On the Improvement of the BDF Attack on LSBS-RSA

  • Authors:
  • Hung-Min Sun;Mu-En Wu;Huaxiong Wang;Jian Guo

  • Affiliations:
  • Department of Computer Science, National Tsing Hua University, Taiwan;Department of Computer Science, National Tsing Hua University, Taiwan and School of Physical & Mathematical Sciences, Nanyang Technological University, Singapore;School of Physical & Mathematical Sciences, Nanyang Technological University, Singapore and Centre for Advanced Computing - Algorithms and Cryptography Department of Computing, Macquarie Universit ...;School of Physical & Mathematical Sciences, Nanyang Technological University, Singapore

  • Venue:
  • ACISP '08 Proceedings of the 13th Australasian conference on Information Security and Privacy
  • Year:
  • 2008

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Abstract

An $\left( \alpha ,\beta ,\gamma \right) $-LSBS RSA denotes an RSA system with primes sharing 驴least significant bits, private exponent dwith βleast significant bits leaked, and public exponent ewith bit-length 驴. Steinfeld and Zheng showed that LSBS-RSA with small eis inherently resistant to the BDF attack, but LSBS-RSA with large eis more vulnerable than standard RSA. In this paper, we improve the BDF attack on LSBS-RSA by reducing the cost of exhaustive search for k, where kis the parameter in RSA equation: $ed=k\cdot \varphi \left( N\right) +1$. Consequently, the complexity of the BDF attacks on LSBS-RSA can be further reduced. Denote 驴as the multiplicity of 2 in k. Our method gives the improvements, which depend on the two cases:1In the case $\gamma \leq \min \left\{ \beta ,2\alpha \right\} -\sigma $, the cost of exhaustive search for kin LSBS-RSA can be simplified to searching kin polynomial time. Thus, the complexity of the BDF attack is independent of 驴, but it still increases as 驴increases.1In the case $\gamma \min \left\{ \beta ,2\alpha \right\} -\sigma $, the complexity of the BDF attack on LSBS-RSA can be further reduced with increasing 驴or β.More precisely, we show that an LSBS-RSA is more vulnerable under the BDF attack as $\max \left\{ 2\alpha ,\beta \right\} $ increases proportionally with the size of N. In the last, we point out that although LSBS-RSA benefits the computational efficiency in some applications, one should be more careful in using LSBS-RSA.