Cryptology ePrint Archive: Report 2007/071

Knowledge-Binding Commitments with Applications in Time-Stamping (Full Version)

Ahto Buldas and Sven Laur

Abstract: We prove in a non-black-box way that every bounded list and set commitment scheme is knowledge-binding. This is a new and rather strong security condition, which makes the security definitions for time-stamping much more natural compared to the previous definitions, which assume unpredictability of adversaries. As a direct consequence, list and set commitment schemes with partial opening property are sufficient for secure time-stamping if the number of elements has an explicit upper bound N. On the other hand, white-box reductions are in a sense strictly weaker than black-box reductions. Therefore, we also extend and generalize the previously known reductions. The corresponding new reductions are Theta(sqrt(N)) times more efficient, which is important for global-scale time-stamping schemes where N is very large.

Category / Keywords: cryptographic protocols / bit commitment, white-box reductions, time-stamping

Publication Info: A shortened version of this work will be published in PKC2007.

Date: received 23 Feb 2007

Contact author: ahto buldas at ut ee

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Version: 20070228:111258 (All versions of this report)

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