Improving Local Collisions: New Attacks on Reduced SHA-256

Florian Mendel; Tomislav Nad; Martin Schläffer

Paper 2015/350

Improving Local Collisions: New Attacks on Reduced SHA-256

Florian Mendel, Tomislav Nad, and Martin Schläffer

Abstract

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.

Metadata

Available format(s): PDF
Category: Secret-key cryptography
Publication info: Published by the IACR in EUROCRYPT 2013
DOI: 10.1007/978-3-642-38348-9_16
Keywords: hash functions SHA-2 cryptanalysis collisions semi-free-start collisions differential characteristics automatic search tool
Contact author(s): florian mendel @ iaik tugraz at
History: 2015-04-23: received
Short URL: https://ia.cr/2015/350
License: CC BY

BibTeX

@misc{cryptoeprint:2015/350,
      author = {Florian Mendel and Tomislav Nad and Martin Schläffer},
      title = {Improving Local Collisions: New Attacks on Reduced {SHA}-256},
      howpublished = {Cryptology {ePrint} Archive, Paper 2015/350},
      year = {2015},
      doi = {10.1007/978-3-642-38348-9_16},
      url = {https://eprint.iacr.org/2015/350}
}