diff --git a/src/api.ml b/src/api.ml
index a7678cf4..058a2a59 100644
--- a/src/api.ml
+++ b/src/api.ml
@@ -154,7 +154,7 @@ module FutureDenom = struct
     fee_deposit: Amount.t;
     fee_refresh: Amount.t;
     fee_refund: Amount.t;
-    denom_secmod_sig: EddsaSignature.t;
+    denom_secmod_sig: Bin_signature.DenominationKeyAnnouncementPS.Sig.t;
   }
 
   let jsont =
@@ -201,7 +201,9 @@ module FutureDenom = struct
     |> mem "fee_deposit" Amount.jsont ~enc:fee_deposit
     |> mem "fee_refresh" Amount.jsont ~enc:fee_refresh
     |> mem "fee_refund" Amount.jsont ~enc:fee_refund
-    |> mem "denom_secmod_sig" EddsaSignature.jsont ~enc:denom_secmod_sig
+    |> mem "denom_secmod_sig"
+         Bin_signature.DenominationKeyAnnouncementPS.Sig.jsont
+         ~enc:denom_secmod_sig
     |> finish
 end
 
diff --git a/src/bin_signature.ml b/src/bin_signature.ml
index 84fdbac4..0bd95eaa 100644
--- a/src/bin_signature.ml
+++ b/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 use B32.jsont, for others types too *)
+    let jsont = Jsont.string
+  end
 end
 
 module SigningKeyAnnouncementPS = struct
diff --git a/src/crypto.ml b/src/crypto.ml
index 249d051e..8fda65f3 100644
--- a/src/crypto.ml
+++ b/src/crypto.ml
@@ -52,6 +52,7 @@ module EddsaSignature : sig
   type t
 
   val sign : key:Mirage_crypto_ec.Ed25519.priv -> string -> t
+  val sign_as_string : key:Mirage_crypto_ec.Ed25519.priv -> string -> string
   val of_b32 : string -> (t, string) result
   val to_b32 : t -> string
   val to_octets : t -> string
@@ -81,6 +82,8 @@ end = struct
     (* mirage_crypto: "The result is the concatenation of r and s, as specified in RFC 8032." *)
     Mirage_crypto_ec.Ed25519.sign ~key s
 
+  let sign_as_string ~key s = sign ~key s
+
   let check_size t =
     match String.length t = 64 with
     | false -> Error "EddsaSignature: invalid string length"
diff --git a/src/management.ml b/src/management.ml
index c338635d..1aff93fe 100644
--- a/src/management.ml
+++ b/src/management.ml
@@ -31,8 +31,8 @@ let mk_future_denom denom_key_signf
     let duration_withdraw =
       Timestamp.diff stamp_start stamp_expire_withdraw |> Timestamp.of_span_exn
     in
-    let ps = { h_denom_pub; h_section_name; anchor_time; duration_withdraw } in
-    ps |> Bin.to_string bin |> denom_key_signf
+    Sig.sign denom_key_signf
+      { h_denom_pub; h_section_name; anchor_time; duration_withdraw }
   in
   FutureDenom.
     {
@@ -60,8 +60,9 @@ let mk_future_signkey signkey_signf
     let duration =
       Timestamp.diff stamp_start stamp_expire |> Timestamp.of_span_exn
     in
-    let ps = { exchange_pub; anchor_time; duration } in
-    ps |> Bin.to_string bin |> signkey_signf
+    { exchange_pub; anchor_time; duration }
+    |> Bin.to_string bin
+    |> signkey_signf
   in
   FutureSignKey.
     { key= pub; stamp_start; stamp_expire; stamp_end; signkey_secmod_sig }
@@ -73,7 +74,8 @@ let mk_future_keys_response (secmod_signkey : Secmod_signkey.t)
     |> List.filter (fun k -> Option.is_none k.Denomination.master_sig)
     |> List.map (fun denom ->
         let signf s =
-          Crypto.EddsaSignature.sign ~key:secmod_denom.sm_key.Signkey.priv s
+          Crypto.EddsaSignature.sign_as_string
+            ~key:secmod_denom.sm_key.Signkey.priv s
         in
         mk_future_denom signf denom)
   in