## CryptoDB

### Nai-Hui Chia

#### Publications

**Year**

**Venue**

**Title**

2022

CRYPTO

Post-Quantum Simulatable Extraction with Minimal Assumptions: Black-Box and Constant-Round
📺
Abstract

From the minimal assumption of post-quantum semi-honest oblivious transfers, we build the first $\epsilon$-simulatable two-party computation (2PC) against quantum polynomial-time (QPT) adversaries that is both constant-round and black-box (for both the construction and security reduction). A recent work by Chia, Chung, Liu, and Yamakawa (FOCS'21) shows that post-quantum 2PC with standard simulation-based security is impossible in constant rounds, unless either $NP \subseteq BQP$ or relying on non-black-box simulation. The $\epsilon$-simulatability we target is a relaxation of the standard simulation-based security that allows for an arbitrarily small noticeable simulation error $\epsilon$. Moreover, when quantum communication is allowed, we can further weaken the assumption to post-quantum secure one-way functions (PQ-OWFs), while maintaining the constant-round and black-box property.
Our techniques also yield the following set of constant-round and black-box two-party protocols secure against QPT adversaries, only assuming black-box access to PQ-OWFs:
- extractable commitments for which the extractor is also an $\epsilon$-simulator;
- $\epsilon$-zero-knowledge commit-and-prove whose commit stage is extractable with $\epsilon$-simulation;
- $\epsilon$-simulatable coin-flipping;
- $\epsilon$-zero-knowledge arguments of knowledge for $NP$ for which the knowledge extractor is also an $\epsilon$-simulator;
- $\epsilon$-zero-knowledge arguments for $QMA$.
At the heart of the above results is a black-box extraction lemma showing how to efficiently extract secrets from QPT adversaries while disturbing their quantum state in a controllable manner, i.e., achieving $\epsilon$-simulatability of the after-extraction state of the adversary.

2021

CRYPTO

A Black-Box Approach to Post-Quantum Zero-Knowledge in Constant Rounds
📺
Abstract

In a recent seminal work, Bitansky and Shmueli (STOC '20) gave the first construction of a constant round zero-knowledge argument for NP secure against quantum attacks. However, their construction has several drawbacks compared to the classical counterparts. Specifically, their construction only achieves computational soundness, requires strong assumptions of quantum hardness of learning with errors (QLWE assumption) and the existence of quantum fully homomorphic encryption (QFHE), and relies on non-black-box simulation. In this paper, we resolve these issues at the cost of weakening the notion of zero-knowledge to what is called ϵ-zero-knowledge. Concretely, we construct the following protocols:
- We construct a constant round interactive proof for NP that satisfies statistical soundness and black-box ϵ-zero-knowledge against quantum attacks assuming the existence of collapsing hash functions, which is a quantum counterpart of collision-resistant hash functions. Interestingly, this construction is just an adapted version of the classical protocol by Goldreich and Kahan (JoC '96) though the proof of ϵ-zero-knowledge property against quantum adversaries requires novel ideas.
- We construct a constant round interactive argument for NP that satisfies computational soundness and black-box ϵ-zero-knowledge against quantum attacks only assuming the existence of post-quantum one-way functions.
At the heart of our results is a new quantum rewinding technique that enables a simulator to extract a committed message of a malicious verifier while simulating verifier's internal state in an appropriate sense.

2020

TCC

Classical Verification of Quantum Computations with Efficient Verifier
📺
Abstract

In this paper, we extend the protocol of classical verification of quantum computations (CVQC) recently proposed by Mahadev to make the verification efficient.
Our result is obtained in the following three steps:
\begin{itemize}
\item We show that parallel repetition of Mahadev's protocol has negligible soundness error. This gives the first constant round CVQC protocol with negligible soundness error. In this part, we only assume the quantum hardness of the learning with error (LWE) problem similar to Mahadev's work.
\item We construct a two-round CVQC protocol in the quantum random oracle model (QROM) where a cryptographic hash function is idealized to be a random function.
This is obtained by applying the Fiat-Shamir transform to the parallel repetition version of Mahadev's protocol.
\item We construct a two-round CVQC protocol with an efficient verifier in the CRS+QRO model where both prover and verifier can access a (classical) common reference string generated by a trusted third party in addition to quantum access to QRO.
Specifically, the verifier can verify a $\mathsf{QTIME}(T)$ computation in time $\mathsf{poly}(\lambda,\log T)$ where $\lambda$ is the security parameter.
For proving soundness, we assume that a standard model instantiation of our two-round protocol with a concrete hash function (say, SHA-3) is sound and the existence of post-quantum indistinguishability obfuscation and post-quantum fully homomorphic encryption in addition to the quantum hardness of the LWE problem.
\end{itemize}

#### Coauthors

- Kai-Min Chung (3)
- Xiaohui Liang (1)
- Takashi Yamakawa (3)