/* $OpenBSD: kex.c,v 1.187 2024/08/23 04:51:00 deraadt 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 HAVE_POLL_H
#include <poll.h>
#endif

#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 "monitor.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, u_int32_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, u_int32_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("");
}

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, 3)) != 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) {
                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
                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, u_int32_t seq, struct ssh *ssh)
{
        struct kex *kex = ssh->kex;
        const int max_ext_info = kex->server ? 1 : 2;
        u_int32_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, u_int32_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;
        free(kex->name);
        kex->name = NULL;
        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, u_int32_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;
        }
        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);
#ifdef OPENSSL_HAS_ECC
        EC_KEY_free(kex->ec_client_key);
#endif /* OPENSSL_HAS_ECC */
#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);
}

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);
        free(remote_version);
        if (r == SSH_ERR_SYSTEM_ERROR)
                errno = oerrno;
        return r;
}