International Association
for Cryptologic Research

Event date: 30 November 2023
Submission deadline: 25 June 2023
Notification: 5 August 2023

Event date: 31 October to 2 November 2023
Submission deadline: 1 July 2023
Notification: 10 August 2023

For standard \LWE samples $(\mathbf{A},\mathbf{b = sA + e})$, $\mathbf{A}$ is typically uniformly over $\mathbb{Z}_q^{n \times m}$, and under the \LWE assumption, the conditional distribution of $\mathbf{s}$ given $\mathbf{b}$ and $\mathbf{s}$ should be consistent. However, when $\mathbf{A}$ is chosen by an adversary, the gap between the two may be larger. In this work, we are mainly interested in quantifying $\tilde{H}_\infty(\mathbf{s}|\mathbf{sA + e})$, while $\mathbf{A}$ is chosen by an adversary. Brakerski and D\"{o}ttling answered the question in one case : they proved that when $\mathbf{s}$ was uniformly chosen from $\mathbb{Z}_q^n$, it holds that $\tilde{H}_\infty(\mathbf{s}|\mathbf{sA + e}) \varpropto \rho_\sigma(\Lambda_q(\mathbf{A}))$. We prove that for any $d < q$ and $\mathbf{s}$ is uniformly chosen from $\mathbb{Z}_d^n$, the above result still holds.

In addition, as an independent result, we have also proved the regularity of the hash function mapped to the prime-order group and its Cartesian product.

As an application of the above results, we improved the multi-key fully homomorphic encryption\cite{TCC:BraHalPol17} and answered the question raised at the end of their work in positive way : we have GSW type ciphertext rather than Dual-GSW, and the improved scheme has shorter keys and ciphertexts

We propose a quantum algorithm that crucially involves the receiver's public-key to establish secure communication of an intended message string, using shared entangled-qubits. The public-key in question is a random bit string that proclaims the sequence of measurement basis used by the receiver. As opposed to known quantum key distribution protocols, wherein a random key string is generated at the end of the communication cycle, here the sender's intended bit string itself is communicated across securely. The quantum outlay for the proposed protocol is limited to the sender and receiver sharing pairs of entangled qubits, prepared in ? ?????? known states, besides unitary manipulations and measurements that the sender and receiver individually perform on their respective qubits, within their confines.

Abstract. Non-fungible tokens (NFTs) are digital representations of assets stored on a blockchain. It allows content creators to certify authenticity of their digital assets and transfer ownership in a transparent and decentralized way. Popular choices of NFT marketplaces infrastructure include blockchains with smart contract functionality or layer-2 solutions. Surprisingly, researchers have largely avoided building NFT schemes over Bitcoin-like blockchains, most likely due to high transaction fees in the BTC network and the belief that Bitcoin lacks enough programmability to implement fair exchanges. In this work we fill this gap. We propose an NFT scheme where trades are settled in a single Bitcoin transaction as opposed to executing complex smart contracts. We use zero-knowledge proofs (concretely, recursive SNARKs) to prove that two Bitcoin transactions, the issuance transaction $tx_0$ and the current trade transaction $tx_n$, are linked through a unique chain of transactions. Indeed, these proofs function as “off-chain receipts” of ownership that can be transferred from the current owner to the new owner using an insecure channel. The size of the proof receipt is short, independent of the total current number of trades $n$, and can be updated incrementally by anyone at anytime. Marketplaces typically require some degree of token ownership delegation, e.g., escrow accounts, to execute the trade between sellers and buyers that are not online concurrently, and to alleviate transaction fees they resort to off-chain trades. This raises concerns on the transparency and purportedly honest behaviour of marketplaces. We achieve fair and non-custodial trades by leveraging our off-chain receipts and letting the involved parties carefully sign the trade transaction with appropriate combinations of sighash flags.

In this paper we propose a new construction for building universal hash functions, a specific instance called multi-265, and provide proofs for their universality. Our construction follows the key-then-hash parallel paradigm. In a first step it adds a variable length input message to a secret key and splits the result in blocks. Then it applies a fixed-length public function to each block and adds their results to form the output. The innovation presented in this work lies in the public function: we introduce the multiply-transform-multiply-construction that makes use of field multiplication and linear transformations. We prove upper bounds for the universality of key-then-hash parallel hash functions making use of a public function with our construction provided the linear transformation are maximum-distance-separable (MDS). We additionally propose a concrete instantiation of our construction multi-265, where the underlying public function uses a near-MDS linear transformation and prove it to be $2^{-154}$-universal. We also make the reference code for multi-265 available.

Zero-knowledge and succinctness are two important properties that arise in the study of non-interactive arguments. Previously, Kitagawa et al. (TCC 2020) showed how to obtain a non-interactive zero-knowledge (NIZK) argument for NP from a succinct non-interactive argument (SNARG) for NP. In particular, their work demonstrates how to leverage the succinctness property from an argument system and transform it into a zero-knowledge property.

In this work, we study a similar question of leveraging succinctness for zero-knowledge. Our starting point is a batch argument for NP, a primitive that allows a prover to convince a verifier of $T$ NP statements $x_1, \ldots, x_T$ with a proof whose size scales sublinearly with $T$. Unlike SNARGs for NP, batch arguments for NP can be built from group-based assumptions in both pairing and pairing-free groups and from lattice-based assumptions. The challenge with batch arguments is that the proof size is only amortized over the number of instances, but can still encode full information about the witness to a small number of instances.

We show how to combine a batch argument for NP with a local pseudorandom generator (i.e., a pseudorandom generator where each output bit only depends on a small number of input bits) and a dual-mode commitment scheme to obtain a NIZK for NP. Our work provides a new generic approach of realizing zero-knowledge from succinctness and highlights a new connection between succinctness and zero-knowledge.

Decentralization, verifiability, and privacy-preserving are three fundamental properties of modern e-voting. In this paper, we conduct extensive investigations into them and present a novel e-voting scheme, VeriVoting, which is the first to satisfy these properties. More specifically, decentralization is realized through blockchain technology and the distribution of decryption power among competing entities, such as candidates. Furthermore, verifiability is satisfied when the public verifies the ballots and decryption keys. And finally, bidirectional unlinkability is achieved to help preserve privacy by decoupling voter identity from ballot content. Following the ideas above, we first leverage linear homomorphic encryption schemes and non-interactive zero-knowledge argument systems to construct a voting primitive, SemiVoting, which meets decentralization, decryption-key verifiability, and ballot privacy. To further achieve ballot ciphertext verifiability and anonymity, we extend this primitive with blockchain and verifiable computation to finally arrive at VeriVoting. Through security analysis and per-formance evaluations, VeriVoting offers a new trade-off between security and efficiency that differs from all previous e-voting schemes and provides a radically novel practical ap-proach to large-scale elections.

The threat of physical side-channel attacks and their countermeasures is a widely researched field. Most physical side-channel attacks rely on the unavoidable influence of computation or storage on voltage or current fluctuations. Such data-dependent influence can be exploited by, for instance, power or electromagnetic analysis. In this work, we introduce a novel non-invasive physical side-channel attack, which exploits the data-dependent changes in the impedance of the chip. Our attack relies on the fact that the temporarily stored contents in registers alter the physical characteristics of the circuit, which results in changes in the die's impedance. To sense such impedance variations, we deploy a well-known RF/microwave method called scattering parameter analysis, in which we inject sine wave signals with high frequencies into the system's power distribution network (PDN) and measure the echo of the signals. We demonstrate that according to the content bits and physical location of a register, the reflected signal is modulated differently at various frequency points enabling the simultaneous and independent probing of individual registers. Such side-channel leakage violates the $t$-probing security model assumption used in masking, which is a prominent side-channel countermeasure. To validate our claims, we mount non-profiled and profiled impedance analysis attacks on hardware implementations of unprotected and high-order masked AES. We show that in the case of profiled attack, only a single trace is required to recover the secret key. Finally, we discuss how a specific class of hiding countermeasures might be effective against impedance leakage.

Indistinguishability obfuscation (IO) is at the frontier of cryptography research for several years. LV16/Lin17 obfuscation schemes are famous progresses towards simplifying obfuscation mechanism. In fact, these two schemes only constructed two compact functional encryption (CFE) algorithms, while other things were taken to AJ15 IO frame or BV15 IO frame. That is, CFE algorithms are inserted into AJ15 IO frame or BV15 IO frame to form a complete IO scheme. The basic structure of two CFE algorithms can be described in the following way. The polynomial-time-computable Boolean function is transformed into a group of low-degree low-locality component functions by using randomized encoding, while some public combination of values of component functions is the value of original Boolean function. The encryptor uses constant-degree multilinear maps (rather than polynomial-degree multilinear maps) to encrypt independent variables of component functions. The decryptor uses zero-testing tool of multilinear maps to obtain values of component functions (rather than to obtain values of independent variables), and then uses public combination to obtain the value of original Boolean function.

In this paper we restrict IO to be a real white box (RWB). Under such restriction we point out that LV16/Lin17 CFE algorithms being inserted into AJ15 IO frame are invalid. More detailedly, such insertion makes the adversary gradually learn the shape of the function, therefore the scheme is not secure. In other words, such scheme is not a real IO scheme, but rather a garbling scheme. It needs to be said that RWB restriction is reasonable, which means the essential contribution of IO for cryptography research.

A flurry of excitement amongst researchers and practitioners has produced modern proof systems built using novel technical ideas and seeing rapid deployment, especially in cryptocurrencies. Most of these modern proof systems use the Fiat-Shamir (F-S) transformation, a seminal method of removing interaction from a protocol with a public-coin verifier. Some prior work has shown that incorrectly applying F-S (i.e., using the so-called "weak" F-S transformation) can lead to breaks of classic protocols like Schnorr's discrete log proof; however, little is known about the risks of applying F-S incorrectly for modern proof systems seeing deployment today.

In this paper, we fill this knowledge gap via a broad theoretical and practical study of F-S in implementations of modern proof systems. We perform a survey of open-source implementations and find 36 weak F-S implementations affecting 12 different proof systems. For four of these---Bulletproofs, Plonk, Spartan, and Wesolowski's VDF---we develop novel knowledge soundness attacks accompanied by rigorous proofs of their efficacy. We perform case studies of applications that use vulnerable implementations, and demonstrate that a weak F-S vulnerability could have led to the creation of unlimited currency in a private blockchain protocol. Finally, we discuss possible mitigations and takeaways for academics and practitioners.

The construction of invertible non-linear layers over $\mathbb F_p^n$ that minimize the multiplicative cost is crucial for the design of symmetric primitives targeting Multi Party Computation (MPC), Zero-Knowledge proofs (ZK), and Fully Homomorphic Encryption (FHE). At the current state of the art, only few non-linear functions are known to be invertible over $\mathbb F_p$, as the power maps $x\mapsto x^d$ for $\gcd(d,p-1)=1$. When working over $\mathbb F_p^n$ for $n\ge2$, a possible way to construct invertible non-linear layers $\mathcal S$ over $\mathbb F_p^n$ is by making use of a local map $F:\mathbb F_p^m\rightarrow \mathbb F_p$ for $m\le n$, that is, $\mathcal S_F(x_0, x_1, \ldots, x_{n-1}) = y_0\|y_1\|\ldots \|y_{n-1}$ where $y_i = F(x_i, x_{i+1}, \ldots, x_{i+m-1})$. This possibility has been recently studied by Grassi, Onofri, Pedicini and Sozzi at FSE/ToSC 2022. Given a quadratic local map $F:\mathbb F_p^m \rightarrow \mathbb F_p$ for $m\in\{1,2,3\}$, they proved that the shift-invariant non-linear function $\mathcal S_F$ over $\mathbb F_p^n$ defined as before is never invertible for any $n\ge 2\cdot m-1$. In this paper, we face the problem by generalizing such construction. Instead of a single local map, we admit multiple local maps, and we study the creation of nonlinear layers that can be efficiently verified and implemented by a similar shift-invariant lifting. After formally defining the construction, we focus our analysis on the case $\mathcal S_{F_0, F_1}(x_0, x_1, \ldots, x_{n-1}) = y_0\|y_1\|\ldots \|y_{n-1}$ for $F_0, F_1 :\mathbb F_p^2\rightarrow \mathbb F_p$ of degree at most 2. This is a generalization of the previous construction using two alternating functions $F_0,F_1$ instead of a single $F$. As main result, we prove that (i) if $n\ge3$, then $\mathcal S_{F_0, F_1}$ is never invertible if both $F_0$ and $F_1$ are quadratic, and that (ii) if $n\ge 4$, then $\mathcal S_{F_0, F_1}$ is invertible if and only if it is a Type-II Feistel scheme.

Protocols for state-machine replication (SMR) often trade off performance for resilience to network delay. In particular, protocols for asynchronous SMR tolerate arbitrary network delay but sacrifice throughput/latency when the network is fast, while partially synchronous protocols have good performance in a fast network but fail to make progress if the network experiences high delay. Existing hybrid protocols are resilient to arbitrary network delay and have good performance when the network is fast, but suffer from high overhead (``thrashing'') if the network repeatedly switches between being fast and slow (e.g., in a network that is typically fast but has intermittent message delays).

We propose Abraxas, a generic approach for constructing a hybrid protocol based on any protocol $\Pi_\mathsf{fast}$ and any asynchronous protocol $\Pi_\mathsf{slow}$ to achieve (1)~security and performance equivalent to $\Pi_\mathsf{slow}$ under arbitrary network behavior; (2)~performance equivalent to $\Pi_\mathsf{fast}$ when conditions are favorable. We instantiate Abraxas with the best existing protocols for $\Pi_\mathsf{fast}$ (Jolteon) and $\Pi_\mathsf{slow}$ (2-chain VABA), and show experimentally that the resulting protocol significantly outperforms Ditto, the previous state-of-the-art hybrid protocol.

A whistleblower is a person who leaks sensitive information on a prominent individual or organisation engaging in an unlawful or immoral activity. Whistleblowing has the potential to mitigate corruption and fraud by identifying the misuse of capital. In extreme cases whistleblowing can also raise awareness about unethical practices to individuals by highlighting dangerous working conditions. Obtaining and sharing the sensitive information associated with whistleblowing can carry great risk to the individual or party revealing the data. In this paper we extend the notion of timed-release encryption to include a new security property which we term implicit authentication, with the goal of making the practice of whistleblowing safer.

We formally define the new primitive of timed-release encryption with implicit authentication (TRE-IA), providing rigorous game-base definitions. We then build a practical TRE-IA construction that satisfies the security requirements of this primitive, using repeated squaring in an RSA group, and the RSA-OAEP encryption scheme. We formally prove our construction secure and provide a performance analysis of our implementation in Python along with recommendations for practical deployment and integration with an existing whistleblowing tool SecureDrop.

In this work, we provide a systematisation of knowledge of delay-based cryptography, in which we discuss and compare the existing primitives within cryptography that utilise a time-delay. We start by considering the role of time within cryptography, explaining broadly what a delay aimed to achieve at its inception and now, in the modern age. We then move on to describing the underlying assumptions used to achieve these goals, and analyse topics including trust, decentralisation and concrete methods to implement a delay. We then survey the existing primitives, discussing their security properties, instantiations and applications. We make explicit the relationships between these primitives, identifying a hierarchy and the theoretical gaps that exist. We end this systematisation of knowledge by highlighting relevant future research directions within the field of delay-based cryptography, from which this area would greatly benefit.

The Number Theoretic Transform (NTT) is used to efficiently execute polynomial multiplication. It has become an important part of lattice-based post-quantum methods and the subsequent generation of standard cryptographic systems. However, implementing post-quantum schemes is challenging since they rely on intricate structures. This paper demonstrates how to develop a high-speed NTT multiplier highly optimized for FPGAs with few logical resources. We describe a novel architecture for NTT that leverages unique precomputation. Our method efficiently maps these specific pre-computed values into the built-in Block RAMs (BRAMs), which greatly reduces the area and time required for implementation when compared to previous works. We have chosen Kyber parameters to implement the proposed architectures. Compared to the most well-known approach for implementing Kyber’s polynomial multiplication using NTT, the time is reduced by 31%, and AT (area × time) is improved by 25% as a result of the pre computation we suggest in this study. It is worth mentioning that we obtained these improvements while our method does not require DSP.

Private set intersection (PSI) enables two parties, each holding a private set to compute their intersection without revealing other information in the process. We introduce a generalized extension of conventional PSI termed as third-party PSI, whereby the intersection output of the two parties is only known to an inputless third party. In this setting, the two parties who participate in the protocol have no knowledge of the intersection result or any information of the set content of the other party. In general, third-party PSI settings arise where there is a need for an external party to obtain the intersection outcome without leakage of additional information to any other party. This setting is motivated by an increasing importance in several real-world applications. We describe protocols which achieve this functionality with minimal communication overhead. To the best of knowledge, our work is the first of its kind to explore this variant of PSI.

We show that the key agreement scheme [Comput. Commun., 2021(170): 1--18] is insecure against impersonation attacks, because there is a trivial equality which results in the loss of data confidentiality.

The bottleneck-complexity (BC) of secure multiparty computation (MPC) protocols is a measure of the maximum number of bits which are sent and received by any party in a protocol. As the name suggests, the goal of studying BC-efficient protocols is to increase overall efficiency by making sure that the workload in the protocol is somehow "amortized'' by the protocol participants.

Orlandi et al. (PKC 2022) initiated the study of BC-efficient protocols from simple assumptions in the correlated randomness model and for semi-honest adversaries. In this work, we extend the study of Orlandi et al. in two primary directions: (a) to a larger and more general class of functions and (b) to the information-theoretic setting.

In particular, we offer semi-honest secure protocols for the useful function classes of abelian programs, 'read-$k$' non-abelian programs, and 'read-$k$' generalized formulas.

Our constructions use a novel abstraction, called 'incremental function secret-sharing' (IFSS), that can be instantiated with unconditional security or from one-way functions (with different efficiency trade-offs).

The division property introduced by Todo in Crypto 2015 is one of the most versatile tools in the arsenal of a cryptanalyst which has given new insights into many ciphers primarily from an algebraic perspective. On the other end of the spectrum we have fault attacks which have evolved into the deadliest of all physical attacks on cryptosystems. The current work aims to combine these seemingly distant tools to come up with a new type of fault attack. We show how fault invariants are formed under special input division multi-sets and are independent of the fault injection location. It is further shown that the same division trail can be exploited as a multi-round Zero-Sum distinguisher to reduce the key-space to practical limits. As a proof of concept division trails of PRESENT and GIFT are exploited to mount practical key-recovery attacks based on the random nibble fault model. For GIFT-64, we are able to recover the unique master-key with 30 nibble faults with faults injected at rounds 21 and 19. For PRESENT-80, DiFA reduces the key-space from $2^{80}$ to $2^{16}$ with 15 faults in round 25 while for PRESENT-128, the unique key is recovered with 30 faults in rounds 25 and 24. This constitutes the best fault attacks on these ciphers in terms of fault injection rounds. We also report an interesting property pertaining to fault induced division trails which shows its inapplicability to attack GIFT-128. Overall, the usage of division trails in fault based cryptanalysis showcases new possibilities and reiterates the applicability of classical cryptanalytic tools in physical attacks.