/* $OpenBSD: kexgen.c,v 1.12 2026/03/03 09:57:25 dtucker Exp $ */
/*
 * Copyright (c) 2019 Markus Friedl.  All rights reserved.
 * Copyright (c) 2019-2024 Roumen Petrov.  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 "includes.h"

#include <sys/types.h>

#include <stdarg.h>
#include <stdio.h>
#include <string.h>

#include "sshxkey.h"
#include "kex.h"
#include "log.h"
#include "packet.h"
#include "ssh2.h"
#include "sshbuf.h"
#include "digest.h"
#include "ssherr.h"

static int input_kex_gen_init(int, uint32_t, struct ssh *);
static int input_kex_gen_reply(int type, uint32_t seq, struct ssh *ssh);

static int
kex_new_sshbuf(u_char *kbuf, size_t klen, struct sshbuf **retp) {
	struct sshbuf *buf;
	int r;

	if ((buf = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;

	r = sshbuf_put_string(buf, kbuf, klen);
	if (r == 0)
		*retp = buf;
	else
		sshbuf_free(buf);
	return r;
}

int
kex_digest_buffer(int hash_alg, struct sshbuf *buf, struct sshbuf **retp) {
	u_char hash[SSH_DIGEST_MAX_LENGTH];
	int r;

	*retp = NULL;

	r = ssh_digest_buffer(hash_alg, buf, hash, sizeof(hash));
	if (r != 0) goto out;

	r = kex_new_sshbuf(hash, ssh_digest_bytes(hash_alg), retp);

out:
	explicit_bzero(hash, sizeof(hash));
	return r;
}

static int
kex_gen_hash(
    int hash_alg,
    const struct sshbuf *client_version,
    const struct sshbuf *server_version,
    const struct sshbuf *client_kexinit,
    const struct sshbuf *server_kexinit,
    const struct sshbuf *server_host_key_blob,
    const struct sshbuf *client_pub,
    const struct sshbuf *server_pub,
    const struct sshbuf *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, sshbuf_len(client_kexinit) + 1)) != 0 ||
	    (r = sshbuf_put_u8(b, SSH2_MSG_KEXINIT)) != 0 ||
	    (r = sshbuf_putb(b, client_kexinit)) != 0 ||
	    (r = sshbuf_put_u32(b, sshbuf_len(server_kexinit) + 1)) != 0 ||
	    (r = sshbuf_put_u8(b, SSH2_MSG_KEXINIT)) != 0 ||
	    (r = sshbuf_putb(b, server_kexinit)) != 0 ||
	    (r = sshbuf_put_stringb(b, server_host_key_blob)) != 0 ||
	    (r = sshbuf_put_stringb(b, client_pub)) != 0 ||
	    (r = sshbuf_put_stringb(b, server_pub)) != 0 ||
	    (r = sshbuf_putb(b, shared_secret)) != 0) {
		sshbuf_free(b);
		return r;
	}
#ifdef DEBUG_KEX
	dump_digestb("hash-input", b);
#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;
}

static int
kex_gen_client(struct ssh *ssh)
{
	struct kex *kex = ssh->kex;
	int r;

	if (kex->impl != NULL) {
		r = kex->impl->funcs->keypair(kex);
		if (r != 0)
			kex_reset_keys(kex);
	} else
		r = SSH_ERR_INVALID_ARGUMENT;
	if (r != 0) return r;

	if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_ECDH_INIT)) != 0 ||
	    (r = sshpkt_put_stringb(ssh, kex->client_pub)) != 0 ||
	    (r = sshpkt_send(ssh)) != 0)
		return r;
	debug("expecting SSH2_MSG_KEX_ECDH_REPLY");
	ssh_dispatch_set(ssh, SSH2_MSG_KEX_ECDH_REPLY, &input_kex_gen_reply);
	return 0;
}

static int
input_kex_gen_reply(int type, uint32_t seq, struct ssh *ssh)
{
	struct kex *kex = ssh->kex;
	struct sshkey *server_host_key = NULL;
	struct sshbuf *shared_secret = NULL;
	struct sshbuf *server_blob = NULL;
	struct sshbuf *server_host_key_blob = NULL;
	u_char *signature = NULL;
	u_char hash[SSH_DIGEST_MAX_LENGTH];
	size_t slen, hashlen;
	int r;

	UNUSED(type);
	UNUSED(seq);

	debug("SSH2_MSG_KEX_ECDH_REPLY received");
	ssh_dispatch_set(ssh, SSH2_MSG_KEX_ECDH_REPLY, &kex_protocol_error);

	/* hostkey */
	r = sshpkt_getb_froms(ssh, &server_host_key_blob);
	if (r != 0) goto out;

	r = Xkey_from_blob(kex->hostkey_alg,
	    sshbuf_ptr(server_host_key_blob), sshbuf_len(server_host_key_blob),
	    &server_host_key);
	if (r != SSH_ERR_SUCCESS) goto out;

	if ((r = kex_verify_host_key(ssh, server_host_key)) != 0)
		goto out;

	/* Q_S, server public key */
	/* signed H */
	if ((r = sshpkt_getb_froms(ssh, &server_blob)) != 0 ||
	    (r = sshpkt_get_string(ssh, &signature, &slen)) != 0 ||
	    (r = sshpkt_get_end(ssh)) != 0)
		goto out;

	/* compute shared secret */
	if (kex->impl != NULL)
		r = kex->impl->funcs->dec(kex, server_blob, &shared_secret);
	else
		r = SSH_ERR_INVALID_ARGUMENT;
	if (r != 0) goto out;

	/* calc and verify H */
	hashlen = sizeof(hash);
	if ((r = kex_gen_hash(
	    kex->impl->hash_alg,
	    kex->client_version,
	    kex->server_version,
	    kex->my,
	    kex->peer,
	    server_host_key_blob,
	    kex->client_pub,
	    server_blob,
	    shared_secret,
	    hash, &hashlen)) != 0)
		goto out;

{	ssh_verify_ctx ctx = { kex->hostkey_alg, server_host_key, &ssh->compat };

	r = Xkey_verify(&ctx, signature, slen, hash, hashlen);
	if (r != 0) goto out;
}

	if ((r = kex_derive_keys(ssh, hash, hashlen, shared_secret)) == 0)
		r = kex_send_newkeys(ssh);
out:
	explicit_bzero(hash, sizeof(hash));
	kex_reset_keys(kex);
	sshbuf_free(server_host_key_blob);
	free(signature);
	sshkey_free(server_host_key);
	sshbuf_free(server_blob);
	sshbuf_free(shared_secret);
	sshbuf_free(kex->client_pub);
	kex->client_pub = NULL;
	return r;
}

static int
kex_gen_server(struct ssh *ssh)
{
	debug("expecting SSH2_MSG_KEX_ECDH_INIT");
	ssh_dispatch_set(ssh, SSH2_MSG_KEX_ECDH_INIT, &input_kex_gen_init);
	return 0;
}

static int
input_kex_gen_init(int type, uint32_t seq, struct ssh *ssh)
{
	struct kex *kex = ssh->kex;
	struct sshkey *server_host_private, *server_host_public;
	struct sshbuf *shared_secret = NULL;
	struct sshbuf *server_pubkey = NULL;
	struct sshbuf *client_pubkey = NULL;
	struct sshbuf *server_host_key_blob = NULL;
	u_char *signature = NULL, hash[SSH_DIGEST_MAX_LENGTH];
	size_t slen, hashlen;
	int r;

	UNUSED(type);
	UNUSED(seq);

	debug("SSH2_MSG_KEX_ECDH_INIT received");
	ssh_dispatch_set(ssh, SSH2_MSG_KEX_ECDH_INIT, &kex_protocol_error);

	if ((r = kex_load_host_keys(ssh, &server_host_public,
	    &server_host_private)) != 0)
		goto out;

	if ((r = sshpkt_getb_froms(ssh, &client_pubkey)) != 0 ||
	    (r = sshpkt_get_end(ssh)) != 0)
		goto out;

	/* compute shared secret */
	if (kex->impl != NULL)
		r = kex->impl->funcs->enc(kex, client_pubkey, &server_pubkey,
		    &shared_secret);
	else
		r = SSH_ERR_INVALID_ARGUMENT;
	if (r != 0) goto out;

	/* calc H */
	if ((server_host_key_blob = sshbuf_new()) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	r = Xkey_putb(kex->hostkey_alg, server_host_public, server_host_key_blob);
	if (r != 0) goto out;

	hashlen = sizeof(hash);
	if ((r = kex_gen_hash(
	    kex->impl->hash_alg,
	    kex->client_version,
	    kex->server_version,
	    kex->peer,
	    kex->my,
	    server_host_key_blob,
	    client_pubkey,
	    server_pubkey,
	    shared_secret,
	    hash, &hashlen)) != 0)
		goto out;

	/* sign H */
{	ssh_sign_ctx ctx = { kex->hostkey_alg, server_host_private, &ssh->compat, NULL, NULL };

	r = kex->xsign(ssh, &ctx, server_host_public, &signature, &slen, hash, hashlen);
	if (r != 0) goto out;
}

	/* send server hostkey, ECDH pubkey 'Q_S' and signed H */
	if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_ECDH_REPLY)) != 0 ||
	    (r = sshpkt_put_stringb(ssh, server_host_key_blob)) != 0 ||
	    (r = sshpkt_put_stringb(ssh, server_pubkey)) != 0 ||
	    (r = sshpkt_put_string(ssh, signature, slen)) != 0 ||
	    (r = sshpkt_send(ssh)) != 0)
		goto out;

	if ((r = kex_derive_keys(ssh, hash, hashlen, shared_secret)) == 0)
		r = kex_send_newkeys(ssh);
out:
	explicit_bzero(hash, sizeof(hash));
	kex_reset_keys(kex);
	sshbuf_free(server_host_key_blob);
	free(signature);
	sshbuf_free(shared_secret);
	sshbuf_free(client_pubkey);
	sshbuf_free(server_pubkey);
	return r;
}

int
kex_init_gen(struct ssh *ssh) {
	struct kex *kex = ssh->kex;

	return kex->server
		? kex_gen_server(ssh)
		: kex_gen_client(ssh);
}
