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In the final RFC 5903 the computation for the DH shared secret changed.
Instead of the full point, only the X point is included.

The member g_xy is always the shared secret but so far its buffer has
been allocated using the size of the public points.  Since this is a
different size now, as the shared secret for EC Groups should only store
the x point, we need another member to specify the length of g_xy.

Since this is a backwards incompatible change older isakmpds won't be
able to negotiate if you use EC groups.  Bump the version of our own
vendor tag so peers can try to keep compatibility based on the presen-
ted tag.  This could be used to implement backwards compatibility to
older isakmpds.

Prompted by and ok mpi@

/*	$OpenBSD: vendor.c,v 1.6 2017/11/08 13:33:49 patrick Exp $	*/
/*
 * Copyright (c) 2006 Hans-Joerg Hoexer <hshoexer@openbsd.org>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

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

#include "exchange.h"
#include "hash.h"
#include "log.h"
#include "message.h"
#include "vendor.h"

static struct vendor_cap openbsd_vendor_cap[] = {
	{ "OpenBSD-6.3", NULL, 0 },
};

#define NUMVIDS	(sizeof openbsd_vendor_cap / sizeof openbsd_vendor_cap[0])

static int
setup_vendor_hashes(void)
{
	struct hash	*hash;
	int		 i, n = NUMVIDS;

	hash = hash_get(HASH_MD5);
	if (!hash) {
		log_print("setup_vendor_hashes: could not find MD5 hash");
		return -1;
	}

	for (i = 0; i < n; i++) {
		openbsd_vendor_cap[i].hashsize = hash->hashsize;
		openbsd_vendor_cap[i].hash = calloc(hash->hashsize,
		    sizeof(u_int8_t));
		if (openbsd_vendor_cap[i].hash == NULL) {
			log_error("setup_vendor_hashes: calloc failed");
			goto errout;
		}

		hash->Init(hash->ctx);
		hash->Update(hash->ctx,
		    (unsigned char *)openbsd_vendor_cap[i].text,
		    strlen(openbsd_vendor_cap[i].text));
		hash->Final(openbsd_vendor_cap[i].hash, hash->ctx);

		LOG_DBG((LOG_EXCHANGE, 50, "setup_vendor_hashes: "
		    "MD5(\"%s\") (%lu bytes)", openbsd_vendor_cap[i].text,
		    (unsigned long)hash->hashsize));
		LOG_DBG_BUF((LOG_EXCHANGE, 50, "setup_vendor_hashes",
		    openbsd_vendor_cap[i].hash, hash->hashsize));
	}
	return 0;

errout:
	for (i = 0; i < n; i++)
		free(openbsd_vendor_cap[i].hash);
	return -1;
}

void
vendor_init(void)
{
	setup_vendor_hashes();
}

int
add_vendor_openbsd(struct message *msg)
{
	u_int8_t	*buf;
	size_t		 buflen;
	int		 i, n = NUMVIDS;

	for (i = 0; i < n; i++) {
		buflen = openbsd_vendor_cap[i].hashsize + ISAKMP_GEN_SZ;
		if ((buf = calloc(buflen, sizeof(char))) == NULL) {
			log_error("add_vendor_payload: calloc(%lu) failed",
			    (unsigned long)buflen);
			return -1;
		}

		SET_ISAKMP_GEN_LENGTH(buf, buflen);
		memcpy(buf + ISAKMP_VENDOR_ID_OFF, openbsd_vendor_cap[i].hash,
		    openbsd_vendor_cap[i].hashsize);
		if (message_add_payload(msg, ISAKMP_PAYLOAD_VENDOR, buf,
		    buflen, 1)) {
			free(buf);
			return -1;
		}
	}

	return 0;
}

void
check_vendor_openbsd(struct message *msg, struct payload *p)
{
	u_int8_t	*pbuf = p->p;
	ssize_t		 vlen;
	int		 i, n = NUMVIDS;

	if (msg->exchange->flags & EXCHANGE_FLAG_OPENBSD) {
		p->flags |= PL_MARK;
		return;
	}

	vlen = GET_ISAKMP_GEN_LENGTH(pbuf) - ISAKMP_GEN_SZ;

	for (i = 0; i < n; i++) {
		if (vlen != openbsd_vendor_cap[i].hashsize) {
			LOG_DBG((LOG_EXCHANGE, 90,
			    "check_vendor_openbsd: bad size %lu != %lu",
			    (unsigned long)vlen,
			    (unsigned long)openbsd_vendor_cap[i].hashsize));
			continue;
		}
		if (memcmp(openbsd_vendor_cap[i].hash, pbuf + ISAKMP_GEN_SZ,
		    vlen) == 0) {
			msg->exchange->flags |= EXCHANGE_FLAG_OPENBSD;
			LOG_DBG((LOG_EXCHANGE, 10, "check_vendor_openbsd: "
			    "OpenBSD (%s)", openbsd_vendor_cap[i].text));
		}
		p->flags |= PL_MARK;
	}
}