Some lower bounds on the algebraic immunity of functions given by their trace forms

  • Authors:
  • V. V. Bayev

  • Affiliations:
  • Faculty of Computational Mathematics and Cybernetics, Lomonosov Moscow State University, Moscow, Russia

  • Venue:
  • Problems of Information Transmission
  • Year:
  • 2008

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Abstract

The algebraic immunity of a Boolean function is a parameter that characterizes the possibility to bound this function from above or below by a nonconstant Boolean function of a low algebraic degree. We obtain lower bounds on the algebraic immunity for a class of functions expressed through the inversion operation in the field GF(2 n ), as well as for larger classes of functions defined by their trace forms. In particular, for n 驴 5, the algebraic immunity of the function Tr n (x 驴1) has a lower bound 驴2驴n + 4驴 驴 4, which is close enough to the previously obtained upper bound 驴驴n驴 + 驴n/驴驴n驴驴 驴 2. We obtain a polynomial algorithm which, give a trace form of a Boolean function f, computes generating sets of functions of degree 驴 d for the following pair of spaces. Each function of the first (linear) space bounds f from below, and each function of the second (affine) space bounds f from above. Moreover, at the output of the algorithm, each function of a generating set is represented both as its trace form and as a polynomial of Boolean variables.