International Association
for Cryptologic Research

In response to side-channel attacks on masked implementations of post-quantum cryptographic algorithms, a new bitsliced higher-order masked implementation of CRYSTALS-Kyber has been presented at CHES'2022. The bitsliced implementations are typically more difficult to break by side-channel analysis because they execute a single instruction across multiple bits in parallel. However, in this paper, we reveal new vulnerabilities in the masked Boolean to arithmetic conversion procedure of this implementation that make the shared and secret key recovery possible. We also present a new chosen ciphertext construction method which maximizes secret key recovery probability for a given message bit recovery probability. We demonstrate practical shared and secret key recovery attacks on the first-, second- and third-order masked implementations of Kyber-768 in ARM Cortex-M4 using profiled deep learning-based power analysis.

Suppose two parties have hash functions $h_1$ and $h_2$ respectively, but each only trusts the security of their own. We wish to build a hash combiner $C^{h_1, h_2}$ which is secure so long as either one of the underlying hash functions is. This question has been well-studied in the regime of collision resistance. In this case, concatenating the two hash outputs clearly works. Unfortunately, a long series of works (Boneh and Boyen, CRYPTO'06; Pietrzak, Eurocrypt'07; Pietrzak, CRYPTO'08) showed no (noticeably) shorter combiner for collision resistance is possible.

We revisit this pessimistic state of affairs, motivated by the observation that collision-resistance is insufficient for many applications of cryptographic hash functions anyway. We argue the right formulation of the "hash combiner" is what we call random oracle (RO) combiners.

Indeed, we circumvent the previous lower bounds for collision resistance by constructing a simple length-preserving RO combiner $$\widetilde{C}_{\mathcal{Z}_1,\mathcal{Z}_2}^{h_1,h_2}(M) = h_1(M, \mathcal{Z}_1) \oplus h_2(M, \mathcal{Z}_2),$$ where $\mathcal{Z}_1, \mathcal{Z}_2$ are random salts of appropriate length. We show that this extra randomness is necessary for RO combiners, and indeed our construction is somewhat tight with this lower bound.

On the negative side, we show that one cannot generically apply the composition theorem to further replace "monolithic" hashes $h_1$ and $h_2$ by some simpler indifferentiable construction (such as the Merkle-Damgård transformation) from smaller components, such as fixed-length compression functions. Despite this issue, we directly prove collision resistance of the Merkle-Damgård variant of our combiner, where $h_1$ and $h_2$ are replaced by iterative Merkle-Damgård hashes applied to fixed-length compression functions. Thus, we can still subvert the concatenation barrier for collision-resistance combiners using practically small components.

Homomorphic encryption (HE) enables computation delegation to untrusted third-party while maintaining data confidentiality. Hybrid encryption (a.k.a Transciphering) allows a reduction in the number of ciphertexts and storage size, which makes HE solutions practical for a variety of modern applications. Still, modern transciphering has two main drawbacks: 1) lack of standardization or bad performance of symmetric decryption under FHE; 2) lack of input data integrity. In this paper, we discuss the concept of Authenticated Transciphering (AT), which like Authenticated Encryption (AE) provides some integrity guarantees for the transciphered data. For that, we report on the first implementations of AES-GCM decryption and Ascon decryption under CKKS. Moreover, we report and demonstrate the first end-to-end process that uses transciphering for real-world applications i.e., running deep neural network inference (ResNet50 over ImageNet) under encryption.

Modern security systems depend fundamentally on the ability of users to authenticate their communications to other parties in a network. Unfortunately, cryptographic authentication can substantially undermine the privacy of users. One possible solution to this problem is to use privacy-preserving cryptographic authentication. These protocols allow users to authenticate their communications without revealing their identity to the verifier. In the non-interactive setting, the most common protocols include blind, ring, and group signatures, each of which has been the subject of enormous research in the security and cryptography literature. These primitives are now being deployed at scale in major applications, including Intel's SGX software attestation framework. The depth of the research literature and the prospect of large-scale deployment motivate us to systematize our understanding of the research in this area. This work provides an overview of these techniques, focusing on applications and efficiency.

The advent of quantum computers poses a serious challenge to the security of cloud infrastructures and services, as they can potentially break the existing public-key cryptosystems, such as Rivest–Shamir–Adleman (RSA) and Elliptic Curve Cryptography (ECC). Even though the gap between today’s quantum computers and the threats they pose to current public-key cryptography is large, the cloud landscape should act proactively and initiate the transition to the post-quantum era as early as possible. To comply with that, the U.S. government issued a National Security Memorandum in May 2022 that mandated federal agencies to migrate to post-quantum cryptosystems (PQC) by 2035. To ensure the long-term security of cloud computing, it is imperative to develop and deploy PQC resistant to quantum attacks. A promising class of post-quantum cryptosystems is based on lattice problems, which require polynomial arithmetic. In this paper, we propose and implement a scalable number-theoretic transform (NTT) architecture that significantly enhances the performance of polynomial multiplication. Our proposed design exploits multi-levels of parallelism to accelerate the NTT computation on reconfigurable hardware. We use the high-level synthesis (HLS) method to implement our design, which allows us to describe the NTT algorithm in a high-level language and automatically generate optimized hardware code. HLS facilitates rapid prototyping and enables us to explore different design spaces and trade-offs on the hardware platforms. Our experimental results show that our design achieves 11$\times$ speedup compared to the state-of-the-art requiring only 14 clock cycles for an NTT computation over a polynomial of degree 256. To demonstrate the applicability of our design, we also present a coprocessor architecture for Kyber, a key encapsulation mechanism (KEM) chosen by the NIST post-quantum standardization process, that utilizes our scalable NTT core.

Modern system-on-chip (SoC) designs are becoming prone to numerous security threats due to their critical applications and ever-growing complexity and size. Therefore, the early stage of the design flow requires comprehensive security verification. The control flow of an SoC, generally implemented using finite state machines (FSMs), is not an exception to this requirement. Any deviations from the desired flow of FSMs can cause serious security issues. On the other hand, the control FSMs may be prone to fault-injection and denial-of-service (DoS) attacks or have inherent information leakage and access control issues at the gate-level netlist abstraction. Therefore, defining a set of security rules (guidelines) for obtaining FSM implementations free from particular security vulnerabilities after performing logic synthesis is crucial. Unfortunately, as of today, no solution exists in the state-of-the-art domain to verify the security of control FSMs. In this paper, we propose a set of such security rules for control FSM design and a verification framework called ARC-FSM-G to check for those security rule violations at pre-silicon to prevent any security vulnerabilities of FSM against fault-injection, access control, and information leakage threats. Experimental results on several benchmarks varying in size and complexity illustrate that ARC-FSM-G can effectively check for violations of all the proposed rules within a few seconds.

Perfect ciphers have been a very attractive cryptographic tool ever since C. Shannon described them. Note that, by definition, if a perfect cipher is used, no one can get any information about the encrypted message without knowing the secret key. We consider the problem of reducing the key length of perfect ciphers, because in many applications the length of the secret key is a crucial parameter. This paper describes a simple method of key length reduction. This method gives a perfect cipher and is based on the use of data compression and randomisation, and the average key length can be made close to Shannon entropy (which is the key length limit). It should be noted that the method can effectively use readily available data compressors (archivers).

We introduce a new candidate post-quantum digital signature scheme from the regular syndrome decoding (RSD) assumption, an established variant of the syndrome decoding assumption which asserts that it is hard to find $w$-regular solutions to systems of linear equations over $\mathbb{F}_2$ (a vector is regular if it is a concatenation of $w$ unit vectors). Our signature is obtained by introducing and compiling a new 5-round zero-knowledge proof system constructed using the MPC-in-the-head paradigm. At the heart of our result is an efficient MPC protocol in the preprocessing model that checks the correctness of a regular syndrome decoding instance by using a share ring-conversion mechanism.

The analysis of our construction is non-trivial and forms a core technical contribution of our work. It requires careful combinatorial analysis and combines several new ideas, such as analyzing soundness in a relaxed setting where a cheating prover is allowed to use any witness sufficiently close to a regular vector. We complement our analysis with an in-depth overview of existing attacks against RSD.

Our signatures are competitive with the best-known code-based signatures, ranging from $12.52$ KB (fast setting, with a signing time of the order of a few milliseconds on a single core of a standard laptop) to about $9$ KB (short setting, with estimated signing time of the order of 15ms).

With the growing number of decentralized finance applications, transaction fairness in blockchains has gained intensive research interest. As a broad concept in the distributed systems and blockchain literature, fairness has been used in different contexts, varying from ones related to the system's liveness to ones that focus on the received order of transactions. In this work, we revisit the fairness definitions known so far and provide a more generic fairness definition called verifiable fairness. Our definition relaxes the ordering rules that are inherently embedded in prior definitions to a predicate defined by concrete applications. Our notion thus gains flexibility and generality, capturing all existing fairness definitions. We provide a solution that achieves our new fairness definition, leveraging trusted hardware. Unlike prior works that usually design a dedicated consensus protocol to achieve fairness goals, our solution is modular and can be integrated with any blockchain system. We implement a prototype using Go Ethereum (Geth) as the blockchain and OpenSGX as the trusted hardware. Evaluation results reveal that our construction imposes only minimal overhead on existing blockchain systems.

Our research focuses on achieving perfect provable encryption by drawing inspiration from the principles of a one-time pad. We explore the potential of leveraging the unique properties of the one-time pad to design effective one-way functions. Our methodology involves the application of the exclusive-or (xor) operation to two randomly chosen strings. To address concerns related to preimage mappings, we incorporate error detection codes. Additionally, we utilize permutations to overcome linearity issues in the computation process.

In order to enhance the security of our approach, we propose the integration of a secret-sharing scheme based on a linear polynomial. This helps mitigate collisions and adds an additional layer of perfect security. We thoroughly investigate the interactions between different aspects of one-way functions to strengthen the reliability of commitments. Lastly, we explore the possibility of nesting one-way functions as a countermeasure against potential backdoors.

Through our study, we aim to contribute to the advancement of secure encryption techniques by leveraging the inherent strengths of the one-time pad and carefully considering the interplay of various components in the design of one-way functions.

This paper's primary purpose is to provide a source of introductory information into building a zero knowledge proof system for general computation. We review how to build such a system with a polynomial commitment scheme, and how to implement a fully functional command set in terms of zero knowledge primitives.

In 2022, Hosny et al. introduce an image encryption scheme that employs a fractional-order chaotic system. Their approach uses the hyper-chaotic system to generate the system's main parameter, namely a secret permutation which is dependent on the size and the sum of the pixels of the source image. According to the authors, their scheme offers adequate security (i.e. $498$ bits) for transmitting color images over unsecured channels. Nevertheless, in this paper we show that the scheme's security is independent on the secret parameters used to initialize the hyper-chaotic system. More precisely, we provide a brute-force attack whose complexity is $\mathcal O(2^{10.57}(WH)^3)$ and needs $2^{9.57}WH$ oracle queries, where $W$ and $H$ are the width and the height of the encrypted image. For example, for an image of size $4000 \times 30000$ ($12$ megapixels image) we obtain a security margin of $81.11$ bits, which is six times lower than the claimed bound. To achieve this result, we present two cryptanalytic attacks, namely a chosen plaintext attack and a chosen ciphertext attack.

The development of quantum computers, which employ a different paradigm of computation, is posing a threat to the security of cryptography. Narrowing down the scope to symmetric-key cryptography, the Grover search algorithm is probably the most influential in terms of its impact on security. Recently, there have been efforts to estimate the complexity of the Grover’s key search for symmetric key ciphers and evaluate their post-quantum security. In this paper, we present a depth-optimized implementation of a quantum circuit for ASCON, which is a symmetric key cipher that has recently been standardized in the NIST (National Institute of Standards and Technology) Lightweight Cryptography standardization. As far as we know, this is the first implementation of a quantum circuit for the ASCON AEAD (Authenticated Encryption with Associated Data) scheme. To our understanding, reducing the depth of the quantum circuit for the target cipher is the most effective approach for Grover’s key search. We demonstrate the optimal Grover’s key search cost for ASCON, along with a proposed depth-optimized quantum circuit. Further, based on the estimated cost, we evaluate the post-quantum security strength of ASCON according to relevant evaluation criteria and state-of-the-art research.

The hodlCoin game is a competitive zero-sum massively multiplayer financial game where the goal is to hodl an asset for long periods of time. By hodling, a player deposits coins of a given asset in a common reserve and receives a proportional amount of hodlCoins. Players who un-hodl pay a fee that is accumulated in the common reserve. Thus, the longer a player hodls, in comparison with other players, the more the player will benefit from fees paid by the players who are un-hodling earlier. Surprisingly, we prove here that, thanks to the accumulation of fees, the price of hodlCoins in comparison with the underlying asset never decreases.

In public key encryption (PKE), anonymity is essential to ensure privacy by preventing the ciphertext from revealing the recipient’s identity. However, the literature has addressed the anonymity of PKE under different attack scenarios to a limited extent. Benhamouda et al. (TCC 2020) introduced the first formal definition of anonymity for PKE under corruption, and Huang et al. (ASIACRYPT 2022) made further extensions and provided a generic framework. In this paper, we introduce a new security notion named enhanced decryption key exposure resistance (En-DKER) for revocable identity-based encryption (RIBE). This notion ensures that the exposure of decryption keys within any time period will not compromise the confidentiality and anonymity of ciphertexts encrypted during different periods. Meanwhile, we construct the first RIBE scheme with En-DKER and prove its security under the learning with errors (LWE) assumption. Our scheme offers several advantages. Firstly, the periodic workload of the key generation center (KGC) in our scheme is nearly zero. Secondly, the encryptor does not need to handle real-time revocation information of users within the system. Thirdly, the size of user secret keys remains constant in multi-bit encryption. Additionally, we present a novel approach to delegate a lattice basis. Diverging from the work of Cash et al. (J CRYPTOL 2012), our approach allows for the outsourcing of subsequent sampling operations to an untrusted server. Leveraging this approach, our scheme significantly reduces the periodic workload for users to generate decryption keys. Finally, we efficiently implemented our scheme using the number theory library (NTL) and multi-threaded parallel program. The experimental results confirm the advantages of our scheme.

With the ever greater adaptation of blockchain systems, smart contract based ecosystems have formed to provide financial services and other utility. This results in an ever increasing demand for transactions on blockchains, however, the amount of transactions per second on a given ledger is limited. Layer-2 systems attempt to improve scalability by taking transactions off-chain, with building blocks that are two party channels which are concatenated to form networks. Interaction between two parties requires (1) routing such a network, (2) interaction with and collateral from all intermediaries on the routed path and (3) interactions are often more limited compared to what can be done on the ledger. In contrast to that design, recent constructions such as Hydra Heads (FC’21) are both multi-party and isomorphic, allowing interactions to have the same expressiveness as on the ledger making it akin to a ledger located on Layer-2. The follow up Interhead Construction (MARBLE’22) further extends the protocol to connect Hydra Heads into networks by means of a “virtual” Hydra Head construction. This work puts forth an even greater generalization of the Interhead Protocol, allowing for inter- action across different Layer-2 ledgers with a multitude of improvements. As concrete example, our design is modular and lightweight, which makes it viable for both full virtual ledger constructions as well as straightfor- ward one-time interactions and payments systems.

In this paper we provide the implementation details and performance analysis of the lattice-based post-quantum commitment scheme introduced by Martínez and Morillo in their work titled «RLWE-Based Zero-Knowledge Proofs for Linear and Multiplicative Relations» together with the corresponding Zero-Knowledge Proofs of Knowledge (ZKPoK) of valid openings, linear and multiplicative relations among committed elements. We bridge the gap between the existing theoretical proposals and practical applications, thoroughly revisiting the security proofs of the aforementioned paper to obtain tight conditions that allow us to find the best sets of parameters for actual instantiations of the commitment scheme and its companion ZKPoK. Our implementation is very flexible and its parameters can be adjusted to obtain a trade-off between speed and memory usage, analyzing how suitable for practical use are the underlying lattice-based techniques. Moreover, our implementation further extends the literature of exact Zero-Knowledge proofs, providing ZKPoK of committed elements without any soundness slack.

The rising popularity of computational integrity protocols has led to an increased focus on efficient domain-specific hash functions, which are one of the core components in these use cases. For example, they are used for polynomial commitments or membership proofs in the context of Merkle trees. Indeed, in modern proof systems the computation of hash functions is a large part of the entire proof's complexity. In the recent years, authors of these hash functions have focused on components which are verifiable with low-degree constraints. This led to constructions like Poseidon, Rescue, Griffin, Reinforced Concrete, and Tip5, all of which showed significant improvements compared to classical hash functions such as SHA-3 when used inside the proof systems. In this paper, we focus on lookup-based computations, a specific component which allows to verify that a particular witness is contained in a lookup table. We work over 31-bit and 64-bit finite fields $\mathbb F_p$, both of which are used in various modern proof systems today and allow for fast implementations. We propose a new 2-to-1 compression function and a SAFE hash function, instantiated by the Monolith permutation. The permutation is significantly more efficient than its competitors, both in terms of circuit friendliness and plain performance, which has become one of the main bottlenecks in various use cases. This includes Reinforced Concrete and Tip5, the first two hash functions using lookup computations internally. Moreover, in Monolith we instantiate the lookup tables as functions defined over $\mathbb F_2$ while ensuring that the outputs are still elements in $\mathbb F_p$. Contrary to Reinforced Concrete and Tip5, this approach allows efficient constant-time plain implementations which mitigates the risk of side-channel attacks potentially affecting competing lookup-based designs. Concretely, our constant time 2-to-1 compression function is faster than a constant time version of Poseidon2 by a factor of 7. Finally, it is also the first arithmetization-oriented function with a plain performance comparable to SHA3-256, essentially closing the performance gap between circuit-friendly hash functions and traditional ones.

Secret sharing has been a promising tool in cryptographic schemes for decades. It allows a dealer to split a secret into some pieces of shares that carry no sensitive information on their own when being treated individually but lead to the original secret when having a sufficient number of them together. Existing schemes lack considering a guaranteed delay prior to secret reconstruction and implicitly assume once the dealer shares the secret, a sufficient number of shareholders will get together and recover the secret at their wish. This, however, may lead to security breaches when a timely reconstruction of the secret matters as the early knowledge of a single revealed share is catastrophic assuming a threshold adversary.

This paper presents the notion of timed secret sharing (TSS), providing lower and upper time bounds for secret reconstruction with the use of time-based cryptography. The recent advances in the literature including short-lived proofs [Asiacrypt 2022], enable us to realize an upper time bound shown to be useful in breaking public goods game, an inherent issue in secret sharing-based systems. Moreover, we establish an interesting trade-off between time and fault tolerance in a secret sharing scheme by having dealer gradually release additional shares over time, offering another approach with the same goal. We propose several constructions that offer a range of security properties while maintaining practical efficiency. Our constructions leverage a variety of techniques and state-of-the-art primitives.

The substitution box (S-box) is an important nonlinear component in most symmetric cryptosystems and thus should have good properties. Its difference distribution table (DDT) and linear approximation table (LAT) affect the security of the cipher against differential and linear cryptanalysis. In most previous work, differential uniformity and linearity of an S-box are two primary cryptographic properties to impact the resistance against differential and linear attacks. In some cases, the branch number and fixed point are also be considered. However, other important cryptographic properties such as the frequency of differential uniformity (resp. linearity) and the number of Bad Input and Bad Output (BIBO) patterns in DDT (resp. LAT) are often ignored. These properties substantially affect lightweight cryptography based on substitution bit permutation networks (SbPN) such as PRESENT, GIFT and RECTANGLE. This paper introduces a new method to search for S-boxes satisfying all above criteria simultaneously. In our strategy, we transform the process of searching for S-boxes under certain constraints on cryptographic properties into a satisfiability (SAT) problem. As applications, we use our new approach to search out 4-bit and 5-bit S-boxes with the same or better cryptographic properties compared with the S-boxes from well-known ciphers. Finally, we also utilize our method to verify a conjecture proposed by Boura et al. in the case of all 3-bit and 4-bit S-boxes. We propose a proposition and two corollaries to reduce the search space in this verification.