diff --git a/src/binary_formats.ml b/src/binary_formats.ml
index 6d42bd03..d833311f 100644
--- a/src/binary_formats.ml
+++ b/src/binary_formats.ml
@@ -2,6 +2,8 @@
 
    - numeric values are in network byte order (big endian) *)
 
+open Include
+
 (* TODO
    less boilerplate?
      - type t abstract
@@ -272,7 +274,8 @@ module Token_public_key_p = MAKE_EDDSA_PUB ()
 
 (* -- Signatures -- *)
 
-module Ecc_signature_purpose = struct
+(* EccSignaturePurpose *)
+module Purpose = struct
   type t = {
     (* This field equals the number of bytes being signed,
        namely 'sizeof (struct Data)'. *)
@@ -284,6 +287,8 @@ module Ecc_signature_purpose = struct
     purpose: int32;
   }
 
+  let size = 4 + 4
+
   let bin =
     let open Bin in
     record (fun size purpose -> { size; purpose })
@@ -291,7 +296,7 @@ module Ecc_signature_purpose = struct
     |+ field beint32 (fun t -> t.purpose)
     |> sealr
 
-  let size = 4 + 4
+  let make size purpose = { size= Int32.of_int size; purpose }
 end
 
 (* This is the running SHA512-hash over all
@@ -418,7 +423,7 @@ module Withdraw_request_ps = struct
   type t = {
     (* TODO set it in encode/decode *)
     (* Purpose is #TALER_SIGNATURE_WALLET_RESERVE_WITHDRAW *)
-    purpose: Ecc_signature_purpose.t;
+    purpose: Purpose.t;
     (* Amount to withdraw, excluding fees, i.e.
        the total sum of the denominations of the coins.
        Note that the reserve must have a value of at least amount+fee. *)
@@ -472,7 +477,7 @@ module Withdraw_request_ps = struct
           max_age_group;
           mask;
         })
-    |+ field Ecc_signature_purpose.bin (fun t -> t.purpose)
+    |+ field Purpose.bin (fun t -> t.purpose)
     |+ field Amount.bin (fun t -> t.amount)
     |+ field Amount.bin (fun t -> t.fee)
     |+ field Hash_planchets_p.bin (fun t -> t.h_planchets)
@@ -483,10 +488,9 @@ module Withdraw_request_ps = struct
 end
 
 module Withdraw_confirmation_ps = struct
+  (* Purpose is #TALER_SIGNATURE_EXCHANGE_CONFIRM_WITHDRAW.
+     Signed by a `struct TALER_ExchangePrivateKeyP` using EdDSA. *)
   type t = {
-    (* Purpose is #TALER_SIGNATURE_EXCHANGE_CONFIRM_WITHDRAW.
-       Signed by a `struct TALER_ExchangePrivateKeyP` using EdDSA. *)
-    (*purpose: Ecc_signature_purpose.t;*)
     (* Commitment made in the /withdraw request.
        Also needed for the /reveal-withdraw endpoint (in case
        of required proof of age restriction) and for /recoup *)
@@ -502,20 +506,14 @@ module Withdraw_confirmation_ps = struct
     noreveal_index: int32;
   }
 
-  let size = Ecc_signature_purpose.size + Hash_planchets_p.size + 4
-
-  let purpose =
-    Ecc_signature_purpose.
-      {
-        size= Int32.of_int size;
-        purpose= Include.Taler_signatures.exchange_confirm_withdraw;
-      }
+  let size = Purpose.size + Hash_planchets_p.size + 4
+  let purpose = Purpose.make size Taler_signatures.exchange_confirm_withdraw
 
   let bin =
     let open Bin in
     record (fun _purpose h_planchets noreveal_index ->
         { h_planchets; noreveal_index })
-    |+ field Ecc_signature_purpose.bin (Fun.const purpose)
+    |+ field Purpose.bin (Fun.const purpose)
     |+ field Hash_planchets_p.bin (fun t -> t.h_planchets)
     |+ field beint32 (fun t -> t.noreveal_index)
     |> sealr