untested: rsa pub binary format

src/binary_formats.ml

@@ -22,6 +22,61 @@
      exchange and gana master branch are not in sync
      and we should use a specific git tag instead *)
 
+(* -- Crypto keys -- *)
+
+module RsaPublicKey = struct
+  (* libgnuutil format:
+     https://docs.gnunet.org/doxygen/d9/dbe/structGNUNET__CRYPTO__RsaPublicKeyHeaderP.html
+     https://docs.gnunet.org/doxygen/d6/d70/group__libgnunetutil.html#ga9c99a81e8cd649c1c925d23211a0738f
+     https://www.gnupg.org/documentation/manuals/gcrypt/MPI-formats.html
+
+    format:
+     - rsa header
+     - modulus
+     - public_exponent
+
+    integer in big-endian format (MSB first).
+    Leading zeroes are stripped unless they are required to keep a value positive.
+  *)
+  type header = {
+    n_len: int;
+    e_len: int;
+  }
+
+  type t = {
+    header: header;
+    n: Z.t;
+    e: Z.t;
+  }
+
+  (* need to strip leading zeros or something? *)
+  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
+
+  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
+         Z.to_bits is in little endian but we need it in big endian *)
+    |+ field cstring (fun t -> z_to_bigendian_bits t.n)
+    |+ field cstring (fun t -> z_to_bigendian_bits t.e)
+    |> sealr
+end
+
 open Include
 
 let int32_size = 4