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SAE: Use constant time operations in sae_test_pwd_seed_ffc()

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Try to avoid showing externally visible timing or memory access
differences regardless of whether the derived pwd-value is smaller than
the group prime.

This is related to CVE-2019-9494.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
This commit is contained in:
Jouni Malinen 2019-03-02 16:05:56 +02:00 committed by Jouni Malinen
parent f8f20717f8
commit cff138b074
1 changed files with 46 additions and 29 deletions
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75
src/common/sae.c
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@ -334,14 +334,17 @@ static int sae_test_pwd_seed_ecc(struct sae_data *sae, const u8 *pwd_seed,
}
/* Returns -1 on fatal failure, 0 if PWE cannot be derived from the provided
* pwd-seed, or 1 if a valid PWE was derived from pwd-seed. */
static int sae_test_pwd_seed_ffc(struct sae_data *sae, const u8 *pwd_seed,
struct crypto_bignum *pwe)
{
u8 pwd_value[SAE_MAX_PRIME_LEN];
size_t bits = sae->tmp->prime_len * 8;
u8 exp[1];
struct crypto_bignum *a, *b;
int res;
struct crypto_bignum *a, *b = NULL;
int res, is_val;
u8 pwd_value_valid;
wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-seed", pwd_seed, SHA256_MAC_LEN);
@ -353,16 +356,29 @@ static int sae_test_pwd_seed_ffc(struct sae_data *sae, const u8 *pwd_seed,
wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-value", pwd_value,
sae->tmp->prime_len);
if (os_memcmp(pwd_value, sae->tmp->dh->prime, sae->tmp->prime_len) >= 0)
{
wpa_printf(MSG_DEBUG, "SAE: pwd-value >= p");
return 0;
}
/* Check whether pwd-value < p */
res = const_time_memcmp(pwd_value, sae->tmp->dh->prime,
sae->tmp->prime_len);
/* pwd-value >= p is invalid, so res is < 0 for the valid cases and
* the negative sign can be used to fill the mask for constant time
* selection */
pwd_value_valid = const_time_fill_msb(res);
/* If pwd-value >= p, force pwd-value to be < p and perform the
* calculations anyway to hide timing difference. The derived PWE will
* be ignored in that case. */
pwd_value[0] = const_time_select_u8(pwd_value_valid, pwd_value[0], 0);
/* PWE = pwd-value^((p-1)/r) modulo p */
res = -1;
a = crypto_bignum_init_set(pwd_value, sae->tmp->prime_len);
if (!a)
goto fail;
/* This is an optimization based on the used group that does not depend
* on the password in any way, so it is fine to use separate branches
* for this step without constant time operations. */
if (sae->tmp->dh->safe_prime) {
/*
* r = (p-1)/2 for the group used here, so this becomes:
@ -376,33 +392,34 @@ static int sae_test_pwd_seed_ffc(struct sae_data *sae, const u8 *pwd_seed,
b = crypto_bignum_init_set(exp, sizeof(exp));
if (b == NULL ||
crypto_bignum_sub(sae->tmp->prime, b, b) < 0 ||
crypto_bignum_div(b, sae->tmp->order, b) < 0) {
crypto_bignum_deinit(b, 0);
b = NULL;
}
crypto_bignum_div(b, sae->tmp->order, b) < 0)
goto fail;
}
if (a == NULL || b == NULL)
res = -1;
else
res = crypto_bignum_exptmod(a, b, sae->tmp->prime, pwe);
if (!b)
goto fail;
crypto_bignum_deinit(a, 0);
crypto_bignum_deinit(b, 0);
res = crypto_bignum_exptmod(a, b, sae->tmp->prime, pwe);
if (res < 0)
goto fail;
if (res < 0) {
wpa_printf(MSG_DEBUG, "SAE: Failed to calculate PWE");
return -1;
}
/* There were no fatal errors in calculations, so determine the return
* value using constant time operations. We get here for number of
* invalid cases which are cleared here after having performed all the
* computation. PWE is valid if pwd-value was less than prime and
* PWE > 1. Start with pwd-value check first and then use constant time
* operations to clear res to 0 if PWE is 0 or 1.
*/
res = const_time_select_u8(pwd_value_valid, 1, 0);
is_val = crypto_bignum_is_zero(pwe);
res = const_time_select_u8(const_time_is_zero(is_val), res, 0);
is_val = crypto_bignum_is_one(pwe);
res = const_time_select_u8(const_time_is_zero(is_val), res, 0);
/* if (PWE > 1) --> found */
if (crypto_bignum_is_zero(pwe) || crypto_bignum_is_one(pwe)) {
wpa_printf(MSG_DEBUG, "SAE: PWE <= 1");
return 0;
}
wpa_printf(MSG_DEBUG, "SAE: PWE found");
return 1;
fail:
crypto_bignum_deinit(a, 1);
crypto_bignum_deinit(b, 1);
return res;
}