Open-Key Distinguishers for the Internal Block Cipher of Tweaked Lesamnta

Yu SASAKI  Kazumaro AOKI  

Publication
IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences   Vol.E96-A   No.1   pp.141-149
Publication Date: 2013/01/01
Online ISSN: 1745-1337
Print ISSN: 0916-8508
Type of Manuscript: Special Section PAPER (Special Section on Cryptography and Information Security)
Category: Hash Functions
Keyword: 
integral attack,  middletext distinguisher,  known-key,  chosen-key,  Lesamnta,  hash,  SHA-3,  

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Summary: 
This paper shows a known-key distinguisher on the internal block cipher of tweaked Lesamnta reduced to 31 (out of 32) rounds, which is one of the hash functions submitted to the SHA-3 competition. Moreover, the paper presents a distinguisher for full internal block cipher of Lesamnta with stronger assumption. For its tweaked version, all previous cryptanalysis can work no more than 24 rounds. We search for a new integral characteristic for the internal block cipher, and discover a 19-round integral characteristic for forward direction. We then search for an integral characteristic for backward direction, and the characteristics can be combined to full rounds with some assumption. The distinguisher for the internal block cipher of Lesamnta-256 requires 2192 query complexity and negligible memory. This is the best attack on Lesamnta compression function and its internal block cipher after the tweak.