Linear feedback shift register based stream ciphers

Linear Feedback Shift Register (LFSR) based stream ciphers are popular because of their low hardware implementation costs. The nonlinear combination generators and clock-controlled generators are two very commonly used schemes in LFSR based stream ciphers. FPGA implementation of these two schemes has been done to obtain an idea about their hardware complexity. The fast correlation attack and edit distance attack, are among the fastest of the reported attacks on the nonlinear combination generators and clock-controlled generators respectively. These two attacks have been implemented. This paper compares the time for successful cryptanalytic attacks on both systems so as to compare their levels of security and hardware complexity.