Loading...

Distinguishing Attacks on Stream Ciphers

Noferesti, Zainab | 2010

661 Viewed
  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 40941 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Aref, Mohammad Reza; Mohajeri, Javad
  7. Abstract:
  8. Stream ciphers are cryptographic schemes being widely used in communications as a means of providing security of data. Hence, scrutinizing their level of security against different cryptanalytic methods has obviously turned to be of great importance. Randomness of the keystream is an important requirement for stream ciphers. The cipher should be designed in such a way that the keystream is free of any statistical biases that would let attackers distinguish it from a random sequence. A distinguishing attack on a stream cipher is one that determines whether a given stream of symbols is a truly random sequence or generated by a specific cipher with an unknown key. In this thesis, we focus on the distinguishing attacks and present three new attacks of this type. These attacks have been performed on eSTREAM project finalists, Grain and Trivium, and the stream cipher Bivium, a truncated version of the latter. Our attack on Grain, with a complexity of O(2102) is the first distinguishing attack on this version of the cipher. This is while applying the same method on the initial version has also resulted in the best ever-since applied attack, with a complexity of O(239.1). Furthermore, our attack on Trivium, with a complexity of O(269.1), is the first successful attack on the full version of this cipher. Moreover, applying the same method on three reduced-round versions of this cipher, we could achieve the best complexity among similar previous attacks. Our attack on Bivium, which has the least complexity among all the attacks against this cipher until now, utilizes the same method. The complexity of this attack is O(230.79). Non-randomness properties of the output of the nonlinear function, low nonlinearity and existence of linear approximations with good bias are the most important factors causing these attacks to be successful.
  9. Keywords:
  10. Stream Cipher ; Distinguish Attack ; Grain Algorithm ; Distinguisher ; Trivium Algorithm ; Bivium Algorithm

 Digital Object List

 Bookmark

No TOC