diff --git a/dune-project b/dune-project
index 1aadd32c..d4fc79ff 100644
--- a/dune-project
+++ b/dune-project
@@ -35,6 +35,7 @@
    bin
    angstrom
    mirage-crypto
+   kdf
    digestif
    duration
    jsont
diff --git a/mte.opam b/mte.opam
index 53820528..3d706521 100644
--- a/mte.opam
+++ b/mte.opam
@@ -22,6 +22,7 @@ depends: [
   "bin"
   "angstrom"
   "mirage-crypto"
+  "kdf"
   "digestif"
   "duration"
   "jsont"
diff --git a/src/crypto.ml b/src/crypto.ml
index 92d1dadb..e43727e0 100644
--- a/src/crypto.ml
+++ b/src/crypto.ml
@@ -64,6 +64,7 @@ module Binary_format_rsa = struct
         pub
     | _ -> assert false
 
+  (* custom private key binary format <> than gcrypt *)
   let priv_to_octets ({ e; d; n; p; q; dp; dq; q' } : Mirage_crypto_pk.Rsa.priv)
       =
     z_array_to_octets [| e; d; n; p; q; dp; dq; q' |]
@@ -259,15 +260,10 @@ end
 module RsaSignature : sig
   type t
 
-  val sign : key:Mirage_crypto_pk.Rsa.priv -> string -> t
   val jsont : t Jsont.t
 end = struct
   type t = string
 
-  (* TODO rsa sign *)
-  let sign ~key s =
-    Mirage_crypto_pk.Rsa.decrypt ~crt_hardening:true ~mask:`Yes ~key s
-
   let jsont =
     let of_b32 s = B32.decode s in
     let to_b32 t = B32.encode t in
@@ -282,3 +278,61 @@ type rsa_priv = RsaPrivateKey.t
 type rsa_pub = RsaPublicKey.t
 type rsa_sig = RsaSignature.t
 type denomination_hash = Hash.DenominationHash.t
+
+(* WIP *)
+module FDH_RSA = struct
+  module Kdf = struct
+    module XTR = Hkdf.Make (Digestif.SHA512)
+    module PRF = Hkdf.Make (Digestif.SHA256)
+
+    let kdf ~xts ~ikm ~ctx ~len =
+      let prk = XTR.extract ~salt:xts ikm in
+      let okm = PRF.expand ~prk ~info:ctx len in
+      okm
+  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 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_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
+    Mirage_crypto_pk.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
+  *)
+end
diff --git a/src/dune b/src/dune
index a9322eed..0a6fa0ea 100644
--- a/src/dune
+++ b/src/dune
@@ -17,6 +17,7 @@
   caqti-driver-pgx
   bin
   mirage-crypto
+  kdf.hkdf
   digestif
   duration
   vif
diff --git a/test/crypto.ml b/test/crypto.ml
index 9b7578e7..bdf2cd95 100644
--- a/test/crypto.ml
+++ b/test/crypto.ml
@@ -73,3 +73,48 @@ let () =
   in
 
   ()
+
+let () =
+  (* kdf *)
+  let salt = "94KPT83PCNS7J83KC5P78Y8" in
+  let ikm = "94KPT83MD1JJ0WV5CDS6AX10D5Q70XBM41NPAY90DNGQ8SBJD5GPR" in
+  let ctx =
+    "94KPT83141HPYVKMCNW78833D1TPWTSC41GPRWVF41NPWVVQDRG62WS04XMPWSKF4WG6JVH0EHM6A82J8S1G"
+  in
+  let out_len = 64 in
+  let out =
+    "GTMR4QT05Z9WF5HKVG0WK9RPXGHSMHJNW377G9GJXCA8B0FEKPF4D27RJMSJZYWSQNTBJ5EYVV7ZW18B48Z0JVJJ80RHB706Y96Q358"
+  in
+
+  let xts = salt |> decode in
+  let ikm = ikm |> decode in
+  let ctx = ctx |> decode in
+  let okm = Crypto.FDH_RSA.Kdf.kdf ~xts ~ikm ~ctx ~len:out_len in
+  let okm = okm |> encode in
+  assert (okm = out);
+
+  ()
+
+let () =
+  let message_hash =
+    "XKQMJ4CNTXBFE1V2WR6JS063J7PZQE4XMB5JH3RS5X0THQ1JQSQ69Y7KDBC9TYRJEZH48MEPY2SF4QHQ4VHXC0YQX5935MQEGP0AX6R"
+  in
+  let rsa_public_key =
+    "040000YRN1NVJ68RS6RJF52PGRCQG19ZKWQPSTJX2G7ZDCKSZFE2VW3HHA81YF5C639JHJF5TX8YTEE2FW2WQCG1PTKNBSPPJEJGA032CN3E8QZ27VWY0K6JFT8ZSYWRH2SKDMXW56A4QKY46JJBWJ6T0ZRVBW6S1HTHXVE2RW8MXRW5T801077MDY13N5F8Z1JZVKBJ06TK3S0YPEDBXK0VEHRHEQJ5X5XYKR4KQTFAZNBMKXY8836VCHBXTK4YNX6AJ1CK29SMJH3Z3QRM16A2TNQGFR0HSMV446BF7FMT2E379ZAT5ST4G3BM2NWZYW545S2SW5MG5S6M88XZZ7SKFD48YVXNZ205GGSEYJPVBMR76WG4ZG30WBCPC1N54XE12RMAG81D8C09WG22PKGGDHYXX68N04002"
+  in
+  let blinding_key_secret =
+    "3SWF49XZPHQMENTSBZQR7Z0B8ZSZ2JRARE79Q4VXZMQ7W6QABXMG"
+  in
+  let blinded_message =
+    "3KHKZJZ30ABB4E56MA2V0EQWGCWH0QQG9P2ZHYHR186C5HZXJMM4N9WXAQTKS94QSV9Y17GGNXN5MB1PZZFG7Q0FY88QPKKRG4MYCPSMTZK5W59R0MJVNJ4P4AQM96TDG5W7RV8GSNR1QQZ1GNHW3CX6D6ZRTMXB2NKB5SSYTDJS79F5ZFBRZ4HVED9JBBPWSR79KVV5QQ4APBGHBCKGMF9NJJS53A1BVYHDEVYAGFYF2SNEP827ZP50FKJ5GKGV8NQ15ESEZ69AT7GJG0T3TZVENY2YN9CVR98W3BKEZ53J7VTANARG8SJS8AMJQ7S23P5HRJ7XE9KTNRNXKH49MXV9JHHYE5535N7AGWEKR47SBCGNF44Z7XJ9RV5BQV12ZRJKN4HBZQHDNCMH3QKX9Z6G64"
+  in
+  let open Crypto in
+  let msg = message_hash |> decode in
+  let pub =
+    rsa_public_key |> decode |> RsaPublicKey.of_octets |> Result.get_ok
+  in
+  let bks = blinding_key_secret |> decode in
+  let s = FDH_RSA.rsa_blind pub ~bks ~msg |> encode in
+  assert (s = blinded_message);
+
+  ()