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).