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path: root/main/hostapd/0018-EAP-pwd-Remove-unused-checks-for-cofactor-1-cases.patch
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From 8b093db2c3f489a74b67f687becf750d24fcf626 Mon Sep 17 00:00:00 2001
From: Jouni Malinen <j@w1.fi>
Date: Sat, 13 Apr 2019 17:30:22 +0300
Subject: [PATCH] EAP-pwd: Remove unused checks for cofactor > 1 cases

None of the ECC groups supported in the implementation had a cofactor
greater than 1, so these checks are unreachable and for all cases, the
cofactor is known to be 1. Furthermore, RFC 5931 explicitly disallow use
of ECC groups with cofactor larger than 1, so this checks cannot be
needed for any curve that is compliant with the RFC.

Remove the unneeded group cofactor checks to simplify the
implementation.

Signed-off-by: Jouni Malinen <j@w1.fi>
---
 src/eap_common/eap_pwd_common.c | 53 ++-------------------------------
 src/eap_peer/eap_pwd.c          | 23 ++------------
 src/eap_server/eap_server_pwd.c | 23 ++------------
 3 files changed, 7 insertions(+), 92 deletions(-)

diff --git a/src/eap_common/eap_pwd_common.c b/src/eap_common/eap_pwd_common.c
index 00f85a390..884150e6c 100644
--- a/src/eap_common/eap_pwd_common.c
+++ b/src/eap_common/eap_pwd_common.c
@@ -151,7 +151,7 @@ int compute_password_element(EAP_PWD_group *grp, u16 num,
 	u8 found = 0; /* 0 (false) or 0xff (true) to be used as const_time_*
 		       * mask */
 	size_t primebytelen = 0, primebitlen;
-	struct crypto_bignum *x_candidate = NULL, *cofactor = NULL;
+	struct crypto_bignum *x_candidate = NULL;
 	const struct crypto_bignum *prime;
 	u8 mask, found_ctr = 0, is_odd = 0;
 
@@ -161,21 +161,15 @@ 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);
-	cofactor = crypto_bignum_init();
 	grp->pwe = crypto_ec_point_init(grp->group);
 	tmp1 = crypto_bignum_init();
 	pm1 = crypto_bignum_init();
 	one = crypto_bignum_init_set((const u8 *) "\x01", 1);
-	if (!cofactor || !grp->pwe || !tmp1 || !pm1 || !one) {
+	if (!grp->pwe || !tmp1 || !pm1 || !one) {
 		wpa_printf(MSG_INFO, "EAP-pwd: unable to create bignums");
 		goto fail;
 	}
 
-	if (crypto_ec_cofactor(grp->group, cofactor) < 0) {
-		wpa_printf(MSG_INFO, "EAP-pwd: unable to get cofactor for "
-			   "curve");
-		goto fail;
-	}
 	primebitlen = crypto_ec_prime_len_bits(grp->group);
 	primebytelen = crypto_ec_prime_len(grp->group);
 	if ((prfbuf = os_malloc(primebytelen)) == NULL) {
@@ -340,19 +334,6 @@ int compute_password_element(EAP_PWD_group *grp, u16 num,
 		goto fail;
 	}
 
-	if (!crypto_bignum_is_one(cofactor)) {
-		/* make sure the point is not in a small sub-group */
-		if (crypto_ec_point_mul(grp->group, grp->pwe, cofactor,
-					grp->pwe) != 0) {
-			wpa_printf(MSG_INFO,
-				   "EAP-pwd: cannot multiply generator by order");
-			goto fail;
-		}
-		if (crypto_ec_point_is_at_infinity(grp->group, grp->pwe)) {
-			wpa_printf(MSG_INFO, "EAP-pwd: point is at infinity");
-			goto fail;
-		}
-	}
 	wpa_printf(MSG_DEBUG, "EAP-pwd: found a PWE in %02d tries", found_ctr);
 
 	if (0) {
@@ -362,7 +343,6 @@ int compute_password_element(EAP_PWD_group *grp, u16 num,
 		ret = 1;
 	}
 	/* cleanliness and order.... */
-	crypto_bignum_deinit(cofactor, 1);
 	crypto_bignum_deinit(x_candidate, 1);
 	crypto_bignum_deinit(pm1, 0);
 	crypto_bignum_deinit(tmp1, 1);
@@ -464,7 +444,6 @@ struct crypto_ec_point * eap_pwd_get_element(EAP_PWD_group *group,
 	struct crypto_ec_point *element;
 	const struct crypto_bignum *prime;
 	size_t prime_len;
-	struct crypto_bignum *cofactor = NULL;
 
 	prime = crypto_ec_get_prime(group->group);
 	prime_len = crypto_ec_prime_len(group->group);
@@ -489,35 +468,7 @@ struct crypto_ec_point * eap_pwd_get_element(EAP_PWD_group *group,
 		goto fail;
 	}
 
-	cofactor = crypto_bignum_init();
-	if (!cofactor || crypto_ec_cofactor(group->group, cofactor) < 0) {
-		wpa_printf(MSG_INFO,
-			   "EAP-pwd: Unable to get cofactor for curve");
-		goto fail;
-	}
-
-	if (!crypto_bignum_is_one(cofactor)) {
-		struct crypto_ec_point *point;
-		int ok = 1;
-
-		/* check to ensure peer's element is not in a small sub-group */
-		point = crypto_ec_point_init(group->group);
-		if (!point ||
-		    crypto_ec_point_mul(group->group, element,
-					cofactor, point) != 0 ||
-		    crypto_ec_point_is_at_infinity(group->group, point))
-			ok = 0;
-		crypto_ec_point_deinit(point, 0);
-
-		if (!ok) {
-			wpa_printf(MSG_INFO,
-				   "EAP-pwd: Small sub-group check on peer element failed");
-			goto fail;
-		}
-	}
-
 out:
-	crypto_bignum_deinit(cofactor, 0);
 	return element;
 fail:
 	crypto_ec_point_deinit(element, 0);
diff --git a/src/eap_peer/eap_pwd.c b/src/eap_peer/eap_pwd.c
index 4be4fcf35..46894a52f 100644
--- a/src/eap_peer/eap_pwd.c
+++ b/src/eap_peer/eap_pwd.c
@@ -309,7 +309,7 @@ eap_pwd_perform_commit_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 				const u8 *payload, size_t payload_len)
 {
 	struct crypto_ec_point *K = NULL;
-	struct crypto_bignum *mask = NULL, *cofactor = NULL;
+	struct crypto_bignum *mask = NULL;
 	const u8 *ptr = payload;
 	u8 *scalar, *element;
 	size_t prime_len, order_len;
@@ -527,21 +527,14 @@ eap_pwd_perform_commit_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 
 	data->private_value = crypto_bignum_init();
 	data->my_element = crypto_ec_point_init(data->grp->group);
-	cofactor = crypto_bignum_init();
 	data->my_scalar = crypto_bignum_init();
 	mask = crypto_bignum_init();
-	if (!data->private_value || !data->my_element || !cofactor ||
+	if (!data->private_value || !data->my_element ||
 	    !data->my_scalar || !mask) {
 		wpa_printf(MSG_INFO, "EAP-PWD (peer): scalar allocation fail");
 		goto fin;
 	}
 
-	if (crypto_ec_cofactor(data->grp->group, cofactor) < 0) {
-		wpa_printf(MSG_INFO, "EAP-pwd (peer): unable to get cofactor "
-			   "for curve");
-		goto fin;
-	}
-
 	if (eap_pwd_get_rand_mask(data->grp, data->private_value, mask,
 				  data->my_scalar) < 0)
 		goto fin;
@@ -595,17 +588,8 @@ eap_pwd_perform_commit_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 		goto fin;
 	}
 
-	/* ensure that the shared key isn't in a small sub-group */
-	if (!crypto_bignum_is_one(cofactor)) {
-		if (crypto_ec_point_mul(data->grp->group, K, cofactor, K) < 0) {
-			wpa_printf(MSG_INFO, "EAP-PWD (peer): cannot multiply "
-				   "shared key point by order");
-			goto fin;
-		}
-	}
-
 	/*
-	 * This check is strictly speaking just for the case above where
+	 * This check is strictly speaking just for the case where
 	 * co-factor > 1 but it was suggested that even though this is probably
 	 * never going to happen it is a simple and safe check "just to be
 	 * sure" so let's be safe.
@@ -644,7 +628,6 @@ eap_pwd_perform_commit_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 
 fin:
 	crypto_bignum_deinit(mask, 1);
-	crypto_bignum_deinit(cofactor, 1);
 	crypto_ec_point_deinit(K, 1);
 	if (data->outbuf == NULL)
 		eap_pwd_state(data, FAILURE);
diff --git a/src/eap_server/eap_server_pwd.c b/src/eap_server/eap_server_pwd.c
index 9799c8197..81ecd773f 100644
--- a/src/eap_server/eap_server_pwd.c
+++ b/src/eap_server/eap_server_pwd.c
@@ -648,7 +648,6 @@ eap_pwd_process_commit_resp(struct eap_sm *sm, struct eap_pwd_data *data,
 			    const u8 *payload, size_t payload_len)
 {
 	const u8 *ptr;
-	struct crypto_bignum *cofactor = NULL;
 	struct crypto_ec_point *K = NULL;
 	int res = 0;
 	size_t prime_len, order_len;
@@ -667,20 +666,13 @@ eap_pwd_process_commit_resp(struct eap_sm *sm, struct eap_pwd_data *data,
 	}
 
 	data->k = crypto_bignum_init();
-	cofactor = crypto_bignum_init();
 	K = crypto_ec_point_init(data->grp->group);
-	if (!data->k || !cofactor || !K) {
+	if (!data->k || !K) {
 		wpa_printf(MSG_INFO, "EAP-PWD (server): peer data allocation "
 			   "fail");
 		goto fin;
 	}
 
-	if (crypto_ec_cofactor(data->grp->group, cofactor) < 0) {
-		wpa_printf(MSG_INFO, "EAP-PWD (server): unable to get "
-			   "cofactor for curve");
-		goto fin;
-	}
-
 	/* element, x then y, followed by scalar */
 	ptr = payload;
 	data->peer_element = eap_pwd_get_element(data->grp, ptr);
@@ -718,18 +710,8 @@ eap_pwd_process_commit_resp(struct eap_sm *sm, struct eap_pwd_data *data,
 		goto fin;
 	}
 
-	/* ensure that the shared key isn't in a small sub-group */
-	if (!crypto_bignum_is_one(cofactor)) {
-		if (crypto_ec_point_mul(data->grp->group, K, cofactor,
-					K) != 0) {
-			wpa_printf(MSG_INFO, "EAP-PWD (server): cannot "
-				   "multiply shared key point by order!\n");
-			goto fin;
-		}
-	}
-
 	/*
-	 * This check is strictly speaking just for the case above where
+	 * This check is strictly speaking just for the case where
 	 * co-factor > 1 but it was suggested that even though this is probably
 	 * never going to happen it is a simple and safe check "just to be
 	 * sure" so let's be safe.
@@ -748,7 +730,6 @@ eap_pwd_process_commit_resp(struct eap_sm *sm, struct eap_pwd_data *data,
 
 fin:
 	crypto_ec_point_deinit(K, 1);
-	crypto_bignum_deinit(cofactor, 1);
 
 	if (res)
 		eap_pwd_state(data, PWD_Confirm_Req);
-- 
2.22.0