add binary_formats.ml

src/dune

@@ -13,6 +13,7 @@
   ;
   include
   ;
+  bin
   angstrom
   zarith ;
   mirage-crypto

src/signatures.ml

@@ -1,18 +1,29 @@
-(* https://docs.taler.net/core/api-common.html#id7 *)
+(* https://docs.taler.net/core/api-common.html#id7
+
+   - numeric values are in network byte order (big endian) *)
 
 module Purpose = struct
   (* defined in gnunet_crypto_lib.h *)
   type t = {
     (* This field equals the number of bytes being signed,
        namely 'sizeof (struct Data)'. *)
-    size: Int32.t; (* = uint32_t *)
+    size: int32; (* = uint32_t *)
     (* This field is used to express the context in
        which the signature is made, ensuring that a
        signature cannot be lifted from one part of the protocol
-       to another. See `src/include/taler/taler_signatures.h` within the
-       exchange's codebase (git://taler.net/exchange). *)
-    purpose: Int32.t; (* = uint32_t *)
+       to another. *)
+    purpose: int32; (* = uint32_t *)
   }
+
+  (* TODO endianess? *)
+  let bin_t =
+    let open Bin in
+    record (fun size purpose -> { size; purpose })
+    |+ field beint32 (fun t -> t.size)
+    |+ field beint32 (fun t -> t.purpose)
+    |> sealr
+
+  let to_string t = Bin.to_string bin_t t
 end
 
 module Data = struct