Real-world Universal zkSNARKs are non-malleable
https://eprint.iacr.org/2024/721

Simulation extractability is a strong security notion of zkSNARKs that guarantees that an attacker who produces a valid proof must know the corresponding witness, even if the attacker had prior access to proofs generated by other users. Notably, simulation extractability implies that proofs are non-malleable and is of fundamental importance for applications of zkSNARKs in distributed systems. In this work, we study sufficient and necessary conditions for constructing simulation-extractable universal zkSNARKs via the popular design approach based on compiling polynomial interactive oracle proofs (PIOP). Our main result is the first security proof that popular universal zkSNARKs, such as PLONK and Marlin, as deployed in the real world, are simulation-extractable. Our result fills a gap left from previous work (Faonio et al. TCC’23, and Kohlweiss et al. TCC’23) which could only prove the simulation extractability of the “textbook” versions of these schemes and does not capture their optimized variants, with all the popular optimization tricks in place, that are eventually implemented and deployed in software libraries.

Ultrametric integral cryptanalysis
https://eprint.iacr.org/2024/722

A systematic method to analyze \emph{divisibility properties} is proposed.
In integral cryptanalysis, divisibility properties interpolate between bits that sum to zero (divisibility by two) and saturated bits (divisibility by $2^{n - 1}$ for $2^n$ inputs).
From a theoretical point of view, we construct a new cryptanalytic technique that is a non-Archimedean multiplicative analogue of linear cryptanalysis. It lifts integral cryptanalysis to characteristic zero in the sense that, if all quantities are reduced modulo two, then one recovers the algebraic theory of integral cryptanalysis.
The new technique leads to a theory of trails. We develop a tool based on off-the-shelf solvers that automates the analysis of these trails and use it to show that many integral distinguishers on PRESENT and SIMON are stronger than expected.

$\mathsf{OPA}$: One-shot Private Aggregation with Single Client Interaction and its Applications to Federated Learning
https://eprint.iacr.org/2024/723

Our work aims to minimize interaction in secure computation due to the high cost and challenges associated with communication rounds, particularly in scenarios with many clients. In this work, we revisit the problem of secure aggregation in the single-server setting where a single evaluation server can securely aggregate client-held individual inputs. Our key contribution is One-shot Private Aggregation ($\mathsf{OPA}$) where clients speak only once (or even choose not to speak) per aggregation evaluation. Since every client communicates just once per aggregation, this streamlines the management of dropouts and dynamic participation of clients, contrasting with multi-round state-of-the-art protocols for each aggregation.

We initiate the study of $\mathsf{OPA}$ in several ways. First, we formalize the model and present a security definition. Second, we construct $\mathsf{OPA}$ protocols based on class groups, DCR, and LWR assumptions. Third, we demonstrate $\mathsf{OPA}$ with two applications: private stream aggregation and privacy-preserving federated learning. Specifically, $\mathsf{OPA}$ can be used as a key building block to enable privacy-preserving federated learning and critically, where client speaks once. This is a sharp departure from prior multi-round protocols whose study was initiated by Bonawitz et al. (CCS, 2017). Moreover, unlike the YOSO (You Only Speak Once) model for general secure computation, $\mathsf{OPA}$ eliminates complex committee selection protocols to achieve adaptive security. Beyond asymptotic improvements, $\mathsf{OPA}$ is practical, outperforming state-of-the-art solutions. We leverage $\mathsf{OPA}$ to develop a streaming variant named $\mathsf{SOPA}$, serving as the building block for privacy-preserving federated learning. We utilize $\mathsf{SOPA}$ to construct logistic regression classifiers for two datasets.

A new distributed key homomorphic PRF is at the core of our construction of $\mathsf{OPA}$. This key component addresses shortcomings observed in previous works that relied on DDH and LWR in the work of Boneh et al. (CRYPTO, 2013), marking it as an independent contribution to our work. Moreover, we also present new distributed key homomorphic PRFs based on class groups or DCR or the LWR assumption.

IACR Statement On the War in Gaza
https://iacr.org/news/item/23119

Announcement: IACR Statement On the War in Gaza

https://www.iacr.org/petitions/gaza_war.html

AFT '24: Advances in Financial Technologies
https://iacr.org/news/item/23122

Event Calendar: AFT '24: Advances in Financial Technologies

Wien, Österreich, 23 September - 25 September 2024
Event date: 23 September to 25 September 2024

Submission deadline: 15 May 2024

Notification: 3 July 2024

UbiSec 2024: The 4th International Conference on Ubiquitous Security
https://iacr.org/news/item/23121

Event Calendar: UbiSec 2024: The 4th International Conference on Ubiquitous Security

changsha, China, 29 December - 31 December 2024
Event date: 29 December to 31 December 2024

Submission deadline: 15 July 2024

Notification: 15 August 2024

PRDC 2024: 29th IEEE Pacific Rim International Conference on Dependable Computing
https://iacr.org/news/item/23120

Event Calendar: PRDC 2024: 29th IEEE Pacific Rim International Conference on Dependable Computing

Osaka, Japan, 13 November - 15 November 2024
Event date: 13 November to 15 November 2024

Submission deadline: 31 July 2024

Notification: 31 August 2024

zkSNARKs in the ROM with Unconditional UC-Security
https://eprint.iacr.org/2024/724

The universal composability (UC) framework is a “gold standard” for security in cryptography. UC-secure protocols achieve strong security guarantees against powerful adaptive adversaries, and retain these guarantees when used as part of larger protocols. Zero knowledge succinct non-interactive arguments of knowledge (zkSNARKs) are a popular cryptographic primitive that are often used within larger protocols deployed in dynamic environments, and so UC-security is a highly desirable, if not necessary, goal.
In this paper we prove that there exist zkSNARKs in the random oracle model (ROM) that unconditionally achieve UC-security. Here, “unconditionally” means that security holds against adversaries that make a bounded number of queries to the random oracle, but are otherwise computationally unbounded.
Prior work studying UC-security for zkSNARKs obtains transformations that rely on computational assumptions and, in many cases, lose most of the succinctness property of the zkSNARK. Moreover, these transformations make the resulting zkSNARK more expensive and complicated.
In contrast, we prove that widely used zkSNARKs in the ROM are UC-secure without modifications. We prove that the Micali construction, which is the canonical construction of a zkSNARK, is UC-secure. Moreover, we prove that the BCS construction, which many zkSNARKs deployed in practice are based on, is UC-secure. Our results confirm the intuition that these natural zkSNARKs do not need to be augmented to achieve UC-security, and give confidence that their use in larger real-world systems is secure.

Let Attackers Program Ideal Models: Modularity and Composability for Adaptive Compromise
https://iacr.org/news/item/23126

ePrint Report: Let Attackers Program Ideal Models: Modularity and Composability for Adaptive Compromise

Joseph Jaeger
We show that the adaptive compromise security definitions of Jaeger and Tyagi (Crypto '20) cannot be applied in several natural use-cases. These include proving multi-user security from single-user security, the security of the cascade PRF, and the security of schemes sharing the same ideal primitive. We provide new variants of the definitions and show that they resolve these issues with composition. Extending these definitions to the asymmetric settings, we establish the security of the modular KEM/DEM and Fujisaki-Okamoto approaches to public key encryption in the full adaptive compromise setting. This allows instantiations which are more efficient and standard than prior constructions.

Научный семинар по криптографии (заседание 9) (Астана)

https://cryptography.kz/ru/%D0%BD%D0%B0%D1%83%D1%87%D0%BD%D1%8B%D0%B9-%D1%81%D0%B5%D0%BC%D0%B8%D0%BD%D0%B0%D1%80-%D0%BF%D0%BE-%D0%BA%D1%80%D0%B8%D0%BF%D1%82%D0%BE%D0%B3%D1%80%D0%B0%D1%84%D0%B8%D0%B8-(%D0%B7%D0%B0%D1%81%D0%B5%D0%B4%D0%B0%D0%BD%D0%B8%D0%B5-9)-(%D0%B0%D1%81%D1%82%D0%B0%D0%BD%D0%B0).html

Improved Conditional Cube Attacks on Ascon AEADs in Nonce-Respecting Settings -- with a Break-Fix Strategy
https://iacr.org/news/item/23142

ePrint Report: Improved Conditional Cube Attacks on Ascon AEADs in Nonce-Respecting Settings -- with a Break-Fix Strategy

Kai Hu
The best-known distinguisher on 7-round Ascon-128 and Ascon-128a AEAD uses a 60-dimensional cube where the nonce bits are set to be equal in the third and fourth rows of the Ascon state during initialization (Rohit et al. ToSC 2021/1).
It was not known how to use this distinguisher to mount key-recovery attacks.
In this paper, we investigate this problem using a new strategy called \textit{break-fix} for the conditional cube attack. The idea is to introduce slightly-modified cubes which increase the degrees of 7-round output bits to be more than 59 (break phase) and then find key conditions which can bring the degree back to 59 (fix phase).
Using this idea, key-recovery attacks on 7-round Ascon-128, Ascon-128a and Ascon-80pq are proposed.
The attacks have better time/memory complexities than the existing attacks, and in some cases improve the weak-key attacks as well.

Efficient Universally-Verifiable Electronic Voting with Everlasting Privacy
https://iacr.org/news/item/23141

ePrint Report: Efficient Universally-Verifiable Electronic Voting with Everlasting Privacy

David Pointcheval
Universal verifiability is a must-to-have for electronic voting schemes. It is essential to ensure honest behavior of all the players during the whole process, together with the eligibility. However, it should not endanger the privacy of the individual votes, which is another major requirement.
Whereas the first property prevents attacks during the voting process, privacy of the votes should hold forever, which has been called everlasting privacy.


A classical approach for universal verifiability is to add some proofs together with the encrypted votes, which requires publication of the latter, while eligibility needs a link between the votes and the voters: it definitely excludes long-term privacy. An alternative is the use of perfectly-hiding commitments, on which proofs are published, while ciphertexts are kept private for computing the tally.


In this paper, we show how recent linearly-homomorphic signatures can be exploited for all the proofs, leading to very efficient procedures towards universal verifiability with both strong receipt-freeness and everlasting privacy.
Privacy will indeed be unconditional, after the publication of the results and the proofs, whereas the soundness of the proofs holds in the algebraic group model and the random oracle model.

A Deniability Analysis of Signal's Initial Handshake PQXDH
https://iacr.org/news/item/23140

ePrint Report: A Deniability Analysis of Signal's Initial Handshake PQXDH

Rune Fiedler, Christian Janson
Many use messaging apps such as Signal to exercise their right to private communication. To cope with the advent of quantum computing, Signal employs a new initial handshake protocol called PQXDH for post-quantum confidentiality, yet keeps guarantees of authenticity and deniability classical. Compared to its predecessor X3DH, PQXDH includes a KEM encapsulation and a signature on the ephemeral key. In this work we show that PQXDH does not meet the same deniability guarantees as X3DH due to the signature on the ephemeral key. Our analysis relies on plaintext awareness of the KEM, which Signal's implementation of PQXDH does not provide. As for X3DH, both parties (initiator and responder) obtain different deniability guarantees due to the asymmetry of the protocol.


For our analysis of PQXDH, we introduce a new model for deniability of key exchange that allows a more fine-grained analysis. Our deniability model picks up on the ideas of prior work and facilitates new combinations of deniability notions, such as deniability against malicious adversaries in the big brother model, i.e. where the distinguisher knows all secret keys. Our model may be of independent interest.

Multi-Client Functional Encryption with Public Inputs and Strong Security
https://iacr.org/news/item/23139

ePrint Report: Multi-Client Functional Encryption with Public Inputs and Strong Security

Ky Nguyen, Duong Hieu Phan, David Pointcheval
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 by the FE and 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.


In this paper, 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.


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 and with adaptive security. This also leads to MIFE for inner products, public-key single-input inner-product FE with LSSS key-policy and KPABE for LSSS, with adaptive security while the previous AB-MCFE construction of Agrawal et al. from CRYPTO '23 considers a slightly larger functionality of average weighted sum but with selective security only.

BGJ15 Revisited: Sieving with Streamed Memory Access
https://iacr.org/news/item/23138

ePrint Report: BGJ15 Revisited: Sieving with Streamed Memory Access

Ziyu Zhao, Jintai Ding, Bo-Yin Yang
The focus of this paper is to tackle the issue of memory access within sieving algorithms for lattice problems. We have conducted an in-depth analysis of an optimized BGJ sieve (Becker-Gama-Joux 2015), and our findings suggest that its inherent structure is significantly more memory-efficient compared to the asymptotically fastest BDGL sieve (Becker-Ducas-Gama-Laarhoven 2016). Specifically, it necessitates merely $2^{0.2075n + o(n)}$ streamed (non-random) main memory accesses for the execution of an $n$-dimensional sieving. We also provide evidence that the time complexity of this refined BGJ sieve could potentially be $2^{0.292n + o(n)}$, or at least something remarkably close to it. Actually, it outperforms the BDGL sieve in all dimensions that are practically achievable. We hope that this study will contribute to the resolution of the ongoing debate regarding the measurement of RAM access overhead in large-scale, sieving-based lattice attacks.


The concept above is also supported by our implementation. Actually, we provide a highly efficient, both in terms of time and memory, CPU-based implementation of the refined BGJ sieve within an optimized sieving framework. This implementation results in approximately 40% savings in RAM usage and is at least $2^{4.5}$ times more efficient in terms of gate count compared to the previous 4-GPU implementation (Ducas-Stevens-Woerden 2021). Notably, we have successfully solved the 183-dimensional SVP Darmstadt Challenge in 30 days using a 112-core server and approximately 0.87TB of RAM. The majority of previous sieving-based SVP computations relied on the HK3-sieve (Herold-Kirshanova 2017), hence this implementation could offer further insights into the behavior of these asymptotically faster sieving algorithms when applied to large-scale problems. Moreover, our refined cost estimation of SVP based on this implementation suggests that some of the NIST PQC candidates, such as Falcon-512, are unlikely to achieve NIST's security requirements.

Quantum Key-Revocable Dual-Regev Encryption, Revisited
https://iacr.org/news/item/23137

ePrint Report: Quantum Key-Revocable Dual-Regev Encryption, Revisited

Prabhanjan Ananth, Zihan Hu, Zikuan Huang
Quantum information can be used to achieve novel cryptographic primitives that are impossible to achieve classically. A recent work by Ananth, Poremba, Vaikuntanathan (TCC 2023) focuses on equipping the dual-Regev encryption scheme, introduced by Gentry, Peikert, Vaikuntanathan (STOC 2008), with key revocation capabilities using quantum information. They further showed that the key-revocable dual-Regev scheme implies the existence of fully homomorphic encryption and pseudorandom functions, with both of them also equipped with key revocation capabilities. Unfortunately, they were only able to prove the security of their schemes based on new conjectures and left open the problem of basing the security of key revocable dual-Regev encryption on well-studied assumptions.


In this work, we resolve this open problem. Assuming polynomial hardness of learning with errors (over sub-exponential modulus), we show that key-revocable dual-Regev encryption is secure. As a consequence, for the first time, we achieve the following results:
1. Key-revocable public-key encryption and key-revocable fully-homomorphic encryption satisfying classical revocation security and based on polynomial hardness of learning with errors. Prior works either did not achieve classical revocation or were based on sub-exponential hardness of learning with errors.
2. Key-revocable pseudorandom functions satisfying classical revocation from the polynomial hardness of learning with errors. Prior works relied upon unproven conjectures.

Mutable Batch Arguments and Applications
https://iacr.org/news/item/23136

ePrint Report: Mutable Batch Arguments and Applications

Rishab Goyal
Non-interactive batch arguments (BARGs) let a prover compute a single proof $\pi$ proving validity of a `batch' of $k$ $\mathbf{NP}$ statements $x_1, \ldots, x_{k}$. The two central features of BARGs are succinctness and soundness. Succinctness states that proof size, $|\pi|$ does not grow with $k$; while soundness states a polytime cheating prover cannot create an accepting proof for any invalid batch of statements.


In this work, we put forth a new concept of mutability for batch arguments, called mutable batch arguments. Our goal is to re-envision how we think about and use BARGs. Traditionally, a BARG proof string $\pi$ is an immutable encoding of $k$ $\mathbf{NP}$ witness $\omega_1, \ldots, \omega_{k}$. In a mutable BARG system, each proof string $\pi$ is a mutable encoding of original witnesses. Thus, a mutable BARG captures and enables computations over a batch proof $\pi$. We also study new privacy notions for mutable BARGs, guaranteeing that a mutated proof hides all non-trivial information. Such mutable BARGs are a naturally good fit for many privacy sensitive applications.


Our main contributions can be summarized as introducing the general concept of mutable BARGs, identifying new non-trivial classes of feasible mutations over BARGs, designing new constructions for mutable BARGs satisfying mutation privacy for these classes from standard cryptographic assumptions, and developing applications of mutable BARGs to advanced signatures such as homomorphic signatures, redactable signatures, and aggregate signatures. Our results improve state-of-the-art known for many such signature systems either in terms of functionality, efficiency, security, or versatility (in terms of cryptographic assumptions).

Secret Sharing with Certified Deletion
https://iacr.org/news/item/23135

ePrint Report: Secret Sharing with Certified Deletion

James Bartusek, Justin Raizes
Secret sharing allows a user to split a secret into many shares so that the secret can be recovered if, and only if, an authorized set of shares is collected. Although secret sharing typically does not require any computational hardness assumptions, its security does require that an adversary cannot collect an authorized set of shares. Over long periods of time where an adversary can benefit from multiple data breaches, this may become an unrealistic assumption.
We initiate the systematic study of secret sharing with certified deletion in order to achieve security even against an adversary that eventually collects an authorized set of shares. In secret sharing with certified deletion, a (classical) secret $s$ is split into quantum shares that can be destroyed in a manner verifiable by the dealer.
We put forth two natural definitions of security. No-signaling security roughly requires that if multiple non-communicating adversaries delete sufficiently many shares, then their combined view contains negligible information about $s$, even if the total set of corrupted parties forms an authorized set. Adaptive security requires privacy of $s$ against an adversary that can continuously and adaptively corrupt new shares and delete previously-corrupted shares, as long as the total set of corrupted shares minus deleted shares remains unauthorized.
Next, we show that these security definitions are achievable: we show how to construct (i) a secret sharing scheme with no-signaling certified deletion for any monotone access structure, and (ii) a threshold secret sharing scheme with adaptive certified deletion. Our first construction uses Bartusek and Khurana's (CRYPTO 2023) 2-out-of-2 secret sharing scheme with certified deletion as a building block, while our second construction is built from scratch and requires several new technical ideas. For example, we significantly generalize the ``XOR extractor'' of Agarwal, Bartusek, Khurana, and Kumar (EUROCRYPT 2023) in order to obtain better seedless extraction from certain quantum sources of entropy, and show how polynomial interpolation can double as a high-rate randomness extractor in our context of threshold sharing with certified deletion.

Secure Multiparty Computation in the Presence of Covert Adaptive Adversaries
https://iacr.org/news/item/23134

ePrint Report: Secure Multiparty Computation in the Presence of Covert Adaptive Adversaries

Isheeta Nargis, Anwar Hasan
We design a new MPC protocol for arithmetic circuits secure against erasure-free covert adaptive adversaries with deterrence 1/2. The new MPC protocol has the same asymptotic communication cost, the number of PKE operations and the number of exponentiation operations as the most efficient MPC protocol for arithmetic circuits secure against covert static adversaries. That means, the new MPC protocol improves security from covert static security to covert adaptive adversary almost for free. For MPC problems where the number of parties n is much larger than the number of multiplication gates M, the new MPC protocol asymptotically improves communication complexity over the most efficient MPC protocol for arithmetic circuits secure against erasure-free active adaptive adversaries.

Proof of Stake and Activity: Rewarding On-Chain Activity Through Consensus
https://iacr.org/news/item/23133

ePrint Report: Proof of Stake and Activity: Rewarding On-Chain Activity Through Consensus

Aram Jivanyan, Karen Terjanian
We are introducing a novel consensus protocol for
blockchain, called Proof of Stake and Activity (PoSA) which can
augment the traditional Proof of Stake methods by integrating
a unique Proof of Activity system. PoSA offers a compelling
economic model that promotes decentralization by rewarding
validators based on their staked capital and also the business
value they contribute to the chain. This protocol has been
implemented already into a fully-fledged blockchain platform
called Bahamut (www.bahamut.io) which boasts hundreds of thousands of active users already.