* Track incoming and outgoing data sizes and time, for
     * size-based rekeys.
     */
    unsigned long incoming_data_size, outgoing_data_size, deferred_data_size;
    unsigned long max_data_size;
    int kex_in_progress;
    long next_rekey, last_rekey;
    char *deferred_rekey_reason;    /* points to STATIC string; don't free */
};

#define logevent(s) logevent(ssh->frontend, s)

/* logevent, only printf-formatted. */
static void logeventf(Ssh ssh, const char *fmt, ...)
{

	void *hkey;		       /* actual host key */
	unsigned char exchange_hash[20];
	int n_preferred_kex;
	const struct ssh_kex *preferred_kex[KEX_MAX];
	int n_preferred_ciphers;
	const struct ssh2_ciphers *preferred_ciphers[CIPHER_MAX];
	const struct ssh_compress *preferred_comp;
	int got_session_id, activated_authconn;
	struct Packet *pktout;
    };
    crState(do_ssh2_transport_state);

    crBegin(ssh->do_ssh2_transport_crstate);

    s->cscipher_tobe = s->sccipher_tobe = NULL;
    s->csmac_tobe = s->scmac_tobe = NULL;
    s->cscomp_tobe = s->sccomp_tobe = NULL;

    s->got_session_id = s->activated_authconn = FALSE;

    /*
     * Be prepared to work around the buggy MAC problem.
     */
    if (ssh->remote_bugs & BUG_SSH2_HMAC)
	s->maclist = buggymacs, s->nmacs = lenof(buggymacs);
    else
	s->maclist = macs, s->nmacs = lenof(macs);

  begin_key_exchange:
    {
	int i, j, commalist_started;

	/*
	 * Set up the preferred key exchange. (NULL => warn below here)
	 */
	s->n_preferred_kex = 0;
	for (i = 0; i < KEX_MAX; i++) {
	    switch (ssh->cfg.ssh_kexlist[i]) {
	      case KEX_DHGEX:

		 * the list. */
		if (i < KEX_MAX - 1) {
		    s->preferred_kex[s->n_preferred_kex++] = NULL;
		}
		break;
	    }
	}




	/*
	 * Set up the preferred ciphers. (NULL => warn below here)
	 */
	s->n_preferred_ciphers = 0;
	for (i = 0; i < CIPHER_MAX; i++) {
	    switch (ssh->cfg.ssh_cipherlist[i]) {
	      case CIPHER_BLOWFISH:

		 * the list. */
		if (i < CIPHER_MAX - 1) {
		    s->preferred_ciphers[s->n_preferred_ciphers++] = NULL;
		}
		break;
	    }
	}


	/*
	 * Set up preferred compression.
	 */
	if (ssh->cfg.compression)
	    s->preferred_comp = &ssh_zlib;
	else
	    s->preferred_comp = &ssh_comp_none;













	/*
	 * Enable queueing of outgoing auth- or connection-layer
	 * packets while we are in the middle of a key exchange.
	 */
	ssh->queueing = TRUE;

     * checked the signature of the exchange hash.)
     */
    s->keystr = ssh->hostkey->fmtkey(s->hkey);
    s->fingerprint = ssh->hostkey->fingerprint(s->hkey);
    verify_ssh_host_key(ssh->frontend,
			ssh->savedhost, ssh->savedport, ssh->hostkey->keytype,
			s->keystr, s->fingerprint);
    if (!s->got_session_id) {     /* don't bother logging this in rekeys */
	logevent("Host key fingerprint is:");
	logevent(s->fingerprint);
    }
    sfree(s->fingerprint);
    sfree(s->keystr);
    ssh->hostkey->freekey(s->hkey);

    /*
     * The exchange hash from the very first key exchange is also
     * the session id, used in session key construction and
     * authentication.
     */
    if (!s->got_session_id) {
	memcpy(ssh->v2_session_id, s->exchange_hash,
	       sizeof(s->exchange_hash));
	s->got_session_id = TRUE;
    }

    /*
     * Send SSH2_MSG_NEWKEYS.
     */
    s->pktout = ssh2_pkt_init(SSH2_MSG_NEWKEYS);
    ssh2_pkt_send_noqueue(ssh, s->pktout);
    ssh->outgoing_data_size = 0;       /* start counting from here */

    freebn(s->K);
    if (ssh->kex == &ssh_diffiehellman_gex) {
	freebn(s->g);
	freebn(s->p);
    }

    /*
     * Key exchange is over. Loop straight back round if we have a
     * deferred rekey reason.
     */
    if (ssh->deferred_rekey_reason) {
	logevent(ssh->deferred_rekey_reason);
	pktin = NULL;
	ssh->deferred_rekey_reason = NULL;
	goto begin_key_exchange;
    }

    /*
     * Otherwise, schedule a timer for our next rekey.
     */
    ssh->kex_in_progress = FALSE;
    ssh->last_rekey = GETTICKCOUNT();
    if (ssh->cfg.ssh_rekey_time != 0)
	ssh->next_rekey = schedule_timer(ssh->cfg.ssh_rekey_time*60*TICKSPERSEC,
					 ssh2_timer, ssh);

    /*
     * If this is the first key exchange phase, we must pass the
     * SSH2_MSG_NEWKEYS packet to the next layer, not because it
     * wants to see it but because it will need time to initialise
     * itself before it sees an actual packet. In subsequent key
     * exchange phases, we don't pass SSH2_MSG_NEWKEYS on, because
     * it would only confuse the layer above.
     */
    if (s->activated_authconn) {
	crReturn(1);
    }
    s->activated_authconn = TRUE;

    /*
     * Now we're encrypting. Begin returning 1 to the protocol main
     * function so that other things can run on top of the
     * transport. If we ever see a KEXINIT, we must go back to the
     * start.
     * 

    ssh->mainchan = NULL;
    ssh->throttled_all = 0;
    ssh->v1_stdout_throttling = 0;
    ssh->queue = NULL;
    ssh->queuelen = ssh->queuesize = 0;
    ssh->queueing = FALSE;
    ssh->qhead = ssh->qtail = NULL;
    ssh->deferred_rekey_reason = NULL;

    *backend_handle = ssh;

#ifdef MSCRYPTOAPI
    if (crypto_startup() == 0)
	return "Microsoft high encryption pack not installed!";
#endif

/*
 * Reconfigure the SSH backend.
 */
static void ssh_reconfig(void *handle, Config *cfg)
{
    Ssh ssh = (Ssh) handle;
    char *rekeying = NULL, rekey_mandatory = FALSE;
    unsigned long old_max_data_size;

    pinger_reconfig(ssh->pinger, &ssh->cfg, cfg);
    ssh_setup_portfwd(ssh, cfg);

    if (ssh->cfg.ssh_rekey_time != cfg->ssh_rekey_time &&
	cfg->ssh_rekey_time != 0) {

    if (old_max_data_size != ssh->max_data_size &&
	ssh->max_data_size != 0) {
	if (ssh->outgoing_data_size > ssh->max_data_size ||
	    ssh->incoming_data_size > ssh->max_data_size)
	    rekeying = "Initiating key re-exchange (data limit lowered)";
    }

    if (ssh->cfg.compression != cfg->compression) {
	rekeying = "Initiating key re-exchange (compression setting changed)";
	rekey_mandatory = TRUE;
    }

    if (ssh->cfg.ssh2_des_cbc != cfg->ssh2_des_cbc ||
	memcmp(ssh->cfg.ssh_cipherlist, cfg->ssh_cipherlist,
	       sizeof(ssh->cfg.ssh_cipherlist))) {
	rekeying = "Initiating key re-exchange (cipher settings changed)";
	rekey_mandatory = TRUE;
    }

    ssh->cfg = *cfg;		       /* STRUCTURE COPY */

    if (rekeying) {
	if (!ssh->kex_in_progress) {
	    do_ssh2_transport(ssh, rekeying, -1, NULL);
	} else if (rekey_mandatory) {
	    ssh->deferred_rekey_reason = rekeying;
	}
    }
}

/*
 * Called to send data down the Telnet connection.
 */
static int ssh_send(void *handle, char *buf, int len)
{