/* $OpenBSD: kexgexc.c,v 1.43 2026/09/16 00:31:27 djm 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 <sys/types.h>

#include <openssl/bn.h>

#include <openssl/dh.h>

#include <stdarg.h>

#include <stdio.h>

#include <string.h>

#include <signal.h>

#include "sshkey.h"

#include "digest.h"

#include "kex.h"

#include "log.h"

#include "packet.h"

#include "dh.h"

#include "ssh2.h"

#include "compat.h"

#include "dispatch.h"

#include "ssherr.h"

#include "sshbuf.h"

#include "misc.h"

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

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

int

kexgex_client(struct ssh *ssh)

{

  struct kex *kex = ssh->kex;

  int r;

  u_int nbits;

  nbits = dh_estimate(kex->dh_need * 8);

  kex->min = DH_GRP_MIN;

  kex->max = DH_GRP_MAX;

  kex->nbits = nbits;

  if (ssh->compat & SSH_BUG_DHGEX_LARGE)

    kex->nbits = MINIMUM(kex->nbits, 4096);

  /* New GEX request */

  if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_DH_GEX_REQUEST)) != 0 ||

      (r = sshpkt_put_u32(ssh, kex->min)) != 0 ||

      (r = sshpkt_put_u32(ssh, kex->nbits)) != 0 ||

      (r = sshpkt_put_u32(ssh, kex->max)) != 0 ||

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

    goto out;

  debug("SSH2_MSG_KEX_DH_GEX_REQUEST(%u<%u<%u) sent",

      kex->min, kex->nbits, kex->max);

#ifdef DEBUG_KEXDH

  fprintf(stderr, "\nmin = %d, nbits = %d, max = %d\n",

      kex->min, kex->nbits, kex->max);

#endif

  debug("expecting SSH2_MSG_KEX_DH_GEX_GROUP");

  ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_GROUP,

      &input_kex_dh_gex_group);

  r = 0;

 out:

  return r;

}

static int

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

{

  struct kex *kex = ssh->kex;

  BIGNUM *p = NULL, *g = NULL;

  const BIGNUM *pub_key;

  int r, bits;

  debug("SSH2_MSG_KEX_DH_GEX_GROUP received");

  ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_GROUP, &kex_protocol_error);

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

      (r = sshpkt_get_bignum2(ssh, &g)) != 0 ||

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

    goto out;

  if ((bits = BN_num_bits(p)) < 0 ||

      (u_int)bits < kex->min || (u_int)bits > kex->max) {

    r = SSH_ERR_DH_GEX_OUT_OF_RANGE;

    goto out;

  }

  if ((kex->dh = dh_new_group(g, p)) == NULL) {

    r = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  p = g = NULL; /* belong to kex->dh now */

  /* generate and send 'e', client DH public key */

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

    goto out;

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

  if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_DH_GEX_INIT)) != 0 ||

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

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

    goto out;

  debug("SSH2_MSG_KEX_DH_GEX_INIT sent");

#ifdef DEBUG_KEXDH

  DHparams_print_fp(stderr, kex->dh);

  fprintf(stderr, "pub= ");

  BN_print_fp(stderr, pub_key);

  fprintf(stderr, "\n");

#endif

  debug("expecting SSH2_MSG_KEX_DH_GEX_REPLY");

  ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_REPLY, &input_kex_dh_gex_reply);

  r = 0;

out:

  BN_clear_free(p);

  BN_clear_free(g);

  return r;

}

static int

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

{

  struct kex *kex = ssh->kex;

  BIGNUM *dh_server_pub = NULL;

  const BIGNUM *pub_key, *dh_p, *dh_g;

  struct sshbuf *shared_secret = NULL;

  struct sshbuf *tmp = NULL, *server_host_key_blob = NULL;

  struct sshkey *server_host_key = NULL;

  u_char *signature = NULL;

  u_char hash[SSH_DIGEST_MAX_LENGTH];

  size_t slen, hashlen;

  int r;

  debug("SSH2_MSG_KEX_DH_GEX_REPLY received");

  ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_REPLY, &kex_protocol_error);

  /* key, cert */

  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_allowlist(tmp, &server_host_key,

      kex->hostkey_alg, NULL)) != 0 ||

      (r = kex_verify_host_key(ssh, server_host_key)) != 0)

    goto out;

  /* DH parameter f, server public DH key, signed H */

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

      (r = sshpkt_get_string(ssh, &signature, &slen)) != 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_server_pub, shared_secret)) != 0)

    goto out;

  if (ssh->compat & SSH_OLD_DHGEX)

    kex->min = kex->max = -1;

  /* calc and verify 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->my,

      kex->peer,

      server_host_key_blob,

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

      dh_p, dh_g,

      pub_key,

      dh_server_pub,

      sshbuf_ptr(shared_secret), sshbuf_len(shared_secret),

      hash, &hashlen)) != 0)

    goto out;

  if ((r = sshkey_verify(server_host_key, signature, slen, hash,

      hashlen, kex->hostkey_alg, ssh->compat, NULL)) != 0)

    goto out;

  if ((r = kex_derive_keys(ssh, hash, hashlen, shared_secret)) != 0 ||

      (r = kex_send_newkeys(ssh)) != 0)

    goto out;

  /* save initial signature and hostkey */

  if ((kex->flags & KEX_INITIAL) != 0) {

    if (kex->initial_hostkey != NULL || kex->initial_sig != NULL) {

      r = SSH_ERR_INTERNAL_ERROR;

      goto out;

    }

    if ((kex->initial_sig = sshbuf_new()) == NULL) {

      r = SSH_ERR_ALLOC_FAIL;

      goto out;

    }

    if ((r = sshbuf_put(kex->initial_sig, signature, slen)) != 0)

      goto out;

    kex->initial_hostkey = server_host_key;

    server_host_key = NULL;

  }

  /* success */

 out:

  explicit_bzero(hash, sizeof(hash));

  DH_free(kex->dh);

  kex->dh = NULL;

  BN_clear_free(dh_server_pub);

  sshbuf_free(shared_secret);

  sshkey_free(server_host_key);

  sshbuf_free(tmp);

  sshbuf_free(server_host_key_blob);

  free(signature);

  return r;

}

#endif /* WITH_OPENSSL */