Linear feedback shift register (aka LFSR) is a finite-state machine made of a linear shift register in which the output is fed back into itself as part of the input. LFSRs are typically made of an adder, shift register, and a constant multiplier (often a simple XOR gate). LFSRs are also called Autonomous LFSR or ALFSR because besides the clock, it doesn't need any other input. LFSRs are the basic components of many keystream generators.