hostapd/src/common/sae.c

/*
 * Simultaneous authentication of equals
 * Copyright (c) 2012, Jouni Malinen <j@w1.fi>
 *
 * This software may be distributed under the terms of the BSD license.
 * See README for more details.
 */

#include "includes.h"
/* TODO: move OpenSSL dependencies into crypto/crypto_openssl.c */
#include <openssl/bn.h>
#include <openssl/ec.h>
#include <openssl/obj_mac.h>

#include "common.h"
#include "crypto/sha256.h"
#include "crypto/random.h"
#include "ieee802_11_defs.h"
#include "sae.h"


static const u8 group19_prime[] = {
	0xFF, 0xFF, 0xFF, 0xFF, 0x00, 0x00, 0x00, 0x01,
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
	0x00, 0x00, 0x00, 0x00, 0xFF, 0xFF, 0xFF, 0xFF,
	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
};

static const u8 group19_order[] = {
	0xFF, 0xFF, 0xFF, 0xFF, 0x00, 0x00, 0x00, 0x00,
	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
	0xBC, 0xE6, 0xFA, 0xAD, 0xA7, 0x17, 0x9E, 0x84,
	0xF3, 0xB9, 0xCA, 0xC2, 0xFC, 0x63, 0x25, 0x51
};


static int val_zero_or_one(const u8 *val, size_t len)
{
	size_t i;

	for (i = 0; i < len - 1; i++) {
		if (val[i])
			return 0;
	}

	return val[len - 1] <= 1;
}


static int val_zero(const u8 *val, size_t len)
{
	size_t i;
	for (i = 0; i < len; i++) {
		if (val[i])
			return 0;
	}
	return 1;
}


static int sae_get_rand(u8 *val)
{
	int iter = 0;

	do {
		if (random_get_bytes(val, sizeof(group19_prime)) < 0)
			return -1;
		if (iter++ > 100)
			return -1;
	} while (os_memcmp(val, group19_order, sizeof(group19_prime)) >= 0 ||
		 val_zero_or_one(val, sizeof(group19_prime)));

	return 0;
}


static EC_POINT * alloc_elem(EC_GROUP *group, const u8 *val, size_t len)
{
	BIGNUM *x, *y;
	EC_POINT *elem;

	x = BN_bin2bn(val, len, NULL);
	y = BN_bin2bn(val + len, len, NULL);
	elem = EC_POINT_new(group);
	if (x == NULL || y == NULL || elem == NULL) {
		BN_free(x);
		BN_free(y);
		EC_POINT_free(elem);
		return NULL;
	}

	if (!EC_POINT_set_affine_coordinates_GFp(group, elem, x, y, NULL)) {
		EC_POINT_free(elem);
		elem = NULL;
	}

	BN_free(x);
	BN_free(y);

	return elem;
}


static void sae_bn_to_bin(const BIGNUM *bn, u8 *bin, size_t len)
{
	int offset = len - BN_num_bytes(bn);
	os_memset(bin, 0, offset);
	BN_bn2bin(bn, bin + offset);
}


static int sae_ec_point_to_bin(BN_CTX *bnctx, EC_GROUP *group, EC_POINT *point,
			       u8 *bin)
{
	BIGNUM *x, *y;
	int ret = -1;

	x = BN_new();
	y = BN_new();

	if (x && y &&
	    EC_POINT_get_affine_coordinates_GFp(group, point, x, y, bnctx)) {
		sae_bn_to_bin(x, bin, 32);
		sae_bn_to_bin(y, bin + 32, 32);
		ret = 0;
	}

	BN_free(x);
	BN_free(y);
	return ret;
}


static void sae_pwd_seed_key(const u8 *addr1, const u8 *addr2, u8 *key)
{
	wpa_printf(MSG_DEBUG, "SAE: PWE derivation - addr1=" MACSTR
		   " addr2=" MACSTR, MAC2STR(addr1), MAC2STR(addr2));
	if (os_memcmp(addr1, addr2, ETH_ALEN) > 0) {
		os_memcpy(key, addr1, ETH_ALEN);
		os_memcpy(key + ETH_ALEN, addr2, ETH_ALEN);
	} else {
		os_memcpy(key, addr2, ETH_ALEN);
		os_memcpy(key + ETH_ALEN, addr1, ETH_ALEN);
	}
}


static int sae_test_pwd_seed(BN_CTX *bnctx, EC_GROUP *group, const u8 *pwd_seed,
			     EC_POINT *pwe, u8 *pwe_bin)
{
	u8 pwd_value[32];
	BIGNUM *x;
	int y_bit;

	wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-seed", pwd_seed, 32);

	/* pwd-value = KDF-z(pwd-seed, "SAE Hunting and Pecking", p) */
	sha256_prf(pwd_seed, 32, "SAE Hunting and Pecking",
		   group19_prime, sizeof(group19_prime),
		   pwd_value, sizeof(pwd_value));
	wpa_hexdump_key(MSG_DEBUG, "SAE: pwd-value",
			pwd_value, sizeof(pwd_value));

	if (os_memcmp(pwd_value, group19_prime, sizeof(group19_prime)) >= 0)
		return 0;

	y_bit = pwd_seed[SHA256_MAC_LEN - 1] & 0x01;

	x = BN_bin2bn(pwd_value, sizeof(pwd_value), NULL);
	if (x == NULL)
		return -1;
	if (!EC_POINT_set_compressed_coordinates_GFp(group, pwe, x, y_bit,
						     bnctx) ||
	    !EC_POINT_is_on_curve(group, pwe, bnctx)) {
		BN_free(x);
		wpa_printf(MSG_DEBUG, "SAE: No solution found");
		return 0;
	}
	BN_free(x);

	wpa_printf(MSG_DEBUG, "SAE: PWE found");

	if (sae_ec_point_to_bin(bnctx, group, pwe, pwe_bin) < 0)
		return -1;

	wpa_hexdump_key(MSG_DEBUG, "SAE: PWE x", pwe_bin, 32);
	wpa_hexdump_key(MSG_DEBUG, "SAE: PWE y", pwe_bin + 32, 32);
	return 1;
}


static int sae_derive_pwe(BN_CTX *bnctx, EC_GROUP *group, const u8 *addr1,
			  const u8 *addr2, const u8 *password,
			  size_t password_len, EC_POINT *pwe, u8 *pwe_bin)
{
	u8 counter, k = 4;
	u8 addrs[2 * ETH_ALEN];
	const u8 *addr[2];
	size_t len[2];
	int found = 0;
	EC_POINT *pwe_tmp;
	u8 pwe_bin_tmp[2 * 32];

	pwe_tmp = EC_POINT_new(group);
	if (pwe_tmp == NULL)
		return -1;

	wpa_hexdump_ascii_key(MSG_DEBUG, "SAE: password",
			      password, password_len);

	/*
	 * H(salt, ikm) = HMAC-SHA256(salt, ikm)
	 * pwd-seed = H(MAX(STA-A-MAC, STA-B-MAC) || MIN(STA-A-MAC, STA-B-MAC),
	 *              password || counter)
	 */
	sae_pwd_seed_key(addr1, addr2, addrs);

	addr[0] = password;
	len[0] = password_len;
	addr[1] = &counter;
	len[1] = sizeof(counter);

	/*
	 * Continue for at least k iterations to protect against side-channel
	 * attacks that attempt to determine the number of iterations required
	 * in the loop.
	 */
	for (counter = 1; counter < k || !found; counter++) {
		u8 pwd_seed[SHA256_MAC_LEN];
		int res;

		wpa_printf(MSG_DEBUG, "SAE: counter = %u", counter);
		if (hmac_sha256_vector(addrs, sizeof(addrs), 2, addr, len,
				       pwd_seed) < 0)
			break;
		res = sae_test_pwd_seed(bnctx, group, pwd_seed,
					found ? pwe_tmp : pwe,
					found ? pwe_bin_tmp : pwe_bin);
		if (res < 0)
			break;
		if (res == 0)
			continue;
		if (found) {
			wpa_printf(MSG_DEBUG, "SAE: Ignore this PWE (one was "
				   "already selected)");
		} else {
			wpa_printf(MSG_DEBUG, "SAE: Use this PWE");
			found = 1;
		}

		if (counter > 200) {
			/* This should not happen in practice */
			wpa_printf(MSG_DEBUG, "SAE: Failed to derive PWE");
			break;
		}
	}

	EC_POINT_clear_free(pwe_tmp);

	return found ? 0 : -1;
}


static int sae_derive_commit(struct sae_data *sae, BN_CTX *bnctx,
			     EC_GROUP *group, EC_POINT *pwe)
{
	BIGNUM *x, *bn_rand, *bn_mask, *order;
	EC_POINT *elem;
	u8 mask[32];
	int ret = -1;

	if (sae_get_rand(sae->sae_rand) < 0 || sae_get_rand(mask) < 0)
		return -1;
	wpa_hexdump_key(MSG_DEBUG, "SAE: rand",
			sae->sae_rand, sizeof(sae->sae_rand));
	wpa_hexdump_key(MSG_DEBUG, "SAE: mask", mask, sizeof(mask));

	x = BN_new();
	bn_rand = BN_bin2bn(sae->sae_rand, 32, NULL);
	bn_mask = BN_bin2bn(mask, sizeof(mask), NULL);
	order = BN_bin2bn(group19_order, sizeof(group19_order), NULL);
	elem = EC_POINT_new(group);
	if (x == NULL || bn_rand == NULL || bn_mask == NULL || order == NULL ||
	    elem == NULL)
		goto fail;

	/* commit-scalar = (rand + mask) modulo r */
	BN_add(x, bn_rand, bn_mask);
	BN_mod(x, x, order, bnctx);
	sae_bn_to_bin(x, sae->own_commit_scalar, 32);
	wpa_hexdump(MSG_DEBUG, "SAE: commit-scalar",
		    sae->own_commit_scalar, 32);

	/* COMMIT-ELEMENT = inverse(scalar-op(mask, PWE)) */
	if (!EC_POINT_mul(group, elem, NULL, pwe, bn_mask, bnctx) ||
	    !EC_POINT_invert(group, elem, bnctx) ||
	    sae_ec_point_to_bin(bnctx, group, elem, sae->own_commit_element) <
	    0)
		goto fail;

	wpa_hexdump(MSG_DEBUG, "SAE: commit-element x",
		    sae->own_commit_element, 32);
	wpa_hexdump(MSG_DEBUG, "SAE: commit-element y",
		    sae->own_commit_element + 32, 32);

	ret = 0;
fail:
	EC_POINT_free(elem);
	BN_free(order);
	BN_clear_free(bn_mask);
	os_memset(mask, 0, sizeof(mask));
	BN_clear_free(bn_rand);
	BN_clear_free(x);
	return ret;
}


int sae_prepare_commit(const u8 *addr1, const u8 *addr2,
		       const u8 *password, size_t password_len,
		       struct sae_data *sae)
{
	BN_CTX *bnctx;
	EC_POINT *pwe;
	EC_GROUP *group;
	int ret = 0;

	bnctx = BN_CTX_new();
	group = EC_GROUP_new_by_curve_name(NID_X9_62_prime256v1);
	pwe = EC_POINT_new(group);
	if (bnctx == NULL || group == NULL || pwe == NULL ||
	    sae_derive_pwe(bnctx, group, addr1, addr2, password, password_len,
			   pwe, sae->pwe) < 0 ||
	    sae_derive_commit(sae, bnctx, group, pwe) < 0)
		ret = -1;

	EC_POINT_clear_free(pwe);
	EC_GROUP_free(group);
	BN_CTX_free(bnctx);

	return ret;
}


static int sae_check_peer_commit(struct sae_data *sae)
{
	/* 0 < scalar < r */
	if (val_zero(sae->peer_commit_scalar, 32) ||
	    os_memcmp(sae->peer_commit_scalar, group19_order,
		      sizeof(group19_prime)) >= 0) {
		wpa_printf(MSG_DEBUG, "SAE: Invalid peer scalar");
		return -1;
	}

	/* element x and y coordinates < p */
	if (os_memcmp(sae->peer_commit_element, group19_prime,
		      sizeof(group19_prime)) >= 0 ||
	    os_memcmp(sae->peer_commit_element + 32, group19_prime,
		      sizeof(group19_prime)) >= 0) {
		wpa_printf(MSG_DEBUG, "SAE: Invalid coordinates in peer "
			   "element");
		return -1;
	}

	return 0;
}


static int sae_derive_k(struct sae_data *sae, u8 *k, BN_CTX *bnctx,
			EC_GROUP *group)
{
	EC_POINT *pwe, *peer_elem, *K;
	BIGNUM *k_bn, *rand_bn, *peer_scalar;
	int ret = -1;

	pwe = alloc_elem(group, sae->pwe, 32);
	peer_scalar = BN_bin2bn(sae->peer_commit_scalar, 32, NULL);
	peer_elem = alloc_elem(group, sae->peer_commit_element, 32);
	K = EC_POINT_new(group);
	k_bn = BN_new();
	rand_bn = BN_bin2bn(sae->sae_rand, 32, NULL);
	if (pwe == NULL || peer_elem == NULL || peer_scalar == NULL ||
	    K == NULL || k_bn == NULL || rand_bn == NULL)
		goto fail;

	if (!EC_POINT_is_on_curve(group, peer_elem, NULL)) {
		wpa_printf(MSG_DEBUG, "SAE: Peer element is not on curve");
		goto fail;
	}

	/*
	 * K = scalar-op(rand, (elem-op(scalar-op(peer-commit-scalar, PWE),
	 *                                        PEER-COMMIT-ELEMENT)))
	 * If K is identity element (point-at-infinity), reject
	 * k = F(K) (= x coordinate)
	 */

	if (!EC_POINT_mul(group, K, NULL, pwe, peer_scalar, bnctx) ||
	    !EC_POINT_add(group, K, K, peer_elem, bnctx) ||
	    !EC_POINT_mul(group, K, NULL, K, rand_bn, bnctx) ||
	    EC_POINT_is_at_infinity(group, K) ||
	    !EC_POINT_get_affine_coordinates_GFp(group, K, k_bn, NULL, bnctx)) {
		wpa_printf(MSG_DEBUG, "SAE: Failed to calculate K and k");
		goto fail;
	}

	sae_bn_to_bin(k_bn, k, 32);
	wpa_hexdump_key(MSG_DEBUG, "SAE: k", k, 32);

	ret = 0;
fail:
	EC_POINT_free(pwe);
	EC_POINT_free(peer_elem);
	EC_POINT_clear_free(K);
	BN_free(k_bn);
	BN_free(rand_bn);
	return ret;
}


static int sae_derive_keys(struct sae_data *sae, const u8 *k, BN_CTX *bnctx)
{
	u8 null_key[32], val[32];
	u8 keyseed[SHA256_MAC_LEN];
	u8 keys[32 + 32];
	BIGNUM *order, *own_scalar, *peer_scalar, *tmp;
	int ret = -1;

	order = BN_bin2bn(group19_order, sizeof(group19_order), NULL);
	own_scalar = BN_bin2bn(sae->own_commit_scalar, 32, NULL);
	peer_scalar = BN_bin2bn(sae->peer_commit_scalar, 32, NULL);
	tmp = BN_new();
	if (order == NULL || own_scalar == NULL || peer_scalar == NULL ||
	    tmp == NULL)
		goto fail;

	/* keyseed = H(<0>32, k)
	 * KCK || PMK = KDF-512(keyseed, "SAE KCK and PMK",
	 *                      (commit-scalar + peer-commit-scalar) modulo r)
	 * PMKID = L((commit-scalar + peer-commit-scalar) modulo r, 0, 128)
	 */

	os_memset(null_key, 0, sizeof(null_key));
	hmac_sha256(null_key, sizeof(null_key), k, 32, keyseed);
	wpa_hexdump_key(MSG_DEBUG, "SAE: keyseed", keyseed, sizeof(keyseed));

	BN_add(tmp, own_scalar, peer_scalar);
	BN_mod(tmp, tmp, order, bnctx);
	sae_bn_to_bin(tmp, val, sizeof(group19_prime));
	wpa_hexdump(MSG_DEBUG, "SAE: PMKID", val, 16);
	sha256_prf(keyseed, sizeof(keyseed), "SAE KCK and PMK",
		   val, sizeof(val), keys, sizeof(keys));
	os_memcpy(sae->kck, keys, 32);
	os_memcpy(sae->pmk, keys + 32, 32);
	wpa_hexdump_key(MSG_DEBUG, "SAE: KCK", sae->kck, 32);
	wpa_hexdump_key(MSG_DEBUG, "SAE: PMK", sae->pmk, 32);

	ret = 0;
fail:
	BN_free(order);
	BN_free(own_scalar);
	BN_free(tmp);
	return ret;
}


int sae_process_commit(struct sae_data *sae)
{
	BN_CTX *bnctx;
	EC_GROUP *group;
	int ret = 0;
	u8 k[32];

	if (sae_check_peer_commit(sae) < 0)
		return -1;

	bnctx = BN_CTX_new();
	group = EC_GROUP_new_by_curve_name(NID_X9_62_prime256v1);
	if (bnctx == NULL || group == NULL ||
	    sae_derive_k(sae, k, bnctx, group) < 0 ||
	    sae_derive_keys(sae, k, bnctx) < 0)
		ret = -1;

	EC_GROUP_free(group);
	BN_CTX_free(bnctx);

	return ret;
}


void sae_write_commit(struct sae_data *sae, struct wpabuf *buf)
{
	wpabuf_put_le16(buf, 19); /* Finite Cyclic Group */
	/* TODO: Anti-Clogging Token (if requested) */
	wpabuf_put_data(buf, sae->own_commit_scalar, 32);
	wpabuf_put_data(buf, sae->own_commit_element, 2 * 32);
}


u16 sae_parse_commit(struct sae_data *sae, const u8 *data, size_t len)
{
	const u8 *pos = data, *end = data + len;
	size_t val_len;

	wpa_hexdump(MSG_DEBUG, "SAE: Commit fields", data, len);

	/* Check Finite Cyclic Group */
	if (pos + 2 > end)
		return WLAN_STATUS_UNSPECIFIED_FAILURE;
	if (WPA_GET_LE16(pos) != 19) {
		wpa_printf(MSG_DEBUG, "SAE: Unsupported Finite Cyclic Group %u",
			   WPA_GET_LE16(pos));
		return WLAN_STATUS_FINITE_CYCLIC_GROUP_NOT_SUPPORTED;
	}
	pos += 2;
	val_len = 32;

	if (pos + val_len > end) {
		wpa_printf(MSG_DEBUG, "SAE: Not enough data for scalar");
		return WLAN_STATUS_UNSPECIFIED_FAILURE;
	}
	os_memcpy(sae->peer_commit_scalar, pos, val_len);
	wpa_hexdump(MSG_DEBUG, "SAE: Peer commit-scalar",
		    sae->peer_commit_scalar, val_len);
	pos += val_len;

	if (pos + 2 * val_len > end) {
		wpa_printf(MSG_DEBUG, "SAE: Not enough data for "
			   "commit-element");
		return WLAN_STATUS_UNSPECIFIED_FAILURE;
	}
	os_memcpy(sae->peer_commit_element, pos, 2 * val_len);
	wpa_hexdump(MSG_DEBUG, "SAE: Peer commit-element(x)",
		    sae->peer_commit_element, val_len);
	wpa_hexdump(MSG_DEBUG, "SAE: Peer commit-element(y)",
		    sae->peer_commit_element + val_len, val_len);
	pos += 2 * val_len;

	if (end > pos) {
		wpa_hexdump(MSG_DEBUG, "SAE: Unexpected extra data in commit",
			    pos, end - pos);
	}

	return WLAN_STATUS_SUCCESS;
}


void sae_write_confirm(struct sae_data *sae, struct wpabuf *buf)
{
	const u8 *sc;
	const u8 *addr[5];
	size_t len[5];

	/* Send-Confirm */
	sc = wpabuf_put(buf, 0);
	wpabuf_put_le16(buf, sae->send_confirm);
	sae->send_confirm++;

	/* Confirm
	 * CN(key, X, Y, Z, ...) =
	 *    HMAC-SHA256(key, D2OS(X) || D2OS(Y) || D2OS(Z) | ...)
	 * confirm = CN(KCK, send-confirm, commit-scalar, COMMIT-ELEMENT,
	 *              peer-commit-scalar, PEER-COMMIT-ELEMENT)
	 */
	addr[0] = sc;
	len[0] = 2;
	addr[1] = sae->own_commit_scalar;
	len[1] = 32;
	addr[2] = sae->own_commit_element;
	len[2] = 2 * 32;
	addr[3] = sae->peer_commit_scalar;
	len[3] = 32;
	addr[4] = sae->peer_commit_element;
	len[4] = 2 * 32;
	hmac_sha256_vector(sae->kck, sizeof(sae->kck), 5, addr, len,
			   wpabuf_put(buf, SHA256_MAC_LEN));
}