Bits Security of the Elliptic Curve Diffie---Hellman Secret Keys

  • Authors:
  • Dimitar Jetchev;Ramarathnam Venkatesan

  • Affiliations:
  • Dept. of Mathematics, University of California at Berkeley, Berkeley, CA 94720;Microsoft Research, One Microsoft Way, Redmond, WA 98052 and Microsoft Research India Private Limited, "Scientia", Bangalore, India 560080

  • Venue:
  • CRYPTO 2008 Proceedings of the 28th Annual conference on Cryptology: Advances in Cryptology
  • Year:
  • 2008

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Abstract

We show that the least significant bits (LSB) of the elliptic curve Diffie---Hellman secret keys are hardcore. More precisely, we prove that if one can efficiently predict the LSB with non-negligible advantage on a polynomial fraction of all the curves defined over a given finite field $\mathbb{F}_p$, then with polynomial factor overhead, one can compute the entire Diffie---Hellman secret on a polynomial fraction of all the curves over the same finite field. Our approach is based on random self-reducibility (assuming GRH) of the Diffie---Hellman problem among elliptic curves of the same order. As a part of the argument, we prove a refinement of H. W. Lenstra's lower bounds on the sizes of the isogeny classes of elliptic curves, which may be of independent interest.