Question 90. What are Shift Register Cascades?

A shift register cascade is a set of LFSRs (see Question 89) connected together in such a way that the behavior of one particular LFSR depends on the behavior of the previous LFSRs in the cascade. This dependent behavior is usually achieved by using one LFSR to control the clock of the following LFSR. For instance one register might be advanced by one step if the preceding register output is 1 and advanced by two steps otherwise. Many different configurations are possible and certain parameter choices appear to offer very good security . For more detail, see an excellent survey article by Gollman and Chambers [GC89].