Cryptology ePrint Archive: Report 2006/004

Provably Secure Subsitution of Cryptographic Tools

Lea Kissner and David Molnar

Abstract: Many cryptographic protocols secure against malicious players use specially designed cryptographic tools. Essentially, these special tools function much like less-expensive tools, but give extra `powers' to a reduction or simulation algorithm. Using these powers, cryptographers can construct a proof of security using standard techniques. However, these powers are not available to either the honest parties or the adversary. In a large class of protocols, by replacing the expensive, specially designed cryptographic tool with a corresponding less-expensive tool, we can improve the protocol's efficiency without changing the functionality available to either the adversary or the honest parties. The key motivating question we address in this paper is whether the new, `substituted' protocol is still secure.

We introduce a framework for reasoning about this question. Our framework uses translators: special purpose oracles that map outputs of one cryptographic tool to corresponding outputs of a different tool. Translators are similar to, but generally weaker than, the ``angels'' of Prabhakaran and Sahai. We introduce the notion of substitution-friendly protocols and show that such protocols remain secure after substitution in our framework. We also leverage existing proofs of security; there is no need to re-prove security from scratch. We demonstrate our framework with a non-interactive non-malleable bit commitment protocol.

Category / Keywords: foundations / provable security, efficiency, translation, foundations

Date: received 3 Jan 2006, last revised 23 Oct 2006

Contact author: leak at cs cmu edu

Available format(s): PDF | BibTeX Citation

Note: We've added new examples and revised explanations for better clarity.

Version: 20061023:130648 (All versions of this report)

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