Backport of:

From 9010d1576e278a4274ad3f4aa15776c28f6ba965 Mon Sep 17 00:00:00 2001
From: NIIBE Yutaka <gniibe@fsij.org>
Date: Wed, 13 Jun 2018 15:28:58 +0900
Subject: [PATCH] ecc: Add blinding for ECDSA.

* cipher/ecc-ecdsa.c (_gcry_ecc_ecdsa_sign): Blind secret D with
randomized nonce B.

--

Reported-by: Keegan Ryan <Keegan.Ryan@nccgroup.trust>
CVE-id: CVE-2018-0495
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>
---
 cipher/ecc-ecdsa.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

Index: libgcrypt11-1.5.3/cipher/ecc.c
===================================================================
--- libgcrypt11-1.5.3.orig/cipher/ecc.c	2018-06-18 10:01:30.602096607 -0400
+++ libgcrypt11-1.5.3/cipher/ecc.c	2018-06-18 10:09:06.655078752 -0400
@@ -715,11 +715,25 @@ sign (gcry_mpi_t input, ECC_secret_key *
   gpg_err_code_t err = 0;
   gcry_mpi_t k, dr, sum, k_1, x;
   mpi_point_t I;
+  unsigned int qbits;
   mpi_ec_t ctx;
+  gcry_mpi_t b;                /* Random number needed for blinding.  */
+  gcry_mpi_t bi;               /* multiplicative inverse of B.        */
 
   if (DBG_CIPHER)
     log_mpidump ("ecdsa sign hash  ", input );
 
+  qbits = mpi_get_nbits (skey->E.n);
+
+  b  = mpi_snew (qbits);
+  bi = mpi_snew (qbits);
+  do
+    {
+      _gcry_mpi_randomize (b, qbits, GCRY_WEAK_RANDOM);
+      mpi_mod (b, b, skey->E.n);
+    }
+  while (!mpi_invm (bi, b, skey->E.n));
+
   k = NULL;
   dr = mpi_alloc (0);
   sum = mpi_alloc (0);
@@ -752,8 +766,11 @@ sign (gcry_mpi_t input, ECC_secret_key *
             }
           mpi_mod (r, x, skey->E.n);  /* r = x mod n */
         }
-      mpi_mulm (dr, skey->d, r, skey->E.n); /* dr = d*r mod n  */
-      mpi_addm (sum, input, dr, skey->E.n); /* sum = hash + (d*r) mod n  */
+      mpi_mulm (dr, b, skey->d, skey->E.n);
+      mpi_mulm (dr, dr, r, skey->E.n);      /* dr = d*r mod n (blinded with b) */
+      mpi_mulm (sum, b, input, skey->E.n);
+      mpi_addm (sum, sum, dr, skey->E.n);   /* sum = hash + (d*r) mod n  (blinded with b) */
+      mpi_mulm (sum, bi, sum, skey->E.n);   /* undo blinding by b^-1 */
       mpi_invm (k_1, k, skey->E.n);         /* k_1 = k^(-1) mod n  */
       mpi_mulm (s, k_1, sum, skey->E.n);    /* s = k^(-1)*(hash+(d*r)) mod n */
     }
@@ -765,6 +782,8 @@ sign (gcry_mpi_t input, ECC_secret_key *
     }
 
  leave:
+  mpi_free (b);
+  mpi_free (bi);
   _gcry_mpi_ec_free (ctx);
   point_free (&I);
   mpi_free (x);