Function two_adic_complexity_of_bytes 

pub fn two_adic_complexity_of_bytes(data: &[u8]) -> usize

The 2-adic complexity of a byte string (its LSB-first bit expansion) — the size of the shortest FCSR (feedback-with-carry / add-with-carry) generating it. Low relative to the bit count ⇒ a carry-based keystream that fools every linear-complexity test (Berlekamp–Massey over any field sees high complexity; the carry is nonlinear over GF(2)). The certified weakness the linear tools miss.