diff --git a/src/crypto.ml b/src/crypto.ml
index bf437fce..bdb9a247 100644
--- a/src/crypto.ml
+++ b/src/crypto.ml
@@ -14,17 +14,9 @@ module Binary_format_rsa = struct
 
   let rev_string len s = String.init len (fun i -> s.[len - 1 - i])
 
-  (* we reverse bytes because Z.of_bits reads bytes in little endian *)
-  let z_of_bits_be src pos len =
-    String.sub src pos len |> rev_string len |> Z.of_bits
-
-  let z_to_bits_be z =
-    let bits = Z.to_bits z in
-    rev_string (String.length bits) bits
-
   let z_array_to_octets (arr : Z.t array) =
     let nb = Array.length arr in
-    let bits_arr = Array.map z_to_bits_be arr in
+    let bits_arr = Array.map Mirage_crypto_pk.Z_extra.to_octets_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
@@ -63,7 +55,8 @@ module Binary_format_rsa = struct
     let z_arr =
       Array.init nb (fun i ->
           let len = len_arr.(i) in
-          let z = z_of_bits_be s !pos len in
+          let s = String.sub src pos len in
+          let z = Mirage_crypto_pk.Z_extra.of_octets_be s in
           pos := !pos + len;
           z)
     in