diff --git a/src/api.ml b/src/api.ml
index f2fbf58e..18903c50 100644
--- a/src/api.ml
+++ b/src/api.ml
@@ -340,3 +340,33 @@ module SignkeyRevocationSignature = struct
     |> mem "master_sig" MasterSigningKeyRevocationPS.jsont ~enc
     |> finish
 end
+
+module AuditorSetupMessage = struct
+  type t = {
+    auditor_url: string;
+    auditor_name: string;
+    auditor_pub: EddsaPublicKey.t;
+    master_sig: MasterAddAuditorPS.t;
+    (* TODO time
+       something about using monotonic system time here! *)
+    validity_start: Timestamp.t;
+  }
+
+  let jsont =
+    let make auditor_url auditor_name auditor_pub master_sig validity_start =
+      { auditor_url; auditor_name; auditor_pub; master_sig; validity_start }
+    in
+    let auditor_url v = v.auditor_url in
+    let auditor_name v = v.auditor_name in
+    let auditor_pub v = v.auditor_pub in
+    let master_sig v = v.master_sig in
+    let validity_start v = v.validity_start in
+    let open Jsont.Object in
+    map ~kind:"AuditorSetupMessage" make
+    |> mem "auditor_url" Jsont.string ~enc:auditor_url
+    |> mem "auditor_name" Jsont.string ~enc:auditor_name
+    |> mem "auditor_pub" EddsaPublicKey.jsont ~enc:auditor_pub
+    |> mem "master_sig" MasterAddAuditorPS.jsont ~enc:master_sig
+    |> mem "validity_start" Timestamp.jsont ~enc:validity_start
+    |> finish
+end
diff --git a/src/bin_signature.ml b/src/bin_signature.ml
index 76b6bc2b..ebd5d006 100644
--- a/src/bin_signature.ml
+++ b/src/bin_signature.ml
@@ -245,6 +245,31 @@ module MasterSigningKeyRevocationPS = struct
   include MK (R)
 end
 
+module MasterAddAuditorPS = struct
+  module R = struct
+    (* purpose.purpose = TALER_SIGNATURE_MASTER_ADD_AUDITOR *)
+    type r = {
+      start_date: TimeAbsoluteNBO.t;
+      auditor_pub: AuditorPublicKeyP.t;
+      h_auditor_url: Hash_64_cstr.t;
+    }
+
+    let bin =
+      let open Bin in
+      Purpose.make_bin Taler_signatures.master_add_auditor @@ fun purpose ->
+      record (fun _purpose start_date auditor_pub h_auditor_url ->
+          { start_date; auditor_pub; h_auditor_url })
+      |+ Purpose.field purpose
+      |+ field TimeAbsoluteNBO.bin (fun t -> t.start_date)
+      |+ field AuditorPublicKeyP.bin (fun t -> t.auditor_pub)
+      |+ field Hash_64_cstr.bin (fun t -> t.h_auditor_url)
+      |> sealr
+  end
+
+  include R
+  include MK (R)
+end
+
 (* ### BIN IMPL END ### *)
 
 module WithdrawRequestPS = struct
@@ -749,15 +774,6 @@ module CoinPurseRefundConfirmationPS = struct
   }
 end
 
-module MasterAddAuditorPS = struct
-  (* purpose.purpose = TALER_SIGNATURE_MASTER_ADD_AUDITOR *)
-  type t = {
-    start_date: TimeAbsoluteNBO.t;
-    auditor_pub: AuditorPublicKeyP.t;
-    h_auditor_url: Hash_64_cstr.t;
-  }
-end
-
 module MasterDelAuditorPS = struct
   (* purpose.purpose = TALER_SIGNATURE_MASTER_DEL_AUDITOR *)
   type t = {