## CryptoDB

### Susumu Kiyoshima

#### Publications

**Year**

**Venue**

**Title**

2021

TCC

Black-Box Impossibilities of Obtaining 2-Round Weak ZK and Strong WI from Polynomial Hardness
📺
Abstract

We study the problem of obtaining 2-round interactive arguments for NP with weak zero-knowledge (weak ZK) [Dwork et al., 2003] or with strong witness indistinguishability (strong WI) [Goldreich, 2001] under polynomially hard falsifiable assumptions. We consider both the delayed-input setting [Jain et al., 2017] and the standard non-delayed-input setting, where in the delayed-input setting, (i) prover privacy is only required to hold against delayed-input verifiers (which learn statements in the last round of the protocol) and (ii) soundness is required to hold even against adaptive provers (which choose statements in the last round of the protocol).
Concretely, we show the following black-box (BB) impossibility results by relying on standard cryptographic primitives.
1. It is impossible to obtain 2-round delayed-input weak ZK arguments under polynomially hard falsifiable assumptions if BB reductions are used to prove soundness. This result holds even when non-black-box techniques are used to prove weak ZK.
2. It is impossible to obtain 2-round non-delayed-input strong WI arguments and 2-round publicly verifiable delayed-input strong WI arguments under polynomially hard falsifiable assumptions if a natural type of BB reductions, called "oblivious" BB reductions, are used to prove strong WI.
3. It is impossible to obtain 2-round delayed-input strong WI arguments under polynomially hard falsifiable assumptions if BB reductions are used to prove both soundness and strong WI (the BB reductions for strong WI are required to be oblivious as above). Compared with the above result, this result no longer requires public verifiability in the delayed-input setting.

2020

CRYPTO

Round-optimal Black-box Commit-and-prove with Succinct Communication
📺
Abstract

We give a four-round black-box construction of a commit-and-prove protocol with succinct communication. Our construction is WI and has constant soundness error, and it can be upgraded into a one that is ZK and has negligible soundness error by relying on a round-preserving transformation of Khurana et al. (TCC 2018). Our construction is obtained by combining the MPC-in-the-head technique of Ishai et al. (SICOMP 2009) with the two-round succinct argument of Kalai et al. (STOC 2014), and the main technical novelty lies in the analysis of the soundness---we show that, although the succinct argument of Kalai et al. does not necessarily provide soundness for NP statements, it can be used in the MPC-in-the-head technique for proving the consistency of committed MPC views. Our construction is based on sub-exponentially hard collision-resistant hash functions, two-round PIRs, and two-round OTs.

2020

JOFC

Statistical Concurrent Non-Malleable Zero-Knowledge from One-Way Functions
Abstract

Concurrent non-malleable zero-knowledge ( $$\mathrm {CNMZK}$$ CNMZK ) protocols are zero-knowledge protocols that provides security even when adversaries interact with multiple provers and verifiers simultaneously. It is known that $$\mathrm {CNMZK}$$ CNMZK arguments for $$\mathcal {NP}$$ NP can be constructed in the plain model. Furthermore, it was recently shown that statistical $$\mathrm {CNMZK}$$ CNMZK arguments for $$\mathcal {NP}$$ NP can also be constructed in the plain model. However, although the former requires only the existence of one-way functions, the latter requires the DDH assumption. In this paper, we construct a statistical $$\mathrm {CNMZK}$$ CNMZK argument for $$\mathcal {NP}$$ NP assuming only the existence of one-way functions. The security is proven via black-box simulation, and the round complexity is $$\mathsf {poly}(n)$$ poly ( n ) . Under the existence of collision-resistant hash functions, the round complexity is reduced to $$\omega (\log n)$$ ω ( log n ) , which is essentially optimal for black-box concurrent zero-knowledge protocols.

2019

JOFC

Non-black-box Simulation in the Fully Concurrent Setting, Revisited
Abstract

We give a new proof of the existence of $$O(n^{\epsilon })$$ O ( n ϵ ) -round public-coin concurrent zero-knowledge arguments for $$\mathcal {NP}$$ NP , where $$\epsilon >0$$ ϵ > 0 is an arbitrary constant. The security is proven in the plain model under the assumption that collision-resistant hash functions exist. The existence of such concurrent zero-knowledge arguments was previously proven by Goyal (STOC’13) in the plain model under the same assumption. In the proof, we use a new variant of the non-black-box simulation technique of Barak (FOCS’01). An important property of our simulation technique is that the simulator runs in a “straight-line” manner in the fully concurrent setting. Compared with the simulation technique of Goyal, which also has such a property, the analysis of our simulation technique is (arguably) simpler.

2019

JOFC

Round-Efficient Black-Box Construction of Composable Multi-Party Computation
Abstract

We present a round-efficient black-box construction of a general multi-party computation (MPC) protocol that satisfies composability in the plain model. The security of our protocol is proven in the angel-based UC framework [Prabhakaran and Sahai, STOC’04] under the minimal assumption of the existence of semi-honest oblivious transfer protocols. The round complexity of our protocol is $$\max (\widetilde{O}(\log ^2n), O(R_{{{{\mathsf {O}}}{{\mathsf {T}}}}}))$$ max ( O ~ ( log 2 n ) , O ( R O T ) ) when the round complexity of the underlying oblivious transfer protocol is $$R_{{{{\mathsf {O}}}{{\mathsf {T}}}}}$$ R O T . Since constant-round semi-honest oblivious transfer protocols can be constructed under standard assumptions (such as the existence of enhanced trapdoor permutations), our result gives a $$\widetilde{O}(\log ^2n)$$ O ~ ( log 2 n ) -round protocol under these assumptions. Previously, only an $$O(\max (n^{\epsilon }, R_{{{{\mathsf {O}}}{{\mathsf {T}}}}}))$$ O ( max ( n ϵ , R O T ) ) -round protocol was shown, where $$\epsilon >0$$ ϵ > 0 is an arbitrary constant. We obtain our MPC protocol by constructing a $$\widetilde{O}(\log ^2n)$$ O ~ ( log 2 n ) -round CCA-secure commitment scheme in a black-box way under the assumption of the existence of one-way functions.

2018

TCC

No-signaling Linear PCPs
Abstract

In this paper, we give a no-signaling linear probabilistically checkable proof (PCP) system for polynomial-time deterministic computation, i.e., a PCP system for $${\mathcal {P}}$$P such that (1) the PCP oracle is a linear function and (2) the soundness holds against any (computational) no-signaling cheating prover, who is allowed to answer each query according to a distribution that depends on the entire query set in a certain way. To the best of our knowledge, our construction is the first PCP system that satisfies these two properties simultaneously.As an application of our PCP system, we obtain a 2-message scheme for delegating computation by using a known transformation. Compared with existing 2-message delegation schemes based on standard cryptographic assumptions, our scheme requires preprocessing but has a simpler structure and makes use of different (possibly cheaper) standard cryptographic primitives, namely additive/multiplicative homomorphic encryption schemes.

#### Program Committees

- Eurocrypt 2020
- TCC 2019

#### Coauthors

- Sanjam Garg (2)
- Carmen Kempka (1)
- Ryo Kikuchi (1)
- Huijia Lin (1)
- Yoshifumi Manabe (1)
- Tatsuaki Okamoto (1)
- Omkant Pandey (2)
- Koutarou Suzuki (1)
- Muthuramakrishnan Venkitasubramaniam (1)