Fixpoint upd_upto (x: SixteenByte * SixteenByte * (SixteenByte * SixteenByte)) i (l:list val):list val :=
  match i with
    O => l
  | S n =>
     match x with (Nonce, C, (Key1, Key2)) =>
     ((upd_Znth (11 + (Z.of_nat n))
     (upd_Znth(6 + (Z.of_nat n))
        (upd_Znth (1 + (Z.of_nat n))
           (upd_Znth (5 * (Z.of_nat n)) (upd_upto x n l)
              (Vint (littleendian (Select16Q C (Z.of_nat n)))))
           (Vint (littleendian (Select16Q Key1 (Z.of_nat n)))))
        (Vint (littleendian (Select16Q Nonce (Z.of_nat n)))))
     (Vint (littleendian (Select16Q Key2 (Z.of_nat n))))))
     end
  end.

Lemma upd_upto_Vint data: forall n, 0<=n<16 -> exists i, Znth n (upd_upto data 4 (list_repeat 16 Vundef)) = Vint i.