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Reverse Cycle Walking and Its Applications

  • Sarah Miracle
  • Scott Yilek
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 10031)

Abstract

We study the problem of constructing a block-cipher on a “possibly-strange” set \(\mathcal{S}\) using a block-cipher on a larger set \(\mathcal{T}\). Such constructions are useful in format-preserving encryption, where for example the set \(\mathcal{S}\) might contain “valid 9-digit social security numbers” while \(\mathcal{T}\) might be the set of 30-bit strings. Previous work has solved this problem using a technique called cycle walking, first formally analyzed by Black and Rogaway. Assuming the size of \(\mathcal{S}\) is a constant fraction of the size of \(\mathcal{T}\), cycle walking allows one to encipher a point \(x \in \mathcal{S}\) by applying the block-cipher on \(\mathcal{T}\) a small expected number of times and O(N) times in the worst case, where \(N = |\mathcal{T}|\), without any degradation in security. We introduce an alternative to cycle walking that we call reverse cycle walking, which lowers the worst-case number of times we must apply the block-cipher on \(\mathcal{T}\) from O(N) to \(O(\log N)\). Additionally, when the underlying block-cipher on \(\mathcal{T}\) is secure against \(q = (1-\epsilon )N\) adversarial queries, we show that applying reverse cycle walking gives us a cipher on \(\mathcal{S}\) secure even if the adversary is allowed to query all of the domain points. Such fully secure ciphers have been the the target of numerous recent papers.

Keywords

Format-preserving encryption Small-domain block ciphers Markov chains 

Notes

Acknowledgements

We thank Tom Ristenpart for his very helpful comments on an earlier draft of this paper. We also thank the anonymous Asiacrypt reviewers for their detailed feedback.

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Copyright information

© International Association for Cryptologic Research 2016

Authors and Affiliations

  1. 1.University of St. ThomasSt. PaulUSA

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