diff --git a/src/util.ml b/src/util.ml
index c8fb197e..35fad636 100644
--- a/src/util.ml
+++ b/src/util.ml
@@ -34,32 +34,28 @@ module Bin_rsa = struct
     let check = function false -> Error (`Msg "invalid data") | true -> Ok ()
 
     let z_array_to_octets (arr : Z.t array) =
-      let arr =
-        Array.map
-          (fun z ->
-            let bits = z_to_bits_be z in
-            (String.length bits, bits))
-          arr
-      in
-      let len = Array.fold_left (fun acc (n, _) -> acc + n) 0 arr in
+      let nb = Array.length arr in
+      (*Fmt.epr "z_array_to_octets, nb=%d@." nb;*)
+      let bits_arr = Array.map z_to_bits_be arr in
+      let len_arr = Array.map String.length bits_arr in
+      let len = (2 * nb) + Array.fold_left ( + ) 0 len_arr in
       let b = Bytes.make len '\x00' in
-      let bits_start_pos =
-        Array.fold_left
-          (fun pos (len, _bits) ->
-            Bytes.set_uint16_be b pos len;
-            pos + 2)
-          0 arr
-      in
-      let _end : int =
-        Array.fold_left
-          (fun pos (len, bits) ->
-            Bytes.blit_string bits 0 b pos len;
-            pos + len)
-          bits_start_pos arr
-      in
+      let pos = ref 0 in
+      Array.iter
+        (fun len ->
+          Bytes.set_uint16_be b !pos len;
+          pos := !pos + 2)
+        len_arr;
+      Array.iteri
+        (fun i bits ->
+          let len = len_arr.(i) in
+          Bytes.blit_string bits 0 b !pos len;
+          pos := !pos + len)
+        bits_arr;
       Bytes.unsafe_to_string b
 
     let z_array_of_octets ~nb s =
+      (*Fmt.epr "z_array_of_octets, nb=%d@." nb;*)
       let s_len = String.length s in
       let* () = check (s_len <= 2 * nb) in
       let pos = ref 0 in