use Jsont.Error for unsupported CS denom + wip type binary signature

src/bin_signature.ml

@@ -88,7 +88,7 @@ module DenominationKeyAnnouncementPS = struct
   (* TODO taler_signatures purpose
      we use TALER_SIGNATURE_SM_RSA_DENOMINATION_KEY instead here *)
   (* purpose.purpose = TALER_SIGNATURE_SM_DENOMINATION_KEY *)
-  type t = {
+  type r = {
     h_denom_pub: DenominationHash.t;
     h_section_name: Hash_64_cstr.t;
     anchor_time: TimeAbsoluteNBO.t;
@@ -107,6 +107,24 @@ module DenominationKeyAnnouncementPS = struct
     |+ field TimeAbsoluteNBO.bin (fun t -> t.anchor_time)
     |+ field TimeRelativeNBO.bin (fun t -> t.duration_withdraw)
     |> sealr
+
+  module type SIG = sig
+    type t
+
+    val sign : (string -> string) -> r -> t
+    val verify : (string -> msg:string -> bool) -> t -> r -> bool
+    val jsont : t Jsont.t
+  end
+
+  module Sig : SIG = struct
+    type t = string
+
+    let sign f r = f @@ Bin.to_string bin r
+    let verify f s r = f s ~msg:(Bin.to_string bin r)
+
+    (* TODO B32.jsont, for others types too *)
+    let jsont = Jsont.string
+  end
 end
 
 module SigningKeyAnnouncementPS = struct