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Implement Elliptic Curve Cryptography modes for key exchange (ECDH) and
host/user keys (ECDSA) as specified by RFC5656. ECDH and ECDSA offer
better performance than plain DH and DSA at the same equivalent symmetric
key length, as well as much shorter keys.

Only the mandatory sections of RFC5656 are implemented, specifically the
three REQUIRED curves nistp256, nistp384 and nistp521 and only ECDH and
ECDSA. Point compression (optional in RFC5656 is NOT implemented).

Certificate host and user keys using the new ECDSA key types are supported.

Note that this code has not been tested for interoperability and may be
subject to change.

feedback and ok markus@

/* $OpenBSD: kexecdhs.c,v 1.1 2010/08/31 11:54:45 djm Exp $ */
/*
 * Copyright (c) 2001 Markus Friedl.  All rights reserved.
 * Copyright (c) 2010 Damien Miller.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <sys/types.h>
#include <string.h>
#include <signal.h>

#include <openssl/ecdh.h>

#include "xmalloc.h"
#include "buffer.h"
#include "key.h"
#include "cipher.h"
#include "kex.h"
#include "log.h"
#include "packet.h"
#include "dh.h"
#include "ssh2.h"
#ifdef GSSAPI
#include "ssh-gss.h"
#endif
#include "monitor_wrap.h"

void
kexecdh_server(Kex *kex)
{
	EC_POINT *client_public;
	EC_KEY *server_key;
	const EC_GROUP *group;
	BIGNUM *shared_secret;
	Key *server_host_private, *server_host_public;
	u_char *server_host_key_blob = NULL, *signature = NULL;
	u_char *kbuf, *hash;
	u_int klen, slen, sbloblen, hashlen;
	int curve_nid;

	curve_nid = kex_ecdh_name_to_nid(kex->name);
	if ((server_key = EC_KEY_new_by_curve_name(curve_nid)) == NULL)
		fatal("%s: EC_KEY_new_by_curve_name failed", __func__);
	if (EC_KEY_generate_key(server_key) != 1)
		fatal("%s: EC_KEY_generate_key failed", __func__);
	group = EC_KEY_get0_group(server_key);

#ifdef DEBUG_KEXECDH
	fputs("server private key:\n", stderr);
	key_dump_ec_key(server_key);
#endif

	if (kex->load_host_public_key == NULL ||
	    kex->load_host_private_key == NULL)
		fatal("Cannot load hostkey");
	server_host_public = kex->load_host_public_key(kex->hostkey_type);
	if (server_host_public == NULL)
		fatal("Unsupported hostkey type %d", kex->hostkey_type);
	server_host_private = kex->load_host_private_key(kex->hostkey_type);
	if (server_host_private == NULL)
		fatal("Missing private key for hostkey type %d",
		    kex->hostkey_type);

	debug("expecting SSH2_MSG_KEX_ECDH_INIT");
	packet_read_expect(SSH2_MSG_KEX_ECDH_INIT);
	if ((client_public = EC_POINT_new(group)) == NULL)
		fatal("%s: EC_POINT_new failed", __func__);
	packet_get_ecpoint(group, client_public);
	packet_check_eom();

	if (key_ec_validate_public(group, client_public) != 0)
		fatal("%s: invalid client public key", __func__);

#ifdef DEBUG_KEXECDH
	fputs("client public key:\n", stderr);
	key_dump_ec_point(group, client_public);
#endif

	/* Calculate shared_secret */
	klen = (EC_GROUP_get_degree(group) + 7) / 8;
	kbuf = xmalloc(klen);
	if (ECDH_compute_key(kbuf, klen, client_public,
	    server_key, NULL) != (int)klen)
		fatal("%s: ECDH_compute_key failed", __func__);

#ifdef DEBUG_KEXDH
	dump_digest("shared secret", kbuf, klen);
#endif
	if ((shared_secret = BN_new()) == NULL)
		fatal("%s: BN_new failed", __func__);
	if (BN_bin2bn(kbuf, klen, shared_secret) == NULL)
		fatal("%s: BN_bin2bn failed", __func__);
	memset(kbuf, 0, klen);
	xfree(kbuf);

	/* calc H */
	key_to_blob(server_host_public, &server_host_key_blob, &sbloblen);
	kex_ecdh_hash(
	    kex->evp_md,
	    group,
	    kex->client_version_string,
	    kex->server_version_string,
	    buffer_ptr(&kex->peer), buffer_len(&kex->peer),
	    buffer_ptr(&kex->my), buffer_len(&kex->my),
	    server_host_key_blob, sbloblen,
	    client_public,
	    EC_KEY_get0_public_key(server_key),
	    shared_secret,
	    &hash, &hashlen
	);
	EC_POINT_clear_free(client_public);

	/* save session id := H */
	if (kex->session_id == NULL) {
		kex->session_id_len = hashlen;
		kex->session_id = xmalloc(kex->session_id_len);
		memcpy(kex->session_id, hash, kex->session_id_len);
	}

	/* sign H */
	if (PRIVSEP(key_sign(server_host_private, &signature, &slen,
	    hash, hashlen)) < 0)
		fatal("kexdh_server: key_sign failed");

	/* destroy_sensitive_data(); */

	/* send server hostkey, ECDH pubkey 'Q_S' and signed H */
	packet_start(SSH2_MSG_KEX_ECDH_REPLY);
	packet_put_string(server_host_key_blob, sbloblen);
	packet_put_ecpoint(group, EC_KEY_get0_public_key(server_key));
	packet_put_string(signature, slen);
	packet_send();

	xfree(signature);
	xfree(server_host_key_blob);
	/* have keys, free server key */
	EC_KEY_free(server_key);

	kex_derive_keys(kex, hash, hashlen, shared_secret);
	BN_clear_free(shared_secret);
	kex_finish(kex);
}