git.librecmc.org Git - librecmc/librecmc.git/blob

   1 From a25b48118d75f3c2d7cb1b2c3b4cffb13091a34c Mon Sep 17 00:00:00 2001
   2 From: Jouni Malinen <j@w1.fi>
   3 Date: Mon, 24 Jun 2019 23:01:06 +0300
   4 Subject: [PATCH 4/6] SAE: Run through prf result processing even if it >=
   5  prime
   6
   7 This reduces differences in timing and memory access within the
   8 hunting-and-pecking loop for ECC groups that have a prime that is not
   9 close to a power of two (e.g., Brainpool curves).
  10
  11 Signed-off-by: Jouni Malinen <j@w1.fi>
  12 (cherry picked from commit 147bf7b88a9c231322b5b574263071ca6dbb0503)
  13 ---
  14  src/common/sae.c | 15 ++++++++++++---
  15  1 file changed, 12 insertions(+), 3 deletions(-)
  16
  17 --- a/src/common/sae.c
  18 +++ b/src/common/sae.c
  19 @@ -304,6 +304,8 @@ static int sae_test_pwd_seed_ecc(struct
  20         struct crypto_bignum *y_sqr, *x_cand;
  21         int res;
  22         size_t bits;
  23 +       int cmp_prime;
  24 +       unsigned int in_range;
  25
  26         wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-seed", pwd_seed, SHA256_MAC_LEN);
  27
  28 @@ -317,8 +319,13 @@ static int sae_test_pwd_seed_ecc(struct
  29         wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-value",
  30                         pwd_value, sae->tmp->prime_len);
  31
  32 -       if (const_time_memcmp(pwd_value, prime, sae->tmp->prime_len) >= 0)
  33 -               return 0;
  34 +       cmp_prime = const_time_memcmp(pwd_value, prime, sae->tmp->prime_len);
  35 +       /* Create a const_time mask for selection based on prf result
  36 +        * being smaller than prime. */
  37 +       in_range = const_time_fill_msb((unsigned int) cmp_prime);
  38 +       /* The algorithm description would skip the next steps if
  39 +        * cmp_prime >= 0 (reutnr 0 here), but go through them regardless to
  40 +        * minimize externally observable differences in behavior. */
  41
  42         x_cand = crypto_bignum_init_set(pwd_value, sae->tmp->prime_len);
  43         if (!x_cand)
  44 @@ -330,7 +337,9 @@ static int sae_test_pwd_seed_ecc(struct
  45
  46         res = is_quadratic_residue_blind(sae, prime, bits, qr, qnr, y_sqr);
  47         crypto_bignum_deinit(y_sqr, 1);
  48 -       return res;
  49 +       if (res < 0)
  50 +               return res;
  51 +       return const_time_select_int(in_range, res, 0);
  52  }
  53
  54