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Propagation of Truncated Differentials in GOST

Authors:
Nicolas Courtois
Theodosis Mourouzis

Keywords: differential cryptanalysis, block ciphers, GOST, Sboxes, diffusion, optimization problems, truncated differentials, aggregated differentials

Abstract:
GOST 28147-89 is a well-known block cipher with 256-bit keys. Its excessively low implementation cost makes it a plausible alternative for major industrial cryptographic algorithms such as 3-DES and AES-256. In 2010, GOST was submitted to ISO to become a part of the international encryption standard ISO/IEC 18033-3. This stimulated intense research by the cryptographic community and lots of new attacks were developed which reduce its 256-bit security level. These recent attacks against full GOST belong to two main categories: complexity reduction attacks and advanced differential attacks. In differential cryptanalysis, the essential task is the exploration of the exponentially large space of differentials in a systematic way and the construction of complex distinguisher attacks. In this paper, we study the GOST cipher in the well-known theory framework of Markov cipher, which is a basis of many works on differential cryptanalysis. However, we prove that GOST is NOT a Markov cipher though in approximation it still seems to behave like one. We propose a heuristic black-box methodology for efficient discovery of interesting sets of differentials in GOST and we show that results better than any previously known can be obtained with this methodology. However, different sets will be the best possible solutions for various numbers of rounds and more work is needed in order to improve the best known singlekey attacks on GOST and adapt them to other sets of S-boxes.

Pages: 156 to 161

Copyright: Copyright (c) IARIA, 2013

Publication date: August 25, 2013

Published in: conference

ISSN: 2162-2116

ISBN: 978-1-61208-298-1

Location: Barcelona, Spain

Dates: from August 25, 2013 to August 31, 2013