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Erschienen in:
2007 | OriginalPaper | Buchkapitel
How to Enrich the Message Space of a Cipher
verfasst von : Thomas Ristenpart, Phillip Rogaway
Erschienen in: Fast Software Encryption
Verlag: Springer Berlin Heidelberg
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Given (deterministic) ciphers
${\mathcal E}$
and
E
that can encipher messages of
l
and
n
bits, respectively, we construct a cipher
${\mathcal E}^*={XLS}[{\mathcal E},E]$
that can encipher messages of
l
+
s
bits for any
s
<
n
. Enciphering such a string will take one call to
${\mathcal E}$
and two calls to
E
. We prove that
${\mathcal E}^*$
is a strong pseudorandom permutation as long as
${\mathcal E}$
and
E
are. Our construction works even in the tweakable and VIL (variable-input-length) settings. It makes use of a multipermutation (a pair of orthogonal Latin squares), a combinatorial object not previously used to get a provable-security result.