Definition fpad (msg : Blist) : Blist :=
  bytesToBits (fpad_inner (bitsToBytes msg)).

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).