diff --git a/GNUmakefile b/GNUmakefile
index 49a9445f..91548fda 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -28,7 +28,7 @@ pins:
 	opam pin --no-action --yes add caqti-miou "git+https://github.com/swrup/ocaml-caqti.git#7838c29ffa9095d32b95affee26b22ef2ad302fd"
 	opam pin --no-action --yes add caqti-mnet "git+https://github.com/swrup/ocaml-caqti.git#7838c29ffa9095d32b95affee26b22ef2ad302fd"
 	opam pin --no-action --yes add caqti-driver-pgx "git+https://github.com/swrup/ocaml-caqti.git#7838c29ffa9095d32b95affee26b22ef2ad302fd"
-	opam pin --no-action --yes "git+https://github.com/swrup/mfat.git#3a98411c507fb38d9442cbf3fc079bd8be97fee6"
+	opam pin --no-action --yes "git+https://github.com/robur-coop/mfat.git#5b1204d914e853f0139c6d2776511f530b753550"
 
 
 .PHONY: vendors
diff --git a/src/fat.ml b/src/fat.ml
index cb9889ec..4d1cada4 100644
--- a/src/fat.ml
+++ b/src/fat.ml
@@ -1,3 +1,24 @@
+module Path = struct
+  let trim_left s =
+    let s = String.trim s in
+    if String.starts_with ~prefix:"/" s then String.sub s 1 (String.length s - 1)
+    else s
+
+  let trim_right s =
+    let s = String.trim s in
+    if String.ends_with ~suffix:"/" s then String.sub s 0 (String.length s - 1)
+    else s
+
+  let add a b =
+    let a = trim_right a in
+    let b = trim_left b in
+    Fmt.str "%s/%s" a b
+
+  (*   let ( / ) = add *)
+  let compare a b = String.compare (trim_right a) (trim_right b)
+  let equal a b = 0 = compare a b
+end
+
 module Fat = Mfat.Make (struct
   include Mkernel.Block
 
@@ -5,12 +26,10 @@ module Fat = Mfat.Make (struct
   let write = atomic_write
 end)
 
-module Sfn = Mfat.Sfn
-module Spath = Mfat.Spath
 include Fat
 
 type entry = Mfat.entry = {
-  name: Sfn.t;
+  name: string;
   is_dir: bool;
   size: int32;
 }
diff --git a/src/secmod_eddsa.ml b/src/secmod_eddsa.ml
index ed5c63c0..a19bf55f 100644
--- a/src/secmod_eddsa.ml
+++ b/src/secmod_eddsa.ml
@@ -1,6 +1,5 @@
 (* TODO
    ! use lock
-   !? Scanf not thread-safe
    schedule tasks
    sign: check timestamps before signing *)
 (* IMPROVE
@@ -10,7 +9,7 @@
    list_issue_date: save timestamp of key generation
    key validity period:
      more checks + do not exceed lookahead
-   refacto common parts with secmod_eddsa *)
+   refacto common parts with secmod_rsa *)
 let src = Logs.Src.create "mte.secmod_eddsa"
 
 module Log = (val Logs.src_log src : Logs.LOG)
@@ -18,19 +17,12 @@ module Log = (val Logs.src_log src : Logs.LOG)
 (* - *)
 open Syntax
 open Time
-module Sfn = Mfat.Sfn
-module Spath = Mfat.Spath
 
 module Cfg = struct
   include Config.Exchange_secmod_eddsa
 
-  let to_spath s =
-    let open Fat.Spath in
-    match of_string s with Error (`Msg e) -> Fmt.failwith "%s" e | Ok v -> v
-
-  (* 8.3 filenames for FAT *)
-  let key_dir = to_spath "/EDDSA"
-  let sm_key = to_spath "/SM_EDDSA"
+  let key_dir = "/EDDSA"
+  let sm_key = "/SM_EDDSA"
 end
 
 type key = {
@@ -60,25 +52,23 @@ let key_bin =
 
 (* for sm_key only *)
 let read_eddsa fs spath =
-  Log.debug (fun m -> m "reading key file `%a`" Spath.pp spath);
+  Log.debug (fun m -> m "reading key file `%s`" spath);
   let* data = Fat.read fs spath |> unwrap_msg in
   let* priv = Eddsa.priv_of_octets data in
   let pub = Eddsa.pub_of_priv priv in
   Ok (priv, pub)
 
 let write_eddsa fs spath priv =
-  Log.debug (fun m -> m "writing key file `%a`" Spath.pp spath);
+  Log.debug (fun m -> m "writing key file `%s`" spath);
   let data = Eddsa.priv_to_octets priv in
   Fat.write fs spath data |> unwrap_msg
 
 let key_spath k =
-  let sfn_res = String.sub (Eddsa.pub_to_b32 k.pub) 0 8 |> Sfn.of_string in
-  match sfn_res with
-  | Error _ -> failwith "not possible"
-  | Ok sfn -> Spath.(Cfg.key_dir / sfn)
+  let sfn = String.sub (Eddsa.pub_to_b32 k.pub) 0 8 in
+  Fat.Path.add Cfg.key_dir sfn
 
 let read_key fs spath =
-  Log.debug (fun m -> m "reading key file `%a`" Spath.pp spath);
+  Log.debug (fun m -> m "reading key file `%s`" spath);
   let* data = Fat.read fs spath |> unwrap_msg in
   (* todo: catch failure *)
   let k = Bin.decode key_bin data (ref 0) in
@@ -86,12 +76,12 @@ let read_key fs spath =
 
 let write_key fs k =
   let spath = key_spath k in
-  Log.debug (fun m -> m "writing key file `%a`" Spath.pp spath);
+  Log.debug (fun m -> m "writing key file `%s`" spath);
   let data = Bin.to_string key_bin k in
   Fat.write fs spath data |> unwrap_msg
 
 let delete_file fs spath =
-  Log.debug (fun m -> m "delete key file `%a`" Spath.pp spath);
+  Log.debug (fun m -> m "delete key file `%s`" spath);
   let+ () = Fat.remove fs spath |> unwrap_msg in
   ()
 
@@ -137,8 +127,8 @@ let load fs =
     else Fat.mkdir fs Cfg.key_dir |> unwrap_msg
   in
   let* l = Fat.ls fs Cfg.key_dir |> unwrap_msg in
-  let l = List.map (fun entry -> Spath.add Cfg.key_dir entry.Fat.name) l in
-  let l = List.filter (fun spath -> not @@ Spath.equal spath Cfg.sm_key) l in
+  let l = List.map (fun entry -> Fat.Path.add Cfg.key_dir entry.Fat.name) l in
+  let l = List.filter (fun spath -> not @@ String.equal spath Cfg.sm_key) l in
   let* keys = list_map (read_key fs) l in
   match keys with
   | [] -> Ok None
diff --git a/src/secmod_rsa.ml b/src/secmod_rsa.ml
index 5b198aa9..de386492 100644
--- a/src/secmod_rsa.ml
+++ b/src/secmod_rsa.ml
@@ -5,8 +5,6 @@ module Log = (val Logs.src_log src : Logs.LOG)
 (* - *)
 open Syntax
 open Time
-module Sfn = Mfat.Sfn
-module Spath = Mfat.Spath
 module DenominationHash = Hash.DenominationHash
 
 module Coin_config = struct
@@ -27,12 +25,8 @@ end
 module Cfg = struct
   include Config.Exchange_secmod_rsa
 
-  let to_spath s =
-    let open Fat.Spath in
-    match of_string s with Error (`Msg e) -> Fmt.failwith "%s" e | Ok v -> v
-
-  let key_dir = to_spath "/RSA"
-  let sm_key = to_spath "/SM_RSA"
+  let key_dir = "/RSA"
+  let sm_key = "/SM_RSA"
   let coin_config_list = List.map Coin_config.of_coin Config.Coin.all_coins
 
   let get_coin_config ~section_name =
@@ -74,39 +68,35 @@ let key_bin =
   |> sealr
 
 let key_spath k =
-  let sfn_res =
-    String.sub (DenominationHash.to_b32 k.h_pub) 0 8 |> Sfn.of_string
-  in
-  match sfn_res with
-  | Error _ -> failwith "not possible"
-  | Ok sfn -> Spath.(Cfg.key_dir / sfn)
+  let sfn = String.sub (DenominationHash.to_b32 k.h_pub) 0 8 in
+  Fat.Path.add Cfg.key_dir sfn
 
 let read_eddsa fs spath =
-  Log.debug (fun m -> m "reading key file `%a`" Spath.pp spath);
+  Log.debug (fun m -> m "reading key file `%s`" spath);
   let* data = Fat.read fs spath |> unwrap_msg in
   let* priv = Eddsa.priv_of_octets data in
   let pub = Eddsa.pub_of_priv priv in
   Ok (priv, pub)
 
 let write_eddsa fs spath priv =
-  Log.debug (fun m -> m "writing key file `%a`" Spath.pp spath);
+  Log.debug (fun m -> m "writing key file `%s`" spath);
   let data = Eddsa.priv_to_octets priv in
   Fat.write fs spath data |> unwrap_msg
 
 let read_key fs spath =
-  Log.debug (fun m -> m "reading key file `%a`" Spath.pp spath);
+  Log.debug (fun m -> m "reading key file `%s`" spath);
   let* data = Fat.read fs spath |> unwrap_msg in
   let k = Bin.decode key_bin data (ref 0) in
   Ok k
 
 let write_key fs k =
   let spath = key_spath k in
-  Log.debug (fun m -> m "writing key file `%a`" Spath.pp spath);
+  Log.debug (fun m -> m "writing key file `%s`" spath);
   let data = Bin.to_string key_bin k in
   Fat.write fs spath data |> unwrap_msg
 
 let delete_file fs spath =
-  Log.debug (fun m -> m "delete key file `%a`" Spath.pp spath);
+  Log.debug (fun m -> m "delete key file `%s`" spath);
   let+ () = Fat.remove fs spath |> unwrap_msg in
   ()
 
@@ -156,8 +146,8 @@ let load fs =
     else Fat.mkdir fs Cfg.key_dir |> unwrap_msg
   in
   let* l = Fat.ls fs Cfg.key_dir |> unwrap_msg in
-  let l = List.map (fun entry -> Spath.add Cfg.key_dir entry.Fat.name) l in
-  let l = List.filter (fun spath -> not @@ Spath.equal spath Cfg.sm_key) l in
+  let l = List.map (fun entry -> Fat.Path.add Cfg.key_dir entry.Fat.name) l in
+  let l = List.filter (fun spath -> not @@ String.equal Cfg.sm_key spath) l in
   let* keys = list_map (read_key fs) l in
   match keys with
   | [] -> Ok None