Lemma Snuffle_sub_simpl data x: Snuffle20 (prepare_data data) = Some x -> exists s, Snuffle 20 (prepare_data data) = Some s /\ forall i (I:0 <= i < 16) v, Znth i (prepare_data data) = v -> littleendian_invert (Int.sub (Znth i x) v) = littleendian_invert (Znth i s).