diff --git a/src/assets.ml b/src/assets.ml
index 1826b27e..9313ee49 100644
--- a/src/assets.ml
+++ b/src/assets.ml
@@ -1,5 +1,4 @@
 (* https://docs.taler.net/design-documents/003-tos-rendering.html
-   https://docs.taler.net/design-documents/003-tos-rendering.html
 
    must support `text/plain` and `text/markdown` *)
 
diff --git a/src/crypto.ml b/src/crypto.ml
index e43727e0..6e2d12fa 100644
--- a/src/crypto.ml
+++ b/src/crypto.ml
@@ -281,58 +281,101 @@ type denomination_hash = Hash.DenominationHash.t
 
 (* WIP *)
 module FDH_RSA = struct
+  open Mirage_crypto_pk
+
   module Kdf = struct
     module XTR = Hkdf.Make (Digestif.SHA512)
     module PRF = Hkdf.Make (Digestif.SHA256)
 
-    let kdf ~xts ~ikm ~ctx ~len =
+    let kdf =
+     fun ~xts ~ikm ~ctx ~len ->
       let prk = XTR.extract ~salt:xts ikm in
       let okm = PRF.expand ~prk ~info:ctx len in
       okm
+
+    let kdf_mod_n ~n ~xts ~ikm ~ctx =
+      let nbits = Z.numbits n in
+      let len = ((nbits - 1) / 8) + 1 in
+      assert (8 * len = nbits);
+      let rec go ctr =
+        (* cat ctx ctr_be *)
+        let ctx =
+          let ctx_len = String.length ctx in
+          let b = Bytes.create (ctx_len + 2) in
+          Bytes.blit_string ctx 0 b 0 ctx_len;
+          Bytes.set_uint16_be b ctx_len ctr;
+          Bytes.unsafe_to_string b
+        in
+        let okm = kdf ~xts ~ikm ~ctx ~len in
+        assert (String.length okm = len);
+        let r = Z_extra.of_octets_be okm in
+        if Z.gt r n then go (succ ctr) else r
+      in
+      go 0
   end
 
-  let kdf_mod_n ~n ~xts ~ikm ~ctx =
-    let nbits = Z.numbits n in
-    let len = ((nbits - 1) / 8) + 1 in
-    assert (8 * len = nbits);
-    let rec go ctr =
-      let ctr_be =
-        let b = Bytes.create 2 in
-        Bytes.set_uint16_be b 0 ctr;
-        Bytes.unsafe_to_string b
-      in
-      let ctx = String.cat ctx ctr_be in
-      let okm = Kdf.kdf ~xts ~ikm ~ctx ~len in
-      assert (String.length okm = len);
-      let r = Mirage_crypto_pk.Z_extra.of_octets_be okm in
-      if Z.gt r n then go (succ ctr) else r
-    in
-    go 0
+  let gcd_validate r n =
+    match Z.equal (Z.gcd r n) Z.one with
+    | true -> ()
+    | false -> Fmt.failwith "RSA key is malicious"
 
   let rsa_full_domain_hash pub msg =
     let xts = RsaPublicKey.to_octets pub in
     let ctx = "RSA-FDA FTpsW!" in
-    let r = kdf_mod_n ~n:pub.n ~xts ~ikm:msg ~ctx in
-    r
+    let r = Kdf.kdf_mod_n ~n:pub.n ~xts ~ikm:msg ~ctx in
+    gcd_validate r pub.n; r
 
   let rsa_blinding_key_derive (pub : RsaPublicKey.t) bks =
     let xts = "Blinding KDF extractor HMAC key" in
     let ctx = "Blinding KDF" in
-    let r = kdf_mod_n ~n:pub.n ~xts ~ikm:bks ~ctx in
-    r
+    let r = Kdf.kdf_mod_n ~n:pub.n ~xts ~ikm:bks ~ctx in
+    gcd_validate r pub.n; r
 
   let rsa_blind pub ~bks ~msg =
     let data = rsa_full_domain_hash pub msg in
     let bkey = rsa_blinding_key_derive pub bks in
-    let r_e = Z.powm bkey pub.e pub.n in
+    (* can we just use [powm] here instead? *)
+    let r_e = Z.powm_sec bkey pub.e pub.n in
     let data_r_e = Z.rem (Z.mul data r_e) pub.n in
-    Mirage_crypto_pk.Z_extra.to_octets_be data_r_e
+    Z_extra.to_octets_be data_r_e
 
-  (*
-let rsa_sign_fdh priv msg =
-  let pub = Mirage_crypto_pk.Rsa.pub_of_priv priv in
-  let v = rsa_full_domain_hash pub msg in
-  let v = Z.to_bits v in
-  Mirage_crypto_pk.Rsa.PKCS1.sig_encode ~crt_hardening:true ~key:priv v
-  *)
+  (* -- WIP crypto -- *)
+
+  (* TODO crypto
+     not sure about signature scheme used by taler
+     libgnunetutil crypto_rsa.c use "(flags raw)" => no padding *)
+  (* decrypt <=> sign *)
+  let rsa_sign_z priv r =
+    let data = Z_extra.to_octets_be r in
+    Rsa.decrypt ~crt_hardening:true ~key:priv data
+
+  (* TODO crypto
+     look into mirage-crypto for this
+     use Eqaf for constant time string compare *)
+  let rsa_verify_z pub r sig_ =
+    let data = Z_extra.to_octets_be r in
+    let sig_' = Rsa.encrypt ~key:pub data in
+    match String.equal sig_ sig_' with
+    | false -> Fmt.error "RSA signature verification failed"
+    | true -> Ok ()
+
+  let rsa_sign_fdh priv msg =
+    let pub = Rsa.pub_of_priv priv in
+    let r = rsa_full_domain_hash pub msg in
+    rsa_sign_z priv r
+
+  let rsa_unblind pub ~bks ~sig_ =
+    let bkey = rsa_blinding_key_derive pub bks in
+    let r_inv =
+      try Z.invert bkey pub.n
+      with Division_by_zero ->
+        (* => gcd(r,n) <> 1, should be already checked for *)
+        assert false
+    in
+    let ubsig = Z.rem (Z.mul sig_ r_inv) pub.n in
+    ubsig
+
+  let rsa_verify pub ~msg ~sig_ =
+    let r = rsa_full_domain_hash pub msg in
+    rsa_verify_z pub r sig_
 end
diff --git a/test/crypto.ml b/test/crypto.ml
index bdf2cd95..0dbf2a02 100644
--- a/test/crypto.ml
+++ b/test/crypto.ml
@@ -96,6 +96,8 @@ let () =
   ()
 
 let () =
+  (* rsa blinding *)
+  (* rsa_private_key data is given in gcrypt sexpr format.. *)
   let message_hash =
     "XKQMJ4CNTXBFE1V2WR6JS063J7PZQE4XMB5JH3RS5X0THQ1JQSQ69Y7KDBC9TYRJEZH48MEPY2SF4QHQ4VHXC0YQX5935MQEGP0AX6R"
   in