Title: Fair threshold decryption with semi-trusted third parties

Authors: Jeongdae Hong, Jinil Kim, Jihye Kim, Matthew K. Franklin, Kunsoo Park

Addresses: School of Computer Science and Engineering, Seoul National University, Gwanak-gu, Seoul 151-742, South Korea. ' School of Computer Science and Engineering, Seoul National University, Gwanak-gu, Seoul 151-742, South Korea. ' ISaC and Department of Mathematical Sciences, Seoul National University, Gwanak-gu, Seoul 151-742, South Korea. ' Department of Computer Science, University of California Davis, One Shields Avenue, Davis, CA 95616, USA. ' School of Computer Science and Engineering, Seoul National University, Gwanak-gu, Seoul 151-742, South Korea

Abstract: A threshold decryption scheme is a multi-party public key cryptosystem that allows any sufficiently large subset of participants to decrypt a ciphertext, but disallows the decryption otherwise. Many threshold cryptographic schemes have been proposed so far, but fairness is not generally considered in this earlier work. In this paper, we present fair threshold decryption schemes, where either all of the participants can decrypt or none of them can. Our solutions employ semi-trusted third parties (STTP) and offline semi-trusted third parties (OTTP) previously used for fair exchange. We consider a number of variants of our schemes to address realistic alternative trust scenarios. Although we describe our schemes using a simple hashed version of ElGamal encryption, our methods generalise to other threshold decryption schemes and threshold signature schemes as well.

Keywords: threshold decryption; fairness; semi-trusted third parties; STTP; optimistic protocol; ElGamal encryption; public key cryptosystems; cryptography; trust; threshold signature schemes.

DOI: 10.1504/IJACT.2010.038307

International Journal of Applied Cryptography, 2010 Vol.2 No.2, pp.139 - 153

Available online: 28 Jan 2011 *

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