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Cryptanalysis of a Stream Cipher with Large Variables Using Distinguishing Attack

Ahmadian, Zahra | 2008

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 40785 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Salmasizadeh, Mahmoud; Mohajeri, Javad
  7. Abstract:
  8. Distinguishing Attacks are a class of attacks on stream ciphers, which evaluate the random peroperties of the keystream generated by a stream cipher. In fact in these attacks the cryptanalyst tries to determine whether a stream is generated by a specific cipher or it seems random. Most of the Distinguishing attacks use the linear cryptanalysis technique and are called Linear Distinguishing Attack. The focus of this thesis is on linear distinguishing attacks. After a survey on stream ciphers and their cryptanalysis methods, distinguishing attack is introduced in detail. Then a new successful linear distinguishing attack on a recently developed stream cipher, Shannon, is presented. This attack is based on linearization of the nonlinear parts of the algorithm. The next step is to combine proper sets of the linear approximations to achieve a new linear relation of only keystream bits with reasonable bias. The distinguisher exploits this relation to attack the cipher. The proposed attack has a data complexity of words as well as computational complexity. The required memory is only a 32-bit counter. The key length in Shannon is 256 bit so any attack with data and computational complexity less than the key space is theoretically successful. Besides, the designers state that there are no distinguishing attacks on Shannon requiring less than keystream words and less than computations. Additionally this attack gives a significant improvement to the previous distinguishing attack on Shannon proposed by Hakala and Nyberg, which had a data compelexity of and requires an array of 107-bit counter.
  9. Keywords:
  10. Distinguish Attack ; Linear Analysis ; Stream Cipher ; Shanon Stream Cipher

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