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move client/server SSH-* banners to buffers under ssh->kex and factor
out the banner exchange. This eliminates some common code from the
client and server.

Also be more strict about handling \r characters - these should only
be accepted immediately before \n (pointed out by Jann Horn).

Inspired by a patch from Markus Schmidt.
(lots of) feedback and ok markus@

/* $OpenBSD: kexecdh.c,v 1.7 2018/12/27 03:25:25 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 <signal.h>
#include <string.h>

#include <openssl/bn.h>
#include <openssl/evp.h>
#include <openssl/ec.h>
#include <openssl/ecdh.h>

#include "ssh2.h"
#include "sshkey.h"
#include "cipher.h"
#include "kex.h"
#include "sshbuf.h"
#include "digest.h"
#include "ssherr.h"

int
kex_ecdh_hash(
    int hash_alg,
    const EC_GROUP *ec_group,
    const struct sshbuf *client_version,
    const struct sshbuf *server_version,
    const u_char *ckexinit, size_t ckexinitlen,
    const u_char *skexinit, size_t skexinitlen,
    const u_char *serverhostkeyblob, size_t sbloblen,
    const EC_POINT *client_dh_pub,
    const EC_POINT *server_dh_pub,
    const BIGNUM *shared_secret,
    u_char *hash, size_t *hashlen)
{
	struct sshbuf *b;
	int r;

	if (*hashlen < ssh_digest_bytes(hash_alg))
		return SSH_ERR_INVALID_ARGUMENT;
	if ((b = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	if ((r = sshbuf_put_stringb(b, client_version)) < 0 ||
	    (r = sshbuf_put_stringb(b, server_version)) < 0 ||
	    /* kexinit messages: fake header: len+SSH2_MSG_KEXINIT */
	    (r = sshbuf_put_u32(b, ckexinitlen+1)) != 0 ||
	    (r = sshbuf_put_u8(b, SSH2_MSG_KEXINIT)) != 0 ||
	    (r = sshbuf_put(b, ckexinit, ckexinitlen)) != 0 ||
	    (r = sshbuf_put_u32(b, skexinitlen+1)) != 0 ||
	    (r = sshbuf_put_u8(b, SSH2_MSG_KEXINIT)) != 0 ||
	    (r = sshbuf_put(b, skexinit, skexinitlen)) != 0 ||
	    (r = sshbuf_put_string(b, serverhostkeyblob, sbloblen)) != 0 ||
	    (r = sshbuf_put_ec(b, client_dh_pub, ec_group)) != 0 ||
	    (r = sshbuf_put_ec(b, server_dh_pub, ec_group)) != 0 ||
	    (r = sshbuf_put_bignum2(b, shared_secret)) != 0) {
		sshbuf_free(b);
		return r;
	}
#ifdef DEBUG_KEX
	sshbuf_dump(b, stderr);
#endif
	if (ssh_digest_buffer(hash_alg, b, hash, *hashlen) != 0) {
		sshbuf_free(b);
		return SSH_ERR_LIBCRYPTO_ERROR;
	}
	sshbuf_free(b);
	*hashlen = ssh_digest_bytes(hash_alg);
#ifdef DEBUG_KEX
	dump_digest("hash", hash, *hashlen);
#endif
	return 0;
}