an minuscule overhead of 0.28%.

Abstract—Large transformer-based models have realized state- of-the-art performance on lots of real-world tasks such as natural language processing and computer vision. However, with the increasing sensitivity of the data and tasks they handle, privacy has become a major concern during model deployment. In this work, we focus on private inference in two-party settings, where one party holds private inputs and the other holds the model. We introduce BumbleBee, a fast and communication-friendly two- party private transformer inference system. Our contributions are three-fold: First, we propose optimized protocols for matrix multiplication, which significantly reduce communication costs by 80% – 90% compared to previous techniques. Secondly, we develop a methodology for constructing efficient protocols tailored to the non-linear activation functions employed in transformer models. The proposed activation protocols have realized a significant enhancement in processing speed, alongside a remarkable reduction in communication costs by 80% – 95% compared with two prior methods. Lastly, we have performed extensive benchmarks on five transformer models. BumbleBee demonstrates its capability by evaluating the LLaMA-7B model, generating one token in approximately 14 minutes using CPUs. Our results further reveal that BumbleBee outperforms Iron (NeurIPS22) by over an order of magnitude and is three times faster than BOLT (Oakland24) with one-tenth communication.

In STOC 2019 Canetti et al. showed how to soundly instantiate the Fiat-Shamir transform assuming that prover and verifier have access to the key of a 𝑐𝑜𝑟𝑟𝑒𝑙𝑎𝑡𝑖𝑜𝑛 𝑖𝑛𝑡𝑟𝑎𝑐𝑡𝑎𝑏𝑙𝑒 ℎ𝑎𝑠ℎ 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛 𝑓𝑜𝑟 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡𝑙𝑦 𝑠𝑒𝑎𝑟𝑐ℎ𝑎𝑏𝑙𝑒 𝑟𝑒𝑙𝑎𝑡𝑖𝑜𝑛𝑠. The transform requires the starting protocol to be a special 3-round public-coin scheme that Canetti et al. call 𝑡𝑟𝑎𝑝𝑑𝑜𝑜𝑟 𝑠𝑖𝑔𝑚𝑎-𝑝𝑟𝑜𝑡𝑜𝑐𝑜𝑙. One downside of the Canetti et al. approach is that the key of the hash function can be used only once (or a pre-determined bounded number of times). That is, each new zero-knowledge proof requires a freshly generated hash key (i.e., a freshly generated setup). This is in contrast to what happens with the standard Fiat-Shamir transform, where the prover, having access to the same hash function (modeled as a random-oracle), can generate an unbounded number of proofs that are guaranteed to be zero-knowledge and sound. As our main contribution, we extend the results of Canetti et al., by proposing a multi-theorem protocol that follows the Fiat-Shamir paradigm and relies on correlation intractable hash functions. Moreover, our protocol remains zero-knowledge and sound even against adversaries that choose the statement to be proven (and the witness for the case of zero-knowledge) adaptively on the key of the hash function. Our construction is presented in the form of a compiler, that follows the Fiat-Shamir paradigm, which takes as input any trapdoor sigma-protocol for the NP-language $L$ and turns it into a non-interactive zero-knowledge protocol that satisfies the properties we mentioned. To be best of our knowledge, ours is the first compiler that follows the Fiat-Shamir paradigm to obtain a multi-theorem adaptive NIZK relying on correlation intractable hash functions.

Oblivious RAM (ORAM) is a general-purpose technique for hiding memory access patterns. This is a fundamental task underlying many secure computation applications. While known ORAM schemes provide optimal asymptotic complexity, despite extensive efforts, their concrete costs remain prohibitively expensive for many interesting applications. The current state-of-the-art practical ORAM schemes are suitable only for somewhat small memories (Square-Root ORAM or Path ORAM). This work presents a novel concretely efficient ORAM construction based on recent breakthroughs in asymptotic complexity of ORAM schemes (PanORAMa and OptORAMa). We bring these constructions to the realm of practically useful schemes by relaxing the restriction on constant local memory size. Our design provides a factor of at least $6$ to $8$ improvement over an implementation of the original Path ORAM for a set of reasonable memory sizes (e.g., 1GB, 1TB) and with the same local memory size. To our knowledge, this is the first practical implementation of an ORAM based on the full hierarchical ORAM framework. Prior to our work, the belief was that hierarchical ORAM-based constructions were inherently too expensive in practice. We implement our design and provide extensive evaluation and experimental results.

Differential-Linear (DL) cryptanalysis is a well known cryptanalytic technique that combines differential and linear cryptanalysis. Over the years, multiple techniques were proposed to increase its strength and applicability. Two relatively recent ones are: The partitioning technique by Leurent and the use of neutral bits adapted by Beierle et al. to DL cryptanalysis. In this paper we compare these techniques and discuss the possibility of using them together to achieve the best possible DL attacks. We study the combination of these two techniques and show that in many cases they are indeed compatible. We demonstrate the strength of the combination in two ways. First, we present the first DL attack on 4-round Xoodyak and an extension to 5-round in the related key model. We show that the attacks are possible only by using these two techniques simultaneously. In addition, using the combination of the two techniques we improve a DL attack on 9-round DES. We show that the partitioning technique mainly reduces the time complexity, and the use of neutral bits mainly reduces the data complexity, while the combination of them reduces both the time and data complexities.

Post-quantum cryptographic (PQC) algorithms, especially those based on the learning with errors (LWE) problem, have been subjected to several physical attacks in the recent past. Although the attacks broadly belong to two classes -- passive side-channel attacks and active fault attacks, the attack strategies vary significantly due to the inherent complexities of such algorithms. Exploring further attack surfaces is, therefore, an important step for eventually securing the deployment of these algorithms. Also, it is important to test the robustness of the already proposed countermeasures in this regard. In this work, we propose a new fault attack on side-channel secure masked implementation of LWE-based key-encapsulation mechanisms (KEMs) exploiting fault propagation. The attack typically originates due to an algorithmic modification widely used to enable masking, namely the Arithmetic-to-Boolean ($\mathtt{A2B}$) conversion. We exploit the data dependency of the adder carry chain in $\mathtt{A2B}$ and extract sensitive information, albeit masking (of arbitrary order) being present. As a practical demonstration of the exploitability of this information leakage, we show key recovery attacks of Kyber, although the leakage also exists for other schemes like Saber. The attack on Kyber targets the decapsulation module and utilizes Belief Propagation (BP) for key recovery. To the best of our knowledge, it is the first attack exploiting an algorithmic component introduced to ease masking rather than only exploiting the randomness introduced by masking to obtain desired faults (as done by Delvaux). Finally, we performed both simulated and electromagnetic (EM) fault-based practical validation of the attack for an open-source first-order secure Kyber implementation running on an STM32 platform.

Sponge based constructions have gained significant popularity for designing lightweight authenticated encryption modes. Most of the authenticated ciphers following the Sponge paradigm can be viewed as variations of the Transform-then-permute construction. It is known that a construction following the Transform-then-permute paradigm provides security against any adversary having data complexity $D$ and time complexity $T$ as long as $DT \ll 2^{b-r}$. Here, $b$ represents the size of the underlying permutation, while $r$ pertains to the rate at which the message is injected. The above result demonstrates that an increase in the rate leads to a degradation in the security of the constructions, with no security guaranteed to constructions operating at the full rate, where $r=b$. This present study delves into the exploration of whether adding some auxiliary states could potentially improve the security of the Transform-then-permute construction. Our investigation yields an affirmative response, demonstrating that a special class of full rate Transform-then-permute with additional states, dubbed frTtP+, can indeed attain security when operated under a suitable feedback function and properly initialized additional state. To be precise, we prove that frTtP+ provides security as long as $D \ll 2^{s/2}$ and $T \ll 2^{s}$, where $s$ denotes the size of the auxiliary state in terms of bits. To demonstrate the applicability of this result, we show that the construction $Orange-Zest_{mod}$ belongs to this class, thereby obtaining the desired security. In addition, we propose a family of full-rate Transform-then-permute construction with a Beetle-like feedback function, dubbed \textsf{fr-Beetle}, which also achieves the same level of security.

Recently, Servan-Schreiber et al. (S&P 2023) proposed a new notion called private access control lists (PACL) for function secret sharing (FSS), where the FSS evaluators can ensure that the FSS dealer is authorized to share the given function with privacy assurance. In particular, for the secret sharing of a point function $f_{\alpha, \beta}$, namely distributed point function (DPF), the authors showed how to efficiently restrict the choice of $\alpha$ via a specific PACL scheme from verifiable DPF. In this work, we show their scheme is insecure due to the lack of assessment of $\beta$, and we fix it using an auxiliary output. We then propose more fine-grained policy constraints for DPF. Our schemes allow an attribute-based access control w.r.t. $\alpha$, and a template restriction for $\beta$. Furthermore, we show how to reduce the storage size of the constraint representation from $O(N)$ to $O(\log N)$, where $N$ is the number of constraints. Our benchmarks show that the amortized running time of our attribute-based scheme and logarithmic storage scheme is $2.5\times$ - $3\times$ faster than the state-of-the-art with $2^{15}$ constraints.

We show that the Cherbal-Benchetioui key agreement scheme [Comput. Electr. Eng., 109, 108759 (2023)] fails to keep user anonymity, not as claimed. The scheme simply thinks that user anonymity is equivalent to protecting the user's real identity. But the true anonymity means that the adversary cannot attribute different sessions to target entities, which relates to entity-distinguishable, not just identity-revealable.

Private Set Intersection (PSI) is a well-studied secure two-party computation problem in which a client and a server want to compute the intersection of their input sets without revealing additional information to the other party. With this work, we present nested Cuckoo hashing, a novel hashing approach that can be combined with additively homomorphic encryption (AHE) to construct an efficient PSI protocol for unbalanced input sets. We formally prove the security of our protocol against semi-honest adversaries in the standard model. Our protocol yields client computation and communication complexity that is sublinear in the server’s set size and is thus of interest to clients with limited resources. The implementation and empirical evaluation of our protocol using the exponential ElGamal and BGV/BFV encryption schemes attests to state-of-the-art practical performance.

An $(n, t)$-Verifiable Secret Sharing (VSS) scheme allows a dealer to share a secret among $n$ parties, s.t. all the parties can verify the validity of their shares and only a set of them, i.e., more than $t$, can access the secret. In this paper, we present $\Pi$, as a unified framework for constructing computational VSS schemes in the honest-majority setting, based on Shamir secret sharing. Notably, $\Pi$ does not rely on homomorphic commitments; instead requires a random oracle and any commitment scheme that extra to its core attributes hiding and binding, it might be homomorphic and/or Post-Quantum (PQ) secure. - When employing Discrete Logarithm (DL)-based commitments, $\Pi$ enables the construction of two novel VSS schemes in the RO model, named $\Pi_P$ and $\Pi_F$. Compared to the well-known Pedersen and Feldman VSS schemes, both $\Pi_P$ and $\Pi_F$ require $O(1)$ (resp. $O(t)$) exponentiations in the verification (resp. reconstruction) process, as opposed to $O(t)$ (resp. $O(t^2)$), albeit at the expense of a constant factor slower sharing and increased communication. - By instantiating $\Pi$ with a hash-based commitment, we obtain a novel PQ-secure VSS scheme, labeled $\Pi_{LA}$. $\Pi_{LA}$ outperforms the recent protocol by Atapoor, Baghery, Cozzo, and Pedersen from Asiacrypt'23 by a constant factor in all metrics. $\Pi_{LA}$ can also be seen as an amplified version of the simple VSS scheme, proposed by Gennaro, Rabin, and Rabin at PODC'98. - Building upon $\Pi_F$, we construct a Publicly VSS (PVSS) scheme, labeled $\Pi_S$, that can be seen as a new variant of Schoenmakers' scheme from Crypto'99. To this end, we first define the Polynomial Discrete Logarithm (PDL) problem, as a generalization of DL and then build a variant of the Schnorr Proof of Knowledge (PoK) scheme based on the new hardness assumption. We think the PDL relation and the associated PoK scheme can be independently interesting for Shamir-based threshold protocols. We believe $\Pi$ is general enough to be employed in various contexts such as lattices, isogenies, and an extensive array of practical use cases.

We design a SNARKs/STARKs-optimized AEAD scheme based on the $\texttt{MonkeySpongeWrap}$ (ToSC 2023(2)) and the RPO permutation (ePrint 2022/1577).

QARMAv2 represents a family of lightweight block ciphers introduced in ToSC 2023. This new iteration, QARMAv2, is an evolution of the original QARMA design, specifically constructed to accommodate more extended tweak values while simultaneously enhancing security measures. This family of ciphers is available in two distinct versions, referred to as QARMAv2-$b$-$s$, where ‘$b$’ signifies the block length, with options for both 64-bit and 128-bit blocks, and ‘$c$’ signifies the key length. In this paper, for the first time, we present differential fault analysis (DFA) of all the QARMAv2 variants- QARMAv2-64, and QARMAv2-128 by introducing an approach to utilize the fault propagation patterns at the nibble level, with the goal of identifying relevant faulty ciphertexts and vulnerable fault positions. This technique highlights a substantial security risk for the practical implementation of QARMAv2. By strategically introducing six random nibble faults into the input of the $(r − 1)$-th and $(r − 2)$-th backward rounds within the $r$-round QARMAv2-64, our attack achieves a significant reduction in the secret key space, diminishing it from the expansive $2^{128}$ to a significantly more smaller set of size $2^{32}$. Additionally, when targeting QARMAv2-128-128, it demands the introduction of six random nibble faults to effectively reduce the secret key space from $2^{128}$ to a remarkably reduced $2^{24}$. To conclude, we also explore the potential extension of our methods to conduct DFA on various other iterations and adaptations of the QARMAv2 cryptographic scheme. To the best of our knowledge, this marks the first instance of a differential fault attack targeting the QARMAv2 tweakable block cipher family, signifying an important direction in cryptographic analysis.

Since 2016’s NIST call for standardization of post-quantum cryptographic primitives, developing efficient post-quantum secure digital signature schemes has become a highly active area of research. The difficulty in constructing such schemes is evidenced by NIST reopening the call in 2022 for digital signature schemes, because of missing diversity in existing proposals. In this work, we introduce the new post-quantum digital signature scheme MiRitH. As direct successor of a scheme recently developed by Adj, Rivera-Zamarripa and Verbel (Africacrypt ’23), it is based on the hardness of the MinRank problem and follows the MPC-in-the-Head paradigm. We revisit the initial proposal, incorporate design-level improvements and provide more efficient parameter sets. We also provide the missing justification for the quantum security of all parameter sets following NIST metrics. In this context we design a novel Grover-amplified quantum search algorithm for solving the MinRank problem that outperforms a naive quantum brute-force search for the solution. MiRitH obtains signatures of size 5.7 kB for NIST category I security and therefore competes for the smallest signatures among any post-quantum signature following the MPCitH paradigm. At the same time MiRitH offers competitive signing and verification timings compared to the state of the art. To substantiate those claims we provide extensive implementations. This includes a reference implementation as well as optimized constant-time implementations for Intel processors (AVX2), and for the ARM (NEON) architecture. The speed-up of our optimized AVX2 implementation relies mostly on a redesign of the finite field arithmetic, improving over existing implementations as well as an improved memory management.

Classic MLaaS solutions suffer from privacy-related risks since the user is required to send unencrypted data to the server hosting the MLaaS. To alleviate this problem, a thriving line of research has emerged called Privacy-Preserving Machine Learning (PPML) or secure MLaaS solutions that use cryptographic techniques to preserve the privacy of both the input of the client and the output of the server. However, these implementations do not take into consideration the possibility of transferability of known attacks in classic MLaaS settings to PPML settings. In this paper, we demonstrate that it is possible to transfer existing model-extraction attacks using adversarial examples to PPML settings due to relaxed constraints on the abilities of the adversary. We show a working example of an end-to-end attack on an image processing application built using a popular FHE-based framework, namely Concrete-ML. We successfully create a cloned model with just 5000 queries, which is, in fact, 10× less than the size of the training set of the victim model, while incurring only a 7% loss in accuracy. Further, we incorporate the well-known defense strategy against such attacks and show that our attack is still able to clone the model. Finally, we evaluate the different defense rationales that exist in literature and observe that such model stealing attacks are difficult to prevent in secure MLaaS settings.

[IMPORTANT NOTE] This work was mostly done whilst I as a Master's student at Royal Holloway. Unfortunately, I do not believe it is of sufficient quality or rigor, and maybe even worse, is very sloppy and at times, confusing. Hence, I am leaving this note to point readers in the direction of other publications that also concern themselves with the complexity of the Crossbred algorithm. These works have done a much better job, and I do not believe that I have the profile at the moment to continue working on this particular topic. [1] J. Baena, D. Cabarcas, S. K. Tiwari, J. Verbel, and L. Villota, ‘Admissible parameters for the Crossbred algorithm and semi-regular sequences over finite fields’, Designs, Codes and Cryptography, Mar. 2025. [2] D. Vidal, C. Delaplace, and S. Ionica, ‘An analysis of the Crossbred Algorithm for the MQ Problem’, IACR Communications in Cryptology, vol. 1, no. 3, 2024.

In the traditional consensus problem (aka Byzantine agreement), parties are required to agree on a common value despite the malicious behavior of some of them, subject to the condition that if all the honest parties start the execution with the same value, then that should be the outcome. This problem has been extensively studied by both the distributed computing and cryptographic protocols communities. With the advent of blockchains, whose main application—a distributed ledger—essentially requires that miners agree on their views, new techniques have been proposed to solve the problem, and in particular in so-called ``permissionless'' environments, where parties are not authenticated or have access to point-to-point channels and, further, may come and go as they please. So far, the fastest way to achieve consensus in the proof-of-work (PoW)-based setting of Bitcoin, takes O(polylog $\kappa$) number of rounds, where $\kappa$ is the security parameter. We present the first protocol in this setting that requires expected-constant number of rounds. Furthermore, we show how to apply securely sequential composition in order to yield a fast distributed ledger protocol that settles all transactions in expected-constant time. Our result is based on a novel instantiation of ``m-for-1 PoWs'' on parallel chains that facilitates our basic building block, Chain-King Consensus. The techniques we use, via parallel chains, to port classical protocol design elements (such as Phase-King Consensus, super-phase sequential composition and others) into the permissionless setting may be of independent interest.

This paper presents a procedure to construct parameterized families of prime-order endomorphism-equipped elliptic curves that are defined over the scalar field of pairing-friendly elliptic curve families such as Barreto–Lynn–Scott (BLS), Barreto–Naehrig (BN) and Kachisa–Schaefer–Scott (KSS), providing general formulas derived from the curves’ seeds. These so-called “embedded curves” are of major interest in SNARK applications that prove statements involving elliptic curve arithmetic i.e. digital signatures. In this paper, the mathematical groundwork is laid, and advantages of these embeddings are discussed. Additionally, practical examples in the case of BN and BLS families are included and impossibility results regarding KSS families are explained.

We present a publicly-detectable watermarking scheme for LMs: the detection algorithm contains no secret information, and it is executable by anyone. We embed a publicly-verifiable cryptographic signature into LM output using rejection sampling and prove that this produces unforgeable and distortion-free (i.e., undetectable without access to the public key) text output. We make use of error-correction to overcome periods of low entropy, a barrier for all prior watermarking schemes. We implement our scheme and find that our formal claims are met in practice.

The Byzantine Fault Tolerance (BFT) protocol is a long-standing topic. Recently, a lot of efforts have been made in the research of asynchronous BFT. However, the existing solutions cannot adapt well to the flexible network environment, and suffer from problems such as high communication complexity or long latency. To improve the efficiency of BFT consensus in flexible networks, we propose FaBFT. FaBFT's clients can make their own assumptions about the network conditions, and make the most of their networks based on different network assumptions. We also use the BlockDAG structure and an efficient consistent broadcast protocol to improve the concurrency and reduce the number of steps in FaBFT. The comparison with other asynchronous BFT protocols shows that FaBFT has lower complexity and cancels the dependency on the view change. We prove that FaBFT is an atomic broadcast protocol in the flexible networks.

The partial sums cryptanalytic technique was introduced in 2000 by Ferguson et al., who used it to break 6-round AES with time complexity of $2^{52}$ S-box computations -- a record that has not been beaten ever since. In 2014, Todo and Aoki showed that for 6-round AES, partial sums can be replaced by a technique based on the Fast Fourier Transform (FFT), leading to an attack with a comparable complexity. In this paper we show that the partial sums technique can be combined with an FFT-based technique, to get the best of the two worlds. Using our combined technique, we obtain an attack on 6-round AES with complexity of about $2^{46.4}$ additions. We fully implemented the attack experimentally, along with the partial sums attack and the Todo-Aoki attack, and confirmed that our attack improves the best known attack on 6-round AES by a factor of more than 32. We expect that our technique can be used to significantly enhance numerous attacks that exploit the partial sums technique. To demonstrate this, we use our technique to improve the best known attack on 7-round Kuznyechik by a factor of more than 80, and to reduce the complexity of the best known attack on the full MISTY1 from $2^{69.5}$ to $2^{67}$.

In TCHES’22, Shen et al. proposed Triplex, a single-pass leakage-resistant authenticated encryption scheme based on Tweakable Block Ciphers (TBCs) with 2n-bit tweaks. Triplex enjoys beyond-birthday-bound ciphertext integrity in the CIML2 setting and birthday-bound confidentiality in the CCAmL1 notion. Despite its strengths, Triplex’s operational efficiency was hindered by its sequential nature, coupled with a rate limit of 2/3. In an endeavor to surmount these efficiency challenges, Peters et al. proposed Multiplex, a variant of Triplex with increased parallelism and a flexible rate of d/(d+1) that retains similar security guarantees. However, the innovation came at the price of requiring TBCs with dn-bit tweaks, which are unusual and potentially costly for d > 3. In this paper, we investigate the limits of generalized Triplex- and Multiplex-type constructions for single-pass leakage-resilient authenticated encryption. Our contributions are threefold. First, we show that such constructions cannot provide CIML2 integrity for any tweak lengths below dn/2 bits. Second, we provide a birthday-bound attack for constructions with TBCs of tweak lengths between dn/2 and (d − 1)n + n/2 bits. Finally, on the constructive side, we propose a family of single-pass leakage-resilient authenticated ciphers, dubbed Tweplex, that uses tweaks of dn/2 bits and provides a rate of d/(d + 1) while providing n/2-bit CIML2 integrity and CCAmL1 confidentiality.

In recent years, the elliptic curve Qu-Vanstone (ECQV) implicit certificate scheme has found application in security credential management systems (SCMS) and secure vehicle-to-everything (V2X) communication to issue pseudonymous certificates. However, the vulnerability of elliptic-curve cryptography (ECC) to polynomial-time attacks posed by quantum computing raises concerns. In order to enhance resistance against quantum computing threats, various post-quantum cryptography methods have been adopted as standard (e.g. Dilithium) or candidate standard methods (e.g. McEliece cryptography), but state of the art has proven to be challenging to implement implicit certificates using lattice-based cryptography methods. Therefore, this study proposes a post-quantum cryptography McEliece-Chen (PQCMC) based on an efficient random invertible matrix generation method to issue pseudonymous certificates with less computation time. The study provides mathematical models to validate the key expansion process for implicit certificates. Furthermore, comprehensive security evaluations and discussions are conducted to demonstrate that distinct implicit certificates can be linked to the same end entity. In experiments, a comparison is conducted between the certificate length and computation time to evaluate the performance of the proposed PQCMC. This study demonstrates the viability of the implicit certificate scheme based on PQC as a means of countering quantum computing threats.

The global lockdown imposed during the Covid-19 pandemic has resulted in significant social and economic challenges. In an effort to reopen economies and simultaneously control the spread of the disease, the implementation of contact tracing and digital vaccine passport technologies has been introduced. While contact tracing methods have been extensively studied and scrutinized for security concerns through numerous publications, vaccine passports have not received the same level of attention in terms of defining the problems they address, establishing security requirements, or developing efficient systems. Many of the existing methods employed currently suffer from privacy issues. This work introduces PPass, an advanced digital vaccine passport system that prioritizes user privacy. We begin by outlining the essential security requirements for an ideal vaccine passport system. To address these requirements, we present two efficient constructions that enable PPass to function effectively across various environments while upholding user privacy. By estimating its performance, we demonstrate the practical feasibility of PPass. Our findings suggest that PPass can efficiently verify a passenger’s vaccine passport in just 7 milliseconds, with a modest bandwidth requirement of 480KB.

Suppose a prover, in possession of a large body of valuable evidence, wants to quickly convince a verifier by presenting only a small portion of the evidence. We define an Approximate Lower Bound Argument, or ALBA, which allows the prover to do just that: to succinctly prove knowledge of a large number of elements satisfying a predicate (or, more generally, elements of a sufficient total weight when a predicate is generalized to a weight function). The argument is approximate because there is a small gap between what the prover actually knows and what the verifier is convinced the prover knows. This gap enables very efficient schemes. We present noninteractive constructions of ALBA in the random oracle and uniform reference string models and show that our proof sizes are nearly optimal. We also show how our constructions can be made particularly communication-efficient when the evidence is distributed among multiple provers, which is of practical importance when ALBA is applied to a decentralized setting. We demonstrate two very different applications of ALBAs: for large-scale decentralized signatures and for proving universal composability of succinct proofs.

We present a general framework for polynomial-time lattice Gaussian sampling. Its principles revolve around a systematic study of the discrete Gaussian measure and its samplers under \emph{extensions} of lattices, a setting encompassing that of nested lattices $\Lat'\subset \Lat$. A fundamental result for Gaussian sampling is that we can sample efficiently in $\Lat$ if we know how to do so in $\Lat'$ and the quotient $\Lat/\Lat'$. This is often presented in a context-dependent manner. Our formalism provides general and unifying foundations for this idea, and also shows that this can be done \emph{regardless} of the primitivity of $\Lat'$. As direct illustrations of this unifying theme, we formally recover and extend several methods from the literature: domain restriction and extension (as, efficient sampling in a larger or smaller lattice); a lattice \emph{filtration} based sampler, which can be seen as a broad generalization of the celebrated Klein's sampler. Then, we demonstrate how to sample using a change of bases, or even switching the ambient space, even when the target lattice is not represented as full-rank in the ambient space. We show how to correct the induced distortion with the ``convolution-like'' technique of Peikert (Crypto 2010) (which we encompass as a byproduct). Since our framework aims at modularity and leverages the combinations of smaller samplers to build new ones, we also propose ad-hoc samplers for the so-called \emph{root lattices} $\An_n, \Dn_n, \mathsf{E}_n$ as base cases, extending the state-of-the-art for root lattice sampling, which was limited to $\ZZ^n$. We also show how our framework blends with the so-called $k$ing construction and provides a sampler for the remarkable Leech and Barnes-Wall lattices. As concrete applications of our methods, we obtain novel, quasi-linear samplers for prime and smooth conductor (as $2^\ell 3^k$) cyclotomic rings, achieving essentially optimal Gaussian width. In a practice-oriented application, we showcase the impact of our work on hash-and-sign signatures over \textsc{ntru} lattices. In the best case, we can decrease their sizes by around 200 bytes (which corresponds to an improvement greater than 20\%). We also improve the new gadget-based constructions (Yu, Jia, Wang, Crypto 2023) and gain up to 110 bytes for the resulting signatures. Lastly, we sprinkle our exposition with several new estimates for the smoothing parameter of lattices, stemming from our algorithmic constructions and by novel methods based on series reversion.

QCB is a proposal for a post-quantum secure, rate-one authenticated encryption with associated data scheme (AEAD) based on classical OCB3 and \(\Theta\)CB, which are vulnerable against a quantum adversary in the Q2 setting. The authors of QCB prove integrity under plus-one unforgeability, whereas the proof of the stronger definition of blind unforgeability has been left as an open problem. After a short overview of QCB and the current state of security definitions for authentication, this work proves blind unforgeability of QCB. Finally, the strategy of using tweakable block ciphers in authenticated encryption is generalised to a generic blindly unforgeable AEAD model.

$\Sigma$-protocols are a widely utilized, relatively simple and well understood type of zero-knowledge proofs. However, the well known Schnorr $\Sigma$-protocol for proving knowledge of discrete logarithm in a cyclic group of known prime order, and similar protocols working over this type of groups, are hard to generalize to dealing with other groups. In particular with hidden order groups, due to the inability of the knowledge extractor to invert elements modulo the order. In this paper, we introduce a universal construction of $\Sigma$-protocols designed to prove knowledge of preimages of group homomorphisms for any abelian finite group. In order to do this, we first establish a general construction of a $\Sigma$-protocol for $\mathfrak{R}$-module homomorphism given only a linear secret sharing scheme over the ring $\mathfrak{R}$, where zero knowledge and special soundness can be related to the privacy and reconstruction properties of the secret sharing scheme. Then, we introduce a new construction of 2-out-of-$n$ packed black-box secret sharing scheme capable of sharing $k$ elements of an arbitrary (abelian, finite) group where each share consists of $k+\log n-3$ group elements. From these two elements we obtain a generic ``batch'' $\Sigma$-protocol for proving knowledge of $k$ preimages of elements via the same group homomorphism, which communicates $k+\lambda-3$ elements of the group to achieve $2^{-\lambda}$ knowledge error. For the case of class groups, we show that our $\Sigma$-protocol improves in several aspects on existing proofs for knowledge of discrete logarithm and other related statements that have been used in a number of works. Finally, we extend our constructions from group homomorphisms to the case of ZK-ready functions, introduced by Cramer and Damg\aa rd in Crypto 09, which in particular include the case of proofs of knowledge of plaintext (and randomness) for some linearly homomorphic encryption schemes such as Joye-Libert encryption. However, in the case of Joye-Libert, we show an even better alternative, using Shamir secret sharing over Galois rings, which achieves $2^{-k}$ knowledge soundness by communicating $k$ ciphertexts to prove $k$ statements.

Publicly Verifiable Secret Sharing (PVSS) allows a dealer to publish encrypted shares of a secret so that parties holding the corresponding decryption keys may later reconstruct it. Both dealing and reconstruction are non-interactive and any verifier can check their validity. PVSS finds applications in randomness beacons, distributed key generation (DKG) and in YOSO MPC (Gentry et al. CRYPTO'21), when endowed with suitable publicly verifiable re-sharing as in YOLO YOSO (Cascudo et al. ASIACRYPT'22). We introduce a PVSS scheme over class groups that achieves similar efficiency to state-of-the art schemes that only allow for reconstructing a function of the secret, while our scheme allows the reconstruction of the original secret. Our construction generalizes the DDH-based scheme of YOLO YOSO to operate over class groups, which poses technical challenges in adapting the necessary NIZKs in face of the unknown group order and the fact that efficient NIZKs of knowledge are not as simple to construct in this setting. Building on our PVSS scheme's ability to recover the original secret, we propose two DKG protocols for discrete logarithm key pairs: a biasable 1-round protocol, which improves on the concrete communication/computational complexities of previous works; and a 2-round unbiasable protocol, which improves on the round complexity of previous works. We also add publicly verifiable resharing towards anonymous committees to our PVSS, so that it can be used to efficiently transfer state among committees in the YOSO setting. Together with a recent construction of MPC in the YOSO model based on class groups (Braun et al. CRYPTO'23), this results in the most efficient full realization (i.e without assuming receiver anonymous channels) of YOSO MPC based on the CDN framework with transparent setup.

In this paper, we propose an enhanced variant of the F4 algorithm specifically designed for efficiently solving multivariate quadratic (MQ) problems, which are central to many post-quantum cryptographic schemes. Our approach overcomes a major inefficiency of conventional F4 by integrating a Hilbert-driven strategy that determines the optimal number of S-polynomials to generate at each degree, thereby reducing unnecessary zero reductions. We further introduce refined pair selection techniques that prioritize candidates yielding S-polynomials with smaller leading terms, which in turn minimizes the dimensions of intermediate matrices used during reduction. Experimental results show that our implementation outperforms state-of-the-art systems such as M4GB and Magma's F4 in both single-core and multi-core environments. Notably, our method sets new records in the Fukuoka MQ Challenge for Type VI problems over F(31) with m = 21,22,23,24 demonstrating the robustness and practical impact of our approach in solving highly challenging MQ instances. According to the computational complexity estimation formula], the problem with m = 24 is approximately 47,627 times harder than the previous record case with m = 20.

Homomorphic Encryption (HE) is one of the mainstream cryptographic tools used to enable secure outsourced computation. A typical task is secure matrix computation. Popular HE schemes are all based on the problem of Ring Learning with Errors (RLWE), where the messages are encrypted in a ring. In general, the ring dimension should be large to ensure security, which is often larger than the matrix size. Hence, exploiting the ring structure to make fast homomorphic matrix computation has been an important topic in HE. In this paper, we present a new framework for encoding a matrix and performing multiplication on encrypted matrices. The new framework requires fewer basic homomorphic operations for matrix multiplication. Suppose that the ring dimension is $n$ and the matrix size is $d\times d$ with $d= n^{\rho}$. (1) In the compact case where $\rho \leq \frac{1}{3}$, the multiplication of two encrypted matrices requires $\tilde{O}(1)$ basic homomorphic operations, which include plaintext-ciphertext multiplications, ciphertext-ciphertext multiplications, and homomorphic Galois automorphisms. (2) In the large sized case where $\rho> \frac{1}{3}$, our new method requires $O\big(d^{(1 - \frac{1}{3\rho})\cdot \log_2 7 }\big)$ basic homomorphic operations, which is better than all existing methods. In addition, the new framework reduces the communication cost, since it requires fewer key-switching keys. The number of key-switching keys is reduced from $O(d)$ to $O(\log d)$.

When Satoshi Nakamoto introduced Bitcoin, a central tenet was that the blockchain functions as a timestamping server. In the Ethereum era, smart contracts widely assume on-chain timestamps are mostly accurate. In this paper, we prove this is indeed the case, namely that recorded timestamps do not wildly deviate from real-world time, a property we call timeliness. Assuming a global clock, we prove that all popular mechanisms for constructing blockchains (proof-of-work, longest chain proof-of-stake, and quorum-based proof-of-stake) are timely under honest majority, but a synchronous network is a necessary condition. Next we show that all timely blockchains can be suitably modified, in a black-box fashion, such that all honest parties output exactly the same ledgers at the same round, achieving a property we call supersafety, which may be of independent interest. Conversely, we also show that supersafety implies (perfect) timeliness, completing the circle.

Over the last decades, fault injection attacks have been demonstrated to be an effective method for breaking the security of electronic devices. Some types of fault injection attacks, like clock and voltage glitching, require very few resources by the attacker and are practical and simple to execute. A cost-effective countermeasure against these attacks is the use of a detector circuit which detects timing violations - the underlying effect that glitch attacks rely on. In this paper, we take a closer look at three examples of such detectors that have been presented in the literature. We demonstrate four high-speed clock glitching attacks, which successfully inject faults in systems, where detectors have been implemented to protect. The attacks remain unnoticed by the glitch detectors. We verify our attacks with practical experiments on an FPGA.

Proof-carrying data (PCD) is a powerful cryptographic primitive that allows mutually distrustful parties to perform distributed computation in an efficiently verifiable manner. Real-world deployments of PCD have sparked keen interest within the applied community and industry. Known constructions of PCD are obtained by recursively-composing SNARKs or related primitives. Unfortunately, known security analyses incur expensive blowups, which practitioners have disregarded as the analyses would lead to setting parameters that are prohibitively expensive. In this work we study the concrete security of recursive composition, with the goal of better understanding how to reasonably set parameters for certain PCD constructions of practical interest. Our main result is that PCD obtained from SNARKs with \emph{straightline knowledge soundness} has essentially the same security as the underlying SNARK (i.e., recursive composition incurs essentially no security loss). We describe how straightline knowledge soundness is achieved by SNARKs in several oracle models, which results in a highly efficient security analysis of PCD that makes black-box use of the SNARK's oracle (there is no need to instantiated the oracle to carry out the security reduction). As a notable application, our work offers an idealized model that provides new, albeit heuristic, insights for the concrete security of \emph{recursive STARKs} used in blockchain systems. Our work could be viewed as partial evidence justifying the parameter choices for recursive STARKs made by practitioners.

In 2019, Wan, Liao, Yan and Tsai proposed an article ``Discrete Sliding Mode Control for Chaos Synchronization and Its Application to an Improved ElGamal Cryptosystem''. However, Wan et al. just renamed the variable names without modified the core algorithm. Their paper passed review phase and then published. For this case, it is difficult to detect this situation by computer/digital forensics techniques. In this paper the authors would like to point out this dilemmas.

Domain Generation Algorithms (DGAs) are used by malware to generate pseudorandom domain names to establish communication between infected bots and Command and Control servers. While DGAs can be detected by machine learning (ML) models with great accuracy, offering DGA detection as a service raises privacy concerns when requiring network administrators to disclose their DNS traffic to the service provider. We propose the first end-to-end framework for privacy-preserving classification as a service of domain names into DGA (malicious) or non-DGA (benign) domains. We achieve this through a combination of two privacy-enhancing technologies (PETs), namely secure multi-party computation (MPC) and differential privacy (DP). Through MPC, our framework enables an enterprise network administrator to outsource the problem of classifying a DNS domain as DGA or non-DGA to an external organization without revealing any information about the domain name. Moreover, the service provider's ML model used for DGA detection is never revealed to the network administrator. Furthermore, by using DP, we also ensure that the classification result cannot be used to learn information about individual entries of the training data. Finally, we leverage the benefits of quantization of deep learning models in the context of MPC to achieve efficient, secure DGA detection. We demonstrate that we achieve a significant speed-up resulting in a 15% reduction in detection runtime without reducing accuracy.

In the ever-evolving landscape of Information Tech- nologies, private decentralized computing on an honest yet curious server has emerged as a prominent paradigm. While numerous schemes exist to safeguard data during computation, the focus has primarily been on protecting the confidentiality of the data itself, often overlooking the potential information leakage arising from the function evaluated by the server. Recognizing this gap, this article aims to address the issue by presenting and implementing an innovative solution for ensuring the privacy of both the data and the program. We introduce a novel approach that combines the power of Fully Homomorphic Encryption with the concept of the Turing Machine model, resulting in the first fully secure practical, non-interactive oblivious Turing Machine. Our Oblivious Turing Machine construction is based on only three hypotheses, the hardness of the Ring Learning With Error problem, the ability to homomorphically evaluate non-linear functions and the capacity to blindly rotate elements of a data structure. Only based on those three assumptions, we propose an implementation of an Oblivious Turing Machine relying on the TFHE cryptosystem and present some implementation results.

Fully homomorphic encryption is a promising approach when computing on encrypted data, especially when sensitive data is involved. For BFV, BGV, and CKKS, three state-of-the-art encryption schemes, the most costly homomorphic primitive is the so-called key switching. While a decent amount of research has been devoted to optimizing other aspects of these schemes, key switching has gone largely untouched. One exception has been a recent work [26] introducing a new double-decomposition technique. Their contributions are an important addition to the current state-of-the-art with one flaw: They take a limited perspective on key switching parameters and their asymptotic complexity which leads to incorrect conclusions about how effective their approach really is. In our work, we deep dive into key switching and correct, enhance, and improve the current state-of-the-art. We provide a new perspective on key switching parameters for the single- and double-decomposition techniques, respectively, and show that the former outperforms the latter in most scenarios. Additionally, we revisit an idea by Gentry, Halevi, and Smart [17] and reduce the number of multiplications.

Symmetric Private Information Retrieval (SPIR) is a protocol that protects privacy during data transmission. However, the existing SPIR focuses only on the privacy of the data to be requested on the server, without considering practical factors such as the payload that may be present during data transmission. This could seriously prevent SPIR from being applied to many complex data scenarios and hinder its further expansion. To solve such problems, we propose a primitive (PSKPIR) for symmetric private keyword information retrieval based on private set intersection (PSI) that supports payload transmission and batch keyword search. Specifically, we combine probe-and-XOR of strings (PaXoS) and Oblivious Programmable PRF (OPPRF) to construct PSI with payload (PSI-Payload) not only satisfies client privacy and server privacy, but also facilitates efficient payload transmission. The client can efficiently generate symmetric keys locally using keywords in the intersection, and receive payloads with matching labels in batches. In addition, we provide security definitions for PSKPIR and use the framework of universal composability (UC) to prove security. Finally, we implement PSKPIR with sublinear communication costs in both LAN and WAN settings. Experimental results show that our payload transfer speed is 10× faster than previous work on sufficiently large data sets.

In this work, we consider the problem of secure key leasing, also known as revocable cryptography (Agarwal et. al. Eurocrypt' 23, Ananth et. al. TCC' 23), as a strengthened security notion of its predecessor put forward in Ananth et. al. Eurocrypt' 21. This problem aims to leverage unclonable nature of quantum information to allow a lessor to lease a quantum key with reusability for evaluating a classical functionality. Later, the lessor can request the lessee to provably delete the key and then the lessee will be completely deprived of the capability to evaluate the function. In this work, we construct a secure key leasing scheme to lease a decryption key of a (classical) public-key, homomorphic encryption scheme from standard lattice assumptions. Our encryption scheme is exactly identical to the (primal) version of Gentry-Sahai-Waters homomorphic encryption scheme with a carefully chosen public key matrix. We achieve strong form of security where: * The entire protocol (including key generation and verification of deletion) uses merely classical communication between a classical lessor (client) and a quantum lessee (server). * Assuming standard assumptions, our security definition ensures that every computationally bounded quantum adversary could only simultaneously provide a valid classical deletion certificate and yet distinguish ciphertexts with at most negligible probability. Our security relies on the hardness of learning with errors assumption. Our scheme is the first scheme to be based on a standard assumption and satisfying the two properties mentioned above. The main technical novelty in our work is the design of an FHE scheme that enables us to apply elegant analyses done in the context of classically verifiable proofs of quantumness from LWE (Brakerski et. al.(FOCS'18, JACM'21) and its parallel amplified version in Radian et. al.(AFT'21)) to the setting of secure leasing. This connection leads to a modular construction and arguably simpler proofs than previously known. An important technical component we prove along the way is an amplified quantum search-to-decision reduction: we design an extractor that uses a quantum distinguisher (who has an internal quantum state) for decisional LWE, to extract secrets with success probability amplified to almost one. This technique might be of independent interest.

This paper re-analyzes the algorithm proposed by Guedes, Assis, and Lula in 2012, which they claimed that the algorithm can break Blum-Micali Pseudorandom number generator in polynomial time. We used a 5×5 transformation matrix instead of the original 2×2 transformation matrix, which can include terms that they missed in their analysis. We proved that their proposed algorithm cannot break the pseudorandom number generator, because during the amplitude amplification process, two iterations of the circuit is the same as an identity gate. To solve this problem, we proposed a corrected algorithm based on Grover’s Search Algorithm for NP-complete problems, which breaks the Blum-Micali Pseudorandom number generator in \(\mathcal{O}(n^4 2^{n/2})\). We conclude that the Blum-Micali Pseudorandom number generator is still quantum resistant. This study indicates that the discrete logarithm problem and prime factorization could still be the foundations of quantum-resistant cryptographical applications.

We show that under a mild number-theoretic conjecture, recovering an integer from its Jacobi signature modulo $N = p^2 q$, for primes $p$ and $q$, is as hard as factoring $N$. This relates, for the first time, the one-wayness of a pseudorandom generator that Damgård proposed in 1988, to a standard number-theoretic problem. In addition, we show breaking the Jacobi pseudorandom function is no harder than factoring.

The lattice-based post-quantum cryptosystem NTRU is used by Google for protecting Google’s internal communication. In NTRU, polynomial multiplication is one of bottleneck. In this paper, we explore the interactions between polynomial multiplication, Toeplitz matrix–vector product, and vectorization with architectural insights. For a unital commutative ring $R$, a positive integer $n$, and an element $\zeta \in R$, we reveal the benefit of vector-by-scalar multiplication instructions while multiplying in $R[x] / \langle x^n - \zeta \rangle$. We aim at designing an algorithm exploiting no algebraic and number–theoretic properties of $n$ and $\zeta$. An obvious way is to multiply in $R[x]$ and reduce modulo $x^n - \zeta$. Since the product in $R[x]$ is a polynomial of degree at most $2n − 2$, one usually chooses a polynomial modulus $g$ such that (i) $deg(g) \geq 2n − 1$, and (ii) there exists a well-studied fast polynomial multiplication algorithm f for multiplying in $R[x] / \langle g \rangle$. We deviate from common approaches and point out a novel insight with dual modules and vector-by-scalar multiplications. Conceptually, we relate the module-theoretic dual of $R[x] / \langle x^n - \zeta \rangle$ and $R[x] / \langle g \rangle$ with Toeplitz matrix-vector products, and demonstrate the benefit of Toeplitz matrix-vector products with vector-by-scalar multiplication instructions. It greatly reduces the register pressure, and allows us to multiply with essentially no permutation instructions that are commonly used in vectorized implementation. We implement the ideas for the NTRU parameter sets ntruhps2048677 and ntruhrss701 on a Cortex-A72 implementing the Armv8.0-A architecture with the single-instruction-multiple-data (SIMD) technology Neon. For polynomial multiplications, our implementation is 2.18× and 2.23× for ntruhps2048677 and ntruhrsss701 than the state-of-the-art optimized implementation. We also vectorize the polynomial inversions and sorting network by employing existing techniques and translating AVX2-optimized implementations into Neon. Compared to the state-of-the-art optimized implementation, our key generation, encapsulation, and decapsulation for ntruhps2048677 are 7.67×, 2.48×, and 1.77× faster, respectively. For ntruhrss701, our key generation, encapsulation, and decapsulation are 7.99×, 1.47×, and 1.56× faster, respectively.

Circuit-based Private Set Intersection (circuit-PSI) empowers two parties, a client and a server, each with input sets $X$ and $Y$, to securely compute a function $f$ on the intersection $X \cap Y$ while preserving the confidentiality of $X \cap Y$ from both parties. Despite the recent proposals of computationally efficient circuit-PSI protocols, they primarily focus on the balanced scenario where $|X|$ is similar to $|Y|$. However, in many practical situations, a circuit-PSI protocol may be applied in an unbalanced context, where $|X|$ is significantly smaller than $|Y|$. Directly applying existing protocols to this scenario poses notable efficiency challenges due to the communication complexity of these protocols scaling at least linearly with the size of the larger set, i.e., $\max(|X|, |Y|)$. In this work, we put forth efficient constructions for unbalanced circuit-PSI, demonstrating sublinear communication complexity in the size of the larger set. Our key insight lies in formalizing unbalanced circuit-PSI as the process of obliviously retrieving values corresponding to keys from a set of key-value pairs. To achieve this, we propose a new functionality named Oblivious Key-Value Retrieval (OKVR) and design the OKVR protocol based on a new notion termed sparse Oblivious Key-Value Store (sparse OKVS). We conduct comprehensive experiments and the results showcase substantial improvements over the state-of-the-art circuit-PSI schemes, i.e., $1.84 \sim 48.86 \times$ communication improvement and $1.50 \sim39.81 \times$ faster computation. Compared to a very recent unbalanced circuit-PSI work, our constructions outperform them by $1.18 \sim 15.99 \times$ and $1.22 \sim 10.44 \times$ in communication and computation overhead, respectively, depending on set sizes and network environments.

We consider the problem of creating, or issuing, zero-knowledge proofs obliviously. In this setting, a prover interacts with a verifier to produce a proof, known only to the verifier. The resulting proof is transferable and can be verified non-interactively by anyone. Crucially, the actual proof cannot be linked back to the interaction that produced it. This notion generalizes common approaches to designing blind signatures, which can be seen as the special case of proving "knowledge of a signing key", and extends the seminal work of Camenisch and Stadler ('97). We propose a provably secure construction of oblivious proofs, focusing on discrete-logarithm representation equipped with AND-composition. We also give three applications of our framework. First, we give a publicly verifiable version of the classical Diffie-Hellman based Oblivious PRF. This yields new constructions of blind signatures and publicly verifiable anonymous tokens. Second, we show how to "upgrade" keyed-verification anonymous credentials (Chase et al., CCS'14) to also be concurrently secure blind signatures on the same set of attributes. Crucially, our upgrade maintains the performance and functionality of the credential in the keyed-verification setting, we only change issuance. We observe that the existing issuer proof that the credential is well-formed may be verified by anyone; creating it with our framework makes it a blind signature, adding public verifiability to the credential system. Finally, we provide a variation of the U-Prove credential system that is provably one-more unforgeable with concurrent issuance sessions. This constitutes a fix for the attack illustrated by Benhamouda et al. (EUROCRYPT'21). Beyond these example applications, as our results are quite general, we expect they may enable modular design of new primitives with concurrent security, a goal that has historically been challenging to achieve.

The BUFF transform is a generic transformation for digital signature schemes, with the purpose of obtaining additional security properties beyond standard unforgeability, e.g., exclusive ownership and non-resignability. In the call for additional post-quantum signatures, these were explicitly mentioned by the NIST as ``additional desirable security properties'', and some of the submissions indeed refer to the BUFF transform with the purpose of achieving them, while some other submissions follow the design of the BUFF transform without mentioning it explicitly. In this work, we show the following negative results regarding the non-resignability property in general, and the BUFF transform in particular. In the plain model, we observe by means of a simple attack that any signature scheme for which the message has a high entropy given the signature does not satisfy the non-resignability property (while non-resignability is trivially not satisfied if the message can be efficiently computed from its signature). Given that the BUFF transform has high entropy in the message given the signature, it follows that the BUFF transform does not achieve non-resignability whenever the random oracle is instantiated with a hash function, no matter what hash function. When considering the random oracle model (ROM), the matter becomes slightly more delicate since prior works did not rigorously define the non-resignability property in the ROM. For the natural extension of the definition to the ROM, we observe that our impossibility result still holds, despite there having been positive claims about the non-resignability of the BUFF transform in the ROM. Indeed, prior claims of the non-resignability of the BUFF transform rely on faulty argumentation. On the positive side, we prove that a salted version of the BUFF transform satisfies a slightly weaker variant of non-resignability in the ROM, covering both classical and quantum attacks, if the entropy requirement in the (weakened) definition of non-resignability is statistical; for the computational variant, we show yet another negative result.

Ring signature (RS) allows users to demonstrate to verifiers their membership within a specified group (ring) without disclosing their identities. Based on this, RS can be used as a privacy protection technology for users' identities in blockchain. However, there is currently a lack of RS schemes that are fully applicable to the blockchain applications: Firstly, users can only spend a UTXO once, and the current RS schemes are not yet perfect in a one-time manner. At the same time, the current RS schemes are not sufficiently developed in terms of regulation. Secondly, the size of the current RS is mostly linearly related to the number of ring members. When there are many members, the transaction processing speed is slow. We propose a one-time and revocable ring signature with logarithmic size in blockchain based on the Sigma-Protocols. Our scheme compresses the RS size and enables users to sign in the blockchain transactions. The scheme allows two RS generated with the same private key for a same UTXO to be linked together. Additionally, it allows regulatory authority to recover the signer's identity at any time. A security model was presented, and its security properties, namely, unforgeability, anonymity, one-time, revocability, and non-slanderability were proven in the random oracle model. Our scheme compresses the RS size to where is the number of ring users, enabling blockchain transactions to have better processing speeds. And it can prevent double-spending attacks in blockchain and allows regulatory authority to recover the identity of the signer.

The algebraic degree of a vectorial Boolean function is one of the main parameters driving the cost of its hardware implementation. Thus, finding decompositions of functions into sequences of functions of lower algebraic degrees has been explored to reduce the cost of implementations. In this paper, we consider such decompositions of permutations over $\mathbb{F}_{2^n}$. We prove the existence of decompositions using quadratic and linear power permutations for all permutations when $2^n-1$ is a prime, and we prove the non-existence of such decompositions for power permutations of differential uniformity strictly lower than $16$ when $4|n$. We also prove that any permutation admits a decomposition into quadratic power permutations and affine permutations of the form $ax+b$ if $4 \nmid n$. Furthermore, we prove that any permutation admits a decomposition into cubic power permutations and affine permutations. Finally, we present a decomposition of the PRESENT S-Box using the power permutation $x^7$ and affine permutations.

Symmetric Private Information Retrieval (SPIR) is a stronger PIR protocol that ensures both client and server privacy. In many cases, the client needs authorization from the data subject before querying data. However, this also means that the server can learn the identity of the data subject. To solve such problems, we propose a new SPIR primitive, called authorized symmetric keyword information retrieval protocol (ASKPIR). Specifically, we designed an efficient DID identification algorithm based on the Pedersen Commitment, which is used to solve the identity management and privacy problems of data subject when data is shared by multiple parties in a distributed environment. Then, we present a novel authorization algorithm combining NIZK proof and DID, which can preserve client privacy. Finally, to improve the efficiency of client retrieval, our protocol constructs PSI-Payload with mqRPMT and OTE so as to support batch keyword searches. In addition, we provide a formal security analysis for the anonymity and unforgeability of the protocol and demonstrate that ASKPIR can achieve malicious security under the UC framework. Theoretical analysis and experimental results show that the ASKPIR protocol is more efficient than other related works and solves the problem of incompatibility between data subject authorization and client privacy.

This study presents an efficient recoverable memory encryption mechanism, named Crystalor. Existing memory encryption mechanisms, such as Intel SGX integrity tree, offer neither crash consistency nor recoverability, which results in attack surfaces and causes a non-trivial limitation of practical availability. Although the crash consistency of encrypted memory has been studied in the research field of microarchitecture, existing mechanisms lack formal security analysis and cannot incorporate with metadata optimization mechanisms, which are essential to achieve a practical performance. Crystalor efficiently realizes provably-secure recoverable memory encryption with metadata optimization. To establish Crystalor with provable security and practical performance, we develop a dedicated universal hash function PXOR-Hash and a microarchitecture equipped with PXOR-Hash. Crystalor incurs almost no latency overhead under the nominal operations for the recoverability, while it has a simple construction in such a way as to be compatible with existing microarchitectures. We evaluate its practical performance through both algorithmic analyses and system-level simulation in comparison with the state-of-the-art ones, such as SCUE. Crystalor requires 29–62% fewer clock cycles per memory read/write operation than SCUE for protecting a 4 TB memory. In addition, Crystalor and SCUE require 312 GB and 554 GB memory overheads for metadata, respectively, which indicates that Crystalor achieves a memory overhead reduction of 44%. The results of the system-level simulation using the gem5 simulator indicate that Crystalor achieves a reduction of up to 11.5% in the workload execution time compared to SCUE. Moreover, Crystalor achieves a higher availability and memory recovery several thousand times faster than SCUE, as Crystalor offers lazy recovery.

We show that the Xu et al.'s authentication and key agreement scheme [IEEE Trans. Ind. Informatics, 18(10), 7118-7127, 2022] is flawed. (1) It confused some operations for bilinear maps and presented some inconsistent computations. (2) It failed to keep anonymity, not as claimed. The adversary can use any device's public key stored in the blockchain to test some verification equations so as to reveal the identity of a target device.

The Peregrine signature scheme is one of the candidates in the ongoing Korean post-quantum cryptography competition. It is proposed as a high-speed variant of Falcon, which is a hash-and-sign signature scheme over NTRU lattices and one of the schemes selected by NIST for standardization. To this end, Peregrine replaces the lattice Gaussian sampler in the Falcon signing procedure with a new sampler based on the centered binomial distribution. While this modification offers significant advantages in terms of efficiency and implementation, it does not come with a provable guarantee that signatures do not leak information about the signing key. Unfortunately, lattice based signature schemes in the hash-and-sign paradigm that lack such a guarantee (such as GGH, NTRUSign or DRS) have generally proved insecure. In this paper, we show that Peregrine is no exception, by demonstrating a practical key recovery attack against it. We observe that the distribution of Peregrine signatures is a hidden transformation of some public distribution and still leaks information about the signing key. By adapting the parallelepiped-learning technique of Nguyen and Regev (Eurocrypt 2006), we show that the signing key can be recovered from a relatively small number of signatures. The learning technique alone yields an approximate version of the key, from which we can recover the exact key using a decoding technique due to Thomas Prest (PKC 2023). For the reference implementation (resp. the official specification version) of Peregrine-512, we fully recover the secret key with good probability in a few hours given around 25,000 (resp. 11 million) signature samples.

In an effort to circumvent the high cost of standard countermeasures against side-channel attacks in post-quantum cryptography, some works have developed low-cost detection-based countermeasures. These countermeasures try to detect maliciously generated input ciphertexts and react to them by discarding the ciphertext or secret key. In this work, we take a look at two previously proposed low-cost countermeasures: the ciphertext sanity check and the decapsulation failure check, and demonstrate successful attacks on these schemes. We show that the first countermeasure can be broken with little to no overhead, while the second countermeasure requires a more elaborate attack strategy that relies on valid chosen ciphertexts. Thus, in this work, we propose the first chosen-ciphertext based side-channel attack that only relies on valid ciphertexts for key recovery. As part of this attack, a third contribution of our paper is an improved solver that retrieves the secret key from linear inequalities constructed using side-channel leakage from the decryption procedure. Our solver is an improvement over the state-of-the-art Belief Propagation solvers by Pessl and Prokop, and later Delvaux. Our method is simpler, easier to understand and has lower computational complexity, while needing less than half the inequalities compared to previous methods.

HQC is a code-based key encapsulation mechanism (KEM) that was selected to move to the fourth round of the NIST post-quantum standardization process. While this scheme was previously targeted by side-channel assisted chosen-ciphertext attacks for key recovery, all these attacks have relied on malformed ciphertexts for key recovery. Thus, all these attacks can be easily prevented by deploying a detection based countermeasures for invalid ciphertexts, and refreshing the secret key upon detection of an invalid ciphertext. This prevents further exposure of the secret key to the attacker and thus serves as an attractive option for protection against prior attacks. Thus, in this work, we present a critical analysis of the detection based countermeasure, and present the first side-channel based chosen-ciphertext attack that attempts to utilize only valid ciphertexts for key recovery, thereby defeating the detection based countermeasure. We propose novel attacks exploiting leakage from the ExpandAndSum and FindPeaks operations within the Reed-Muller decoder for full key recovery with 100% success rate. We show that our attacks are quite robust to noise in the side-channel measurements, and we also present novel extensions of our attack to the shuffling countermeasure on both the ExpandAndSum and FindPeaks operation, which renders the shuffling countermeasure ineffective. Our work therefore shows that low-cost detection based countermeasures can be rendered ineffective, and cannot offer standalone protection against CC-based side-channel attacks. Thus, our work encourages more study towards development of new low-cost countermeasures against CC-based side-channel attacks.