[Resource Topic] 2015/567: Key-Recovery Attack on the ASASA Cryptosystem with Expanding S-boxes

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Title:
Key-Recovery Attack on the ASASA Cryptosystem with Expanding S-boxes

Authors: Henri Gilbert, Jérôme Plût, Joana Treger

Abstract:

We present a cryptanalysis of the ASASA public key cipher introduced at Asiacrypt 2014. This scheme alternates three layers of affine transformations A with two layers of quadratic substitutions S. We show that the partial derivatives of the public key polynomials contain information about the intermediate layer. This enables us to present a very simple distinguisher between an ASASA public key and random polynomials. We then expand upon the ideas of the distinguisher to achieve a full secret key recovery. This method uses only linear algebra and has a complexity dominated by the cost of computing the kernels of 2^{26} small matrices with entries in \mathbb F_{16}.

ePrint: https://eprint.iacr.org/2015/567

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