2015 | OriginalPaper | Buchkapitel
Relaxing Full-Codebook Security: A Refined Analysis of Key-Length Extension Schemes
verfasst von : Peter Gaži, Jooyoung Lee, Yannick Seurin, John Steinberger, Stefano Tessaro
Erschienen in: Fast Software Encryption
Verlag: Springer Berlin Heidelberg
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Abstract
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Plain cascades of length \(\ell =2r+1\) are secure whenever \(q_cq_e^r \ll 2^{r(\kappa +n)}\), \(q_c \ll 2^\kappa \) and \(q_e \ll 2^{2\kappa }\). The bound for \(r = 1\) also applies to two-key triple encryption (as used within Triple DES).
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The r-round XOR-cascade is secure as long as \(q_cq_e^r \ll 2^{r(\kappa +n)}\), matching an attack by Gaži (CRYPTO 2013).
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We fully characterize the security of Gaži and Tessaro’s two-call \(\mathsf {2XOR}\) construction (EUROCRYPT 2012) for all values of \(q_c\), and note that the addition of a third whitening step strictly increases security for \(2^{n/4} \le q_c \le 2^{3/4n}\). We also propose a variant of this construction without re-keying and achieving comparable security levels.