kexgen.c - third_party/openssh-portable - Git at Google

 /* $OpenBSD: kexgen.c,v 1.2 2019/01/23 00:30:41 djm Exp $ */
 /*
  * Copyright (c) 2019 Markus Friedl.  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 <stdio.h>
 #include <string.h>
 #include <signal.h>

 #include "sshkey.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, u_int32_t, struct ssh *);
 static int input_kex_gen_reply(int type, u_int32_t seq, struct ssh *ssh);

 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
 	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;
 }

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

 	switch (kex->kex_type) {
 #ifdef WITH_OPENSSL
 	case KEX_DH_GRP1_SHA1:
 	case KEX_DH_GRP14_SHA1:
 	case KEX_DH_GRP14_SHA256:
 	case KEX_DH_GRP16_SHA512:
 	case KEX_DH_GRP18_SHA512:
 		r = kex_dh_keypair(kex);
 		break;
 	case KEX_ECDH_SHA2:
 		r = kex_ecdh_keypair(kex);
 		break;
 #endif
 	case KEX_C25519_SHA256:
 		r = kex_c25519_keypair(kex);
 		break;
 	case KEX_KEM_SNTRUP4591761X25519_SHA512:
 		r = kex_kem_sntrup4591761x25519_keypair(kex);
 		break;
 	default:
 		r = SSH_ERR_INVALID_ARGUMENT;
 		break;
 	}
 	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, u_int32_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 *tmp = NULL, *server_host_key_blob = NULL;
 	u_char *signature = NULL;
 	u_char hash[SSH_DIGEST_MAX_LENGTH];
 	size_t slen, hashlen;
 	int r;

 	/* hostkey */
 	if ((r = sshpkt_getb_froms(ssh, &server_host_key_blob)) != 0)
 		goto out;
 	/* sshkey_fromb() consumes its buffer, so make a copy */
 	if ((tmp = sshbuf_fromb(server_host_key_blob)) == NULL) {
 		r = SSH_ERR_ALLOC_FAIL;
 		goto out;
 	}
 	if ((r = sshkey_fromb(tmp, &server_host_key)) != 0)
 		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 */
 	switch (kex->kex_type) {
 #ifdef WITH_OPENSSL
 	case KEX_DH_GRP1_SHA1:
 	case KEX_DH_GRP14_SHA1:
 	case KEX_DH_GRP14_SHA256:
 	case KEX_DH_GRP16_SHA512:
 	case KEX_DH_GRP18_SHA512:
 		r = kex_dh_dec(kex, server_blob, &shared_secret);
 		break;
 	case KEX_ECDH_SHA2:
 		r = kex_ecdh_dec(kex, server_blob, &shared_secret);
 		break;
 #endif
 	case KEX_C25519_SHA256:
 		r = kex_c25519_dec(kex, server_blob, &shared_secret);
 		break;
 	case KEX_KEM_SNTRUP4591761X25519_SHA512:
 		r = kex_kem_sntrup4591761x25519_dec(kex, server_blob,
 		    &shared_secret);
 		break;
 	default:
 		r = SSH_ERR_INVALID_ARGUMENT;
 		break;
 	}
 	if (r !=0 )
 		goto out;

 	/* calc and verify H */
 	hashlen = sizeof(hash);
 	if ((r = kex_gen_hash(
 	    kex->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;

 	if ((r = sshkey_verify(server_host_key, signature, slen, hash, hashlen,
 	    kex->hostkey_alg, ssh->compat)) != 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));
 	explicit_bzero(kex->c25519_client_key, sizeof(kex->c25519_client_key));
 	explicit_bzero(kex->sntrup4591761_client_key,
 	    sizeof(kex->sntrup4591761_client_key));
 	sshbuf_free(server_host_key_blob);
 	free(signature);
 	sshbuf_free(tmp);
 	sshkey_free(server_host_key);
 	sshbuf_free(server_blob);
 	sshbuf_free(shared_secret);
 	sshbuf_free(kex->client_pub);
 	kex->client_pub = NULL;
 	return r;
 }

 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, u_int32_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;

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

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

 	/* compute shared secret */
 	switch (kex->kex_type) {
 #ifdef WITH_OPENSSL
 	case KEX_DH_GRP1_SHA1:
 	case KEX_DH_GRP14_SHA1:
 	case KEX_DH_GRP14_SHA256:
 	case KEX_DH_GRP16_SHA512:
 	case KEX_DH_GRP18_SHA512:
 		r = kex_dh_enc(kex, client_pubkey, &server_pubkey,
 		    &shared_secret);
 		break;
 	case KEX_ECDH_SHA2:
 		r = kex_ecdh_enc(kex, client_pubkey, &server_pubkey,
 		    &shared_secret);
 		break;
 #endif
 	case KEX_C25519_SHA256:
 		r = kex_c25519_enc(kex, client_pubkey, &server_pubkey,
 		    &shared_secret);
 		break;
 	case KEX_KEM_SNTRUP4591761X25519_SHA512:
 		r = kex_kem_sntrup4591761x25519_enc(kex, client_pubkey,
 		    &server_pubkey, &shared_secret);
 		break;
 	default:
 		r = SSH_ERR_INVALID_ARGUMENT;
 		break;
 	}
 	if (r !=0 )
 		goto out;

 	/* calc H */
 	if ((server_host_key_blob = sshbuf_new()) == NULL) {
 		r = SSH_ERR_ALLOC_FAIL;
 		goto out;
 	}
 	if ((r = sshkey_putb(server_host_public, server_host_key_blob)) != 0)
 		goto out;
 	hashlen = sizeof(hash);
 	if ((r = kex_gen_hash(
 	    kex->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 */
 	if ((r = kex->sign(ssh, server_host_private, server_host_public,
 	     &signature, &slen, hash, hashlen, kex->hostkey_alg)) != 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));
 	sshbuf_free(server_host_key_blob);
 	free(signature);
 	sshbuf_free(shared_secret);
 	sshbuf_free(client_pubkey);
 	sshbuf_free(server_pubkey);
 	return r;
 }
	/* $OpenBSD: kexgen.c,v 1.2 2019/01/23 00:30:41 djm Exp $ */
	/*
	* Copyright (c) 2019 Markus Friedl. 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 <stdio.h>
	#include <string.h>
	#include <signal.h>

	#include "sshkey.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, u_int32_t, struct ssh *);
	static int input_kex_gen_reply(int type, u_int32_t seq, struct ssh *ssh);

	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
	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;
	}

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

	switch (kex->kex_type) {
	#ifdef WITH_OPENSSL
	case KEX_DH_GRP1_SHA1:
	case KEX_DH_GRP14_SHA1:
	case KEX_DH_GRP14_SHA256:
	case KEX_DH_GRP16_SHA512:
	case KEX_DH_GRP18_SHA512:
	r = kex_dh_keypair(kex);
	break;
	case KEX_ECDH_SHA2:
	r = kex_ecdh_keypair(kex);
	break;
	#endif
	case KEX_C25519_SHA256:
	r = kex_c25519_keypair(kex);
	break;
	case KEX_KEM_SNTRUP4591761X25519_SHA512:
	r = kex_kem_sntrup4591761x25519_keypair(kex);
	break;
	default:
	r = SSH_ERR_INVALID_ARGUMENT;
	break;
	}
	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, u_int32_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 tmp = NULL, server_host_key_blob = NULL;
	u_char *signature = NULL;
	u_char hash[SSH_DIGEST_MAX_LENGTH];
	size_t slen, hashlen;
	int r;

	/* hostkey */
	if ((r = sshpkt_getb_froms(ssh, &server_host_key_blob)) != 0)
	goto out;
	/* sshkey_fromb() consumes its buffer, so make a copy */
	if ((tmp = sshbuf_fromb(server_host_key_blob)) == NULL) {
	r = SSH_ERR_ALLOC_FAIL;
	goto out;
	}
	if ((r = sshkey_fromb(tmp, &server_host_key)) != 0)
	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 */
	switch (kex->kex_type) {
	#ifdef WITH_OPENSSL
	case KEX_DH_GRP1_SHA1:
	case KEX_DH_GRP14_SHA1:
	case KEX_DH_GRP14_SHA256:
	case KEX_DH_GRP16_SHA512:
	case KEX_DH_GRP18_SHA512:
	r = kex_dh_dec(kex, server_blob, &shared_secret);
	break;
	case KEX_ECDH_SHA2:
	r = kex_ecdh_dec(kex, server_blob, &shared_secret);
	break;
	#endif
	case KEX_C25519_SHA256:
	r = kex_c25519_dec(kex, server_blob, &shared_secret);
	break;
	case KEX_KEM_SNTRUP4591761X25519_SHA512:
	r = kex_kem_sntrup4591761x25519_dec(kex, server_blob,
	&shared_secret);
	break;
	default:
	r = SSH_ERR_INVALID_ARGUMENT;
	break;
	}
	if (r !=0 )
	goto out;

	/* calc and verify H */
	hashlen = sizeof(hash);
	if ((r = kex_gen_hash(
	kex->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;

	if ((r = sshkey_verify(server_host_key, signature, slen, hash, hashlen,
	kex->hostkey_alg, ssh->compat)) != 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));
	explicit_bzero(kex->c25519_client_key, sizeof(kex->c25519_client_key));
	explicit_bzero(kex->sntrup4591761_client_key,
	sizeof(kex->sntrup4591761_client_key));
	sshbuf_free(server_host_key_blob);
	free(signature);
	sshbuf_free(tmp);
	sshkey_free(server_host_key);
	sshbuf_free(server_blob);
	sshbuf_free(shared_secret);
	sshbuf_free(kex->client_pub);
	kex->client_pub = NULL;
	return r;
	}

	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, u_int32_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;

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

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

	/* compute shared secret */
	switch (kex->kex_type) {
	#ifdef WITH_OPENSSL
	case KEX_DH_GRP1_SHA1:
	case KEX_DH_GRP14_SHA1:
	case KEX_DH_GRP14_SHA256:
	case KEX_DH_GRP16_SHA512:
	case KEX_DH_GRP18_SHA512:
	r = kex_dh_enc(kex, client_pubkey, &server_pubkey,
	&shared_secret);
	break;
	case KEX_ECDH_SHA2:
	r = kex_ecdh_enc(kex, client_pubkey, &server_pubkey,
	&shared_secret);
	break;
	#endif
	case KEX_C25519_SHA256:
	r = kex_c25519_enc(kex, client_pubkey, &server_pubkey,
	&shared_secret);
	break;
	case KEX_KEM_SNTRUP4591761X25519_SHA512:
	r = kex_kem_sntrup4591761x25519_enc(kex, client_pubkey,
	&server_pubkey, &shared_secret);
	break;
	default:
	r = SSH_ERR_INVALID_ARGUMENT;
	break;
	}
	if (r !=0 )
	goto out;

	/* calc H */
	if ((server_host_key_blob = sshbuf_new()) == NULL) {
	r = SSH_ERR_ALLOC_FAIL;
	goto out;
	}
	if ((r = sshkey_putb(server_host_public, server_host_key_blob)) != 0)
	goto out;
	hashlen = sizeof(hash);
	if ((r = kex_gen_hash(
	kex->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 */
	if ((r = kex->sign(ssh, server_host_private, server_host_public,
	&signature, &slen, hash, hashlen, kex->hostkey_alg)) != 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));
	sshbuf_free(server_host_key_blob);
	free(signature);
	sshbuf_free(shared_secret);
	sshbuf_free(client_pubkey);
	sshbuf_free(server_pubkey);
	return r;
	}