Abstract
We give a simple algorithm that can find biased linear approximations of nonlinear building blocks. The algorithm is useful if the building block is relatively small and exhaustive search is possible. Instead of searching all possible linear relations individually, we show how the most biased relation can be found in just a few steps. As an example we show how we can find a biased relation in the output bits of the stream cipher Pomaranch Version 2, a tweaked variant of Pomaranch, resulting in both distinguishing and key recovery attacks. These attacks will break both the 128-bit variant and the 80-bit variant of the cipher with complexity faster than exhaustive key search.
The work described in this paper has been supported in part by the European Commission through the IST Programme under Contract IST-2002-507932 ECRYPT. The information in this document reflects only the author’s views, is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.
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Hell, M., Johansson, T. (2007). On the Problem of Finding Linear Approximations and Cryptanalysis of Pomaranch Version 2. In: Biham, E., Youssef, A.M. (eds) Selected Areas in Cryptography. SAC 2006. Lecture Notes in Computer Science, vol 4356. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-74462-7_16
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DOI: https://doi.org/10.1007/978-3-540-74462-7_16
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