diff --git a/src/crypto.ml b/src/crypto.ml
index 92d1dadb..8b6ab805 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 +278,22 @@ type rsa_priv = RsaPrivateKey.t
 type rsa_pub = RsaPublicKey.t
 type rsa_sig = RsaSignature.t
 type denomination_hash = Hash.DenominationHash.t
+
+let rsa_full_domain_hash pub msg =
+  let xts = RsaPublicKey.to_octets pub in
+  let ctx = "RSA-FDA FTpsW!" in
+  let r = Fdh.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 = Fdh.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
diff --git a/src/fdh.ml b/src/fdh.ml
index 5d4b2157..f9fbd3ca 100644
--- a/src/fdh.ml
+++ b/src/fdh.ml
@@ -7,3 +7,29 @@ module Kdf = struct
     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_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
+  *)
diff --git a/test/crypto.ml b/test/crypto.ml
index 0d09e8a8..3facd609 100644
--- a/test/crypto.ml
+++ b/test/crypto.ml
@@ -94,3 +94,27 @@ let () =
   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 = rsa_blind pub ~bks ~msg |> encode in
+  assert (s = blinded_message);
+
+  ()