Cryptology ePrint Archive: Report 2022/035

Time-Traveling Simulators Using Blockchains and Their Applications

Vipul Goyal and Justin Raizes and Pratik Soni

Abstract: Blockchain technology has the potential of transforming cryptography. We study the problem of round-complexity of zero-knowledge, and more broadly, of secure computation in the blockchain-hybrid model, where all parties can access the blockchain as an oracle.

We study zero-knowledge and secure computation through the lens of a new security notion where the simulator is given the ability to ``time-travel” or more accurately, to look into the future states of the blockchain and use this information to perform simulation. Such a time-traveling simulator gives a novel security guarantee of the following form: whatever the adversary could have learnt from an interaction, it could have computed on its own shortly into the future (e.g., a few hours from now).

We exhibit the power of time-traveling simulators by constructing round-efficient protocols in the blockchain-hybrid model. In particular, we construct: 1. Three-round zero-knowledge (ZK) argument for NP with a polynomial-time black-box time-traveling simulator. 2. Three-round secure two-party computation (2PC) for any functionality with a polynomial-time black-box time-traveling simulator for both parties.

In addition to standard cryptographic assumptions, we rely on natural hardness assumptions for Proof-of-Work based blockchains. In comparison, in the plain model, three-round protocols with black-box simulation are impossible, and constructions with non-black-box simulation for ZK require novel cryptographic assumptions while no construction for three-round 2PC is known. Our three-round 2PC result relies on a new, two-round extractable commitment that admits a time-traveling extractor.

Category / Keywords: foundations / Zero-knowledge, Secure two-party computation, Blockchains

Original Publication (with major differences): ITCS 2022

Date: received 10 Jan 2022, last revised 10 Jan 2022

Contact author: psoni at andrew cmu edu, jraizes at andrew cmu edu, vipul at cmu edu

Available format(s): PDF | BibTeX Citation

Note: A preliminary version of this paper appears at ITCS 2022.

Version: 20220114:072537 (All versions of this report)

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