diff --git a/src/binary_formats.ml b/src/binary_formats.ml
index 88f18f80..fe66ef4e 100644
--- a/src/binary_formats.ml
+++ b/src/binary_formats.ml
@@ -4,7 +4,8 @@
 
 open Include
 
-(* TODO test *)
+let int32_size = 4
+let int64_size = 8
 
 (* -- Time -- *)
 
@@ -13,6 +14,8 @@ module Time = struct
     type t = { timestamp_us: int64 }
     type t_nbo = { abs_value_us__: int64 }
 
+    let size = int64_size
+
     (* not BE here? never used? *)
     let bin =
       let open Bin in
@@ -31,6 +34,8 @@ module Time = struct
     type t = { timestamp_us: int64 }
     type t_nbo = { rel_value_us__: int64 }
 
+    let size = int64_size
+
     let bin =
       let open Bin in
       record (fun timestamp_us -> { timestamp_us })
@@ -142,24 +147,18 @@ module Exchange_signature_p = MK_64 ()
 module Master_public_key_p = MK_32 ()
 module Master_private_key_p = MK_32 ()
 module Master_signature_p = MK_64 ()
-
-module Wire_transfert_identifier_raw_p = struct
-  (* uint8_t raw[32]; *)
-  type t = { raw: string }
-
-  let bin =
-    let open Bin in
-    record (fun raw -> { raw }) |+ field (bytes 32) (fun t -> t.raw) |> sealr
-end
+module Wire_transfert_identifier_raw_p = MK_BASIC_32 ()
 
 module UUID = struct
   (* uint32_t value[4]; *)
   type t = { value: string }
 
+  let size = 4 * int32_size
+
   let bin =
     let open Bin in
     record (fun value -> { value })
-    |+ field (bytes (4 * 4)) (fun t -> t.value)
+    |+ field (bytes size) (fun t -> t.value)
     |> sealr
 end
 
@@ -167,11 +166,11 @@ module Wad_id = struct
   (* uint32_t value[6]; *)
   type t = { raw: string }
 
+  let size = 6 * int32_size
+
   let bin =
     let open Bin in
-    record (fun raw -> { raw })
-    |+ field (bytes (4 * 6)) (fun t -> t.raw)
-    |> sealr
+    record (fun raw -> { raw }) |+ field (bytes size) (fun t -> t.raw) |> sealr
 end
 
 (* TODO not sure what to do of union, probably not needed *)
@@ -227,7 +226,7 @@ module Purpose = struct
     purpose: int32;
   }
 
-  let size = 4 + 4
+  let size = 2 * int32_size
 
   let bin =
     let open Bin in
@@ -261,6 +260,8 @@ module Hash_planchets_p = MK_64 ()
 module Age_mask = struct
   type t = { mask: int32 }
 
+  let size = int32_size
+
   let bin =
     let open Bin in
     record (fun mask -> { mask }) |+ field beint32 (fun t -> t.mask) |> sealr
@@ -331,6 +332,8 @@ module Amount = struct
     currency: string;
   }
 
+  let size = int64_size + int32_size + currency_len
+
   (* TODO BE here? *)
   let bin =
     let open Bin in
@@ -348,6 +351,8 @@ module AmountNBO = struct
     currency: string;
   }
 
+  let size = int64_size + int32_size + currency_len
+
   let bin =
     let open Bin in
     record (fun value fraction currency -> { value; fraction; currency })
@@ -357,6 +362,8 @@ module AmountNBO = struct
     |> sealr
 end
 
+module Blinding_master_seed = MK_BASIC_32 ()
+
 (* Format used for to generate the signature on a request to withdraw
    coins from a reserve. *)
 module Withdraw_request_ps = struct
@@ -390,7 +397,7 @@ module Withdraw_request_ps = struct
        or all zeros, if no denomination of cipher type Clause-Schnorr is used. *)
     (* TODO TALER doc
        `TALER_BlindingMasterSecretP` in doc, but probably TALER_BlindingMasterSeed *)
-    blinding_seed: string;
+    blinding_seed: Blinding_master_seed.t;
     (* If age restriction proof is required, the maximum age _group_
        to commit to, 0 otherwise. Note that in this case, all
        denominations for all coins MUST support age restriction.
@@ -404,6 +411,14 @@ module Withdraw_request_ps = struct
     mask: Age_mask.t;
   }
 
+  let size =
+    Purpose.size
+    + (2 * Amount.size)
+    + Hash_planchets_p.size
+    + Blinding_master_seed.size
+    + int32_size
+    + Age_mask.size
+
   let bin =
     let open Bin in
     record
@@ -421,7 +436,7 @@ module Withdraw_request_ps = struct
     |+ field Amount.bin (fun t -> t.amount)
     |+ field Amount.bin (fun t -> t.fee)
     |+ field Hash_planchets_p.bin (fun t -> t.h_planchets)
-    |+ field cstring (fun t -> t.blinding_seed)
+    |+ field Blinding_master_seed.bin (fun t -> t.blinding_seed)
     |+ field beint32 (fun t -> t.max_age_group)
     |+ field Age_mask.bin (fun t -> t.mask)
     |> sealr
@@ -446,7 +461,7 @@ module Withdraw_confirmation_ps = struct
     noreveal_index: int32;
   }
 
-  let size = Purpose.size + Hash_planchets_p.size + 4
+  let size = Purpose.size + Hash_planchets_p.size + int32_size
   let purpose = Purpose.make size Taler_signatures.exchange_confirm_withdraw
 
   let bin =