Finding smooth integers in short intervals using CRT decoding

  • Authors:
  • Dan Boneh

  • Affiliations:
  • Computer Science Department, Stanford University, Stanford, California

  • Venue:
  • Journal of Computer and System Sciences - Special issue on STOC 2000
  • Year:
  • 2002

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Abstract

We present a new algorithm for CRT list decoding. An instance of the, CRT list decoding problem consists of integers B, 〈p1, ..., pn〉 and 〈r1, ..., rn〉, where p1 2 n is a sequence of relatively prime integers. The CRT list decoding problem is to find all positive integers x such that x = ri mod pi for all but e values of i ∈ {1, ..., n}. Suppose B = Πi=1r pi for some integer k. Goldreich, Ron, and Sudan (in "Proc. of STOC'99", pp. 225-234, 1999) recently gave several applications for this problem and presented the first efficient algorithm that works whenever e (approximately) satisfies e - √2kn log pn/log p1. Our new algorithm achieves the stronger bound e - √kn log pn/log p1 (approximately). The improvement is significant when k is relatively close to n, e.g. k n/3. The bounds we obtain are similar to the bounds obtained by Guruswami and Sudan for Reed-Solomon list decoding. Hence, our algorithm reduces the gap between CRT list decoding and list decoding of Reed-Solomon codes. In addition, we give a new application for CRT list decoding: finding smooth integers in short intervals. Problems of this type come up in several algorithms for factoring large integers. We define and solve a generalized CRT list decoding problem and discuss how it might be used within the quadratic sieve factoring method.