Gradually releasable commitment schemes and their applications

Richard Cleve

Marc Fischlin, Roger Fischlin

Benny Pinkas B

Abstract. We demonstrate a transformation of Yao’s protocol for secure two-party computation to a fair protocol in which neither party gains any substantial advantage by terminating the protocol prematurely. The transformation adds additional steps before and after the execution of the original protocol, but does not change it otherwise, and does not use a trusted third party. It is based on the use of gradual release timed commitments, which are a new variant of timed commitments, and on a novel use of blind signatures for verifying that the committed values are correct. 1

Tatsuaki Okamoto, Kazuo Ohta

The simultaneous secret exchange protocol is the key tool for contract signing protocols and certified mail protocols. This paper proposes efficient simultaneous secret exchange protocols (or gradual secret releasing protocols) that are based on general assumptions such as the existence of one-way permutations and one-way functions, while the existing efficient simultaneous secret exchange protocols are based on more constrained assumptions such as specific number theoretic problems and the existence of oblivious transfer primitives (or trap-door one-way permutations). Moreover, while the existing simultaneous secret exchange protocols have an additional requirement that the underlying commit (encryption) function is “ideal”, the above-mentioned “general assumptions” are provably sufficient for our schemes. Therefore, our protocols are provably secure under the general assumptions. In addition, our protocols are at least as efficient as the existing practical protocols, when efficient one-way permutations and one-way functions are used.

Marc Fischlin, Roger Fischlin

Ou Ruan, Cai Fu, Guohua Cui

Commit-Prove-Fair-Open functionality Ou Ruan , Cai Fu a,* , Guohua Cui a a College of Compute Science & Technology, Huazhong University of Science & Technology, Wuhan, China, 430074 b College of Compute Science & Technology, Hubei University of Technology, Wuhan, China, 430068 ruanou@21cn.com, 12695133@qq.com,cgh3986@126.com *:Corresponding author: Cai Fu , 12695133@qq.com, stand_fucai@126.com Research supported in part by the National Natural Science Funds of China(60903175) Abstract. In TCC 2006, Garay et al. introduced the notion of commit-prove-fair-open functionality in order to achieve what they called resource of secure multi-party computation(MPC) with corrupted majority. The protocol realizing this notion of fairness follows the gradual release approach and, further, it can be proven secure in the simulation paradigm and enjoys composition properties. In this paper, we show a more efficient resource-fair protocol of FCPFO based on a new variant of Garay et al. time-lines and simplified Camenisch-Shoup(sCS) commitment,whose communication and computation complexity are less than 1/5 of Garay et al. construction. In addition, our new protocol allows commitment to value 0, which is not possible in the plain Garay et al. construction.