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New Automatic Meet in the Middle Cryptanalysis of Block Ciphers

Ahmadi, Siavash | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 52778 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Aref, Mohammad Reza
  7. Abstract:
  8. Block ciphers are one of the most necessary primitives for security protocols, and so, cryptanalysis of them is also essential as well. On the other hand, the security assessment of block ciphers is usually manual work with various specific-idea according to the target block cipher type and algorithm. Some basic known methods for cryptanalysis of block ciphers include linear, differential, meet in the middle, and integral attacks. However, cryptanalysis methods are not limited to them, and many new hybrid or innovative methods are employed to provide better results. Recently, automated cryptanalysis of block ciphers and proposing new methods for analyzes of these ciphers are in the concentration. Especially, providing a tool for cryptanalysis can help designers to propose stronger block ciphers, and also help cryptanalyst to analyze previous block ciphers. Here, the approach is also providing new automated cryptanalysis methods for block ciphers. Therefore, cryptanalysis of block ciphers can perform more quickly, and the best results can found in the lowest possible time.In this thesis, meet in the middle attack is chosen as a concentration point, and a new generalized meet in the middle attack on block ciphers is proposed. This attack, which can be performed on most of the block ciphers, is also being automated. It means that a completely automated algorithm for identification of the best situation for the generalized meet in the middle attack along with the method of extracting attack complexities is provided. To propose the automated method, first, a framework for simulation of block cipher diffusion is presented in which the block ciphers can be defined. Then, the automated method is explained with the aid of the mentioned framework. Cryptanalysis results of this new attack on various block ciphers with Feistel and SPN structure show the successful performance of it. Due to the general approach of the proposed attack, the utilization of particular features of each cipher can improve the results.Meet in the middle attack on some kind of block ciphers such as GOST and GOST2 should be turned into the idea-driven attack, and generalized meet in the middle attack could not give appropriate results. In this thesis, after introducing generalized meet in the middle attack and automation method of it, some specific meet in the middle attack on GOST and GOST2 block ciphers are also provided, and then the total algorithms for these attacks are also given. Finally, the obtained results are compared with the best previous results to show the superiority of the proposed attacks
  9. Keywords:
  10. Confidentiality ; Block Cipher ; Meet-in-the Middle Attack ; Cut Sets ; Automated Cryptanalysis

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