Hint Rewrite Znth_map_Vbyte using list_solve : norm entailer_rewrite.
Ltac fold_Vbyte :=
repeat match goal with |- context [Vint (Int.repr (Byte.signed ?c))] =>
fold (Vbyte c)
end.

Lemma Znth_map_Vubyte: forall (i : Z) (l : list byte),
0 <= i < Zlength l -> Znth i (map Vubyte l)  = Vubyte (Znth i l).