IACR News item: 01 May 2015
Peter Gazi, Jooyoung Lee, Yannick Seurin, John Steinberger, Stefano Tessaro
ePrint ReportMore concretely, we show the following results for key-length extension schemes using a block cipher with $n$-bit blocks and $\\kappa$-bit keys:
- Plain cascades of length $\\ell = 2r+1$ are secure whenever $q_c q_e^r \\ll 2^{r(\\kappa+n)}$, $q_c \\ll 2^\\ka$ and $q_e \\ll 2^{2\\ka}$. The bound for $r = 1$ also applies to two-key triple encryption (as used within Triple DES).
- The $r$-round XOR-cascade is secure as long as $q_c q_e^r \\ll 2^{r(\\kappa+n)}$, matching an attack by Gazi (CRYPTO 2013).
- We fully characterize the security of Gazi and Tessaro\'s two-call 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.
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