/* $OpenBSD: kexgexs.c,v 1.51 2026/03/03 09:57:25 dtucker Exp $ */

/*

 * Copyright (c) 2000 Niels Provos.  All rights reserved.

 * Copyright (c) 2001 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"

#ifdef WITH_OPENSSL

#include "openbsd-compat/openssl-compat.h"

#include <stdarg.h>

#include <stdio.h>

#include <string.h>

#include <signal.h>

#include <openssl/bn.h>

#include <openssl/dh.h>

#include "sshkey.h"

#include "cipher.h"

#include "digest.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"

#include "dispatch.h"

#include "ssherr.h"

#include "sshbuf.h"

#include "misc.h"

static int input_kex_dh_gex_request(int, uint32_t, struct ssh *);

static int input_kex_dh_gex_init(int, uint32_t, struct ssh *);

int

kexgex_server(struct ssh *ssh)

{

  ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_REQUEST,

      &input_kex_dh_gex_request);

  debug("expecting SSH2_MSG_KEX_DH_GEX_REQUEST");

  return 0;

}

static int

input_kex_dh_gex_request(int type, uint32_t seq, struct ssh *ssh)

{

  struct kex *kex = ssh->kex;

  int r;

  u_int min = 0, max = 0, nbits = 0;

  const BIGNUM *dh_p, *dh_g;

  debug("SSH2_MSG_KEX_DH_GEX_REQUEST received");

  ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_REQUEST, &kex_protocol_error);

  if ((r = sshpkt_get_u32(ssh, &min)) != 0 ||

      (r = sshpkt_get_u32(ssh, &nbits)) != 0 ||

      (r = sshpkt_get_u32(ssh, &max)) != 0 ||

      (r = sshpkt_get_end(ssh)) != 0)

    goto out;

  kex->nbits = nbits;

  kex->min = min;

  kex->max = max;

  min = MAXIMUM(DH_GRP_MIN, min);

  max = MINIMUM(DH_GRP_MAX, max);

  nbits = MAXIMUM(DH_GRP_MIN, nbits);

  nbits = MINIMUM(DH_GRP_MAX, nbits);

  if (kex->max < kex->min || kex->nbits < kex->min ||

      kex->max < kex->nbits || kex->max < DH_GRP_MIN) {

    r = SSH_ERR_DH_GEX_OUT_OF_RANGE;

    goto out;

  }

  /* Contact privileged parent */

  kex->dh = mm_choose_dh(min, nbits, max);

  if (kex->dh == NULL) {

    (void)sshpkt_disconnect(ssh, "no matching DH grp found");

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  debug("SSH2_MSG_KEX_DH_GEX_GROUP sent");

  DH_get0_pqg(kex->dh, &dh_p, NULL, &dh_g);

  if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_DH_GEX_GROUP)) != 0 ||

      (r = sshpkt_put_bignum2(ssh, dh_p)) != 0 ||

      (r = sshpkt_put_bignum2(ssh, dh_g)) != 0 ||

      (r = sshpkt_send(ssh)) != 0)

    goto out;

  /* Compute our exchange value in parallel with the client */

  if ((r = dh_gen_key(kex->dh, kex->we_need * 8)) != 0)

    goto out;

  debug("expecting SSH2_MSG_KEX_DH_GEX_INIT");

  ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_INIT, &input_kex_dh_gex_init);

  r = 0;

 out:

  return r;

}

static int

input_kex_dh_gex_init(int type, uint32_t seq, struct ssh *ssh)

{

  struct kex *kex = ssh->kex;

  BIGNUM *dh_client_pub = NULL;

  const BIGNUM *pub_key, *dh_p, *dh_g;

  struct sshbuf *shared_secret = NULL;

  struct sshbuf *server_host_key_blob = NULL;

  struct sshkey *server_host_public, *server_host_private;

  u_char *signature = NULL;

  u_char hash[SSH_DIGEST_MAX_LENGTH];

  size_t slen, hashlen;

  int r;

  debug("SSH2_MSG_KEX_DH_GEX_INIT received");

  ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_INIT, &kex_protocol_error);

  if ((r = kex_load_hostkey(ssh, &server_host_private,

      &server_host_public)) != 0)

    goto out;

  /* key, cert */

  if ((r = sshpkt_get_bignum2(ssh, &dh_client_pub)) != 0 ||

      (r = sshpkt_get_end(ssh)) != 0)

    goto out;

  if ((shared_secret = sshbuf_new()) == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if ((r = kex_dh_compute_key(kex, dh_client_pub, shared_secret)) != 0)

    goto out;

  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;

  /* calc H */

  DH_get0_key(kex->dh, &pub_key, NULL);

  DH_get0_pqg(kex->dh, &dh_p, NULL, &dh_g);

  hashlen = sizeof(hash);

  if ((r = kexgex_hash(

      kex->hash_alg,

      kex->client_version,

      kex->server_version,

      kex->peer,

      kex->my,

      server_host_key_blob,

      kex->min, kex->nbits, kex->max,

      dh_p, dh_g,

      dh_client_pub,

      pub_key,

      sshbuf_ptr(shared_secret), sshbuf_len(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, DH pubkey 'f' and signed H */

  if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_DH_GEX_REPLY)) != 0 ||

      (r = sshpkt_put_stringb(ssh, server_host_key_blob)) != 0 ||

      (r = sshpkt_put_bignum2(ssh, pub_key)) != 0 ||     /* f */

      (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)) != 0)

    goto out;

  /* retain copy of hostkey used at initial KEX */

  if (kex->initial_hostkey == NULL &&

      (r = sshkey_from_private(server_host_public,

      &kex->initial_hostkey)) != 0)

    goto out;

  /* success */

 out:

  explicit_bzero(hash, sizeof(hash));

  DH_free(kex->dh);

  kex->dh = NULL;

  BN_clear_free(dh_client_pub);

  sshbuf_free(shared_secret);

  sshbuf_free(server_host_key_blob);

  free(signature);

  return r;

}

#endif /* WITH_OPENSSL */