diff --git a/src/binary_formats.ml b/src/binary_formats.ml
index f03c8c4a..3faa3403 100644
--- a/src/binary_formats.ml
+++ b/src/binary_formats.ml
@@ -264,20 +264,29 @@ module GNUNET_RsaPublicKey = struct
     e: Z.t;
   }
 
+  let rev_string len s = String.init len (fun i -> s.[len - 1 - i])
+
   (* todo: need to strip leading zeros or something? *)
   (* reverse bytes because Z.of_bits reads little endian *)
-  let z_of_be_bits src pos len =
-    let x = String.sub src pos len in
-    let x = String.init len (fun i -> x.[len - 1 - i]) in
-    Z.of_bits x
+  let z_of_bits_be src pos len =
+    String.sub src pos len |> rev_string len |> Z.of_bits
 
   let of_pub ({ n; e } : Mirage_crypto_pk.Rsa.pub) =
     { n_len= Z.size n; e_len= Z.size e; n; e }
 
-  let to_octets _pub = assert false
-  let error = Error "GNUNET_RsaPublicKey.of_octets: invalid data"
+  let to_octets { n_len; e_len; n; e } =
+    let len = 4 + n_len + e_len in
+    let b = Bytes.make len '\x00' in
+    Bytes.set_uint16_be b 0 n_len;
+    Bytes.set_uint16_be b 2 e_len;
+    let n = Z.to_bits n |> rev_string n_len in
+    let e = Z.to_bits e |> rev_string e_len in
+    Bytes.blit_string n 0 b 4 n_len;
+    Bytes.blit_string e 0 b (4 + n_len) e_len;
+    Bytes.unsafe_to_string b
 
   let of_octets s =
+    let error = Error "GNUNET_RsaPublicKey.of_octets: invalid data" in
     let len = String.length s in
     match len >= 4 with
     | false -> error
@@ -287,8 +296,8 @@ module GNUNET_RsaPublicKey = struct
         match len = n_len + e_len + 4 with
         | false -> error
         | true ->
-            let n = z_of_be_bits s 4 n_len in
-            let e = z_of_be_bits s (4 + n_len) e_len in
+            let n = z_of_bits_be s 4 n_len in
+            let e = z_of_bits_be s (4 + n_len) e_len in
             Ok { n_len; e_len; n; e })
 end