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From 362704dda04507e7ebb8035122e83d9f0ae7c320 Mon Sep 17 00:00:00 2001
From: Jouni Malinen <jouni@codeaurora.org>
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 <jouni@codeaurora.org>
---
src/common/sae.c | 64 ++++++++++++++++++++++++++++++++------------------------
1 file changed, 37 insertions(+), 27 deletions(-)
--- a/src/common/sae.c
+++ b/src/common/sae.c
@@ -209,12 +209,14 @@ get_rand_1_to_p_1(const u8 *prime, size_
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
@@ -225,7 +227,7 @@ static int is_quadratic_residue_blind(st
* 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;
@@ -235,41 +237,45 @@ static int is_quadratic_residue_blind(st
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;
@@ -429,6 +435,8 @@ static int sae_derive_pwe_ecc(struct sae
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_*
@@ -453,7 +461,9 @@ static int sae_derive_pwe_ecc(struct sae
* (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",
@@ -504,7 +514,7 @@ static int sae_derive_pwe_ecc(struct sae
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(
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