  In this paper we present a novel switching function for multiplicative
watermarking systems. The switching function is based on the algebraic
structure of elliptic curves over finite fields. The resulting function allows
for both watermarking generator and remover to define appropriate system
parameters, sharing only limited information, namely a private key. Given the
definition of the switching function, we prove that the resulting watermarking
parameters lead to a stable watermarking scheme.
