On the relation between the Ideal Cipher and the Random Oracle Models

Yevgeniy Dodis, Prashant Puniya

Research output: Chapter in Book/Report/Conference proceedingConference contribution


The Random Oracle Model and the Ideal Cipher Model are two of the most popular idealized models in cryptography. It is a fundamentally important practical and theoretical problem to compare the relative strengths of these models and to see how they relate to each other. Recently, Coron et al. [8] proved that one can securely instantiate a random oracle in the ideal cipher model. In this paper, we investigate if it is possible to instantiate an ideal block cipher in the' random oracle model, which is a considerably more challenging question. We conjecture that the Luby-Rackoff construction [19] with a sufficient number of rounds should suffice to show this implication. This does not follow from the famous Luby-Rackoff result [19] showing that 4 rounds are enough to turn a pseudorandom function into a pseudorandom permutation, since the results of the intermediate rounds are known to everybody. As a partial step toward resolving this conjecture, we show that random oracles imply ideal ciphers in the honest-but-curious model, where all the participants are assumed to follow the protocol, but keep all their intermediate results. Namely, we show that the Luby-Rackoff construction with a superlogarithmic number of rounds can be used to instantiate the ideal block cipher in any honest-but-curious cryptosystem, and result in a similar honest-but-curious cryptosystem in the random oracle model. We also show that securely instantiating the ideal cipher using the Luby Rackoff construction with upto a logarithmic number of rounds is equivalent in the honest-but-curious and malicious models.

Original languageEnglish (US)
Title of host publicationTheory of Cryptography
Subtitle of host publicationThird Theory of Cryptography Conference, TCC 2006, Proceedings
Number of pages23
StatePublished - 2006
Event3rd Theory of Cryptography Conference, TCC 2006 - New York, NY, United States
Duration: Mar 4 2006Mar 7 2006

Publication series

NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
Volume3876 LNCS
ISSN (Print)0302-9743
ISSN (Electronic)1611-3349


Other3rd Theory of Cryptography Conference, TCC 2006
Country/TerritoryUnited States
CityNew York, NY

ASJC Scopus subject areas

  • Theoretical Computer Science
  • General Computer Science


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