Security of most significant bits of gx2

  • Authors:
  • Igor E. Shparlinski

  • Affiliations:
  • Department of Computing, Macquarie University, Sydney, Australia

  • Venue:
  • Information Processing Letters
  • Year:
  • 2002

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Abstract

Boneh and Venkatesan have recently proposed a polynomial time algorithm for recovering a "hidden" element α of a finite field Fp = {0,..., p - 1 } of p elements from rather short strings of the most significant bits of the remainder modulo p of αt for several values of t selected uniformly at random from F*p. González Vasco and Shparlinski, using bounds of exponential sums, have generalized this algorithm to the case where t is selected from a subgroup of F*p. In turn, this has allowed to improve one of the statements of the aforementioned work about the security of the most significant bits of the Diffie-Hellman key. Namely, it has been shown that having an oracle which, given gx, gy ∈ F*p for returns about log1/2 p most significant bits of gxy ∈ F*p, one can construct a polynomial time algorithm to compute gxy, provided that the multiplicative order of g is not too small. Here we use exponential sums of a different type to show that a similar statement holds for a much weaker 'diagonal' oracle which which, given gx ∈ F*p, returns about log1/2p most significant bits of gx2 ∈ F*p.