From cd803299ca485eb857e37c88f973fccfbb8600e5 Mon Sep 17 00:00:00 2001
From: Jouni Malinen <j@w1.fi>
Date: Mon, 24 Jun 2019 23:02:51 +0300
Subject: [PATCH] EAP-pwd: 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 <j@w1.fi>
---
src/eap_common/eap_pwd_common.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/src/eap_common/eap_pwd_common.c b/src/eap_common/eap_pwd_common.c
index 5b4f55c28..2b2b8efdb 100644
--- a/src/eap_common/eap_pwd_common.c
+++ b/src/eap_common/eap_pwd_common.c
@@ -137,6 +137,8 @@ int compute_password_element(EAP_PWD_group *grp, u16 num,
struct crypto_bignum *x_candidate = NULL;
const struct crypto_bignum *prime;
u8 found_ctr = 0, is_odd = 0;
+ int cmp_prime;
+ unsigned int in_range;
if (grp->pwe)
return -1;
@@ -205,8 +207,13 @@ int compute_password_element(EAP_PWD_group *grp, u16 num,
if (primebitlen % 8)
buf_shift_right(prfbuf, primebytelen,
8 - primebitlen % 8);
- if (const_time_memcmp(prfbuf, prime_bin, primebytelen) >= 0)
- continue;
+ cmp_prime = const_time_memcmp(prfbuf, prime_bin, primebytelen);
+ /* 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, but go through them regardless to minimize
+ * externally observable differences in behavior. */
crypto_bignum_deinit(x_candidate, 1);
x_candidate = crypto_bignum_init_set(prfbuf, primebytelen);
@@ -237,9 +244,10 @@ int compute_password_element(EAP_PWD_group *grp, u16 num,
goto fail;
found_ctr = const_time_select_u8(found, found_ctr, ctr);
/* found is 0 or 0xff here and res is 0 or 1. Bitwise OR of them
- * (with res converted to 0/0xff) handles this in constant time.
+ * (with res converted to 0/0xff and masked with prf being below
+ * prime) handles this in constant time.
*/
- found |= res * 0xff;
+ found |= (res & in_range) * 0xff;
}
if (found == 0) {
wpa_printf(MSG_INFO,