Cryptology ePrint Archive: Report 2010/282

Pseudo-Linear Approximations for ARX Ciphers: With Application to Threefish

Kerry A. McKay and Poorvi L. Vora

Abstract: The operations addition modulo 2^n and exclusive-or have recently been combined to obtain an efficient mechanism for nonlinearity in block cipher design. In this paper, we show that ciphers using this approach may be approximated by pseudo-linear expressions relating groups of contiguous bits of the round key, round input, and round output. The bias of an approximation can be large enough for known plaintext attacks. We demonstrate an application of this concept to a reduced-round version of the Threefish block cipher, a component of the Skein entry in the secure hash function competition.

Category / Keywords: secret-key cryptography / Threefish, Skein, ARX cipher, linear cryptanalysis

Date: received 12 May 2010, last revised 5 Aug 2010

Contact author: kerry at gwu edu

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Version: 20100806:014231 (All versions of this report)

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