International Association
for Cryptologic Research

Current adaptively secure identity-based encryption (IBE) constructions from lattices are unable to achieve a good balance among the master public key size, secret key size, modulus and reduction loss. All existing IBE schemes are subject to a quadratic restriction of modulus on the trapdoor norm, which harshly increases the modulus. In this work, we remove this restriction and present a new adaptively secure IBE scheme from lattices in the standard model, which improves the state-of-the-art construction proposed by Abla et al. (TCC 2021) and achieves asymptotically better efficiency. More precisely, we achieve the asymptotically minimum number of public vectors among all the previous schemes and a tight security reduction, together with a significantly smaller modulus compared to the scheme by Abla et al. (TCC 2021). Furthermore, our scheme enjoys the smallest Gaussian width of the secret key among all existing schemes. We propose a novel cross-multiplication design for our IBE scheme and several novel tools/techniques including: a) homomorphic computation outputting BGG+-style encoding with two distinct-norm trapdoors; b) sampling algorithm with hybrid Gaussian outputs; c) partial rerandomization. These new tools and techniques are general and could find rich applications in lattice-based cryptography.

The adaptive security of threshold signatures considers an adversary that adaptively corrupts users to learn their secret key shares and states. Crites, Komlo, and Maller (Crypto 2023) proposed Sparkle, the first adaptively secure threshold signature scheme in the pairing-free discrete-log setting, but it requires the algebraic group model (AGM) and is based on an interactive assumption. Bacho, Loss, Tessaro, Wagner, and Zhu (Eurocrypt 2024) proposed Twinkle, whose adaptive security can be proved based on the standard DDH assumption without the AGM. We propose Dazzle and Dazzle-T, adaptively secure threshold signature schemes based on DDH without the AGM, the same assumption and model as Twinkle. Our schemes improve upon Twinkle in signature size, round complexity, and/or security tightness. In particular, Dazzle and Dazzle-T both have signatures that are shorter than Twinkle by one group element. Regarding the round complexity and tightness, Twinkle is three-round and non-tight. Dazzle is two-round and has the same security loss as Twinkle. Dazzle-T is three-round and fully tight. We achieve our improvements by optimizing the underlying single-party signature scheme and showing that the single-party scheme can be transformed to a threshold scheme by a simpler transformation than that of Twinkle.

We introduce an enhanced requirement of deniable public key encryption that we call dual-deniability. It asks that a sender who is coerced should be able to produce fake randomness, which can explain the target ciphertext as the encryption of any alternative message under any valid key she/he desires to deny. Compared with the original notion of deniability (Canetti et al. in CRYPTO ’97, hereafter named message-deniability), this term further provides a shield for the anonymity of the receiver against coercion attacks. We first give a formal definition of dual-deniability, along with its weak-mode variant. For conceptual understanding, we then show dual-deniability implies semantic security and anonymity against CPA, separates full robustness, and even contradicts key-less or mixed robustness, while is (constructively) implied by key-deniability and full robustness with a minor assumption for bits encryption. As for the availability of dual-deniability, our main scheme is a generic approach from ciphertext-simulatable PKE, where we devise a subtle multi-encryption schema to hide the true message within random masking ciphertexts under plan-ahead setting. Further, we leverage the weak model to present a more efficient scheme having negligible detection probability and constant ciphertext size. Besides, we revisit the notable scheme (Sahai and Waters in STOC ’14) and show it is inherently dual-deniable. Finally, we extend the Boneh-Katz transform to capture CCA security, deriving dual-deniable and CCA-secure PKE from any selectively dual-deniable IBE under multi-TA setting. Overall our work mounts the feasibility of anonymous messaging against coercion attacks.

Dynamic Decentralized Functional Encryption (DDFE) is a generalization of Functional Encryption which allows multiple users to join the system dynamically without interaction and without relying on a trusted third party. Users can independently encrypt their inputs for a joint evaluation under functions embedded in functional decryption keys; and they keep control on these functions as they all have to contribute to the generation of the functional keys. In this work, we present new generic compilers which, when instantiated with existing schemes from the literature, improve over the state-of-the-art in terms of security, computational assumptions and functionality. Specifically, we obtain the first adaptively secure DDFE schemes for inner products in both the standard and the stronger function-hiding setting which guarantees privacy not only for messages but also for the evaluated functions. Furthermore, we present the first DDFE for inner products whose security can be proven under the LWE assumption in the standard model. Finally, we give the first construction of a DDFE for the attribute-weighted sums functionality with attribute-based access control (with some limitations). All prior constructions guarantee only selective security, rely on group-based assumptions on pairings, and cannot provide access control.

Dynamic Decentralized Functional Encryption (DDFE), introduced by Chotard et al. (CRYPTO'20), represents a robust generalization of (Multi-Client) Functional Encryption. It allows users to dynamically join and contribute private inputs to individually controlled joint functions without requiring a trusted authority. Recently, Shi and Vanjani (PKC'23) proposed the first Multi-Client Functional Encryption scheme for function-hiding inner products (FH-IP) without relying on random oracles. Unfortunately, their construction still requires a trusted key authority, leaving open the question of whether a full-fledged FH-IP-DDFE can exist in the standard model. In this work, we answer this question affirmatively by introducing Updatable Pseudorandom Zero Sharing, a novel concept that provides both the critical functionality and security properties needed to construct secure DDFE schemes in the standard model. Our second contribution is a novel proof strategy, which preserves adaptive security when transforming any functional encryption scheme for FH-IP into FH-IP-DDFE. Together, these two techniques enable a modular construction of FH-IP-DDFE that is secure against adaptive message and key queries in the standard model. Additionally, our pseudorandom zero-sharing scheme is highly versatile, enabling the first DDFE for attribute-weighted sums in the standard model, complementing the recent ROM-based construction by Agrawal et al. (CRYPTO'23).

Multi-client Attribute-Based Encryption (ABE) is a generalization of key-policy ABE where attributes can be independently encrypted across several ciphertexts w.r.t. labels, and a joint decryption of these ciphertexts is possible if and only if (1) all ciphertexts share the same label, and (2) the combination of attributes satisfies the policy of the decryption key. All encryptors have their own secret key and security is preserved even if some of them are known to the adversary. Very recently, Pointcheval et al. (TCC 2024) presented a semi-generic construction of MC-ABE for restricted function classes, e.g., NC0 and constant-threshold policies. We identify an abstract criterion common to all their policy classes which suffices to present the construction in a fully black-box way and allows for a slight strengthening of the supported policy classes. The construction of Pointcheval et al. is based on pairings. We provide a new lattice-based instantiation from evasive LWE. Furthermore, we revisit existing constructions for policies that can be viewed as a conjunction of local policies (one per encryptor). Existing constructions from MDDH (Agrawal et al., CRYPTO 2023) and LWE (Francati et al., EUROCRYPT 2023) do not support encryption w.r.t. different labels. We show how this feature can be included at the cost of additionally relying on random oracles. Notably, the security model of Francati et al. additionally guarantees attribute-hiding but does not capture collusions. Our new construction is also attribute-hiding and provides resilience against any polynomially bounded number of collusions which must be fixed at the time of setup.

Recent years have witnessed a significant development for functional encryption (FE) in the multi-user setting, particularly with multi-client functional encryption (MCFE). The challenge becomes more important when combined with access control, such as attribute-based encryption (ABE), which was actually not covered syntactically by the public-key FE nor semantically by the secret-key MCFE frameworks. On the other hand, as for complex primitives, many works have studied the admissibility of adversaries to ensure that the security model encompasses all real threats of attacks. 1. At a conceptual level, by adding a public input to FE/MCFE, we cover many previous primitives, notably attribute-based function classes. Furthermore, with the strongest admissibility for inner-product functionality, our framework is quite versatile, as it encrypts multiple sub-vectors, allows repetitions and corruptions, and eventually also encompasses public-key FE and classical ABE, bridging the private setting of MCFE with the public setting of FE and ABE. 2. Finally, we propose an MCFE with public inputs with the class of functions that combines inner-products (on private inputs) and attribute-based access-control (on public inputs) for LSSS policies. We achieve the first AB-MCFE for inner products with strong admissibility (from Nguyen et al., ACNS'23) and with adaptive security. In the end, our concrete MCFE leads to MIFE for inner products, public-key single-input inner-product FE with LSSS key-policy, and KPABE for LSSS, with adaptive security. Previous AB-MCFE constructions are either restricted in terms of weaker admissibility (Nguyen et al., ASIACRYPT'22) or considers a slightly larger functionality of attribute-weighted sum but with only selective security (Agrawal et al., CRYPTO'23).

Garbling is a fundamental cryptographic primitive, with numerous theoretical and practical applications. Since the first construction by Yao (FOCS’82, ’86), a line of work has concerned itself with reducing the communication and computational complexity of that construction. One of the most efficient garbling schemes presently is the ‘Half Gates’ scheme by Zahur, Rosulek, and Evans (Eurocrypt’15). Despite its widespread adoption, the provable security of this scheme has been based on assumptions whose only instantiations are in idealized models. For example, in their original paper, Zahur, Rosulek, and Evans showed that hash functions satisfying a notion called circular correlation robustness (CCR) suffice for this task, and then proved that CCR secure hash functions can be instantiated in the random permutation model. In this work, we show how to securely instantiate the Half Gates scheme in the standard model. To this end, we first show how this scheme can be securely instantiated given a (family of) weak CCR hash function, a notion that we introduce. Furthermore, we show how a weak CCR hash function can be used to securely instantiate other efficient garbling schemes, namely the ones by Rosulek and Roy (Crypto’21) and Heath (Eurocrypt’24). Thus we believe this notion to be of independent interest. Finally, we construct such weak CCR hash functions using indistinguishability obfuscation and one-way functions. The security proof of this construction constitutes our main technical contribution. While our construction is not practical, it serves as a proof of concept supporting the soundness of these garbling schemes, which we regard to be particularly important given the recent initiative by NIST to standardize garbling, and the optimizations in Half Gates being potentially adopted.

Signal recently deployed a new handshake protocol named PQXDH to protect against "harvest-now-decrypt-later" attacks of a future quantum computer. To this end, PQXDH adds a post-quantum KEM to the Diffie-Hellman combinations of the prior X3DH handshake. In this work, we give a reductionist security analysis of Signal's PQXDH handshake in a game-based security model that captures the targeted "maximum-exposure" security against both classical and quantum adversaries, allowing fine-grained compromise of user's long-term, semi-static, and ephemeral key material. We augment prior such models to capture not only the added KEM component but also the signing of public keys, which prior analyses did not capture but which adds an additional flavor of post-quantum security in PQXDH. We then establish fully parameterized, concrete security bounds for the classical and post-quantum session key security of PQXDH, and discuss how design choices in PQXDH make a KEM binding property necessary and how a lack of domain separation reduces the achievable security. Our discussion of KEM binding and domain separation complements the concurrent tool-based analysis of PQXDH by Bhargavan, Jacomme, Kiefer, and Schmidt (USENIX Security 2024), which pointed out a potential re-encapsulation attack if the KEM shared secret does not bind the public key. In contrast to the tool-based analysis, we analyze all protocol modes of PQXDH and its "maximum-exposure" security. We further show that both Kyber (used in PQXDH) and the NIST standard ML-KEM (expected to replace Kyber) satisfy a novel binding notion we introduce and rely on for our PQXDH analysis, which may be of independent interest.

In this work, we focus on Single-Input Functionality (SIF), a specialized case of MPC where only one designated party, called the dealer, holds a private input. SIF enables the dealer to perform computations with other parties without disclosing any additional information about the private input. SIF has a wide range of applications, such as multiple-verifier zero-knowledge and verifiable relation sharing. We propose the \emph{first} 1-round SIF protocol against a dishonest majority in the preprocessing model, achieving high efficiency. Previous works either require at least 2-round online communication (Yang and Wang, Asiacrypt 2022; Baum et al., CCS 2022; Zhou et al., Euro S\&P 2024) or are limited to feasibility results (Lepinski et al., TCC 2005; Applebaum et al., Crypto 2022). We also show the necessity of using the broadcast channels, by formally proving that 1-round SIF is \emph{impossible} to achieve in the preprocessing model, if there are no broadcast channels available. Finally, we implement our protocol and present extensive experimental results, demonstrating its practical efficiency.

Recently, there is a growing need for SNARKs to operate over a broader range of algebraic structures, and one important structure is Galois ring. We present transparent SNARK schemes over arbitrary Galois rings. Compared with Rinocchio scheme in Ganesh et al. (J Cryptol 2023), our SNARK schemes do not require a trusted third party to establish a structured reference string (SRS). In this paper, we present the expander code over arbitrary Galois rings, which can be encoded in $O(n)$ time. Using this expander code, we then extend the Brakedown commitment scheme in Golovnev et al. (CRYPTO 2023) to Galois rings. By combining the Libra framework in Xie et al. (CRYPTO 2019), we present a transparent SNARK for log-space uniform circuits over Galois rings, achieving $O(n)$ prover time, $O(\sqrt{n})$ proof size, and $O(\sqrt{n})$ verifier time. And by combining HyperPlonk in Chen et al. (EUROCRYPT 2023), we present a transparent SNARK for NP circuits over Galois rings, with $O(n\log^2 n)$ prover time, $O(\sqrt{n})$ proof size, and $O(\sqrt{n})$ verifier time.