Range Extension for Weak PRFs; The Good, the Bad, and the Ugly

  • Authors:
  • Krzysztof Pietrzak;Johan Sjödin

  • Affiliations:
  • CWI Amsterdam,;Department of Computer Science, ETH Zurich, CH-8092 Zurich, Switzerland

  • Venue:
  • EUROCRYPT '07 Proceedings of the 26th annual international conference on Advances in Cryptology
  • Year:
  • 2007

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Abstract

We investigate a general class of (black-box) constructions for range extension of weak pseudorandom functions: a construction based on mindependent functions F1,...,Fmis given by a set of strings over {1,...,m}*, where for example $\{\langle{2}\rangle, \langle{1,2}\rangle\}$ corresponds to the function X驴[F2(X),F2(F1(X))]. All efficient constructions for range expansion of weak pseudorandom functions that we are aware of are of this form.We completely classify such constructions as good, bador ugly, where the good constructions are those whose security can be proven via a black-box reduction, the bad constructions are those whose insecurity can be proven via a black-box reduction, and the ugly constructions are those which are neither good nor bad.Our classification shows that the range expansion from [10] is optimal, in the sense that it achieves the best possible expansion (2m驴 1 when using mkeys).Along the way we show that for weak quasirandomfunctions (i.e. in the information theoretic setting), all constructions which are not bad --- in particular all the ugly ones --- are secure.