University of Waterloo
Although block ciphers can be implemented as stream ciphers using OFB and CTR mode, stream ciphers are in favor of many applications for two reasons. Grain and Trivium are the hardware-oriented finalists of the eSTREAM. They are both based on nonlinear feedback shift registers. In this paper, the authors study their generalized classes of nonlinear feedback shift registers with time varying feedback functions, namely, Grain-like and Trivium-like structures. Some interesting results regarding their periods are obtained.