International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: An Improved Distinguisher for Dragon

Joo Yeon Cho
Josef Pieprzyk
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Abstract: 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.
  title={An Improved Distinguisher for Dragon},
  booktitle={IACR Eprint archive},
  keywords={Stream Ciphers, eSTREAM, Dragon, Modular Addition},
  note={ 13704 received 23 Mar 2007, last revised 10 Jul 2007},
  author={Joo Yeon Cho and Josef Pieprzyk},