A low-memory algorithm for finding short product representations in finite groups

  • Authors:
  • Gaetan Bisson;Andrew V. Sutherland

  • Affiliations:
  • LORIA, Vandœuvre-lès-Nancy, France and Eindhoven University of Technology, Eindhoven, The Netherlands;Massachusetts Institute of Technology, Cambridge, USA

  • Venue:
  • Designs, Codes and Cryptography
  • Year:
  • 2012

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Abstract

We describe a space-efficient algorithm for solving a generalization of the subset sum problem in a finite group G, using a Pollard-驴 approach. Given an element z and a sequence of elements S, our algorithm attempts to find a subsequence of S whose product in G is equal to z. For a random sequence S of length d log2 n, where n = #G and d 驴 2 is a constant, we find that its expected running time is $${O(\sqrt{n}\,{\rm log}\,n)}$$ group operations (we give a rigorous proof for d 4), and it only needs to store O(1) group elements. We consider applications to class groups of imaginary quadratic fields, and to finding isogenies between elliptic curves over a finite field.