International Association
for Cryptologic Research

Threshold ECDSA receives interest lately due to its widespread adoption in blockchain applications. A common building block of all leading constructions involves a secure conversion of multiplicative shares into additive ones, which is called the multiplicative-to-additive (MtA) function. MtA dominates the overall complexity of all existing threshold ECDSA constructions. Specifically, $O(n^2)$ invocations of MtA are required in the case of $n$ active signers. Hence, improvement of MtA leads directly to significant improvements for all state-of-the-art threshold ECDSA schemes.

In this paper, we design a novel MtA by revisiting the Joye-Libert (JL) cryptosystem. Specifically, we revisit JL encryption and propose a JL-based commitment, then give efficient zero-knowledge proofs for JL cryptosystem which are the first to have standard soundness. Our new MtA offers the best time-space complexity trade-off among all existing MtA constructions. It outperforms state-of-the-art constructions from Paillier by a factor of $1.85$ to $2$ in bandwidth and $1.2$ to $1.7$ in computation. It is $7\times$ faster than those based on Castagnos-Laguillaumie encryption only at the cost of $2\times$ more bandwidth. While our MtA is slower than OT-based constructions, it saves $18.7\times$ in bandwidth requirement. In addition, we also design a batch version of MtA to further reduce the amotised time and space cost by another $25$%.

This work formally analyzes the anonymity guarantees of continuous stop-and-go mixnets and attempts to answer the above question. Existing mixnet based anonymous communication protocols that aim to provide provable anonymity guarantees rely on round-based communication models --- which requires synchronization among all the nodes and clients, and difficult to achieve in practice. Continuous stop-and-go mixnets (e.g., Loopix and Nym) provide a nice alternative by adding a random delay for each message on every hop independent of all other hops and all other messages. The core anonymization technique of continuous mixnets combined with the fact that the messages are sent by the clients to the mixnet at different times makes it a difficult problem to formally prove security for such mixnet protocols; all existing analyses for such designs provide only experimental evaluations for anonymity.

We are the first to close that gap and provide a formal analysis. We provide two indistinguishability based definitions (of sender anonymity), namely pairwise unlinkability and user unlinkability, tuned specifically for continuous stop-and-go mixnets. We derive the adversarial advantage as a function of the protocol parameters for the two definitions. We show that there is a fundamental lower bound on the adversarial advantage $\delta$ for pairwise unlinkability; however, strong user unlinkability (negligible adversarial advantage) can be achieved if the users message rate ($\lambda_u$) is proportional to message processing rate ($\lambda$) on the nodes.

Fully Homomorphic Encryption (FHE) is a widely used cryptographic primitive for performing arbitrary computations on encrypted data. However, FHE incorporates a computationally intensive mechanism known as "bootstrapping", that resets the noise in the ciphertext to a lower level allowing the computation on circuits of arbitrary depth. This process can take significant time, ranging from several minutes to hours. To address the above issue, in this work, we propose an Electronic Design Automation (EDA) framework FHEDA that generates efficient Boolean representations of circuits compatible with the Torus-FHE (ASIACRYPT 2020) scheme. To the best of our knowledge, this is the first work in the EDA domain of FHE. We integrate logic synthesis tricks and gate optimization techniques into our FHEDA framework for reducing the total number of bootstrapping operations in a Boolean circuit, which leads to a significant (up to 50%) reduction in homomorphic computation time. Our FHEDA is built upon the observation that in Torus-FHE at most one Boolean gate over fresh encryptions does not require bootstrapping. By integrating this observation with logic replacement techniques into FHEDA, we could reduce the total number of bootstrapping operations along with the circuit depth. This eventually reduces the homomorphic evaluation time of Boolean circuits. In order to verify the efficacy of our approach, we assess the performance of the proposed EDA flow on a diverse set of representative benchmarks including privacy-preserving machine learning and different symmetric key block ciphers.

Key-policy attribute-based encryption scheme (KP-ABE) uses a set of attributes as public keys for encryption. It allows homomorphic evaluation of ciphertext into another ciphertext of the same message, which can be decrypted if a certain access policy based on the attributes is satisfied. A lattice-based KP-ABE scheme is reported in several works in the literature, and its software implementation is available in an open-source library called PALISADE. However, as the cryptographic primitives in KP-ABE are overly involved, non-trivial hardware acceleration is needed for its adoption in practical applications.

In this work, we provide GPU-based algorithms for accelerating KP-ABE encryption and homomorphic evaluation functions seamlessly integrated into the open-source library with minor additional build changes needed to run the GPU kernels. Using GPU algorithms, we perform both homomorphic encryption and homomorphic evaluation operations 2.1× and 13.2× faster than the CPU implementations reported in the literature on an Intel i9, respectively. Furthermore, our implementation supports up to 128 attributes for encryption and homomorphic evaluation with fixed and changing access policies. Unlike the reported GPU-based homomorphic operations in the literature, which support only up to 32 attributes and give estimations for a higher number of attributes. We also propose a GPU-based KP-ABE scheme for publish/subscribe messaging applications, in which end-to-end security of the messages is guaranteed. Here, while the exchanged messages are encrypted with as many as 128 attributes by publishers, fewer attributes are needed for homomorphic evaluation. Our fast and memory-efficient GPU implementations of KP-ABE encryption and homomorphic evaluation operations demonstrate that the KP-ABE scheme can be used for practicable publish/subscribe messaging applications.

Clouds have replaced local backup systems due to their stronger reliability and availability guarantees compared to local machines, which are prone to hardware/software failure or can be stolen or lost, especially in the case of portable devices

In recent years, some digital assets are managed solely through the knowledge of cryptographic secrets (e.g., cryptocurrency, encrypted datasets), whose loss results in the permanent loss of the digital asset. Since the security of such systems relies on the assumption that the cryptographic key remains secret, a secret owner Alice cannot simply store a backup copy of such secret on the cloud, since this corresponds to giving away her ownership over the digital assets. Thus Alice must rely on her personal machines to maintain these secrets.

Is it possible to obtain the best of the two worlds, where Alice benefits from the convenience of storing a backup copy of her cryptographic secrets on the cloud such that she can recover them even when she loses her devices and forgets all credentials, while at the same time retaining full ownership of her secrets?

In this paper, we show that this is indeed possible, by revisiting and expanding the concept of Break-glass Encryption pioneered by Scafuro [PKC19].

We provide a secret-recovery mechanism where confidentiality is always guaranteed when Alice has not lost her credentials, even in the presence of a malicious cloud and users ([PKC19] only guarantees that a violation of confidentiality will be {\em detected}, not prevented). Recoverability is achieved in most circumstances.

We design and prove security of a credential-less authentication mechanism, that enables Alice to access her secret, without remembering any credentials. This tool was assumed in [PKC19] but not implemented. We redesign the storage mechanism on the cloud side so that the cloud needs to perform no operations during the storage phase. This is in contrast with [PKC19] where the cloud must re-encrypt the stored file continuously with the help of a secure enclave (regardless of whether a recovery procedure will happen).

Our protocols are proved secure in the Universal Composition framework.

Decision tree evaluation is extensively used in machine learning to construct accurate classification models. Often in the cloud-assisted communication paradigm cloud servers execute remote evaluations of classification models using clients’ data. In this setting, the need for private decision tree evaluation (PDTE) has emerged to guarantee no leakage of information for the client’s input nor the service provider’s trained model i.e., decision tree. In this paper, we propose a private decision tree evaluation protocol based on the three-party replicated secret sharing (RSS) scheme. This enables us to securely classify inputs without any leakage of the provided input or the trained decision tree model. Our protocol only requires constant rounds of communication among servers, which is useful in a network with longer delays.

Ma et al. (NDSS 2021) presented a lightweight PDTE protocol with sublinear communication cost with linear round complexity in the size of the input data. This protocol works well in the low latency network such as LAN while its total execution time is unfavourably increased in the WAN setting. In contrast, Tsuchida et al. (ProvSec 2020) constructed a constant round PDTE protocol at the cost of communication complexity, which works well in the WAN setting. Although their construction still requires 25 rounds, it showed a possible direction on how to make constant round PDTE protocols. Ji et al. (IEEE Transactions on Dependable and Secure Computing) presented a simplified PDTE with constant rounds using the function secret sharing (FSS) at the cost of communication complexity.

Our proposed protocol only requires five rounds among the employed three servers executing secret sharing schemes, which is comparable to previously proposed protocols that are based on garbled circuits and homomorphic encryption. To further demonstrate the efficiency of our protocol, we evaluated it using real-world classification datasets. The evaluation results indicate that our protocol provides better concrete performance in the WAN setting that has a large network delay.

Quantum attacks using superposition queries are known to break many classically secure modes of operation. While these attacks do not necessarily threaten the security of the modes themselves, since they rely on a strong adversary model, they help us to draw limits on the provable security of these modes.

Typically these attacks use the structure of the mode (stream cipher, MAC or authenticated encryption scheme) to embed a period-finding problem, which can be solved with a dedicated quantum algorithm. The hidden period can be recovered with a few superposition queries (e.g., $O(n)$ for Simon's algorithm), leading to state or key-recovery attacks. However, this strategy breaks down if the period changes at each query, e.g., if it depends on a nonce.

In this paper, we focus on this case and give dedicated state-recovery attacks on the authenticated encryption schemes Rocca, Rocca-S, Tiaoxin-346 and AEGIS-128L. These attacks rely on a procedure to find a Boolean hidden shift with a single superposition query, which overcomes the change of nonce at each query. As they crucially depend on such queries, we stress that they do not break any security claim of the authors, and do not threaten the schemes if the adversary only makes classical queries.

Various message authentication codes (MACs), including HMAC-Streebog and Streebog-K, are based on the keyless hash function Streebog. Under the assumption that the compression function of Streebog is resistant to the related key attacks, the security proofs of these algorithms were recently presented at CTCrypt 2022.

We carefully detail the resources of the adversary in the related key settings, revisit the proof, and obtain tight security bounds. Let $n$ be the bit length of the hash function state. If the amount of processed data is less than about $2^{n-k}$ blocks, then for HMAC-Streebog-512 and Streebog-K, the only effective method of forgery (or distinguishing) is guessing the $k$-bit secret key or the tag if it is shorter than the key. So, we can speak about ``$k$-bit security'' without specifying the amount of material, if the key length is no longer than half of a state. The bound for HMAC-Streebog-256 is worse and equal to $2^{\frac{n}{2}-k}$ blocks.

BGV and BFV are among the most widely used fully homomorphic encryption (FHE) schemes. Both schemes have a common plaintext space, with a rich algebraic structure. Our main contribution is to show how this structure can be exploited to more efficiently homomorphically evaluate polynomials. Namely, using Galois automorphisms, we present an algorithm to homomorphically evaluate a polynomial of degree $d$ in only $3\log(d)$ (in some cases only $2\log(d)$) many ciphertext-ciphertext multiplications and automorphism evaluations, where $d$ is bounded by the ring degree. In other words, as long as the degree of the polynomial is bounded, we achieve an exponential speedup compared to the state of the art. In particular, the approach also improves on the theoretical lower bound of $2\sqrt{d}$ many ciphertext-ciphertext multiplications, which would apply if automorphisms were not available.

We investigate how to apply our improved polynomial evaluation to the bootstrapping procedure for BFV, and show that we are able to significantly improve its performance. We demonstrate this by providing an implementation of our improved BFV bootstrapping using the Microsoft SEAL library. More concretely, we obtain a $1.6\times$ speed up compared to the prior implementation given by Chen and Han (Eurocrypt 2018). The techniques are independent of, and can be combined with, the more recent optimisations presented by Geelen \textit{et al}. (Eurocrypt 2023).

As an additional contribution, we show how the bootstrapping approach used in schemes such as FHEW and TFHE can be applied in the BFV context. In particular, we demonstrate that programmable bootstrapping can be achieved for BFV. Moreover, we show how this bootstrapping approach can be improved in the BFV context to make better use of the Galois structure. However, we estimate that its complexity is around three orders of magnitude slower than the classical approach to BFV bootstrapping.

Using the provable security approach, we analyze CRISP – a standardized Russian cryptographic protocol that aims to ensure confidentiality, integrity of transmitted messages, as well as protection against replay attacks. The protocol is considered as a specific mode of authenticated encryption with associated data (AEAD). We take into account that one key can be used by many protocol's participants and in different cipher suites. We impose requirements for the set of the cipher suites used in the protocol and show that the existing ones meet them. Estimates of the maximum allowable amount of data processed using a single key are also given.

The differential meet-in-the-middle (MITM) attack is a new cryptanalysis technique proposed at Crypto 2023 recently. It led to greatly improved attacks on round-reduced SKINNY-128-384 and AES-256. In this paper, we revisit the differential MITM attack and propose several variants by absorbing techniques widely used in the classical differential attack. In particular, we present a new differential MITM attack that generalizes the basic differential MITM attack in several aspects. As for applications, we make refinements to the 24-round attack on SKINNY-128-384; on 12-round AES-256, we show that the classical differential attack and the generalized differential MITM attack perform better than the basic differential MITM attack.

Motivated by proof-of-stake (PoS) blockchains such as Ethereum, two key desiderata have recently been studied for Byzantine-fault tolerant (BFT) state-machine replication (SMR) consensus protocols: Finality means that the protocol retains consistency, as long as less than a certain fraction of validators are malicious, even in partially-synchronous environments that allow for temporary violations of assumed network delay bounds. Accountable safety means that in any case of inconsistency, a certain fraction of validators can be identified to have provably violated the protocol. Earlier works have developed impossibility results and protocol constructions for these properties separately. We show that accountable safety implies finality, thereby unifying earlier results.

Focusing on its cryptographic core, we provide the first formal description of the Matrix secure group messaging protocol. Observing that no existing secure messaging model in the literature captures the relationships (and shared state) between users, their devices and the groups they are a part of, we introduce the Device-Oriented Group Messaging model to capture these key characteristics of the Matrix protocol.

Utilising our new formalism, we determine that Matrix achieves the basic security notions of confidentiality and authentication, provided it introduces authenticated group membership. On the other hand, while the state sharing functionality in Matrix conflicts with advanced security notions in the literature – forward and post-compromise security – it enables features such as history sharing and account recovery, provoking broader questions about how such security notions should be conceptualised.

We propose a new scheme based on ephemeral elliptic curves over the ring $\mathbb{Z}/n\mathbb{Z}$ where $n=pq$ is an RSA modulus with $p=u_p^2+v_p^2$, $q=u_q^2+v_q^2$, $u_p\equiv u_q\equiv 3\pmod 4$. The new scheme is a variant of both the RSA and the KMOV cryptosystems. The scheme can be used for both signature and encryption. We study the security of the new scheme and show that is immune against factorization attacks, discrete logarithm problem attacks, sum of two squares attacks, sum of four squares attacks, isomorphism attacks, and homomorphism attacks. Moreover, we show that the private exponents can be much smaller than the ordinary exponents for RSA and KMOV, which makes the decryption phase in the new scheme more efficient.

In this paper, we present NEV -- a faster and smaller NTRU Encryption using Vector decoding, which is provably IND-CPA secure in the standard model under the decisional NTRU and RLWE assumptions over the cyclotomic ring $R_q = \mathbb{Z}_q[X]/(X^n+1)$. Our main technique is a novel and non-trivial way to integrate a previously known plaintext encoding and decoding mechanism into the provably IND-CPA secure NTRU variant by Stehl\'e and Steinfeld (Eurocrypt 2011). Unlike the original NTRU encryption and its variants which encode the plaintext into the least significant bits of the coefficients of a message polynomial, we encode each plaintext bit into the most significant bits of multiple coefficients of the message polynomial, so that we can use a vector of noised coefficients to decode each plaintext bit in decryption, and significantly reduce the size of $q$ with a reasonably negligible decryption failure.

Concretely, we can use $q = 769$ to obtain public keys and ciphertexts of 615 bytes with decryption failure $\leq 2^{-138}$ at NIST level 1 security, and 1229 bytes with decryption failure $\leq 2^{-152}$ at NIST level 5 security. By applying the Fujisaki-Okamoto transformation in a standard way, we obtain an IND-CCA secure KEM from our basic PKE scheme. Compared to NTRU and Kyber in the NIST Round 3 finalists at the same security levels, our KEM is 33-48% more compact and 5.03-29.94X faster than NTRU in the round-trip time of ephemeral key exchange, and is 21% more compact and 1.42-1.74X faster than Kyber.

We also give an optimized encryption scheme NEV' with better noise tolerance (and slightly better efficiency) based on a variant of the RLWE problem, called Subset-Sum Parity RLWE problem, which we show is polynomially equivalent to the standard decisional RLWE problem (with different parameters), and maybe of independent interest.

In this paper a method to build Secret Agreement algorithms is pre- sented, which only requires an abelian group and at least one automor- phism of the operator of this group. An example of such an algorithm is also presented. Knowledge of entropic quasigroups and Bruck-Murdoch- Toyoda theorem on how to build a quasigroup with these two elements is assumed.

We show that the scheme [Clust. Comput. 25(1): 451-468, 2022] fails to keep anonymity, not as claimed. The scheme simply acknowledges that user anonymity is equivalent to protecting the target user's identity against exposure, while its long-term pseudo-identity can be exposed. We want to clarify that the true anonymity means that an adversary cannot attribute different sessions to different target users, even though the adversary cannot recover the true identifier from the long-term pseudo-identifier. We also clarify some misunderstandings in the scheme.

Recent works have revisited blockcipher structures to achieve MPC- and ZKP-friendly designs. In particular, Albrecht et al. (EUROCRYPT 2015) first pioneered using a novel structure SP networks with partial non-linear layers (P-SPNs) and then (ESORICS 2019) repopularized using multi-line generalized Feistel networks (GFNs). In this paper, we persist in exploring symmetric cryptographic constructions that are conducive to the applications such as MPC. In order to study the minimization of non-linearity in Type-II Generalized Feistel Networks, we generalize the (extended) GFN by replacing the bit-wise shuffle in a GFN with the stronger linear layer in P-SPN and introducing the key in each round. We call this scheme Generalized Extended Generalized Feistel Network (GEGFN). When the block-functions (or S-boxes) are public random permutations or (domain-preserving) functions, we prove CCA security for the 5-round GEGFN. Our results also hold when the block-functions are over the prime fields F_p, yielding blockcipher constructions over (F_p)^*.

Emails have improved our workplace efficiency and communication. However, they are often processed unencrypted by mail servers, leaving them open to data breaches on a single service provider. Public-key based solutions for end-to-end secured email, such as Pretty Good Privacy (PGP) and Secure/Multipurpose Internet Mail Extensions (S/MIME), are available but are not widely adopted due to usability obstacles and also hinder processing of encrypted emails.

We propose PrivMail, a novel approach to secure emails using secret sharing methods. Our framework utilizes Secure Multi-Party Computation techniques to relay emails through multiple service providers, thereby preventing any of them from accessing the content in plaintext. Additionally, PrivMail supports private server-side email processing similar to IMAP SEARCH, and eliminates the need for cryptographic certificates, resulting in better usability than public-key based solutions. An important aspect of our framework is its capability to enable third-party searches on user emails while maintaining the privacy of both the email and the query used to conduct the search.

We integrate PrivMail into the current email infrastructure and provide a Thunderbird plugin to enhance user-friendliness. To evaluate our solution, we benchmarked transfer and search operations using the Enron Email Dataset and demonstrate that PrivMail is an effective solution for enhancing email security.

The theory of finite simple groups is a (rather unexplored) area likely to provide interesting computational problems and modelling tools useful in a cryptographic context. In this note, we review some applications of finite non-abelian simple groups to cryptography and discuss different scenarios in which this theory is clearly central, providing the relevant definitions to make the material accessible to both cryptographers and group theorists, in the hope of stimulating further interaction between these two (non-disjoint) communities. In particular, we look at constructions based on various group-theoretic factorization problems, review group theoretical hash functions, and discuss fully homomorphic encryption using simple groups. The Hidden Subgroup Problem is also briefly discussed in this context.