reflexivity. Qed.

(*cf xsalsa-paper, beginning of Section 2*)
Lemma upd_upto_char data l: Zlength l = 16 ->
upd_upto data 4 l = match data with ((Nonce, C), (Key1, Key2)) =>
match Nonce with (N1, N2, N3, N4) =>
match C with (C1, C2, C3, C4) =>
match Key1 with (K1, K2, K3, K4) =>
match Key2 with (L1, L2, L3, L4) =>
map Vint (map littleendian [C1; K1; K2; K3;
K4; C2; N1; N2;
N3; N4; C3; L1;
L2; L3; L4; C4]) end end end end end.