Cryptology ePrint Archive: Report 2009/526
Improved Related-Key Boomerang Attacks on Round-Reduced Threefish-512
Jiazhe Chen and Keting Jia
Abstract: Hash function Skein is one of the 14 NIST SHA-3 second
round candidates. Threefish is a tweakable block cipher as the core
of Skein, defined with a 256-, 512-, and 1024-bit block size. The
512-bit block size is the primary proposal of the authors. Skein had
been updated after it entered the second round, the only differences
between the original and the new version are the rotation constants.
In this paper we construct related-key boomerang distinguishers on
round-reduced Threefish-512 based on the new rotation constants
using the method of \emph{modular differential}. With these
distinguishers, we mount related-key boomerang key recovery attacks
on Threefish-512 reduced to 32, 33 and 34 rounds. The attack on
32-round Threefish-512 has time complexity $2^{195}$ with memory of
$2^{12}$ bytes. The attacks on Threefish-512 reduced to 33 and 34
rounds has time complexity of $2^{325.56}$ and $2^{483}$ encryptions
respectively, and both with negligible memory. The best key recovery
attack known before is proposed by Aumasson et al. Their attack,
which bases on the old rotation constants, is also a related-key
boomerang attack. For 32-round Threefish-512, their attack requires
$2^{312}$ encryptions and $2^{71}$ bytes of memory.
Category / Keywords: secret-key cryptography / Threefish-512, related-key boomerang attack, modular differential
Date: received 1 Nov 2009, last revised 20 Feb 2010
Contact author: jiazhechen at mail sdu edu cn
Available format(s): PDF | BibTeX Citation
Note: new results are added
Version: 20100220:123300 (All versions of this report)
Short URL: ia.cr/2009/526
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