Figure 7. LFSR
A Linear Feedback Shift Register (LFSR) is a mechanism for generating a sequence of binary bits. The register (see Figure 7) consists of a series of cells that are set by an initialization vector that is, most often, the secret key. The behavior of the register is regulated by a clock and at each clocking instant, the contents of the cells of the register are shifted right by one position, and the exclusive-or of a subset of the cell contents is placed in the leftmost cell. One bit of output is usually derived during this update procedure.
LFSRs are fast and easy to implement in both hardware and software. With a judicious choice of feedback taps the sequences that are generated can have a good statistical appearance. However, the sequences generated by single LFSRs are not secure because a powerful mathematical framework has been developed over the years which allows for their straightforward analysis. However, LFSRs are useful as building blocks in more secure systems (see Question 90, Question 91 and Question 92).