One of the most important methods for cryptanalysis of LFSR-based stream ciphers. Basically, the cryptanalyst attempts to identify some correlation between the output of the stream cipher and the output of one of its internal pieces. By observing the output stream, one can deduce the output of the generator and one of its internal pieces. By using such information, the analyst can then obtain information about the generator's other internal pieces and eventually crack the whole system.