From b85c8d6645039fc9d403791750510e439731d479 Mon Sep 17 00:00:00 2001
From: Werner Koch <wk@gnupg.org>
Date: Mon, 31 Aug 2015 23:13:27 +0200
Subject: [PATCH] rsa: Add verify after sign to avoid Lenstra's CRT attack.

* cipher/rsa.c (rsa_sign): Check the CRT.
--

Failures in the computation of the CRT (e.g. due faulty hardware) can
lead to a leak of the private key.  The standard precaution against
this is to verify the signature after signing.  GnuPG does this itself
and even has an option to disable this.  However, the low performance
impact of this extra precaution suggest that it should always be done
and Libgcrypt is the right place here.  For decryption is not done
because the application will detect the failure due to garbled
plaintext and in any case no key derived material will be send to the
user.

Signed-off-by: Werner Koch <wk@gnupg.org>
---
 cipher/rsa.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

Index: libgcrypt-1.5.4/cipher/rsa.c
===================================================================
--- libgcrypt-1.5.4.orig/cipher/rsa.c	2014-08-07 17:21:31.000000000 +0200
+++ libgcrypt-1.5.4/cipher/rsa.c	2015-09-08 11:20:27.063040413 +0200
@@ -996,6 +996,8 @@ static gcry_err_code_t
 rsa_sign (int algo, gcry_mpi_t *resarr, gcry_mpi_t data, gcry_mpi_t *skey)
 {
   RSA_secret_key sk;
+  RSA_public_key pk;
+  gcry_mpi_t result = NULL;
 
   (void)algo;
 
@@ -1008,6 +1010,19 @@ rsa_sign (int algo, gcry_mpi_t *resarr,
   resarr[0] = mpi_alloc( mpi_get_nlimbs (sk.n));
   secret (resarr[0], data, &sk);
 
+  /* Check that the created signature is good.  This detects a failure
+     of the CRT algorithm  (Lenstra's attack on RSA's use of the CRT).  */
+  result = gcry_mpi_new (0);
+  pk.n = sk.n;
+  pk.e = sk.e;
+  public (result, resarr[0], &pk);
+  if (mpi_cmp (result, data))
+    {
+      gcry_mpi_release (result);
+      return GPG_ERR_BAD_SIGNATURE;
+    }
+
+  gcry_mpi_release (result);
   return GPG_ERR_NO_ERROR;
 }