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

Lemma fold_left_iv_length: forall k (HK: forall iv x, length iv = k -> length (sha_h iv x) = k) l iv x , length iv = k -> length (fold_left sha_h l (sha_h iv x)) = k.