An LLL-reduction algorithm with quasi-linear time complexity: extended abstract

  • Authors:
  • Andrew Novocin;Damien Stehlé;Gilles Villard

  • Affiliations:
  • CNRS, ENS Lyon, INRIA, UCBL, U. Lyon, Lyon, France;CNRS, ENS Lyon, INRIA, UCBL, U. Lyon, Lyon, France;CNRS, ENS Lyon, INRIA, UCBL, U. Lyon, Lyon, France

  • Venue:
  • Proceedings of the forty-third annual ACM symposium on Theory of computing
  • Year:
  • 2011

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Abstract

We devise an algorithm, L1, with the following specifications: It takes as input an arbitrary basis B=(bi)i ∈ Zd x d of a Euclidean lattice L; It computes a basis of L which is reduced for a mild modification of the Lenstra-Lenstra-Lovász reduction; It terminates in time O(d5+ε β + dω+1+ε β1+ε) where β = log max |bi| (for any ε0 and ω is a valid exponent for matrix multiplication). This is the first LLL-reducing algorithm with a time complexity that is quasi-linear in β and polynomial in d. The backbone structure of L1 is able to mimic the Knuth-Schönhage fast gcd algorithm thanks to a combination of cutting-edge ingredients. First the bit-size of our lattice bases can be decreased via truncations whose validity are backed by recent numerical stability results on the QR matrix factorization. Also we establish a new framework for analyzing unimodular transformation matrices which reduce shifts of reduced bases, this includes bit-size control and new perturbation tools. We illustrate the power of this framework by generating a family of reduction algorithms.