= Linear Feedback Shift Register =

A LFSR is set of rules to alter a set of bits. They are useful to psudeo random
number generators, and as key generators for stream ciphers.

All LFSRs are cyclical in nature, and after a set amount of time will repeat
back into themselves. THe initial state of the bits in the LFSR is called the
seed.

The maximum period for a _n_ bit shift register is 

2^n - 1

A *tap* is where a bit is read and fed back into itself.