Nonlinearly Filtered Generators are widely used in modern cryptosystems, and it is remarkably significant to study its cryptographical properties for cryptography and cryptanalysis. The properties of Boolean functions were introduced, Expansion Function and Expansion Spectrum were defined, and the measure of information leak of Nonlinear Filtered Generator was given. The key causation that the output sequence of Nonlinearly Filtered Generator leaks the information of its input sequence was brought out. An attack based on the information leak of the Nonlinearly Filtered Generator was put forward. This method could be used to attack those cryptographic algorithms using Nonlinear Filtered Generator as a part of pseudo random sequence generator. An example was proposed to show how to determine the inputs of Nonlinearly Filtered Generator when its outputs was given.
