## CryptoDB

### Quang Dao

#### Publications

**Year**

**Venue**

**Title**

2024

CRYPTO

Non-Interactive Zero-Knowledge from LPN and MQ
Abstract

We give the first construction of non-interactive zero-knowledge (NIZK) arguments from post-quantum assumptions other than Learning with Errors. In particular, we achieve NIZK under the polynomial hardness of the Learning Parity with Noise (LPN) assumption, and the exponential hardness of solving random underdetermined multivariate quadratic equations (MQ). We also construct NIZK satisfying statistical zero-knowledge assuming a new variant of LPN, Dense-Sparse LPN, introduced by Dao and Jain (ePrint 2024), together with exponentially-hard MQ.
The main technical ingredient of our construction is an extremely natural (but only in hindsight!) construction of correlation-intractable (CI) hash functions from MQ, for a NIZK-friendly sub-class of constant-degree polynomials that we call concatenated constant-degree polynomials. Under exponential security, this hash function also satisfies the stronger notion of approximate CI for concatenated constant-degree polynomials. The NIZK construction then follows from a prior blueprint of Brakerski-Koppula-Mour (CRYPTO 2020). In addition, we also show how to construct (approximate) CI hashing for degree-$d$ polynomials from the (exponential) hardness of solving random degree-$d$ equations, a natural generalization of MQ. To realize NIZK with statistical zero-knowledge, we design a lossy public-key encryption scheme with approximate linear decryption and inverse-polynomial decryption error from Dense-Sparse LPN. These constructions may be of independent interest.
Our work therefore gives a new way to leverage MQ with uniformly random equations, which has found little cryptographic applications to date. Indeed, most applications in the context of encryption and signature schemes make use of structured variants of MQ, where the polynomials are not truly random but posses a hidden planted structure. We believe that the MQ assumption may plausibly find future use in the designing other advanced proof systems.

2024

CRYPTO

Lossy Cryptography from Code-Based Assumptions
Abstract

Over the past few decades, we have seen a proliferation of advanced cryptographic primitives with lossy or homomorphic properties built from various assumptions such as Quadratic Residuosity, Decisional Diffie-Hellman, and Learning with Errors. These primitives imply hard problems in the complexity class $\mathcal{SZK}$ (statistical zero-knowledge); as a consequence, they can only be based on assumptions that are broken in $\mathcal{BPP}^{\mathcal{SZK}}$.
This poses a barrier for building advanced cryptography from code-based assumptions such as Learning Parity with Noise (LPN), as LPN is only known to be in $\mathcal{BPP}^{\mathcal{SZK}}$ under an extremely low noise rate $\frac{\log^2 n}{n}$, for which it is broken in quasi-polynomial time.
In this work, we propose a new code-based assumption: Dense-Sparse LPN, that falls in the complexity class $\mathcal{BPP}^{\mathcal{SZK}}$ and is conjectured to be secure against subexponential time adversaries. Our assumption is a variant of LPN that is inspired by McEliece's cryptosystem and random $k\mbox{-}$XOR in average-case complexity. Roughly, the assumption states that
\[(\mathbf{T}\, \mathbf{M}, \mathbf{s} \,\mathbf{T}\, \mathbf{M} + \mathbf{e}) \quad \text{is indistinguishable from}\quad (\mathbf{T} \,\mathbf{M}, \mathbf{u}),\]
for a random (dense) matrix $\mathbf{T}$, random sparse matrix $\mathbf{M}$, and sparse noise vector $\mathbf{e}$ drawn from the Bernoulli distribution with inverse polynomial noise probability.
We leverage our assumption to build lossy trapdoor functions (Peikert-Waters STOC 08). This gives the first post-quantum alternative to the lattice-based construction in the original paper. Lossy trapdoor functions, being a fundamental cryptographic tool, are known to enable a broad spectrum of both lossy and non-lossy cryptographic primitives; our construction thus implies these primitives in a generic manner. In particular, we achieve collision-resistant hash functions with plausible subexponential security, improving over a prior construction from LPN with noise rate $\frac{\log^2 n}{n}$ that is only quasi-polynomially secure.

2023

EUROCRYPT

Spartan and Bulletproofs are simulation-extractable (for free!)
Abstract

Increasing deployment of advanced zero-knowledge proof systems, especially zkSNARKs, has raised critical questions about their security against real-world attacks. Two classes of attacks of concern in practice are adaptive soundness attacks, where an attacker can prove false statements by choosing its public input after generating a proof, and malleability attacks, where an attacker can use a valid proof to create another valid proof it could not have created itself. Prior work has shown that simulation-extractability (SIM-EXT), a strong notion of security for proof systems, rules out these attacks.
In this paper, we prove that two transparent, discrete-log-based zkSNARKs, Spartan and Bulletproofs, are simulation-extractable (SIM-EXT) in the random oracle model if the discrete logarithm assumption holds in the underlying group. Since these assumptions are required to prove standard security properties for Spartan and Bulletproofs, our results show that SIM-EXT is, surprisingly, ``for free'' with these schemes. Our result is the first SIM-EXT proof for Spartan and encompasses both linear- and sublinear-verifier variants. Our result for Bulletproofs encompasses both the aggregate range proof and arithmetic circuit variants, and is the first to not rely on the algebraic group model (AGM), resolving an open question posed by Ganesh et al. (EUROCRYPT'22). As part of our analysis, we develop a generalization of the tree-builder extraction theorem of Attema et al. (TCC'22), which may be of independent interest.

2023

CRYPTO

Multi-Party Homomorphic Secret Sharing and Sublinear MPC from Sparse LPN
Abstract

Over the past few years, we have seen the powerful emergence of homomorphic secret sharing (HSS) as a compelling alternative to fully homomorphic encryption (FHE), due to its efficiency benefits and its feasibility from an array of standard assumptions. However, all previously known HSS schemes, with the exception of schemes built from FHE or indistinguishability obfuscation (iO), can only support two parties.
In this work, we give the first construction of a \emph{multi-party} HSS scheme for a non-trivial function class, from an assumption not known to imply FHE. In particular, we construct an HSS scheme for an \emph{arbitrary} number of parties with an \emph{arbitrary} corruption threshold, supporting evaluations of $\log / \log \log$-degree polynomials, containing a polynomial number of monomials, over arbitrary finite fields. As a consequence, we obtain an MPC protocol for any number of parties, with (slightly) \emph{sub-linear} communication per party of roughly $O(S / \log \log S)$ bits when evaluating a layered Boolean circuit of size $S$.
Our HSS scheme relies on the \emph{sparse} Learning Parity with Noise (LPN) assumption, a standard variant of LPN with a sparse public matrix that has been studied and used in prior works. Thanks to this assumption, our construction enjoys several unique benefits. In particular, it can be built on top of \emph{any} linear secret sharing scheme, producing noisy output shares that can be error-corrected by the decoder. This yields HSS for low-degree polynomials with optimal download rate. Unlike prior works, our scheme also has a low computation overhead in that the per-party computation of a constant degree polynomial takes $O(M)$ work, where $M$ is the number of monomials.

#### Coauthors

- Paul Grubbs (1)
- Yuval Ishai (1)
- Aayush Jain (3)
- Zhengzhong Jin (1)
- Huijia Lin (1)