Function hash_blocks_bits (b:nat) (B:(0<b)%nat) (hash_block_bit : Blist -> Blist -> Blist) (r: Blist)
(msg: Blist) {measure length msg} : Blist :=
match msg with
| nil => r
| _ => hash_blocks_bits b B hash_block_bit (hash_block_bit r (firstn b msg)) (skipn b msg)
end.

(* Specifications: *)
Lemma hash_blocks_bits_len c b (B:(0<b)%nat) h
(HH: forall x y, length x = c -> length y = b -> length (h x y)  = c)
r l: length r = c ->
InBlocks b l ->
length (hash_blocks_bits b B h r l) = c.