NLFSR

A non-linear 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.

Non-linear feedback shift registers are components in modern stream ciphers, especially in RFID and smartcard applications. NLFSRs are known to be more resistant to cryptanalytic attacks than Linear Feedback Shift Registers (LFSRs).

...
Wikipedia