Cryptology ePrint Archive: Report 2008/246

The Random Oracle Model and the Ideal Cipher Model are Equivalent

Jean-Sebastien Coron and Jacques Patarin and Yannick Seurin

Abstract: The Random Oracle Model and the Ideal Cipher Model are two well known idealised models of computation for proving the security of cryptosystems. At Crypto 2005, Coron et al. showed that security in the random oracle model implies security in the ideal cipher model; namely they showed that a random oracle can be replaced by a block cipher-based construction, and the resulting scheme remains secure in the ideal cipher model. The other direction was left as an open problem, i.e. constructing an ideal cipher from a random oracle. In this paper we solve this open problem and show that the Feistel construction with 6 rounds is enough to obtain an ideal cipher; we also show that 5 rounds are insufficient by providing a simple attack. This contrasts with the classical Luby-Rackoff result that 4 rounds are necessary and sufficient to obtain a (strong) pseudo-random permutation from a pseudo-random function.

Category / Keywords: foundations / random oracle model, ideal cipher model, indifferentiability, Feistel/Luby-Rackoff construction.

Publication Info: An extended abstract of this paper will appear at CRYPTO 2008. This is the full version.

Date: received 30 May 2008, last revised 16 Aug 2008

Contact author: jscoron at gmail com

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Version: 20080816:121712 (All versions of this report)

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