From a25b48118d75f3c2d7cb1b2c3b4cffb13091a34c Mon Sep 17 00:00:00 2001 From: Jouni Malinen Date: Mon, 24 Jun 2019 23:01:06 +0300 Subject: [PATCH 4/6] SAE: Run through prf result processing even if it >= prime 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 (cherry picked from commit 147bf7b88a9c231322b5b574263071ca6dbb0503) --- src/common/sae.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/src/common/sae.c b/src/common/sae.c index 0d56e5505..759e48e22 100644 --- a/src/common/sae.c +++ b/src/common/sae.c @@ -304,6 +304,8 @@ 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); @@ -317,8 +319,13 @@ 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) @@ -330,7 +337,9 @@ static int sae_test_pwd_seed_ecc(struct sae_data *sae, const u8 *pwd_seed, res = is_quadratic_residue_blind(sae, prime, bits, qr, qnr, y_sqr); crypto_bignum_deinit(y_sqr, 1); - return res; + if (res < 0) + return res; + return const_time_select_int(in_range, res, 0); } -- 2.20.1