Definition c := (32 * 8)%nat.
Definition p := (32 * 8)%nat.
Definition b := (c + p)%nat.
Definition BlockSize := 64.
Definition BlockSize_Bits := BlockSize * 8.
Definition sha_iv : Blist :=
intsToBits SHA256.init_registers.
Definition sha_h (regs : Blist) (block : Blist) : Blist :=
intsToBits (SHA256.hash_block (bitsToInts regs) (bitsToInts block)).
Definition sha_splitandpad (msg : Blist) : Blist :=
bytesToBits (sha_padding_lemmas.pad (bitsToBytes msg)).
(* artifact of app_fpad definition *)
Definition fpad_inner (msg : list byte) : list byte :=
(let n := BlockSize + Zlength msg in
[Byte.repr 128%Z]
++ list_repeat (Z.to_nat (-(n + 9) mod 64)) Byte.zero
++ intlist_to_bytelist ([Int.repr (n * 8 / Int.modulus); Int.repr (n * 8)]))%list.
Definition fpad (msg : Blist) : Blist :=
bytesToBits (fpad_inner (bitsToBytes msg)).
unfold fpad. rewrite bytesToBits_len. apply fpad_inner_length.
apply M. (*we're exploiting the fact that c=p here*)
Qed.

(* --------------- *)
Lemma xor_equiv_byte: forall xpad XPAD k K,
bytes_bits_lists xpad (HMAC_SHA256.sixtyfour XPAD) ->
((length K) * 8)%nat = (c + p)%nat ->
bytes_bits_lists k K ->
bytes_bits_lists (BLxor k xpad) (HMAC_SHA256.mkArg K XPAD).