A nonlinear-feedback shift register (NLFSR) is a shift register whose input bit is a non-linear function of its previous state.

For an n-bit shift register r its next state is defined as:

${\displaystyle r_{i+1}(b_{0},b_{1},b_{2},\ldots ,b_{n-1})=r_{i}(b_{1},b_{2},\ldots ,f(b_{0},b_{1},b_{2},\ldots ,b_{n-1}))}$,

where f is the non-linear feedback function.