## CryptoDB

### Shafik Nassar

#### Publications

**Year**

**Venue**

**Title**

2024

EUROCRYPT

Strong Batching for Non-Interactive Statistical Zero-Knowledge
Abstract

In a zero-knowledge proof, a prover needs to convince a verifier that an input x is contained in a language Pi without revealing any additional information. By repeating a zero-knowledge proof k times, it is possible to prove (still in zero-knowledge) that k separate inputs x1,...,xk all belong to Pi. But this increases the communication by a factor of k. Can one do better? In other words, is (non-trivial) zero-knowledge batch verification for Pi possible?
Recent works by Kaslasi et al. (TCC 2020, Eurocrypt 2021) show that any problem possessing a non-interactive statistical zero-knowledge proof (NISZK) has a non-trivial statistical zero-knowledge batch verification protocol. Two major limitations of their results are: (1) the communication in the batch protocol is roughly poly(n,log(k))+O(k), which is better than the naive cost of k*poly(n) but still scales linearly with k, and, (2) the batch protocol requires Omega(k) rounds of interaction.
In this work we remove both of these limitations by showing that any problem in NISZK has a non-interactive statistical zero-knowledge batch verification protocol with communication poly(n,log(k)).

2024

TCC

Monotone Policy BARGs from BARGs and Additively Homomorphic Encryption
Abstract

A monotone policy batch $\mathsf{NP}$ language $\mathcal{L}_{\mathcal{R}, P}$ is parameterized by a monotone policy $P \colon \{0,1\}^k \to \{0,1\}$ and an $\mathsf{NP}$ relation $\mathcal{R}$. A statement $(x_1, \ldots, x_k)$ is a \textsc{yes} instance if there exists $w_1, \ldots, w_k$ where $P(\mathcal{R}(x_1, w_1), \ldots, \mathcal{R}(x_k, w_k)) = 1$. For example, we might say that an instance $(x_1, \ldots, x_k)$ is a \textsc{yes} instance if a majority of the statements are true. A monotone policy batch argument (BARG) for $\mathsf{NP}$ allows a prover to prove that $(x_1, \ldots, x_k) \in \mathcal{L}_{\mathcal{R}, P}$ with a proof of size $\mathsf{poly}(\lambda, |\mathcal{R}|, \log k)$, where $\lambda$ is the security parameter, $|\mathcal{R}|$ is the size of the Boolean circuit that computes $\mathcal{R}$, and $k$ is the number of instances. Recently, Brakerski, Brodsky, Kalai, Lombardi, and Paneth (CRYPTO~2023) gave the first monotone policy BARG for $\mathsf{NP}$ from the learning with errors (LWE) assumption.
In this work, we describe a generic approach for constructing monotone policy BARGs from any BARG for $\mathsf{NP}$ together with an additively homomorphic encryption scheme. This yields the first constructions of monotone policy BARGs from the $k$-$\ms{Lin}$ assumption in prime-order pairing groups as well as the (subexponential) DDH assumption in /pairing-free/ groups. Central to our construction is a notion of a zero-fixing hash function, which is a relaxed version of a predicate-extractable hash function from the work of Brakerski~et~al. Our relaxation enables a direct realization of zero-fixing hash functions from BARGs for $\mathsf{NP}$ and additively homomorphic encryption, whereas the previous notion relied on leveled homomorphic encryption, and by extension, the LWE assumption.
As an application, we also show how to combine a monotone policy BARG with a puncturable signature scheme to obtain a monotone policy aggregate signature scheme. Our work yields the first (statically-secure) monotone policy aggregate signatures that supports general monotone Boolean circuits from standard pairing-based assumptions. Previously, this was only known from LWE.

2022

CRYPTO

Succinct Interactive Oracle Proofs: Applications and Limitations
📺
Abstract

Interactive Oracle Proofs (IOPs) are a new type of proof-system that combines key properties of interactive proofs and PCPs: IOPs enable a verifier to be convinced of the correctness of a statement by interacting with an untrusted prover while reading just a few bits of the messages sent by the prover. IOPs have become very prominent in the design of efficient proof-systems in recent years.
In this work we study succinct IOPs, which are IOPs in which the communication complexity is polynomial (or even linear) in the original witness. While there are strong impossibility results for the existence of succinct PCPs (i.e., PCPs whose length is polynomial in the witness), it is known that the rich class of NP relations that are decidable in small space have succinct IOPs. In this work we show both new applications, and limitations, for succinct IOPs:
1. First, using one-way functions, we show how to compile IOPs into zero-knowledge proofs, while nearly preserving the proof length. This complements a recent line of work, initiated by Ben Sasson et al. (TCC,2016B), who compileIOPs into super-succinct zero-knowledge arguments.
Applying the compiler to the state-of-the-art succinctIOPs yields zero-knowledge proofs for bounded-space NP relations, with communication that is nearly equal to the original witness length. This yields the shortest known zero-knowledge proofs from the minimal assumption of one-way functions.
2. Second, we give a barrier for obtaining succinct IOPs for more general NP relations. In particular, we show that if a language has a succinct IOP, then it can be decided in space that is proportionate only to the witness length, after a bounded-time probabilistic preprocessing. We use this result to show that under a simple and plausible (but to the best of our knowledge, new) complexity-theoretic conjecture, there is no succinct IOP for CSAT.

#### Coauthors

- Changrui Mu (1)
- Shafik Nassar (3)
- Ron D. Rothblum (2)
- Prashant Nalini Vasudevan (1)
- Brent Waters (1)
- David J. Wu (1)