JJ: Description from the destination commit:

wip encode/decode rsapublickey

JJ: Description from source commit:
.

src/binary_formats.ml

@@ -257,58 +257,55 @@ module GNUNET_RsaPublicKey = struct
     integer in big-endian format (MSB first).
     Leading zeroes are stripped unless they are required to keep a value positive.
   *)
-  type header = {
+  type t = {
     n_len: int;
     e_len: int;
-  }
-
-  type t = {
-    header: header;
     n: Z.t;
     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? *)
-  (* Z.to_bits is in little endian but we need it in big endian *)
-  let z_to_bigendian_bits v =
-    let s = Z.to_bits v in
-    let len = String.length s in
-    let s = String.init len (fun i -> s.[len - 1 - i]) in
-    s
-
-  let header_bin =
-    let open Bin in
-    record (fun n_len e_len -> { n_len; e_len })
-    |+ field beint16 (fun t -> t.n_len)
-    |+ field beint16 (fun t -> t.e_len)
-    |> sealr
-
-  (* note: this one has a dynamic sizeof *)
-  let bin =
-    let open Bin in
-    record (fun header n e ->
-        let n = Z.of_bits n in
-        let e = Z.of_bits e in
-        { header; n; e })
-    |+ field header_bin (fun t -> t.header)
-    (* TODO
-       here it should not be [cstring] but [bytes t.header.n_len]
-       -> just parse this without Bin *)
-    |+ field cstring (fun t -> z_to_bigendian_bits t.n)
-    |+ field cstring (fun t -> z_to_bigendian_bits t.e)
-    |> sealr
+  (* reverse bytes because Z.of_bits reads little endian *)
+  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) =
-    let header = { n_len= Z.size n; e_len= Z.size e } in
-    let t = { header; n; e } in
-    t
+    { n_len= Z.size n; e_len= Z.size e; n; e }
+
+  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
+    | true -> (
+        let n_len = String.get_uint16_be s 0 in
+        let e_len = String.get_uint16_be s 2 in
+        match len = n_len + e_len + 4 with
+        | false -> error
+        | true ->
+            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
 
 module DenominationHash = MK_SRC_HASH_32 (struct
   type src = Mirage_crypto_pk.Rsa.pub
 
   let to_octets pub =
-    GNUNET_RsaPublicKey.of_pub pub |> Bin.to_string GNUNET_RsaPublicKey.bin
+    GNUNET_RsaPublicKey.of_pub pub |> GNUNET_RsaPublicKey.to_octets
 end)
 
 module ExchangePublicKeyP = struct