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SAE: Run through prf result processing even if it >= prime

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This reduces differences in timing and memory access within the
hunting-and-pecking loop for ECC groups that have a prime that is not
close to a power of two (e.g., Brainpool curves).

Signed-off-by: Jouni Malinen <j@w1.fi>
This commit is contained in:
Jouni Malinen 2019-06-24 23:01:06 +03:00
parent 91b6eba773
commit 147bf7b88a
1 changed files with 12 additions and 3 deletions
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13
src/common/sae.c
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@ -142,6 +142,8 @@ static int sae_test_pwd_seed_ecc(struct sae_data *sae, const u8 *pwd_seed,
struct crypto_bignum *y_sqr, *x_cand;
int res;
size_t bits;
int cmp_prime;
unsigned int in_range;
wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-seed", pwd_seed, SHA256_MAC_LEN);
@ -155,8 +157,13 @@ static int sae_test_pwd_seed_ecc(struct sae_data *sae, const u8 *pwd_seed,
wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-value",
pwd_value, sae->tmp->prime_len);
if (const_time_memcmp(pwd_value, prime, sae->tmp->prime_len) >= 0)
return 0;
cmp_prime = const_time_memcmp(pwd_value, prime, sae->tmp->prime_len);
/* Create a const_time mask for selection based on prf result
* being smaller than prime. */
in_range = const_time_fill_msb((unsigned int) cmp_prime);
/* The algorithm description would skip the next steps if
* cmp_prime >= 0 (reutnr 0 here), but go through them regardless to
* minimize externally observable differences in behavior. */
x_cand = crypto_bignum_init_set(pwd_value, sae->tmp->prime_len);
if (!x_cand)
@ -169,7 +176,9 @@ static int sae_test_pwd_seed_ecc(struct sae_data *sae, const u8 *pwd_seed,
res = dragonfly_is_quadratic_residue_blind(sae->tmp->ec, qr, qnr,
y_sqr);
crypto_bignum_deinit(y_sqr, 1);
if (res < 0)
return res;
return const_time_select_int(in_range, res, 0);
}