EAP-pwd: Use const_time_memcmp() for pwd_value >= prime comparison

This reduces timing and memory access pattern differences for an
operation that could depend on the used password.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>

diff --git a/src/eap_common/eap_pwd_common.c b/src/eap_common/eap_pwd_common.c
index 884150e6c1eb5620af27060946bbe6ef17014d39..6ca2c8badb880710d884aeab7f088b05c9d54bdb 100644 (file)
--- a/src/eap_common/eap_pwd_common.c
+++ b/src/eap_common/eap_pwd_common.c
@@ -144,6 +144,7 @@ int compute_password_element(EAP_PWD_group *grp, u16 num,
        u8 qnr_bin[MAX_ECC_PRIME_LEN];
        u8 qr_or_qnr_bin[MAX_ECC_PRIME_LEN];
        u8 x_bin[MAX_ECC_PRIME_LEN];
+       u8 prime_bin[MAX_ECC_PRIME_LEN];
        struct crypto_bignum *tmp1 = NULL, *tmp2 = NULL, *pm1 = NULL;
        struct crypto_hash *hash;
        unsigned char pwe_digest[SHA256_MAC_LEN], *prfbuf = NULL, ctr;
@@ -161,6 +162,11 @@ int compute_password_element(EAP_PWD_group *grp, u16 num,
        os_memset(x_bin, 0, sizeof(x_bin));
 
        prime = crypto_ec_get_prime(grp->group);
+       primebitlen = crypto_ec_prime_len_bits(grp->group);
+       primebytelen = crypto_ec_prime_len(grp->group);
+       if (crypto_bignum_to_bin(prime, prime_bin, sizeof(prime_bin),
+                                primebytelen) < 0)
+               return -1;
        grp->pwe = crypto_ec_point_init(grp->group);
        tmp1 = crypto_bignum_init();
        pm1 = crypto_bignum_init();
@@ -170,8 +176,6 @@ int compute_password_element(EAP_PWD_group *grp, u16 num,
                goto fail;
        }
 
-       primebitlen = crypto_ec_prime_len_bits(grp->group);
-       primebytelen = crypto_ec_prime_len(grp->group);
        if ((prfbuf = os_malloc(primebytelen)) == NULL) {
                wpa_printf(MSG_INFO, "EAP-pwd: unable to malloc space for prf "
                           "buffer");
@@ -237,6 +241,8 @@ 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;
 
                crypto_bignum_deinit(x_candidate, 1);
                x_candidate = crypto_bignum_init_set(prfbuf, primebytelen);
@@ -246,9 +252,6 @@ int compute_password_element(EAP_PWD_group *grp, u16 num,
                        goto fail;
                }
 
-               if (crypto_bignum_cmp(x_candidate, prime) >= 0)
-                       continue;
-
                wpa_hexdump_key(MSG_DEBUG, "EAP-pwd: x_candidate",
                                prfbuf, primebytelen);
                const_time_select_bin(found, x_bin, prfbuf, primebytelen,