From 74dfe2da068eab508d5619fbe606e71129100809 Mon Sep 17 00:00:00 2001
From: swrup <swrup@protonmail.com>
Date: Tue, 24 Feb 2026 21:03:47 +0100
Subject: [PATCH]

---
 src/fdh_rsa.ml      | 20 +++++++++-----------
 test/test_crypto.ml | 12 +++++++++---
 2 files changed, 18 insertions(+), 14 deletions(-)

diff --git a/src/fdh_rsa.ml b/src/fdh_rsa.ml
index 011ab302..2b9631a7 100644
--- a/src/fdh_rsa.ml
+++ b/src/fdh_rsa.ml
@@ -64,7 +64,12 @@ let rsa_blind pub ~bks ~msg =
   let data_r_e = Z.rem (Z.mul data r_e) pub.n in
   Z_extra.to_octets_be data_r_e
 
+(* decrypt <=> sign *)
+let rsa_sign_blinded priv ~msg_blind =
+  Mirage_crypto_pk.Rsa.decrypt ~crt_hardening:true ~key:priv msg_blind
+
 let rsa_unblind pub ~bks ~sig_ =
+  let data = Z_extra.of_octets_be sig_ in
   let bkey = rsa_blinding_key_derive pub bks in
   let r_inv =
     try Z.invert bkey pub.n
@@ -72,22 +77,15 @@ let rsa_unblind pub ~bks ~sig_ =
       (* => 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_sign_fdh priv msg =
-  let pub = RsaPrivateKey.pub_of_priv priv in
-  let r = rsa_full_domain_hash pub msg in
-  let data = Z_extra.to_octets_be r in
-  (* decrypt <=> sign *)
-  Mirage_crypto_pk.Rsa.decrypt ~crt_hardening:true ~key:priv data
+  let data = Z.rem (Z.mul data r_inv) pub.n in
+  Z_extra.to_octets_be data
 
 (* TODO crypto
    look into mirage-crypto for this
    at least use Eqaf for constant time string compare *)
 let rsa_verify pub ~msg ~sig_ =
-  let r = rsa_full_domain_hash pub msg in
-  let s1 = Z_extra.to_octets_be r in
+  let msg_fdh = rsa_full_domain_hash pub msg in
+  let s1 = Z_extra.to_octets_be msg_fdh in
   let s2 = Mirage_crypto_pk.Rsa.encrypt ~key:pub sig_ in
   match String.equal s1 s2 with
   | false -> Fmt.error "RSA signature verification failed"
diff --git a/test/test_crypto.ml b/test/test_crypto.ml
index ec16fa9c..62d467e2 100644
--- a/test/test_crypto.ml
+++ b/test/test_crypto.ml
@@ -99,12 +99,18 @@ let () =
     "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);
+  let msg = decode message_hash in
+  let msg_blind = Fdh_rsa.rsa_blind pub ~bks ~msg in
+  assert (encode msg_blind = blinded_message);
+  let msg_fdh =
+    Mirage_crypto_pk.Z_extra.to_octets_be
+    @@ Fdh_rsa.rsa_full_domain_hash pub msg
+  in
+  let msg_fdh' = Fdh_rsa.rsa_unblind pub ~bks ~sig_:msg_blind in
+  assert (msg_fdh' = msg_fdh);
 
   ()