Traceable method and system for broadcasting digital data

Abstract

A method and system for a user to obtain a derived value Kof m bits, in which, given two pseudo-random functions gand gfrom m bits to m bits, said user obtains, on the basis of an input parameter consisting of a word r of n bits, a derived value K=g∘ . . . g∘g(S), where, for i=, . . . , n, g=gif r, and g=gif r, and where S is a master value of m bits which is not disclosed to said user. This method comprises the following steps: a search is conducted, from among a set of words of m bits U, where j=, . . . , p, recorded in a table Tand equal to U=g∘ . . . g∘g(S), where, for i=, . . . , l(j), the indices vare predetermined bits, for a word Uequal to U=g∘ . . . g∘g(S); said derived value Kis thereafter obtained by calculating K=g∘ . . . g(U). Application to the encryption/decryption of pay-per-use digital contents, and to authentication.