Binary sequences generated by nonlinearly filtering maximal length sequences with period 2
n
− 1 are studied in this paper. We focus on the particular class of normal filters and provide improved lower bounds on the linear complexity of generated keystreams. This is achieved by first proving properties
of a special class of determinants which are associated to linearized polynomials over finite fields of characteristic 2 and then by applying the above to simplify generalizations of the root presence test.
Keywords Binary sequences - filter functions - linear complexity - linear feedback shift registers - linearized polynomials - stream ciphers