Cryptanalysis of block ciphers

J. Lu

Research output: ThesisPhd Thesis 4 Research NOT TU/e / Graduation NOT TU/e)

Abstract

The block cipher is one of the most important primitives in modern cryptography, information and network security; one of the primary purposes of such ciphers is to provide confidentiality for data transmitted in insecure communication environments. To ensure that confidentiality is robustly provided, it is essential to investigate the security of a block cipher against a variety of cryptanalytic attacks. In this thesis, we propose a new extension of differential cryptanalysis, which we call the impossible boomerang attack. We describe the early abort technique for (related-key) impossible differential cryptanalysis and rectangle attacks. Finally, we analyse the security of a number of block ciphers that are currently being widely used or have recently been proposed for use in emerging cryptographic applications; our main cryptanalytic results are as follows. An impossible differential attack on 7-round AES when used with 128 or 192 key bits, and an impossible differential attack on 8-round AES when used with 256 key bits. An impossible boomerang attack on 6-round AES when used with 128 key bits, and an impossible boomerang attack on 7-round AES when used with 192 or 256 key bits. A related-key impossible boomerang attack on 8-round AES when used with 192 key bits, and a related-key impossible boomerang attack on 9-round AES when used with 256 key bits, both using two keys. An impossible differential attack on 11-round reduced Camellia when used with 128 key bits, an impossible differential attack on 12-round reduced Camellia when used with 192 key bits, and an impossible differential attack on 13-round reduced Camellia when used with 256 key bits. A related-key rectangle attack on the full Cobra-F64a, and a related-key differential attack on the full Cobra-F64b. A related-key rectangle attack on 44-round SHACAL-2. A related-key rectangle attack on 36-round XTEA. An impossible differential attack on 25-round reduced HIGHT, a related-key rectangle attack on 26-round reduced HIGHT, and a related-key impossible differential attack on 28-round reduced HIGHT. In terms of either the attack complexity or the numbers of attacked rounds, the attacks presented in the thesis are better than any previously published cryptanalytic results for the block ciphers concerned, except in the case of AES; for AES, the presented impossible differential attacks on 7-round AES used with 128 key bits and 8-round AES used with 256 key bits are the best currently published results on AES in a single key attack scenario, and the presented related-key impossible boomerang attacks on 8-round AES used with 192 key bits and 9-round AES used with 256 key bits are the best currently published results on AES in a related-key attack scenario involving two keys.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Royal Holloway University of London
Supervisors/Advisors
  • Mitchell, C., Promotor, External person
Award date1 Jan 2013
Place of PublicationLondon
Publisher
Publication statusPublished - 2013
Externally publishedYes

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