Definition HKDF_extract (salt IKM: list byte): list byte := HMAC256 IKM salt.
Function Ti (PRK info: list byte) n:=
match n with
O => nil
|S m => let prev := Ti PRK info m in
HMAC256 (prev ++ info ++ [Byte.repr (Z.of_nat n)]) PRK
end.
Function T (PRK info: list byte) (n:nat):list byte :=
match n with
O => nil
| S m => (T PRK info m) ++ (Ti PRK info n)
end.
Definition HKDF_expand (PRK info:list byte) (L:Z):list byte :=
if zle L 0 then nil else
let N := Z.of_nat SHA256.DigestLength in
let k := if zeq (L mod N) 0 then Z.div L N else (Z.div L N) + 1 in
floyd.sublist.sublist 0 L (T PRK info (Z.to_nat k)).
Definition HKDF salt IKM info L:=
let PRK := HKDF_extract salt IKM in
HKDF_expand PRK info L.
(************************************ Test vectors**************************)
Definition decode_hex := sha.functional_prog.hexstring_to_bytelist.
Module HKDF_test_rfc5869_A1.
Definition IKM   := decode_hex "0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b".
Definition salt  := decode_hex "000102030405060708090a0b0c".
Definition info  := decode_hex "f0f1f2f3f4f5f6f7f8f9".
Definition L     := 42.
Definition PRK   := decode_hex "077709362c2e32df0ddc3f0dc47bba6390b6c73bb50f9c3122ec844ad7c2b3e5".
Definition OKM   := decode_hex "3cb25f25faacd57a90434f64d0362f2a2d2d0a90cf1a5a4c5db02d56ecc4c5bf34007208d5b887185865".
Goal map Byte.unsigned (HKDF_extract salt IKM) = map Byte.unsigned PRK.
vm_compute; reflexivity. Qed.
Goal map Byte.unsigned (HKDF salt IKM info L) = map Byte.unsigned OKM.
vm_compute. reflexivity. Qed. (*6secs*)
End HKDF_test_rfc5869_A1.
Module HKDF_test_rfc5869_A2.
Definition IKM   := decode_hex "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f404142434445464748494a4b4c4d4e4f".
Definition salt  := decode_hex "606162636465666768696a6b6c6d6e6f707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f909192939495969798999a9b9c9d9e9fa0a1a2a3a4a5a6a7a8a9aaabacadaeaf".
Definition info  := decode_hex "b0b1b2b3b4b5b6b7b8b9babbbcbdbebfc0c1c2c3c4c5c6c7c8c9cacbcccdcecfd0d1d2d3d4d5d6d7d8d9dadbdcdddedfe0e1e2e3e4e5e6e7e8e9eaebecedeeeff0f1f2f3f4f5f6f7f8f9fafbfcfdfeff".
Definition L     := 82.
Definition PRK   := decode_hex "06a6b88c5853361a06104c9ceb35b45cef760014904671014a193f40c15fc244".
Definition OKM   := decode_hex "b11e398dc80327a1c8e7f78c596a49344f012eda2d4efad8a050cc4c19afa97c59045a99cac7827271cb41c65e590e09da3275600c2f09b8367793a9aca3db71cc30c58179ec3e87c14c01d5c1f3434f1d87".
Goal map Byte.unsigned (HKDF_extract salt IKM) = map Byte.unsigned PRK.
vm_compute.  reflexivity. Qed.
Goal map Byte.unsigned (HKDF salt IKM info L) = map Byte.unsigned OKM.
vm_compute. reflexivity. Qed.
End HKDF_test_rfc5869_A2.
Module HKDF_test_rfc5869_A3.
Definition IKM   := decode_hex "0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b".
Definition salt  := decode_hex "".
Definition info  := decode_hex "".
Definition L     := 42.
Definition PRK   := decode_hex "19ef24a32c717b167f33a91d6f648bdf96596776afdb6377ac434c1c293ccb04".
Definition OKM   := decode_hex "8da4e775a563c18f715f802a063c5a31b8a11f5c5ee1879ec3454e5f3c738d2d9d201395faa4b61a96c8".
Goal map Byte.unsigned (HKDF_extract salt IKM) = map Byte.unsigned PRK.
vm_compute.  reflexivity. Qed.
Goal map Byte.unsigned (HKDF salt IKM info L) = map Byte.unsigned OKM.
vm_compute. reflexivity. Qed.
End HKDF_test_rfc5869_A3.
(********************************Lemmas*************************************)

Lemma Zlength_HKDF_expand x y z rest: 0 <= 32 * z -> 0 <= rest < 32 ->
(Zlength (HKDF_expand x y (32*z+rest)) = 32*z+rest)%Z.