Cryptanalysis of the quadratic generator

  • Authors:
  • Domingo Gomez;Jaime Gutierrez;Alvar Ibeas

  • Affiliations:
  • Faculty of Sciences, University of Cantabria, Santander, Spain;Faculty of Sciences, University of Cantabria, Santander, Spain;Faculty of Sciences, University of Cantabria, Santander, Spain

  • Venue:
  • INDOCRYPT'05 Proceedings of the 6th international conference on Cryptology in India
  • Year:
  • 2005

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Abstract

Let p be a prime and let a and c be integers modulo p. The quadratic congruential generator (QCG) is a sequence (vn) of pseudorandom numbers defined by the relation $v_{n+1}\equiv av^{2}_{n}+c mod p$. We show that if sufficiently many of the most significant bits of several consecutive values vn of the QCG are given, one can recover in polynomial time the initial value v0 (even in the case where the coefficient c is unknown), provided that the initial value v0 does not lie in a certain small subset of exceptional values.