diff --git a/src/fdh.ml b/src/fdh.ml
index 5d4b2157..31f4c893 100644
--- a/src/fdh.ml
+++ b/src/fdh.ml
@@ -7,3 +7,49 @@ module Kdf = struct
     let okm = PRF.expand ~prk ~info:ctx len in
     okm
 end
+
+(* WIP
+let kdf_mod_n ~n ~xts ~ikm ~ctx =
+  let nbits = Z.numbits n in
+  let rec go ctr =
+    let len = ((nbits - 1) / 8) + 1 in
+    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
+    let r = Z.of_bits okm in
+    (* we do that because n is odd?
+       TODO
+       gcry_mpi_clear_highbit .. *)
+    if Z.gt r n then go (succ ctr) else r
+  in
+  go 0
+
+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 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
+
+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 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
+  let data_r_e = Z.rem (Z.mul data r_e) pub.n in
+  Z.to_bits data_r_e
+  *)