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Heuristic Cryptanalysis of Stream Ciphers Based on Connection Equations

Fahimnia, Behnam | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41705 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Eghlidos, Taraneh
  7. Abstract:
  8. Stream ciphers are one of the cryptography primitives which have high speed in encryption process and less amount of hardware which they used. Guess and determine attacks are general attacks on stream ciphers which have often been implemented in ad hoc manner. Finding the minimal basis for internal state of a cryptosystem based on connection equations is the main challenge in stream ciphers. In recent years, some efforts for applying this attack in the systematic manner are started. First work in this area is the Simple Guess and Determine attack. The dvanced and Heuristic Guess and Determine attacks then presented, which all of them have heuristic structure. In this thesis, after reviewing of the previous heuristic attack, we improve them. In spite of valuable innovations putting on the proposed algorithms, especially in the recent heuristic one, it seems that these algorithms could still be improved and in this thesis improved algorithms are introduced. Then two new attack algorithms based on connection equations, with use of matrix and graph models are proposed. These models enable us to close the solution of finding minimal basis for the system presented by connection equations. Reducing attack complexity and the complexity of running the attack algorithms are the main advantage of using the matrix and graph models. With applying these algorithms to SOSEMANUK stream cipher and random generators, both of them show higher efficiency than Heuristic Guess and Determine attack. It must be mentioned that the graph model for attack has the best results among all models for Guess and Determine attacks.

  9. Keywords:
  10. Guess and Determine Attack ; Stream Cipher ; Heuristic Guess and Determine Attack ; Connection Equations

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