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Improving local collisions: new attacks on reduced SHA-256. (English) Zbl 1306.94075

Johansson, Thomas (ed.) et al., Advances in cryptology – EUROCRYPT 2013. 32nd annual international conference on the theory and applications of cryptographic techniques, Athens, Greece, May 26–30, 2013. Proceedings. Berlin: Springer (ISBN 978-3-642-38347-2/pbk). Lecture Notes in Computer Science 7881, 262-278 (2013).
Summary: In this paper, we focus on the construction of semi-free-start collisions for SHA-256, and show how to turn them into collisions. We present a collision attack on 28 steps of the hash function with practical complexity. Using a two-block approach we are able to turn a semi-free-start collision into a collision for 31 steps with a complexity of at most \(2^{65.5}\). The main improvement of our work is to extend the size of the local collisions used in these attacks. To construct differential characteristics and confirming message pairs for longer local collisions, we had to improve the search strategy of our automated search tool. To test the limits of our techniques we present a semi-free-start collision for 38 steps.
For the entire collection see [Zbl 1263.94005].

MSC:

94A60 Cryptography
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