Time-lock puzzles from randomized encodings

Nir Bitansky, Shafi Goldwasser, Abhishek Jain, Omer Paneth, Vinod Vaikuntanathan

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

Abstract

Time-lock puzzles are a mechanism for sending messages \to the future". A sender can quickly generate a puzzle with a solution s that remains hidden until a moderately large amount of time t has elapsed. The solution s should be hidden from any adversary that runs in time significantly less than t, including resourceful parallel adversaries with polynomially many processors. While the notion of time-lock puzzles has been around for 22 years, there has only been a single candidate proposed. Fifteen years ago, Rivest, Shamir and Wagner suggested a beautiful candidate time-lock puzzle based on the assump-Tion that exponentiation modulo an RSA integer is an \in-herently sequential" computation. We show that various avors of randomized encodings give rise to time-lock puzzles of varying strengths, whose se-curity can be shown assuming the mere existence of non-parallelizing languages, which are languages that require cir-cuits of depth at least t to decide, in the worst-case. The existence of such languages is necessary for the existence of time-lock puzzles. We instantiate the construction with dif-ferent randomized encodings from the literature, where in-creasingly better efficiency is obtained based on increasingly stronger cryptographic assumptions, ranging from one-way functions to indistinguishability obfuscation. We also ob-serve that time-lock puzzles imply one-way functions, and thus the reliance on some cryptographic assumption is nec-essary. Finally, generalizing the above, we construct other types of puzzles such as proofs of work from randomized encod-ings and a suitable worst-case hardness assumption (that is necessary for such puzzles to exist.

Original languageEnglish
Title of host publicationITCS 2016 - Proceedings of the 2016 ACM Conference on Innovations in Theoretical Computer Science
PublisherAssociation for Computing Machinery, Inc
Pages345-356
Number of pages12
ISBN (Electronic)9781450340571
DOIs
StatePublished - 14 Jan 2016
Externally publishedYes
Event7th ACM Conference on Innovations in Theoretical Computer Science, ITCS 2016 - Cambridge, United States
Duration: 14 Jan 201616 Jan 2016

Publication series

NameITCS 2016 - Proceedings of the 2016 ACM Conference on Innovations in Theoretical Computer Science

Conference

Conference7th ACM Conference on Innovations in Theoretical Computer Science, ITCS 2016
Country/TerritoryUnited States
CityCambridge
Period14/01/1616/01/16

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