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Cryptanalysis of Stream Ciphers By Structural Attacks

Rohani, Neda | 2010

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 40839 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Aref, Mohammad Reza; Mohajeri, Javad
  7. Abstract:
  8. According to the development of communication, cryptography has become a vital issue. Several algorithms have been introduced for cryptography applications. Stream ciphers are such algorithms with high speed and appropriate efficiency. Lots of attacks have been applied to stream ciphers. We concentrate on two kinds of attacks in this thesis. Distinguishing attack is a general attack in which the attacker tries to distinguish the observed output sequence from random. Designers apply this kind of attack to test the statistical probabilities of the output sequence. We applied this attack on Grain family and Trivium family. In the proposed attack, nonlinear parts are replaced with linear ones. The attacks have complexities of order of O(239.1), O(2102), O(230.79) and O(269.1) respectively for Grain, Grain-v1, Bivium and Trivium. Second attack is guess and determine which is a kind of structural attack. This attack consists of some steps. First a part of internal state is guessed. According to update and output functions, the remaining part of internal state is determined. Time complexity is specified according to the number of guessed bits in the first step. Some times for applying the attack, some assumptions are made. For holding the assumptions, some amount of keystream is needed. The number of keystream bits is called data complexity. We applied this attack on Grain, Bivium and Trivium. These attacks have complexities of order of O(270), O(227.75) and O(290.67) respectively. Data complexity of the attacks are of order of O(225.97), O(244) and O(263.5) respectively
  9. Keywords:
  10. Stream Cipher ; Distinguish Attack ; Guess and Determine Attack ; Bias ; Cryptanalysis ; Linear Approximation

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