Cryptology ePrint Archive: Report 2020/1078

Fair and Sound Secret Sharing from Homomorphic Time-Lock Puzzles

Jodie Knapp and Elizabeth A. Quaglia

Abstract: Achieving fairness and soundness in non-simultaneous rational secret sharing schemes has proved to be challenging. On the one hand, soundness can be ensured by providing side information related to the secret as a check, but on the other, this can be used by deviant players to compromise fairness. To overcome this, the idea of incorporating a time delay was suggested in the literature: in particular, time-delay encryption based on memory-bound functions has been put forth as a solution. In this paper, we propose a different approach to achieve such delay, namely using homomorphic time-lock puzzles (HTLPs), introduced at CRYPTO 2019, and construct a fair and sound rational secret sharing scheme in the non-simultaneous setting from HTLPs. HTLPs are used to embed sub-shares of the secret for a predetermined time. This allows to restore fairness of the secret reconstruction phase, despite players having access to information related to the secret which is required to ensure soundness of the scheme. Key to our construction is the fact that the time-lock puzzles are homomorphic so that players can compactly evaluate sub-shares. Without this efficiency improvement, players would have to independently solve each puzzle sent from the other players to obtain a share of the secret, which would be computationally inefficient. We argue that achieving both fairness and soundness in a non-simultaneous scheme using a time delay based on CPU-bound functions rather than memory-bound functions is more cost effective and realistic in relation to the implementation of the construction.

Category / Keywords: cryptographic protocols / secret sharing, fair exchange, threshold cryptography, time-lock puzzles

Original Publication (with major differences): The 14th International Conference on Provable and Practical Security (ProvSec 2020)

Date: received 7 Sep 2020, last revised 17 Sep 2020

Contact author: jodie knapp 2018 at rhul ac uk, elizabeth quaglia@rhul ac uk

Available format(s): PDF | BibTeX Citation

Note: This work is partly supported by the EPSRC and the UK government as part of the Centre for Doctoral Training in Cyber Security at Royal Holloway, University of London (EP/P009301/1).

Version: 20200917:145751 (All versions of this report)

Short URL: ia.cr/2020/1078


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