Paper 2007/108

An Improved Distinguisher for Dragon

Joo Yeon Cho and Josef Pieprzyk


Dragon stream cipher is one of the focus ciphers which have reached Phase 2 of the eSTREAM project. In this paper, we present a new method of building a linear distinguisher for Dragon. The distinguisher is constructed by exploiting the biases of two S-boxes and the modular addition which are basic components of the nonlinear function $F$. The bias of the distinguisher is estimated to be around $2^{-75.32}$ which is better than the bias of the distinguisher presented by Englund and Maximov. We have shown that Dragon is distinguishable from a random cipher by using around $2^{150.6}$ keystream words and $2^{59}$ memory. In addition, we present a very efficient algorithm for computing the bias of linear approximation of modular addition.

Note: A new attack method is described in chapter 4.

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Published elsewhere. Unknown where it was published
Stream CipherseSTREAMDragonModular Addition
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jooyeon cho @ gmail com
2007-07-10: last of 2 revisions
2007-03-26: received
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      author = {Joo Yeon Cho and Josef Pieprzyk},
      title = {An Improved Distinguisher for Dragon},
      howpublished = {Cryptology ePrint Archive, Paper 2007/108},
      year = {2007},
      note = {\url{}},
      url = {}
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