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