From 362704dda04507e7ebb8035122e83d9f0ae7c320 Mon Sep 17 00:00:00 2001 From: Jouni Malinen Date: Tue, 26 Feb 2019 19:34:38 +0200 Subject: [PATCH 06/14] SAE: Avoid branches in is_quadratic_residue_blind() Make the non-failure path in the function proceed without branches based on r_odd and in constant time to minimize risk of observable differences in timing or cache use. (CVE-2019-9494) Signed-off-by: Jouni Malinen Signed-off-by: Adrian Bunk Upstream-Status: Backport CVE: CVE-2019-9494 --- src/common/sae.c | 64 ++++++++++++++++++++++++++++++++------------------------ 1 file changed, 37 insertions(+), 27 deletions(-) diff --git a/src/common/sae.c b/src/common/sae.c index d55323b..5df9b95 100644 --- a/src/common/sae.c +++ b/src/common/sae.c @@ -232,12 +232,14 @@ get_rand_1_to_p_1(const u8 *prime, size_t prime_len, size_t prime_bits, static int is_quadratic_residue_blind(struct sae_data *sae, const u8 *prime, size_t bits, - const struct crypto_bignum *qr, - const struct crypto_bignum *qnr, + const u8 *qr, const u8 *qnr, const struct crypto_bignum *y_sqr) { - struct crypto_bignum *r, *num; + struct crypto_bignum *r, *num, *qr_or_qnr = NULL; int r_odd, check, res = -1; + u8 qr_or_qnr_bin[SAE_MAX_ECC_PRIME_LEN]; + size_t prime_len = sae->tmp->prime_len; + unsigned int mask; /* * Use the blinding technique to mask y_sqr while determining @@ -248,7 +250,7 @@ static int is_quadratic_residue_blind(struct sae_data *sae, * r = a random number between 1 and p-1, inclusive * num = (v * r * r) modulo p */ - r = get_rand_1_to_p_1(prime, sae->tmp->prime_len, bits, &r_odd); + r = get_rand_1_to_p_1(prime, prime_len, bits, &r_odd); if (!r) return -1; @@ -258,41 +260,45 @@ static int is_quadratic_residue_blind(struct sae_data *sae, crypto_bignum_mulmod(num, r, sae->tmp->prime, num) < 0) goto fail; - if (r_odd) { - /* - * num = (num * qr) module p - * LGR(num, p) = 1 ==> quadratic residue - */ - if (crypto_bignum_mulmod(num, qr, sae->tmp->prime, num) < 0) - goto fail; - check = 1; - } else { - /* - * num = (num * qnr) module p - * LGR(num, p) = -1 ==> quadratic residue - */ - if (crypto_bignum_mulmod(num, qnr, sae->tmp->prime, num) < 0) - goto fail; - check = -1; - } + /* + * Need to minimize differences in handling different cases, so try to + * avoid branches and timing differences. + * + * If r_odd: + * num = (num * qr) module p + * LGR(num, p) = 1 ==> quadratic residue + * else: + * num = (num * qnr) module p + * LGR(num, p) = -1 ==> quadratic residue + */ + mask = const_time_is_zero(r_odd); + const_time_select_bin(mask, qnr, qr, prime_len, qr_or_qnr_bin); + qr_or_qnr = crypto_bignum_init_set(qr_or_qnr_bin, prime_len); + if (!qr_or_qnr || + crypto_bignum_mulmod(num, qr_or_qnr, sae->tmp->prime, num) < 0) + goto fail; + /* r_odd is 0 or 1; branchless version of check = r_odd ? 1 : -1, */ + check = const_time_select_int(mask, -1, 1); res = crypto_bignum_legendre(num, sae->tmp->prime); if (res == -2) { res = -1; goto fail; } - res = res == check; + /* branchless version of res = res == check + * (res is -1, 0, or 1; check is -1 or 1) */ + mask = const_time_eq(res, check); + res = const_time_select_int(mask, 1, 0); fail: crypto_bignum_deinit(num, 1); crypto_bignum_deinit(r, 1); + crypto_bignum_deinit(qr_or_qnr, 1); return res; } static int sae_test_pwd_seed_ecc(struct sae_data *sae, const u8 *pwd_seed, - const u8 *prime, - const struct crypto_bignum *qr, - const struct crypto_bignum *qnr, + const u8 *prime, const u8 *qr, const u8 *qnr, u8 *pwd_value) { struct crypto_bignum *y_sqr, *x_cand; @@ -452,6 +458,8 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1, struct crypto_bignum *x = NULL, *qr = NULL, *qnr = NULL; u8 x_bin[SAE_MAX_ECC_PRIME_LEN]; u8 x_cand_bin[SAE_MAX_ECC_PRIME_LEN]; + u8 qr_bin[SAE_MAX_ECC_PRIME_LEN]; + u8 qnr_bin[SAE_MAX_ECC_PRIME_LEN]; size_t bits; int res = -1; u8 found = 0; /* 0 (false) or 0xff (true) to be used as const_time_* @@ -476,7 +484,9 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1, * (qnr) modulo p for blinding purposes during the loop. */ if (get_random_qr_qnr(prime, prime_len, sae->tmp->prime, bits, - &qr, &qnr) < 0) + &qr, &qnr) < 0 || + crypto_bignum_to_bin(qr, qr_bin, sizeof(qr_bin), prime_len) < 0 || + crypto_bignum_to_bin(qnr, qnr_bin, sizeof(qnr_bin), prime_len) < 0) goto fail; wpa_hexdump_ascii_key(MSG_DEBUG, "SAE: password", @@ -527,7 +537,7 @@ static int sae_derive_pwe_ecc(struct sae_data *sae, const u8 *addr1, break; res = sae_test_pwd_seed_ecc(sae, pwd_seed, - prime, qr, qnr, x_cand_bin); + prime, qr_bin, qnr_bin, x_cand_bin); const_time_select_bin(found, x_bin, x_cand_bin, prime_len, x_bin); pwd_seed_odd = const_time_select_u8( -- 2.7.4