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From e8b7f10be275bcedb5fc05ed4837a89bfd605c61 Mon Sep 17 00:00:00 2001
From: NIIBE Yutaka <gniibe@fsij.org>
Date: Tue, 13 Apr 2021 10:00:00 +0900
Subject: [PATCH] cipher: Hardening ElGamal by introducing exponent blinding
too.
* cipher/elgamal.c (do_encrypt): Also do exponent blinding.
--
Base blinding had been introduced with USE_BLINDING. This patch add
exponent blinding as well to mitigate side-channel attack on mpi_powm.
GnuPG-bug-id: 5328
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>
Upstream-Status: Backport
CVE: CVE-2021-33560
Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
---
cipher/elgamal.c | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)
diff --git a/cipher/elgamal.c b/cipher/elgamal.c
index 4eb52d62..9835122f 100644
--- a/cipher/elgamal.c
+++ b/cipher/elgamal.c
@@ -522,8 +522,9 @@ do_encrypt(gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input, ELG_public_key *pkey )
static void
decrypt (gcry_mpi_t output, gcry_mpi_t a, gcry_mpi_t b, ELG_secret_key *skey )
{
- gcry_mpi_t t1, t2, r;
+ gcry_mpi_t t1, t2, r, r1, h;
unsigned int nbits = mpi_get_nbits (skey->p);
+ gcry_mpi_t x_blind;
mpi_normalize (a);
mpi_normalize (b);
@@ -534,20 +535,33 @@ decrypt (gcry_mpi_t output, gcry_mpi_t a, gcry_mpi_t b, ELG_secret_key *skey )
t2 = mpi_snew (nbits);
r = mpi_new (nbits);
+ r1 = mpi_new (nbits);
+ h = mpi_new (nbits);
+ x_blind = mpi_snew (nbits);
/* We need a random number of about the prime size. The random
number merely needs to be unpredictable; thus we use level 0. */
_gcry_mpi_randomize (r, nbits, GCRY_WEAK_RANDOM);
+ /* Also, exponent blinding: x_blind = x + (p-1)*r1 */
+ _gcry_mpi_randomize (r1, nbits, GCRY_WEAK_RANDOM);
+ mpi_set_highbit (r1, nbits - 1);
+ mpi_sub_ui (h, skey->p, 1);
+ mpi_mul (x_blind, h, r1);
+ mpi_add (x_blind, skey->x, x_blind);
+
/* t1 = r^x mod p */
- mpi_powm (t1, r, skey->x, skey->p);
+ mpi_powm (t1, r, x_blind, skey->p);
/* t2 = (a * r)^-x mod p */
mpi_mulm (t2, a, r, skey->p);
- mpi_powm (t2, t2, skey->x, skey->p);
+ mpi_powm (t2, t2, x_blind, skey->p);
mpi_invm (t2, t2, skey->p);
/* t1 = (t1 * t2) mod p*/
mpi_mulm (t1, t1, t2, skey->p);
+ mpi_free (x_blind);
+ mpi_free (h);
+ mpi_free (r1);
mpi_free (r);
mpi_free (t2);
--
2.11.0
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