/src/openssh/kex.c

Source
/* $OpenBSD: kex.c,v 1.196 2026/09/22 00:24:47 dtucker Exp $ */
/*
 * Copyright (c) 2000, 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"

#include <sys/types.h>
#include <errno.h>
#include <signal.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#ifdef WITH_OPENSSL
#include <openssl/crypto.h>
#include <openssl/dh.h>
#endif

#include "ssh.h"
#include "ssh2.h"
#include "atomicio.h"
#include "version.h"
#include "packet.h"
#include "compat.h"
#include "cipher.h"
#include "sshkey.h"
#include "kex.h"
#include "log.h"
#include "mac.h"
#include "match.h"
#include "misc.h"
#include "dispatch.h"
#include "myproposal.h"

#include "ssherr.h"
#include "sshbuf.h"
#include "digest.h"
#include "xmalloc.h"

/* prototype */
static int kex_choose_conf(struct ssh *, uint32_t seq);
static int kex_input_newkeys(int, uint32_t, struct ssh *);

static const char * const proposal_names[PROPOSAL_MAX] = {
  "KEX algorithms",
  "host key algorithms",
  "ciphers ctos",
  "ciphers stoc",
  "MACs ctos",
  "MACs stoc",
  "compression ctos",
  "compression stoc",
  "languages ctos",
  "languages stoc",
};

/*
 * Fill out a proposal array with dynamically allocated values, which may
 * be modified as required for compatibility reasons.
 * Any of the options may be NULL, in which case the default is used.
 * Array contents must be freed by calling kex_proposal_free_entries.
 */
void
kex_proposal_populate_entries(struct ssh *ssh, char *prop[PROPOSAL_MAX],
    const char *kexalgos, const char *ciphers, const char *macs,
    const char *comp, const char *hkalgs)
{
  const char *defpropserver[PROPOSAL_MAX] = { KEX_SERVER };
  const char *defpropclient[PROPOSAL_MAX] = { KEX_CLIENT };
  const char **defprop = ssh->kex->server ? defpropserver : defpropclient;
  u_int i;
  char *cp;

  if (prop == NULL)
    fatal_f("proposal missing");

  /* Append EXT_INFO signalling to KexAlgorithms */
  if (kexalgos == NULL)
    kexalgos = defprop[PROPOSAL_KEX_ALGS];
  if ((cp = kex_names_cat(kexalgos, ssh->kex->server ?
      "ext-info-s,kex-strict-s-v00@openssh.com" :
      "ext-info-c,kex-strict-c-v00@openssh.com")) == NULL)
    fatal_f("kex_names_cat");

  for (i = 0; i < PROPOSAL_MAX; i++) {
    switch(i) {
    case PROPOSAL_KEX_ALGS:
      prop[i] = compat_kex_proposal(ssh, cp);
      break;
    case PROPOSAL_ENC_ALGS_CTOS:
    case PROPOSAL_ENC_ALGS_STOC:
      prop[i] = xstrdup(ciphers ? ciphers : defprop[i]);
      break;
    case PROPOSAL_MAC_ALGS_CTOS:
    case PROPOSAL_MAC_ALGS_STOC:
      prop[i]  = xstrdup(macs ? macs : defprop[i]);
      break;
    case PROPOSAL_COMP_ALGS_CTOS:
    case PROPOSAL_COMP_ALGS_STOC:
      prop[i] = xstrdup(comp ? comp : defprop[i]);
      break;
    case PROPOSAL_SERVER_HOST_KEY_ALGS:
      prop[i] = xstrdup(hkalgs ? hkalgs : defprop[i]);
      break;
    default:
      prop[i] = xstrdup(defprop[i]);
    }
  }
  free(cp);
}

void
kex_proposal_free_entries(char *prop[PROPOSAL_MAX])
{
  u_int i;

  for (i = 0; i < PROPOSAL_MAX; i++)
    free(prop[i]);
}

/* put algorithm proposal into buffer */
int
kex_prop2buf(struct sshbuf *b, char *proposal[PROPOSAL_MAX])
{
  u_int i;
  int r;

  sshbuf_reset(b);

  /*
   * add a dummy cookie, the cookie will be overwritten by
   * kex_send_kexinit(), each time a kexinit is set
   */
  for (i = 0; i < KEX_COOKIE_LEN; i++) {
    if ((r = sshbuf_put_u8(b, 0)) != 0)
      return r;
  }
  for (i = 0; i < PROPOSAL_MAX; i++) {
    if ((r = sshbuf_put_cstring(b, proposal[i])) != 0)
      return r;
  }
  if ((r = sshbuf_put_u8(b, 0)) != 0 || /* first_kex_packet_follows */
      (r = sshbuf_put_u32(b, 0)) != 0)  /* uint32 reserved */
    return r;
  return 0;
}

/* parse buffer and return algorithm proposal */
int
kex_buf2prop(struct sshbuf *raw, int *first_kex_follows, char ***propp)
{
  struct sshbuf *b = NULL;
  u_char v;
  u_int i;
  char **proposal = NULL;
  int r;

  *propp = NULL;
  if ((proposal = calloc(PROPOSAL_MAX, sizeof(char *))) == NULL)
    return SSH_ERR_ALLOC_FAIL;
  if ((b = sshbuf_fromb(raw)) == NULL) {
    r = SSH_ERR_ALLOC_FAIL;
    goto out;
  }
  if ((r = sshbuf_consume(b, KEX_COOKIE_LEN)) != 0) { /* skip cookie */
    error_fr(r, "consume cookie");
    goto out;
  }
  /* extract kex init proposal strings */
  for (i = 0; i < PROPOSAL_MAX; i++) {
    if ((r = sshbuf_get_cstring(b, &(proposal[i]), NULL)) != 0) {
      error_fr(r, "parse proposal %u", i);
      goto out;
    }
    debug2("%s: %s", proposal_names[i], proposal[i]);
  }
  /* first kex follows / reserved */
  if ((r = sshbuf_get_u8(b, &v)) != 0 || /* first_kex_follows */
      (r = sshbuf_get_u32(b, &i)) != 0) { /* reserved */
    error_fr(r, "parse");
    goto out;
  }
  if (first_kex_follows != NULL)
    *first_kex_follows = v;
  debug2("first_kex_follows %d ", v);
  debug2("reserved %u ", i);
  r = 0;
  *propp = proposal;
 out:
  if (r != 0 && proposal != NULL)
    kex_prop_free(proposal);
  sshbuf_free(b);
  return r;
}

void
kex_prop_free(char **proposal)
{
  u_int i;

  if (proposal == NULL)
    return;
  for (i = 0; i < PROPOSAL_MAX; i++)
    free(proposal[i]);
  free(proposal);
}

int
kex_protocol_error(int type, uint32_t seq, struct ssh *ssh)
{
  int r;

  /* If in strict mode, any unexpected message is an error */
  if ((ssh->kex->flags & KEX_INITIAL) && ssh->kex->kex_strict) {
    ssh_packet_disconnect(ssh, "strict KEX violation: "
        "unexpected packet type %u (seqnr %u)", type, seq);
  }
  error_f("type %u seq %u", type, seq);
  if ((r = sshpkt_start(ssh, SSH2_MSG_UNIMPLEMENTED)) != 0 ||
      (r = sshpkt_put_u32(ssh, seq)) != 0 ||
      (r = sshpkt_send(ssh)) != 0)
    return r;
  return 0;
}

static void
kex_reset_dispatch(struct ssh *ssh)
{
  ssh_dispatch_range(ssh, SSH2_MSG_TRANSPORT_MIN,
      SSH2_MSG_TRANSPORT_MAX, &kex_protocol_error);
}

void
kex_set_server_sig_algs(struct ssh *ssh, const char *allowed_algs)
{
  char *alg, *oalgs, *algs, *sigalgs;
  const char *sigalg;

  /*
   * NB. allowed algorithms may contain certificate algorithms that
   * map to a specific plain signature type, e.g.
   * rsa-sha2-512-cert-v01@openssh.com => rsa-sha2-512
   * We need to be careful here to match these, retain the mapping
   * and only add each signature algorithm once.
   */
  if ((sigalgs = sshkey_alg_list(0, 1, 1, ',')) == NULL)
    fatal_f("sshkey_alg_list failed");
  oalgs = algs = xstrdup(allowed_algs);
  free(ssh->kex->server_sig_algs);
  ssh->kex->server_sig_algs = NULL;
  for ((alg = strsep(&algs, ",")); alg != NULL && *alg != '\0';
      (alg = strsep(&algs, ","))) {
    if ((sigalg = sshkey_sigalg_by_name(alg)) == NULL)
      continue;
    if (!kex_has_any_alg(sigalg, sigalgs))
      continue;
    /* Don't add an algorithm twice. */
    if (ssh->kex->server_sig_algs != NULL &&
        kex_has_any_alg(sigalg, ssh->kex->server_sig_algs))
      continue;
    xextendf(&ssh->kex->server_sig_algs, ",", "%s", sigalg);
  }
  free(oalgs);
  free(sigalgs);
  if (ssh->kex->server_sig_algs == NULL)
    ssh->kex->server_sig_algs = xstrdup("");
}

void
kex_set_warn_weak_crypto(struct ssh *ssh, int warn_weak_crypto)
{
  if (ssh != NULL && ssh->kex != NULL)
    ssh->kex->warn_weak_crypto = warn_weak_crypto != 0;
}

void
kex_check_warn_weak_crypto(struct ssh *ssh)
{
  char remote_id[512];
  int rekeyed = 0;

  if (ssh == NULL || ssh->kex == NULL || ssh->kex->name == NULL ||
      !ssh->kex->warn_weak_crypto || ssh->kex->non_pq_kex_warned)
    return;
  if (kex_is_pq_from_name(ssh->kex->name)) {
    ssh->kex->pq_kex_negotiated = 1;
    return;
  }

  if (ssh->kex->pq_kex_negotiated)
    rekeyed = 1;
  if (!rekeyed && !ssh->kex->server)
    return; /* client logs initial KEX warning separately */

  sshpkt_fmt_connection_id(ssh, remote_id, sizeof(remote_id));
  logit("WARNING: %sconnection%s%s is not using a post-quantum "
      "key exchange algorithm: \"%s\"", rekeyed ? "rekeyed " : "",
      ssh->kex->server ? " from " : "",
      ssh->kex->server ? remote_id : "", ssh->kex->name);
  ssh->kex->non_pq_kex_warned = 1;
}

static int
kex_compose_ext_info_server(struct ssh *ssh, struct sshbuf *m)
{
  int r;

  if (ssh->kex->server_sig_algs == NULL &&
      (ssh->kex->server_sig_algs = sshkey_alg_list(0, 1, 1, ',')) == NULL)
    return SSH_ERR_ALLOC_FAIL;
  if ((r = sshbuf_put_u32(m, 4)) != 0 ||
      (r = sshbuf_put_cstring(m, "server-sig-algs")) != 0 ||
      (r = sshbuf_put_cstring(m, ssh->kex->server_sig_algs)) != 0 ||
      (r = sshbuf_put_cstring(m,
      "publickey-hostbound@openssh.com")) != 0 ||
      (r = sshbuf_put_cstring(m, "0")) != 0 ||
      (r = sshbuf_put_cstring(m, "ping@openssh.com")) != 0 ||
      (r = sshbuf_put_cstring(m, "0")) != 0 ||
      (r = sshbuf_put_cstring(m, "agent-forward")) != 0 ||
      (r = sshbuf_put_cstring(m, "0")) != 0) {
    error_fr(r, "compose");
    return r;
  }
  return 0;
}

static int
kex_compose_ext_info_client(struct ssh *ssh, struct sshbuf *m)
{
  int r;

  if ((r = sshbuf_put_u32(m, 1)) != 0 ||
      (r = sshbuf_put_cstring(m, "ext-info-in-auth@openssh.com")) != 0 ||
      (r = sshbuf_put_cstring(m, "0")) != 0) {
    error_fr(r, "compose");
    goto out;
  }
  /* success */
  r = 0;
 out:
  return r;
}

static int
kex_maybe_send_ext_info(struct ssh *ssh)
{
  int r;
  struct sshbuf *m = NULL;

  if ((ssh->kex->flags & KEX_INITIAL) == 0)
    return 0;
  if (!ssh->kex->ext_info_c && !ssh->kex->ext_info_s)
    return 0;

  /* Compose EXT_INFO packet. */
  if ((m = sshbuf_new()) == NULL)
    fatal_f("sshbuf_new failed");
  if (ssh->kex->ext_info_c &&
      (r = kex_compose_ext_info_server(ssh, m)) != 0)
    goto fail;
  if (ssh->kex->ext_info_s &&
      (r = kex_compose_ext_info_client(ssh, m)) != 0)
    goto fail;

  /* Send the actual KEX_INFO packet */
  debug("Sending SSH2_MSG_EXT_INFO");
  if ((r = sshpkt_start(ssh, SSH2_MSG_EXT_INFO)) != 0 ||
      (r = sshpkt_putb(ssh, m)) != 0 ||
      (r = sshpkt_send(ssh)) != 0) {
    error_f("send EXT_INFO");
    goto fail;
  }

  r = 0;

 fail:
  sshbuf_free(m);
  return r;
}

int
kex_server_update_ext_info(struct ssh *ssh)
{
  int r;

  if ((ssh->kex->flags & KEX_HAS_EXT_INFO_IN_AUTH) == 0)
    return 0;

  debug_f("Sending SSH2_MSG_EXT_INFO");
  if ((r = sshpkt_start(ssh, SSH2_MSG_EXT_INFO)) != 0 ||
      (r = sshpkt_put_u32(ssh, 1)) != 0 ||
      (r = sshpkt_put_cstring(ssh, "server-sig-algs")) != 0 ||
      (r = sshpkt_put_cstring(ssh, ssh->kex->server_sig_algs)) != 0 ||
      (r = sshpkt_send(ssh)) != 0) {
    error_f("send EXT_INFO");
    return r;
  }
  return 0;
}

int
kex_send_newkeys(struct ssh *ssh)
{
  int r;

  kex_reset_dispatch(ssh);
  if ((r = sshpkt_start(ssh, SSH2_MSG_NEWKEYS)) != 0 ||
      (r = sshpkt_send(ssh)) != 0)
    return r;
  debug("SSH2_MSG_NEWKEYS sent");
  ssh_dispatch_set(ssh, SSH2_MSG_NEWKEYS, &kex_input_newkeys);
  if ((r = kex_maybe_send_ext_info(ssh)) != 0)
    return r;
  debug("expecting SSH2_MSG_NEWKEYS");
  return 0;
}

/* Check whether an ext_info value contains the expected version string */
static int
kex_ext_info_check_ver(struct kex *kex, const char *name,
    const u_char *val, size_t len, const char *want_ver, u_int flag)
{
  if (memchr(val, '\0', len) != NULL) {
    error("SSH2_MSG_EXT_INFO: %s value contains nul byte", name);
    return SSH_ERR_INVALID_FORMAT;
  }
  debug_f("%s=<%s>", name, val);
  if (strcmp(val, want_ver) == 0)
    kex->flags |= flag;
  else
    debug_f("unsupported version of %s extension", name);
  return 0;
}

static int
kex_ext_info_client_parse(struct ssh *ssh, const char *name,
    const u_char *value, size_t vlen)
{
  int r;

  /* NB. some messages are only accepted in the initial EXT_INFO */
  if (strcmp(name, "server-sig-algs") == 0) {
    /* Ensure no \0 lurking in value */
    if (memchr(value, '\0', vlen) != NULL) {
      error_f("nul byte in %s", name);
      return SSH_ERR_INVALID_FORMAT;
    }
    debug_f("%s=<%s>", name, value);
    free(ssh->kex->server_sig_algs);
    ssh->kex->server_sig_algs = xstrdup((const char *)value);
  } else if (ssh->kex->ext_info_received == 1 &&
      strcmp(name, "publickey-hostbound@openssh.com") == 0) {
    if ((r = kex_ext_info_check_ver(ssh->kex, name, value, vlen,
        "0", KEX_HAS_PUBKEY_HOSTBOUND)) != 0) {
      return r;
    }
  } else if (ssh->kex->ext_info_received == 1 &&
      strcmp(name, "ping@openssh.com") == 0) {
    if ((r = kex_ext_info_check_ver(ssh->kex, name, value, vlen,
        "0", KEX_HAS_PING)) != 0) {
      return r;
    }
  } else if (ssh->kex->ext_info_received == 1 &&
      strcmp(name, "agent-forward") == 0) {
    if ((r = kex_ext_info_check_ver(ssh->kex, name, value, vlen,
        "0", KEX_HAS_NEWAGENT)) != 0) {
      return r;
    }
  } else
    debug_f("%s (unrecognised)", name);

  return 0;
}

static int
kex_ext_info_server_parse(struct ssh *ssh, const char *name,
    const u_char *value, size_t vlen)
{
  int r;

  if (strcmp(name, "ext-info-in-auth@openssh.com") == 0) {
    if ((r = kex_ext_info_check_ver(ssh->kex, name, value, vlen,
        "0", KEX_HAS_EXT_INFO_IN_AUTH)) != 0) {
      return r;
    }
  } else
    debug_f("%s (unrecognised)", name);
  return 0;
}

int
kex_input_ext_info(int type, uint32_t seq, struct ssh *ssh)
{
  struct kex *kex = ssh->kex;
  const int max_ext_info = kex->server ? 1 : 2;
  uint32_t i, ninfo;
  char *name;
  u_char *val;
  size_t vlen;
  int r;

  debug("SSH2_MSG_EXT_INFO received");
  if (++kex->ext_info_received > max_ext_info) {
    error("too many SSH2_MSG_EXT_INFO messages sent by peer");
    return dispatch_protocol_error(type, seq, ssh);
  }
  ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, &kex_protocol_error);
  if ((r = sshpkt_get_u32(ssh, &ninfo)) != 0)
    return r;
  if (ninfo >= 1024) {
    error("SSH2_MSG_EXT_INFO with too many entries, expected "
        "<=1024, received %u", ninfo);
    return dispatch_protocol_error(type, seq, ssh);
  }
  for (i = 0; i < ninfo; i++) {
    if ((r = sshpkt_get_cstring(ssh, &name, NULL)) != 0)
      return r;
    if ((r = sshpkt_get_string(ssh, &val, &vlen)) != 0) {
      free(name);
      return r;
    }
    debug3_f("extension %s", name);
    if (kex->server) {
      if ((r = kex_ext_info_server_parse(ssh, name,
          val, vlen)) != 0)
        return r;
    } else {
      if ((r = kex_ext_info_client_parse(ssh, name,
          val, vlen)) != 0)
        return r;
    }
    free(name);
    free(val);
  }
  return sshpkt_get_end(ssh);
}

static int
kex_input_newkeys(int type, uint32_t seq, struct ssh *ssh)
{
  struct kex *kex = ssh->kex;
  int r, initial = (kex->flags & KEX_INITIAL) != 0;
  char *cp, **prop;

  debug("SSH2_MSG_NEWKEYS received");
  if (kex->ext_info_c && initial)
    ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, &kex_input_ext_info);
  ssh_dispatch_set(ssh, SSH2_MSG_NEWKEYS, &kex_protocol_error);
  ssh_dispatch_set(ssh, SSH2_MSG_KEXINIT, &kex_input_kexinit);
  if ((r = sshpkt_get_end(ssh)) != 0)
    return r;
  if ((r = ssh_set_newkeys(ssh, MODE_IN)) != 0)
    return r;
  if (initial) {
    /* Remove initial KEX signalling from proposal for rekeying */
    if ((r = kex_buf2prop(kex->my, NULL, &prop)) != 0)
      return r;
    if ((cp = match_filter_denylist(prop[PROPOSAL_KEX_ALGS],
        kex->server ?
        "ext-info-s,kex-strict-s-v00@openssh.com" :
        "ext-info-c,kex-strict-c-v00@openssh.com")) == NULL) {
      error_f("match_filter_denylist failed");
      goto fail;
    }
    free(prop[PROPOSAL_KEX_ALGS]);
    prop[PROPOSAL_KEX_ALGS] = cp;
    if ((r = kex_prop2buf(ssh->kex->my, prop)) != 0) {
      error_f("kex_prop2buf failed");
 fail:
      kex_proposal_free_entries(prop);
      free(prop);
      return SSH_ERR_INTERNAL_ERROR;
    }
    kex_proposal_free_entries(prop);
    free(prop);
  }
  kex->done = 1;
  kex->flags &= ~KEX_INITIAL;
  sshbuf_reset(kex->peer);
  kex->flags &= ~(KEX_INIT_SENT|KEX_INIT_RECVD);
  kex_check_warn_weak_crypto(ssh);
  return 0;
}

int
kex_send_kexinit(struct ssh *ssh)
{
  u_char *cookie;
  struct kex *kex = ssh->kex;
  int r;

  if (kex == NULL) {
    error_f("no kex");
    return SSH_ERR_INTERNAL_ERROR;
  }
  if (kex->flags & KEX_INIT_SENT)
    return 0;
  kex->done = 0;

  /* generate a random cookie */
  if (sshbuf_len(kex->my) < KEX_COOKIE_LEN) {
    error_f("bad kex length: %zu < %d",
        sshbuf_len(kex->my), KEX_COOKIE_LEN);
    return SSH_ERR_INVALID_FORMAT;
  }
  if ((cookie = sshbuf_mutable_ptr(kex->my)) == NULL) {
    error_f("buffer error");
    return SSH_ERR_INTERNAL_ERROR;
  }
  arc4random_buf(cookie, KEX_COOKIE_LEN);

  if ((r = sshpkt_start(ssh, SSH2_MSG_KEXINIT)) != 0 ||
      (r = sshpkt_putb(ssh, kex->my)) != 0 ||
      (r = sshpkt_send(ssh)) != 0) {
    error_fr(r, "compose reply");
    return r;
  }
  debug("SSH2_MSG_KEXINIT sent");
  kex->flags |= KEX_INIT_SENT;
  return 0;
}

int
kex_input_kexinit(int type, uint32_t seq, struct ssh *ssh)
{
  struct kex *kex = ssh->kex;
  const u_char *ptr;
  u_int i;
  size_t dlen;
  int r;

  debug("SSH2_MSG_KEXINIT received");
  if (kex == NULL) {
    error_f("no kex");
    return SSH_ERR_INTERNAL_ERROR;
  }
  free(kex->name);
  kex->name = NULL;
  if ((kex->flags & KEX_INIT_RECVD) != 0) {
    ssh_packet_disconnect(ssh,
        "multiple KEXINIT received from peer");
  }
  kex->flags |= KEX_INIT_RECVD;
  ssh_dispatch_set(ssh, SSH2_MSG_KEXINIT, &kex_protocol_error);
  ptr = sshpkt_ptr(ssh, &dlen);
  if ((r = sshbuf_put(kex->peer, ptr, dlen)) != 0)
    return r;

  /* discard packet */
  for (i = 0; i < KEX_COOKIE_LEN; i++) {
    if ((r = sshpkt_get_u8(ssh, NULL)) != 0) {
      error_fr(r, "discard cookie");
      return r;
    }
  }
  for (i = 0; i < PROPOSAL_MAX; i++) {
    if ((r = sshpkt_get_string(ssh, NULL, NULL)) != 0) {
      error_fr(r, "discard proposal");
      return r;
    }
  }
  /*
   * XXX RFC4253 sec 7: "each side MAY guess" - currently no supported
   * KEX method has the server move first, but a server might be using
   * a custom method or one that we otherwise don't support. We should
   * be prepared to remember first_kex_follows here so we can eat a
   * packet later.
   * XXX2 - RFC4253 is kind of ambiguous on what first_kex_follows means
   * for cases where the server *doesn't* go first. I guess we should
   * ignore it when it is set for these cases, which is what we do now.
   */
  if ((r = sshpkt_get_u8(ssh, NULL)) != 0 || /* first_kex_follows */
      (r = sshpkt_get_u32(ssh, NULL)) != 0 || /* reserved */
      (r = sshpkt_get_end(ssh)) != 0)
      return r;

  if (!(kex->flags & KEX_INIT_SENT))
    if ((r = kex_send_kexinit(ssh)) != 0)
      return r;
  if ((r = kex_choose_conf(ssh, seq)) != 0)
    return r;

  if (kex->kex_type < KEX_MAX && kex->kex[kex->kex_type] != NULL)
    return (kex->kex[kex->kex_type])(ssh);

  error_f("unknown kex type %u", kex->kex_type);
  return SSH_ERR_INTERNAL_ERROR;
}

struct kex *
kex_new(void)
{
  struct kex *kex;

  if ((kex = calloc(1, sizeof(*kex))) == NULL ||
      (kex->peer = sshbuf_new()) == NULL ||
      (kex->my = sshbuf_new()) == NULL ||
      (kex->client_version = sshbuf_new()) == NULL ||
      (kex->server_version = sshbuf_new()) == NULL ||
      (kex->session_id = sshbuf_new()) == NULL) {
    kex_free(kex);
    return NULL;
  }
  return kex;
}

void
kex_free_newkeys(struct newkeys *newkeys)
{
  if (newkeys == NULL)
    return;
  if (newkeys->enc.key) {
    explicit_bzero(newkeys->enc.key, newkeys->enc.key_len);
    free(newkeys->enc.key);
    newkeys->enc.key = NULL;
  }
  if (newkeys->enc.iv) {
    explicit_bzero(newkeys->enc.iv, newkeys->enc.iv_len);
    free(newkeys->enc.iv);
    newkeys->enc.iv = NULL;
  }
  free(newkeys->enc.name);
  explicit_bzero(&newkeys->enc, sizeof(newkeys->enc));
  free(newkeys->comp.name);
  explicit_bzero(&newkeys->comp, sizeof(newkeys->comp));
  mac_clear(&newkeys->mac);
  if (newkeys->mac.key) {
    explicit_bzero(newkeys->mac.key, newkeys->mac.key_len);
    free(newkeys->mac.key);
    newkeys->mac.key = NULL;
  }
  free(newkeys->mac.name);
  explicit_bzero(&newkeys->mac, sizeof(newkeys->mac));
  freezero(newkeys, sizeof(*newkeys));
}

void
kex_free(struct kex *kex)
{
  u_int mode;

  if (kex == NULL)
    return;

#ifdef WITH_OPENSSL
  DH_free(kex->dh);
  EC_KEY_free(kex->ec_client_key);
#endif /* WITH_OPENSSL */
  for (mode = 0; mode < MODE_MAX; mode++) {
    kex_free_newkeys(kex->newkeys[mode]);
    kex->newkeys[mode] = NULL;
  }
  sshbuf_free(kex->peer);
  sshbuf_free(kex->my);
  sshbuf_free(kex->client_version);
  sshbuf_free(kex->server_version);
  sshbuf_free(kex->client_pub);
  sshbuf_free(kex->session_id);
  sshbuf_free(kex->initial_sig);
  sshkey_free(kex->initial_hostkey);
  free(kex->failed_choice);
  free(kex->hostkey_alg);
  free(kex->name);
  free(kex->server_sig_algs);
  free(kex);
}

int
kex_ready(struct ssh *ssh, char *proposal[PROPOSAL_MAX])
{
  int r;

  if ((r = kex_prop2buf(ssh->kex->my, proposal)) != 0)
    return r;
  ssh->kex->flags = KEX_INITIAL;
  kex_reset_dispatch(ssh);
  ssh_dispatch_set(ssh, SSH2_MSG_KEXINIT, &kex_input_kexinit);
  return 0;
}

int
kex_setup(struct ssh *ssh, char *proposal[PROPOSAL_MAX])
{
  int r;

  if ((r = kex_ready(ssh, proposal)) != 0)
    return r;
  if ((r = kex_send_kexinit(ssh)) != 0) {   /* we start */
    kex_free(ssh->kex);
    ssh->kex = NULL;
    return r;
  }
  return 0;
}

/*
 * Request key re-exchange, returns 0 on success or a ssherr.h error
 * code otherwise. Must not be called if KEX is incomplete or in-progress.
 */
int
kex_start_rekex(struct ssh *ssh)
{
  if (ssh->kex == NULL) {
    error_f("no kex");
    return SSH_ERR_INTERNAL_ERROR;
  }
  if (ssh->kex->done == 0) {
    error_f("requested twice");
    return SSH_ERR_INTERNAL_ERROR;
  }
  ssh->kex->done = 0;
  return kex_send_kexinit(ssh);
}

static int
choose_enc(struct sshenc *enc, char *client, char *server)
{
  char *name = match_list(client, server, NULL);

  if (name == NULL)
    return SSH_ERR_NO_CIPHER_ALG_MATCH;
  if ((enc->cipher = cipher_by_name(name)) == NULL) {
    error_f("unsupported cipher %s", name);
    free(name);
    return SSH_ERR_INTERNAL_ERROR;
  }
  enc->name = name;
  enc->enabled = 0;
  enc->iv = NULL;
  enc->iv_len = cipher_ivlen(enc->cipher);
  enc->key = NULL;
  enc->key_len = cipher_keylen(enc->cipher);
  enc->block_size = cipher_blocksize(enc->cipher);
  return 0;
}

static int
choose_mac(struct ssh *ssh, struct sshmac *mac, char *client, char *server)
{
  char *name = match_list(client, server, NULL);

  if (name == NULL)
    return SSH_ERR_NO_MAC_ALG_MATCH;
  if (mac_setup(mac, name) < 0) {
    error_f("unsupported MAC %s", name);
    free(name);
    return SSH_ERR_INTERNAL_ERROR;
  }
  mac->name = name;
  mac->key = NULL;
  mac->enabled = 0;
  return 0;
}

static int
choose_comp(struct sshcomp *comp, char *client, char *server)
{
  char *name = match_list(client, server, NULL);

  if (name == NULL)
    return SSH_ERR_NO_COMPRESS_ALG_MATCH;
#ifdef WITH_ZLIB
  if (strcmp(name, "zlib@openssh.com") == 0) {
    comp->type = COMP_DELAYED;
  } else
#endif  /* WITH_ZLIB */
  if (strcmp(name, "none") == 0) {
    comp->type = COMP_NONE;
  } else {
    error_f("unsupported compression scheme %s", name);
    free(name);
    return SSH_ERR_INTERNAL_ERROR;
  }
  comp->name = name;
  return 0;
}

static int
choose_kex(struct kex *k, char *client, char *server)
{
  k->name = match_list(client, server, NULL);

  debug("kex: algorithm: %s", k->name ? k->name : "(no match)");
  if (k->name == NULL)
    return SSH_ERR_NO_KEX_ALG_MATCH;
  if (!kex_name_valid(k->name)) {
    error_f("unsupported KEX method %s", k->name);
    return SSH_ERR_INTERNAL_ERROR;
  }
  k->kex_type = kex_type_from_name(k->name);
  k->hash_alg = kex_hash_from_name(k->name);
  k->ec_nid = kex_nid_from_name(k->name);
  return 0;
}

static int
choose_hostkeyalg(struct kex *k, char *client, char *server)
{
  free(k->hostkey_alg);
  k->hostkey_alg = match_list(client, server, NULL);

  debug("kex: host key algorithm: %s",
      k->hostkey_alg ? k->hostkey_alg : "(no match)");
  if (k->hostkey_alg == NULL)
    return SSH_ERR_NO_HOSTKEY_ALG_MATCH;
  k->hostkey_type = sshkey_type_from_name(k->hostkey_alg);
  if (k->hostkey_type == KEY_UNSPEC) {
    error_f("unsupported hostkey algorithm %s", k->hostkey_alg);
    return SSH_ERR_INTERNAL_ERROR;
  }
  k->hostkey_nid = sshkey_ecdsa_nid_from_name(k->hostkey_alg);
  return 0;
}

static int
proposals_match(char *my[PROPOSAL_MAX], char *peer[PROPOSAL_MAX])
{
  static int check[] = {
    PROPOSAL_KEX_ALGS, PROPOSAL_SERVER_HOST_KEY_ALGS, -1
  };
  int *idx;
  char *p;

  for (idx = &check[0]; *idx != -1; idx++) {
    if ((p = strchr(my[*idx], ',')) != NULL)
      *p = '\0';
    if ((p = strchr(peer[*idx], ',')) != NULL)
      *p = '\0';
    if (strcmp(my[*idx], peer[*idx]) != 0) {
      debug2("proposal mismatch: my %s peer %s",
          my[*idx], peer[*idx]);
      return (0);
    }
  }
  debug2("proposals match");
  return (1);
}

static int
kexalgs_contains(char **peer, const char *ext)
{
  return kex_has_any_alg(peer[PROPOSAL_KEX_ALGS], ext);
}

static int
kex_choose_conf(struct ssh *ssh, uint32_t seq)
{
  struct kex *kex = ssh->kex;
  struct newkeys *newkeys;
  char **my = NULL, **peer = NULL;
  char **cprop, **sprop;
  int nenc, nmac, ncomp;
  u_int mode, ctos, need, dh_need, authlen;
  int r, first_kex_follows;

  debug2("local %s KEXINIT proposal", kex->server ? "server" : "client");
  if ((r = kex_buf2prop(kex->my, NULL, &my)) != 0)
    goto out;
  debug2("peer %s KEXINIT proposal", kex->server ? "client" : "server");
  if ((r = kex_buf2prop(kex->peer, &first_kex_follows, &peer)) != 0)
    goto out;

  if (kex->server) {
    cprop=peer;
    sprop=my;
  } else {
    cprop=my;
    sprop=peer;
  }

  /* Check whether peer supports ext_info/kex_strict */
  if ((kex->flags & KEX_INITIAL) != 0) {
    if (kex->server) {
      kex->ext_info_c = kexalgs_contains(peer, "ext-info-c");
      kex->kex_strict = kexalgs_contains(peer,
          "kex-strict-c-v00@openssh.com");
    } else {
      kex->ext_info_s = kexalgs_contains(peer, "ext-info-s");
      kex->kex_strict = kexalgs_contains(peer,
          "kex-strict-s-v00@openssh.com");
    }
    if (kex->kex_strict) {
      debug3_f("will use strict KEX ordering");
      if (seq != 0)
        ssh_packet_disconnect(ssh,
            "strict KEX violation: "
            "KEXINIT was not the first packet");
    }
  }

  /* Check whether client supports rsa-sha2 algorithms */
  if (kex->server && (kex->flags & KEX_INITIAL)) {
    if (kex_has_any_alg(peer[PROPOSAL_SERVER_HOST_KEY_ALGS],
        "rsa-sha2-256,rsa-sha2-256-cert-v01@openssh.com"))
      kex->flags |= KEX_RSA_SHA2_256_SUPPORTED;
    if (kex_has_any_alg(peer[PROPOSAL_SERVER_HOST_KEY_ALGS],
        "rsa-sha2-512,rsa-sha2-512-cert-v01@openssh.com"))
      kex->flags |= KEX_RSA_SHA2_512_SUPPORTED;
  }

  /* Algorithm Negotiation */
  if ((r = choose_kex(kex, cprop[PROPOSAL_KEX_ALGS],
      sprop[PROPOSAL_KEX_ALGS])) != 0) {
    kex->failed_choice = peer[PROPOSAL_KEX_ALGS];
    peer[PROPOSAL_KEX_ALGS] = NULL;
    goto out;
  }
  if ((r = choose_hostkeyalg(kex, cprop[PROPOSAL_SERVER_HOST_KEY_ALGS],
      sprop[PROPOSAL_SERVER_HOST_KEY_ALGS])) != 0) {
    kex->failed_choice = peer[PROPOSAL_SERVER_HOST_KEY_ALGS];
    peer[PROPOSAL_SERVER_HOST_KEY_ALGS] = NULL;
    goto out;
  }
  for (mode = 0; mode < MODE_MAX; mode++) {
    if ((newkeys = calloc(1, sizeof(*newkeys))) == NULL) {
      r = SSH_ERR_ALLOC_FAIL;
      goto out;
    }
    kex->newkeys[mode] = newkeys;
    ctos = (!kex->server && mode == MODE_OUT) ||
        (kex->server && mode == MODE_IN);
    nenc  = ctos ? PROPOSAL_ENC_ALGS_CTOS  : PROPOSAL_ENC_ALGS_STOC;
    nmac  = ctos ? PROPOSAL_MAC_ALGS_CTOS  : PROPOSAL_MAC_ALGS_STOC;
    ncomp = ctos ? PROPOSAL_COMP_ALGS_CTOS : PROPOSAL_COMP_ALGS_STOC;
    if ((r = choose_enc(&newkeys->enc, cprop[nenc],
        sprop[nenc])) != 0) {
      kex->failed_choice = peer[nenc];
      peer[nenc] = NULL;
      goto out;
    }
    authlen = cipher_authlen(newkeys->enc.cipher);
    /* ignore mac for authenticated encryption */
    if (authlen == 0 &&
        (r = choose_mac(ssh, &newkeys->mac, cprop[nmac],
        sprop[nmac])) != 0) {
      kex->failed_choice = peer[nmac];
      peer[nmac] = NULL;
      goto out;
    }
    if ((r = choose_comp(&newkeys->comp, cprop[ncomp],
        sprop[ncomp])) != 0) {
      kex->failed_choice = peer[ncomp];
      peer[ncomp] = NULL;
      goto out;
    }
    debug("kex: %s cipher: %s MAC: %s compression: %s",
        ctos ? "client->server" : "server->client",
        newkeys->enc.name,
        authlen == 0 ? newkeys->mac.name : "<implicit>",
        newkeys->comp.name);
  }
  need = dh_need = 0;
  for (mode = 0; mode < MODE_MAX; mode++) {
    newkeys = kex->newkeys[mode];
    need = MAXIMUM(need, newkeys->enc.key_len);
    need = MAXIMUM(need, newkeys->enc.block_size);
    need = MAXIMUM(need, newkeys->enc.iv_len);
    need = MAXIMUM(need, newkeys->mac.key_len);
    dh_need = MAXIMUM(dh_need, cipher_seclen(newkeys->enc.cipher));
    dh_need = MAXIMUM(dh_need, newkeys->enc.block_size);
    dh_need = MAXIMUM(dh_need, newkeys->enc.iv_len);
    dh_need = MAXIMUM(dh_need, newkeys->mac.key_len);
  }
  /* XXX need runden? */
  kex->we_need = need;
  kex->dh_need = dh_need;

  /* ignore the next message if the proposals do not match */
  if (first_kex_follows && !proposals_match(my, peer))
    ssh->dispatch_skip_packets = 1;
  r = 0;
 out:
  kex_prop_free(my);
  kex_prop_free(peer);
  return r;
}

static int
derive_key(struct ssh *ssh, int id, u_int need, u_char *hash, u_int hashlen,
    const struct sshbuf *shared_secret, u_char **keyp)
{
  struct kex *kex = ssh->kex;
  struct ssh_digest_ctx *hashctx = NULL;
  char c = id;
  u_int have;
  size_t mdsz;
  u_char *digest;
  int r;

  if ((mdsz = ssh_digest_bytes(kex->hash_alg)) == 0)
    return SSH_ERR_INVALID_ARGUMENT;
  if ((digest = calloc(1, ROUNDUP(need, mdsz))) == NULL) {
    r = SSH_ERR_ALLOC_FAIL;
    goto out;
  }

  /* K1 = HASH(K || H || "A" || session_id) */
  if ((hashctx = ssh_digest_start(kex->hash_alg)) == NULL ||
      ssh_digest_update_buffer(hashctx, shared_secret) != 0 ||
      ssh_digest_update(hashctx, hash, hashlen) != 0 ||
      ssh_digest_update(hashctx, &c, 1) != 0 ||
      ssh_digest_update_buffer(hashctx, kex->session_id) != 0 ||
      ssh_digest_final(hashctx, digest, mdsz) != 0) {
    r = SSH_ERR_LIBCRYPTO_ERROR;
    error_f("KEX hash failed");
    goto out;
  }
  ssh_digest_free(hashctx);
  hashctx = NULL;

  /*
   * expand key:
   * Kn = HASH(K || H || K1 || K2 || ... || Kn-1)
   * Key = K1 || K2 || ... || Kn
   */
  for (have = mdsz; need > have; have += mdsz) {
    if ((hashctx = ssh_digest_start(kex->hash_alg)) == NULL ||
        ssh_digest_update_buffer(hashctx, shared_secret) != 0 ||
        ssh_digest_update(hashctx, hash, hashlen) != 0 ||
        ssh_digest_update(hashctx, digest, have) != 0 ||
        ssh_digest_final(hashctx, digest + have, mdsz) != 0) {
      error_f("KDF failed");
      r = SSH_ERR_LIBCRYPTO_ERROR;
      goto out;
    }
    ssh_digest_free(hashctx);
    hashctx = NULL;
  }
#ifdef DEBUG_KEX
  fprintf(stderr, "key '%c'== ", c);
  dump_digest("key", digest, need);
#endif
  *keyp = digest;
  digest = NULL;
  r = 0;
 out:
  free(digest);
  ssh_digest_free(hashctx);
  return r;
}

#define NKEYS 6
int
kex_derive_keys(struct ssh *ssh, u_char *hash, u_int hashlen,
    const struct sshbuf *shared_secret)
{
  struct kex *kex = ssh->kex;
  u_char *keys[NKEYS];
  u_int i, j, mode, ctos;
  int r;

  /* save initial hash as session id */
  if ((kex->flags & KEX_INITIAL) != 0) {
    if (sshbuf_len(kex->session_id) != 0) {
      error_f("already have session ID at kex");
      return SSH_ERR_INTERNAL_ERROR;
    }
    if ((r = sshbuf_put(kex->session_id, hash, hashlen)) != 0)
      return r;
  } else if (sshbuf_len(kex->session_id) == 0) {
    error_f("no session ID in rekex");
    return SSH_ERR_INTERNAL_ERROR;
  }
  for (i = 0; i < NKEYS; i++) {
    if ((r = derive_key(ssh, 'A'+i, kex->we_need, hash, hashlen,
        shared_secret, &keys[i])) != 0) {
      for (j = 0; j < i; j++)
        free(keys[j]);
      return r;
    }
  }
  for (mode = 0; mode < MODE_MAX; mode++) {
    ctos = (!kex->server && mode == MODE_OUT) ||
        (kex->server && mode == MODE_IN);
    kex->newkeys[mode]->enc.iv  = keys[ctos ? 0 : 1];
    kex->newkeys[mode]->enc.key = keys[ctos ? 2 : 3];
    kex->newkeys[mode]->mac.key = keys[ctos ? 4 : 5];
  }
  return 0;
}

int
kex_load_hostkey(struct ssh *ssh, struct sshkey **prvp, struct sshkey **pubp)
{
  struct kex *kex = ssh->kex;

  *pubp = NULL;
  *prvp = NULL;
  if (kex->load_host_public_key == NULL ||
      kex->load_host_private_key == NULL) {
    error_f("missing hostkey loader");
    return SSH_ERR_INVALID_ARGUMENT;
  }
  *pubp = kex->load_host_public_key(kex->hostkey_type,
      kex->hostkey_nid, ssh);
  *prvp = kex->load_host_private_key(kex->hostkey_type,
      kex->hostkey_nid, ssh);
  if (*pubp == NULL)
    return SSH_ERR_NO_HOSTKEY_LOADED;
  return 0;
}

int
kex_verify_host_key(struct ssh *ssh, struct sshkey *server_host_key)
{
  struct kex *kex = ssh->kex;

  if (kex->verify_host_key == NULL) {
    error_f("missing hostkey verifier");
    return SSH_ERR_INVALID_ARGUMENT;
  }
  if (server_host_key->type != kex->hostkey_type ||
      (kex->hostkey_type == KEY_ECDSA &&
      server_host_key->ecdsa_nid != kex->hostkey_nid))
    return SSH_ERR_KEY_TYPE_MISMATCH;
  if (kex->verify_host_key(server_host_key, ssh) == -1)
    return  SSH_ERR_SIGNATURE_INVALID;
  return 0;
}

#if defined(DEBUG_KEX) || defined(DEBUG_KEXDH) || defined(DEBUG_KEXECDH)
void
dump_digest(const char *msg, const u_char *digest, int len)
{
  fprintf(stderr, "%s\n", msg);
  sshbuf_dump_data(digest, len, stderr);
}
#endif

/*
 * Send a plaintext error message to the peer, suffixed by \r\n.
 * Only used during banner exchange, and there only for the server.
 */
static void
send_error(struct ssh *ssh, char *msg)
{
  char *crnl = "\r\n";

  if (!ssh->kex->server)
    return;

  if (atomicio(vwrite, ssh_packet_get_connection_out(ssh),
      msg, strlen(msg)) != strlen(msg) ||
      atomicio(vwrite, ssh_packet_get_connection_out(ssh),
      crnl, strlen(crnl)) != strlen(crnl))
    error_f("write: %.100s", strerror(errno));
}

/*
 * Sends our identification string and waits for the peer's. Will block for
 * up to timeout_ms (or indefinitely if timeout_ms <= 0).
 * Returns on 0 success or a ssherr.h code on failure.
 */
int
kex_exchange_identification(struct ssh *ssh, int timeout_ms,
    const char *version_addendum)
{
  int remote_major, remote_minor, mismatch, oerrno = 0;
  size_t len, n;
  int r, expect_nl;
  u_char c;
  struct sshbuf *our_version = ssh->kex->server ?
      ssh->kex->server_version : ssh->kex->client_version;
  struct sshbuf *peer_version = ssh->kex->server ?
      ssh->kex->client_version : ssh->kex->server_version;
  char *our_version_string = NULL, *peer_version_string = NULL;
  char *cp, *remote_version = NULL;

  /* Prepare and send our banner */
  sshbuf_reset(our_version);
  if (version_addendum != NULL && *version_addendum == '\0')
    version_addendum = NULL;
  if ((r = sshbuf_putf(our_version, "SSH-%d.%d-%s%s%s\r\n",
      PROTOCOL_MAJOR_2, PROTOCOL_MINOR_2, SSH_VERSION,
      version_addendum == NULL ? "" : " ",
      version_addendum == NULL ? "" : version_addendum)) != 0) {
    oerrno = errno;
    error_fr(r, "sshbuf_putf");
    goto out;
  }

  if (atomicio(vwrite, ssh_packet_get_connection_out(ssh),
      sshbuf_mutable_ptr(our_version),
      sshbuf_len(our_version)) != sshbuf_len(our_version)) {
    oerrno = errno;
    debug_f("write: %.100s", strerror(errno));
    r = SSH_ERR_SYSTEM_ERROR;
    goto out;
  }
  if ((r = sshbuf_consume_end(our_version, 2)) != 0) { /* trim \r\n */
    oerrno = errno;
    error_fr(r, "sshbuf_consume_end");
    goto out;
  }
  our_version_string = sshbuf_dup_string(our_version);
  if (our_version_string == NULL) {
    error_f("sshbuf_dup_string failed");
    r = SSH_ERR_ALLOC_FAIL;
    goto out;
  }
  debug("Local version string %.100s", our_version_string);

  /* Read other side's version identification. */
  for (n = 0; ; n++) {
    if (n >= SSH_MAX_PRE_BANNER_LINES) {
      send_error(ssh, "No SSH identification string "
          "received.");
      error_f("No SSH version received in first %u lines "
          "from server", SSH_MAX_PRE_BANNER_LINES);
      r = SSH_ERR_INVALID_FORMAT;
      goto out;
    }
    sshbuf_reset(peer_version);
    expect_nl = 0;
    for (;;) {
      if (timeout_ms > 0) {
        r = waitrfd(ssh_packet_get_connection_in(ssh),
            &timeout_ms, NULL);
        if (r == -1 && errno == ETIMEDOUT) {
          send_error(ssh, "Timed out waiting "
              "for SSH identification string.");
          error("Connection timed out during "
              "banner exchange");
          r = SSH_ERR_CONN_TIMEOUT;
          goto out;
        } else if (r == -1) {
          oerrno = errno;
          error_f("%s", strerror(errno));
          r = SSH_ERR_SYSTEM_ERROR;
          goto out;
        }
      }

      len = atomicio(read, ssh_packet_get_connection_in(ssh),
          &c, 1);
      if (len != 1 && errno == EPIPE) {
        verbose_f("Connection closed by remote host");
        r = SSH_ERR_CONN_CLOSED;
        goto out;
      } else if (len != 1) {
        oerrno = errno;
        error_f("read: %.100s", strerror(errno));
        r = SSH_ERR_SYSTEM_ERROR;
        goto out;
      }
      if (c == '\r') {
        expect_nl = 1;
        continue;
      }
      if (c == '\n')
        break;
      if (c == '\0' || expect_nl) {
        verbose_f("banner line contains invalid "
            "characters");
        goto invalid;
      }
      if ((r = sshbuf_put_u8(peer_version, c)) != 0) {
        oerrno = errno;
        error_fr(r, "sshbuf_put");
        goto out;
      }
      if (sshbuf_len(peer_version) > SSH_MAX_BANNER_LEN) {
        verbose_f("banner line too long");
        goto invalid;
      }
    }
    /* Is this an actual protocol banner? */
    if (sshbuf_len(peer_version) > 4 &&
        memcmp(sshbuf_ptr(peer_version), "SSH-", 4) == 0)
      break;
    /* If not, then just log the line and continue */
    if ((cp = sshbuf_dup_string(peer_version)) == NULL) {
      error_f("sshbuf_dup_string failed");
      r = SSH_ERR_ALLOC_FAIL;
      goto out;
    }
    /* Do not accept lines before the SSH ident from a client */
    if (ssh->kex->server) {
      verbose_f("client sent invalid protocol identifier "
          "\"%.256s\"", cp);
      free(cp);
      goto invalid;
    }
    debug_f("banner line %zu: %s", n, cp);
    free(cp);
  }
  peer_version_string = sshbuf_dup_string(peer_version);
  if (peer_version_string == NULL)
    fatal_f("sshbuf_dup_string failed");
  /* XXX must be same size for sscanf */
  if ((remote_version = calloc(1, sshbuf_len(peer_version))) == NULL) {
    error_f("calloc failed");
    r = SSH_ERR_ALLOC_FAIL;
    goto out;
  }

  /*
   * Check that the versions match.  In future this might accept
   * several versions and set appropriate flags to handle them.
   */
  if (sscanf(peer_version_string, "SSH-%d.%d-%[^\n]\n",
      &remote_major, &remote_minor, remote_version) != 3) {
    error("Bad remote protocol version identification: '%.100s'",
        peer_version_string);
 invalid:
    send_error(ssh, "Invalid SSH identification string.");
    r = SSH_ERR_INVALID_FORMAT;
    goto out;
  }
  debug("Remote protocol version %d.%d, remote software version %.100s",
      remote_major, remote_minor, remote_version);
  compat_banner(ssh, remote_version);

  mismatch = 0;
  switch (remote_major) {
  case 2:
    break;
  case 1:
    if (remote_minor != 99)
      mismatch = 1;
    break;
  default:
    mismatch = 1;
    break;
  }
  if (mismatch) {
    error("Protocol major versions differ: %d vs. %d",
        PROTOCOL_MAJOR_2, remote_major);
    send_error(ssh, "Protocol major versions differ.");
    r = SSH_ERR_NO_PROTOCOL_VERSION;
    goto out;
  }

  if (ssh->kex->server && (ssh->compat & SSH_BUG_PROBE) != 0) {
    logit("probed from %s port %d with %s.  Don't panic.",
        ssh_remote_ipaddr(ssh), ssh_remote_port(ssh),
        peer_version_string);
    r = SSH_ERR_CONN_CLOSED; /* XXX */
    goto out;
  }
  if (ssh->kex->server && (ssh->compat & SSH_BUG_SCANNER) != 0) {
    logit("scanned from %s port %d with %s.  Don't panic.",
        ssh_remote_ipaddr(ssh), ssh_remote_port(ssh),
        peer_version_string);
    r = SSH_ERR_CONN_CLOSED; /* XXX */
    goto out;
  }
  /* success */
  r = 0;
 out:
  free(our_version_string);
  free(peer_version_string);
  ssh->remote_version = remote_version;  /* transferred */
  if (r == SSH_ERR_SYSTEM_ERROR)
    errno = oerrno;
  return r;
}


Coverage Report

Created: 2026-10-23 07:09