Module HP.
(*SHA256: blocksize = 64bytes corresponds to #define SHA_LBLOCK	16 #define SHA256_CBLOCK	(SHA_LBLOCK*4) *)
Module Type HASH_FUNCTION.
Parameter BlockSize:nat. (*measured in bytes; 64 in SHA256*)
Parameter DigestLength: nat. (*measured in bytes; 32 in SHA256*)
Parameter Hash : list byte -> list byte.
End HASH_FUNCTION.
Module Type HMAC_Module.
Parameter HMAC: byte -> byte -> list byte -> list byte -> list byte.
End HMAC_Module.
Module HMAC_FUN (HF:HASH_FUNCTION)  <: HMAC_Module.
Definition sixtyfour {A} (i:A): list A:= list_repeat HF.BlockSize i.
(*Reading rfc4231 reveals that padding happens on the right*)
Definition zeroPad (k: list byte) : list byte :=
k ++ list_repeat (HF.BlockSize-length k) Byte.zero.
Definition mkKey (l:list byte) : list byte :=
if Z.gtb (Zlength l) (Z.of_nat HF.BlockSize)
then (zeroPad (HF.Hash l))
else zeroPad l.
Definition KeyPreparation (l:list byte) : list byte := mkKey l.
Definition mkArg (key:list byte) (pad:byte): list byte :=
(map (fun p => Byte.xor (fst p) (snd p))
(combine key (sixtyfour pad))).
(* Definition Ipad := P.Ipad. Definition Opad := P.Opad. *)
(*innerArg to be applied to message, (map Byte.repr (mkKey password)))*)
Definition innerArg IP (text: list byte) key : list byte :=
(mkArg key IP) ++ text.
Definition INNER IP k text := HF.Hash (innerArg IP text k).
Definition outerArg OP (innerRes: list byte) key: list byte :=
(mkArg key OP) ++ innerRes.
Definition OUTER OP k innerRes := HF.Hash (outerArg OP innerRes k).
Definition HmacCore IP OP txt (key: list byte): list byte := OUTER OP key (INNER IP key txt).
Definition HASH a txt :=  HF.Hash (a ++ txt).
Definition HmacCore' IP OP txt (key: list byte): list byte :=
HASH (mkArg key OP) (HASH (mkArg key IP) txt).
Goal forall IP OP txt key, HmacCore IP OP txt key = HmacCore' IP OP txt key.
*)

Lemma length_SF {A} (a:A) :length (sixtyfour a) = HF.BlockSize.