kexgex.c - security/openssh-library - Rivoreo Source Code Repositories

 /*
  * Copyright (c) 2000 Niels Provos.  All rights reserved.
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */

 #include "includes.h"
 RCSID("$OpenBSD: kexgex.c,v 1.7 2001/06/23 02:34:29 markus Exp $");

 #include <openssl/bn.h>

 #include "xmalloc.h"
 #include "buffer.h"
 #include "bufaux.h"
 #include "key.h"
 #include "kex.h"
 #include "log.h"
 #include "packet.h"
 #include "dh.h"
 #include "ssh2.h"
 #include "compat.h"

 u_char *
 kexgex_hash(
     char *client_version_string,
     char *server_version_string,
     char *ckexinit, int ckexinitlen,
     char *skexinit, int skexinitlen,
     char *serverhostkeyblob, int sbloblen,
     int min, int wantbits, int max, BIGNUM *prime, BIGNUM *gen,
     BIGNUM *client_dh_pub,
     BIGNUM *server_dh_pub,
     BIGNUM *shared_secret)
 {
 	Buffer b;
 	static u_char digest[EVP_MAX_MD_SIZE];
 	EVP_MD *evp_md = EVP_sha1();
 	EVP_MD_CTX md;

 	buffer_init(&b);
 	buffer_put_cstring(&b, client_version_string);
 	buffer_put_cstring(&b, server_version_string);

 	/* kexinit messages: fake header: len+SSH2_MSG_KEXINIT */
 	buffer_put_int(&b, ckexinitlen+1);
 	buffer_put_char(&b, SSH2_MSG_KEXINIT);
 	buffer_append(&b, ckexinit, ckexinitlen);
 	buffer_put_int(&b, skexinitlen+1);
 	buffer_put_char(&b, SSH2_MSG_KEXINIT);
 	buffer_append(&b, skexinit, skexinitlen);

 	buffer_put_string(&b, serverhostkeyblob, sbloblen);
 	if (min == -1 || max == -1)
 		buffer_put_int(&b, wantbits);
 	else {
 		buffer_put_int(&b, min);
 		buffer_put_int(&b, wantbits);
 		buffer_put_int(&b, max);
 	}
 	buffer_put_bignum2(&b, prime);
 	buffer_put_bignum2(&b, gen);
 	buffer_put_bignum2(&b, client_dh_pub);
 	buffer_put_bignum2(&b, server_dh_pub);
 	buffer_put_bignum2(&b, shared_secret);

 #ifdef DEBUG_KEXDH
 	buffer_dump(&b);
 #endif
 	EVP_DigestInit(&md, evp_md);
 	EVP_DigestUpdate(&md, buffer_ptr(&b), buffer_len(&b));
 	EVP_DigestFinal(&md, digest, NULL);

 	buffer_free(&b);

 #ifdef DEBUG_KEXDH
 	dump_digest("hash", digest, evp_md->md_size);
 #endif
 	return digest;
 }

 /* client */

 void
 kexgex_client(Kex *kex)
 {
 	BIGNUM *dh_server_pub = NULL, *shared_secret = NULL;
 	BIGNUM *p = NULL, *g = NULL;
 	Key *server_host_key;
 	u_char *kbuf, *hash, *signature = NULL, *server_host_key_blob = NULL;
 	u_int klen, kout, slen, sbloblen;
 	int dlen, plen, min, max, nbits;
 	DH *dh;

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

 	if (datafellows & SSH_OLD_DHGEX) {
 		debug("SSH2_MSG_KEX_DH_GEX_REQUEST_OLD sent");

 		/* Old GEX request */
 		packet_start(SSH2_MSG_KEX_DH_GEX_REQUEST_OLD);
 		packet_put_int(nbits);
 		min = DH_GRP_MIN;
 		max = DH_GRP_MAX;
 	} else {
 		debug("SSH2_MSG_KEX_DH_GEX_REQUEST sent");

 		/* New GEX request */
 		min = DH_GRP_MIN;
 		max = DH_GRP_MAX;
 		packet_start(SSH2_MSG_KEX_DH_GEX_REQUEST);
 		packet_put_int(min);
 		packet_put_int(nbits);
 		packet_put_int(max);
 	}
 #ifdef DEBUG_KEXDH
 	fprintf(stderr, "\nmin = %d, nbits = %d, max = %d\n",
 	    min, nbits, max);
 #endif
 	packet_send();

 	debug("expecting SSH2_MSG_KEX_DH_GEX_GROUP");
 	packet_read_expect(&plen, SSH2_MSG_KEX_DH_GEX_GROUP);

 	if ((p = BN_new()) == NULL)
 		fatal("BN_new");
 	packet_get_bignum2(p, &dlen);
 	if ((g = BN_new()) == NULL)
 		fatal("BN_new");
 	packet_get_bignum2(g, &dlen);
 	packet_done();

 	if (BN_num_bits(p) < min || BN_num_bits(p) > max)
 		fatal("DH_GEX group out of range: %d !< %d !< %d",
 		    min, BN_num_bits(p), max);

 	dh = dh_new_group(g, p);
 	dh_gen_key(dh, kex->we_need * 8);

 #ifdef DEBUG_KEXDH
 	DHparams_print_fp(stderr, dh);
 	fprintf(stderr, "pub= ");
 	BN_print_fp(stderr, dh->pub_key);
 	fprintf(stderr, "\n");
 #endif

 	debug("SSH2_MSG_KEX_DH_GEX_INIT sent");
 	/* generate and send 'e', client DH public key */
 	packet_start(SSH2_MSG_KEX_DH_GEX_INIT);
 	packet_put_bignum2(dh->pub_key);
 	packet_send();

 	debug("expecting SSH2_MSG_KEX_DH_GEX_REPLY");
 	packet_read_expect(&plen, SSH2_MSG_KEX_DH_GEX_REPLY);

 	/* key, cert */
 	server_host_key_blob = packet_get_string(&sbloblen);
 	server_host_key = key_from_blob(server_host_key_blob, sbloblen);
 	if (server_host_key == NULL)
 		fatal("cannot decode server_host_key_blob");

 	if (kex->verify_host_key == NULL)
 		fatal("cannot verify server_host_key");
 	if (kex->verify_host_key(server_host_key) == -1)
 		fatal("server_host_key verification failed");

 	/* DH paramter f, server public DH key */
 	dh_server_pub = BN_new();
 	if (dh_server_pub == NULL)
 		fatal("dh_server_pub == NULL");
 	packet_get_bignum2(dh_server_pub, &dlen);

 #ifdef DEBUG_KEXDH
 	fprintf(stderr, "dh_server_pub= ");
 	BN_print_fp(stderr, dh_server_pub);
 	fprintf(stderr, "\n");
 	debug("bits %d", BN_num_bits(dh_server_pub));
 #endif

 	/* signed H */
 	signature = packet_get_string(&slen);
 	packet_done();

 	if (!dh_pub_is_valid(dh, dh_server_pub))
 		packet_disconnect("bad server public DH value");

 	klen = DH_size(dh);
 	kbuf = xmalloc(klen);
 	kout = DH_compute_key(kbuf, dh_server_pub, dh);
 #ifdef DEBUG_KEXDH
 	dump_digest("shared secret", kbuf, kout);
 #endif
 	shared_secret = BN_new();
 	BN_bin2bn(kbuf, kout, shared_secret);
 	memset(kbuf, 0, klen);
 	xfree(kbuf);

 	if (datafellows & SSH_OLD_DHGEX)
 		min = max = -1;

 	/* calc and verify H */
 	hash = kexgex_hash(
 	    kex->client_version_string,
 	    kex->server_version_string,
 	    buffer_ptr(&kex->my), buffer_len(&kex->my),
 	    buffer_ptr(&kex->peer), buffer_len(&kex->peer),
 	    server_host_key_blob, sbloblen,
 	    min, nbits, max,
 	    dh->p, dh->g,
 	    dh->pub_key,
 	    dh_server_pub,
 	    shared_secret
 	);
 	/* have keys, free DH */
 	DH_free(dh);
 	xfree(server_host_key_blob);
 	BN_free(dh_server_pub);

 	if (key_verify(server_host_key, (u_char *)signature, slen, hash, 20) != 1)
 		fatal("key_verify failed for server_host_key");
 	key_free(server_host_key);
 	xfree(signature);

 	/* save session id */
 	if (kex->session_id == NULL) {
 		kex->session_id_len = 20;
 		kex->session_id = xmalloc(kex->session_id_len);
 		memcpy(kex->session_id, hash, kex->session_id_len);
 	}
 	kex_derive_keys(kex, hash, shared_secret);
 	BN_clear_free(shared_secret);

 	kex_finish(kex);
 }

 /* server */

 void
 kexgex_server(Kex *kex)
 {
 	BIGNUM *shared_secret = NULL, *dh_client_pub = NULL;
 	Key *server_host_key;
 	DH *dh = dh;
 	u_char *kbuf, *hash, *signature = NULL, *server_host_key_blob = NULL;
 	u_int sbloblen, klen, kout;
 	int min = -1, max = -1, nbits = -1, type, plen, dlen, slen;

 	if (kex->load_host_key == NULL)
 		fatal("Cannot load hostkey");
 	server_host_key = kex->load_host_key(kex->hostkey_type);
 	if (server_host_key == NULL)
 		fatal("Unsupported hostkey type %d", kex->hostkey_type);

 	type = packet_read(&plen);
 	switch(type){
 	case SSH2_MSG_KEX_DH_GEX_REQUEST:
 		debug("SSH2_MSG_KEX_DH_GEX_REQUEST received");
 		min = packet_get_int();
 		nbits = packet_get_int();
 		max = packet_get_int();
 		min = MAX(DH_GRP_MIN, min);
 		max = MIN(DH_GRP_MAX, max);
 		break;
 	case SSH2_MSG_KEX_DH_GEX_REQUEST_OLD:
 		debug("SSH2_MSG_KEX_DH_GEX_REQUEST_OLD received");
 		nbits = packet_get_int();
 		min = DH_GRP_MIN;
 		max = DH_GRP_MAX;
 		/* unused for old GEX */
 		break;
 	default:
 		fatal("protocol error during kex, no DH_GEX_REQUEST: %d", type);
 	}
 	packet_done();

 	if (max < min || nbits < min || max < nbits)
 		fatal("DH_GEX_REQUEST, bad parameters: %d !< %d !< %d",
 		    min, nbits, max);

 	dh = choose_dh(min, nbits, max);
 	if (dh == NULL)
 		packet_disconnect("Protocol error: no matching DH grp found");

 	debug("SSH2_MSG_KEX_DH_GEX_GROUP sent");
 	packet_start(SSH2_MSG_KEX_DH_GEX_GROUP);
 	packet_put_bignum2(dh->p);
 	packet_put_bignum2(dh->g);
 	packet_send();

 	/* flush */
 	packet_write_wait();

 	/* Compute our exchange value in parallel with the client */
 	dh_gen_key(dh, kex->we_need * 8);

 	debug("expecting SSH2_MSG_KEX_DH_GEX_INIT");
 	packet_read_expect(&plen, SSH2_MSG_KEX_DH_GEX_INIT);

 	/* key, cert */
 	dh_client_pub = BN_new();
 	if (dh_client_pub == NULL)
 		fatal("dh_client_pub == NULL");
 	packet_get_bignum2(dh_client_pub, &dlen);

 #ifdef DEBUG_KEXDH
 	fprintf(stderr, "dh_client_pub= ");
 	BN_print_fp(stderr, dh_client_pub);
 	fprintf(stderr, "\n");
 	debug("bits %d", BN_num_bits(dh_client_pub));
 #endif

 #ifdef DEBUG_KEXDH
 	DHparams_print_fp(stderr, dh);
 	fprintf(stderr, "pub= ");
 	BN_print_fp(stderr, dh->pub_key);
 	fprintf(stderr, "\n");
 #endif
 	if (!dh_pub_is_valid(dh, dh_client_pub))
 		packet_disconnect("bad client public DH value");

 	klen = DH_size(dh);
 	kbuf = xmalloc(klen);
 	kout = DH_compute_key(kbuf, dh_client_pub, dh);
 #ifdef DEBUG_KEXDH
 	dump_digest("shared secret", kbuf, kout);
 #endif
 	shared_secret = BN_new();
 	BN_bin2bn(kbuf, kout, shared_secret);
 	memset(kbuf, 0, klen);
 	xfree(kbuf);

 	key_to_blob(server_host_key, &server_host_key_blob, &sbloblen);

 	if (type == SSH2_MSG_KEX_DH_GEX_REQUEST_OLD)
 		min = max = -1;

 	/* calc H */			/* XXX depends on 'kex' */
 	hash = kexgex_hash(
 	    kex->client_version_string,
 	    kex->server_version_string,
 	    buffer_ptr(&kex->peer), buffer_len(&kex->peer),
 	    buffer_ptr(&kex->my), buffer_len(&kex->my),
 	    (char *)server_host_key_blob, sbloblen,
 	    min, nbits, max,
 	    dh->p, dh->g,
 	    dh_client_pub,
 	    dh->pub_key,
 	    shared_secret
 	);
 	BN_free(dh_client_pub);

 	/* save session id := H */
 	/* XXX hashlen depends on KEX */
 	if (kex->session_id == NULL) {
 		kex->session_id_len = 20;
 		kex->session_id = xmalloc(kex->session_id_len);
 		memcpy(kex->session_id, hash, kex->session_id_len);
 	}

 	/* sign H */
 	/* XXX hashlen depends on KEX */
 	key_sign(server_host_key, &signature, &slen, hash, 20);

 	/* destroy_sensitive_data(); */

 	/* send server hostkey, DH pubkey 'f' and singed H */
 	debug("SSH2_MSG_KEX_DH_GEX_REPLY sent");
 	packet_start(SSH2_MSG_KEX_DH_GEX_REPLY);
 	packet_put_string((char *)server_host_key_blob, sbloblen);
 	packet_put_bignum2(dh->pub_key);	/* f */
 	packet_put_string((char *)signature, slen);
 	packet_send();
 	xfree(signature);
 	xfree(server_host_key_blob);
 	/* have keys, free DH */
 	DH_free(dh);

 	kex_derive_keys(kex, hash, shared_secret);
 	BN_clear_free(shared_secret);

 	kex_finish(kex);
 }

 void
 kexgex(Kex *kex)
 {
 	if (kex->server)
 		kexgex_server(kex);
 	else
 		kexgex_client(kex);
 }
	/*
	* Copyright (c) 2000 Niels Provos. All rights reserved.
	* Copyright (c) 2001 Markus Friedl. All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions
	* are met:
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	* 2. Redistributions in binary form must reproduce the above copyright
	* notice, this list of conditions and the following disclaimer in the
	* documentation and/or other materials provided with the distribution.
	*
	* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
	* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
	* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
	* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
	* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
	* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
	* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
	* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
	* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
	* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	*/

	#include "includes.h"
	RCSID("$OpenBSD: kexgex.c,v 1.7 2001/06/23 02:34:29 markus Exp $");

	#include <openssl/bn.h>

	#include "xmalloc.h"
	#include "buffer.h"
	#include "bufaux.h"
	#include "key.h"
	#include "kex.h"
	#include "log.h"
	#include "packet.h"
	#include "dh.h"
	#include "ssh2.h"
	#include "compat.h"

	u_char *
	kexgex_hash(
	char *client_version_string,
	char *server_version_string,
	char *ckexinit, int ckexinitlen,
	char *skexinit, int skexinitlen,
	char *serverhostkeyblob, int sbloblen,
	int min, int wantbits, int max, BIGNUM prime, BIGNUM gen,
	BIGNUM *client_dh_pub,
	BIGNUM *server_dh_pub,
	BIGNUM *shared_secret)
	{
	Buffer b;
	static u_char digest[EVP_MAX_MD_SIZE];
	EVP_MD *evp_md = EVP_sha1();
	EVP_MD_CTX md;

	buffer_init(&b);
	buffer_put_cstring(&b, client_version_string);
	buffer_put_cstring(&b, server_version_string);

	/* kexinit messages: fake header: len+SSH2_MSG_KEXINIT */
	buffer_put_int(&b, ckexinitlen+1);
	buffer_put_char(&b, SSH2_MSG_KEXINIT);
	buffer_append(&b, ckexinit, ckexinitlen);
	buffer_put_int(&b, skexinitlen+1);
	buffer_put_char(&b, SSH2_MSG_KEXINIT);
	buffer_append(&b, skexinit, skexinitlen);

	buffer_put_string(&b, serverhostkeyblob, sbloblen);
	if (min == -1 \|\| max == -1)
	buffer_put_int(&b, wantbits);
	else {
	buffer_put_int(&b, min);
	buffer_put_int(&b, wantbits);
	buffer_put_int(&b, max);
	}
	buffer_put_bignum2(&b, prime);
	buffer_put_bignum2(&b, gen);
	buffer_put_bignum2(&b, client_dh_pub);
	buffer_put_bignum2(&b, server_dh_pub);
	buffer_put_bignum2(&b, shared_secret);

	#ifdef DEBUG_KEXDH
	buffer_dump(&b);
	#endif
	EVP_DigestInit(&md, evp_md);
	EVP_DigestUpdate(&md, buffer_ptr(&b), buffer_len(&b));
	EVP_DigestFinal(&md, digest, NULL);

	buffer_free(&b);

	#ifdef DEBUG_KEXDH
	dump_digest("hash", digest, evp_md->md_size);
	#endif
	return digest;
	}

	/* client */

	void
	kexgex_client(Kex *kex)
	{
	BIGNUM dh_server_pub = NULL, shared_secret = NULL;
	BIGNUM p = NULL, g = NULL;
	Key *server_host_key;
	u_char kbuf, hash, signature = NULL, server_host_key_blob = NULL;
	u_int klen, kout, slen, sbloblen;
	int dlen, plen, min, max, nbits;
	DH *dh;

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

	if (datafellows & SSH_OLD_DHGEX) {
	debug("SSH2_MSG_KEX_DH_GEX_REQUEST_OLD sent");

	/* Old GEX request */
	packet_start(SSH2_MSG_KEX_DH_GEX_REQUEST_OLD);
	packet_put_int(nbits);
	min = DH_GRP_MIN;
	max = DH_GRP_MAX;
	} else {
	debug("SSH2_MSG_KEX_DH_GEX_REQUEST sent");

	/* New GEX request */
	min = DH_GRP_MIN;
	max = DH_GRP_MAX;
	packet_start(SSH2_MSG_KEX_DH_GEX_REQUEST);
	packet_put_int(min);
	packet_put_int(nbits);
	packet_put_int(max);
	}
	#ifdef DEBUG_KEXDH
	fprintf(stderr, "\nmin = %d, nbits = %d, max = %d\n",
	min, nbits, max);
	#endif
	packet_send();

	debug("expecting SSH2_MSG_KEX_DH_GEX_GROUP");
	packet_read_expect(&plen, SSH2_MSG_KEX_DH_GEX_GROUP);

	if ((p = BN_new()) == NULL)
	fatal("BN_new");
	packet_get_bignum2(p, &dlen);
	if ((g = BN_new()) == NULL)
	fatal("BN_new");
	packet_get_bignum2(g, &dlen);
	packet_done();

	if (BN_num_bits(p) < min \|\| BN_num_bits(p) > max)
	fatal("DH_GEX group out of range: %d !< %d !< %d",
	min, BN_num_bits(p), max);

	dh = dh_new_group(g, p);
	dh_gen_key(dh, kex->we_need * 8);

	#ifdef DEBUG_KEXDH
	DHparams_print_fp(stderr, dh);
	fprintf(stderr, "pub= ");
	BN_print_fp(stderr, dh->pub_key);
	fprintf(stderr, "\n");
	#endif

	debug("SSH2_MSG_KEX_DH_GEX_INIT sent");
	/* generate and send 'e', client DH public key */
	packet_start(SSH2_MSG_KEX_DH_GEX_INIT);
	packet_put_bignum2(dh->pub_key);
	packet_send();

	debug("expecting SSH2_MSG_KEX_DH_GEX_REPLY");
	packet_read_expect(&plen, SSH2_MSG_KEX_DH_GEX_REPLY);

	/* key, cert */
	server_host_key_blob = packet_get_string(&sbloblen);
	server_host_key = key_from_blob(server_host_key_blob, sbloblen);
	if (server_host_key == NULL)
	fatal("cannot decode server_host_key_blob");

	if (kex->verify_host_key == NULL)
	fatal("cannot verify server_host_key");
	if (kex->verify_host_key(server_host_key) == -1)
	fatal("server_host_key verification failed");

	/* DH paramter f, server public DH key */
	dh_server_pub = BN_new();
	if (dh_server_pub == NULL)
	fatal("dh_server_pub == NULL");
	packet_get_bignum2(dh_server_pub, &dlen);

	#ifdef DEBUG_KEXDH
	fprintf(stderr, "dh_server_pub= ");
	BN_print_fp(stderr, dh_server_pub);
	fprintf(stderr, "\n");
	debug("bits %d", BN_num_bits(dh_server_pub));
	#endif

	/* signed H */
	signature = packet_get_string(&slen);
	packet_done();

	if (!dh_pub_is_valid(dh, dh_server_pub))
	packet_disconnect("bad server public DH value");

	klen = DH_size(dh);
	kbuf = xmalloc(klen);
	kout = DH_compute_key(kbuf, dh_server_pub, dh);
	#ifdef DEBUG_KEXDH
	dump_digest("shared secret", kbuf, kout);
	#endif
	shared_secret = BN_new();
	BN_bin2bn(kbuf, kout, shared_secret);
	memset(kbuf, 0, klen);
	xfree(kbuf);

	if (datafellows & SSH_OLD_DHGEX)
	min = max = -1;

	/* calc and verify H */
	hash = kexgex_hash(
	kex->client_version_string,
	kex->server_version_string,
	buffer_ptr(&kex->my), buffer_len(&kex->my),
	buffer_ptr(&kex->peer), buffer_len(&kex->peer),
	server_host_key_blob, sbloblen,
	min, nbits, max,
	dh->p, dh->g,
	dh->pub_key,
	dh_server_pub,
	shared_secret
	);
	/* have keys, free DH */
	DH_free(dh);
	xfree(server_host_key_blob);
	BN_free(dh_server_pub);

	if (key_verify(server_host_key, (u_char *)signature, slen, hash, 20) != 1)
	fatal("key_verify failed for server_host_key");
	key_free(server_host_key);
	xfree(signature);

	/* save session id */
	if (kex->session_id == NULL) {
	kex->session_id_len = 20;
	kex->session_id = xmalloc(kex->session_id_len);
	memcpy(kex->session_id, hash, kex->session_id_len);
	}
	kex_derive_keys(kex, hash, shared_secret);
	BN_clear_free(shared_secret);

	kex_finish(kex);
	}

	/* server */

	void
	kexgex_server(Kex *kex)
	{
	BIGNUM shared_secret = NULL, dh_client_pub = NULL;
	Key *server_host_key;
	DH *dh = dh;
	u_char kbuf, hash, signature = NULL, server_host_key_blob = NULL;
	u_int sbloblen, klen, kout;
	int min = -1, max = -1, nbits = -1, type, plen, dlen, slen;

	if (kex->load_host_key == NULL)
	fatal("Cannot load hostkey");
	server_host_key = kex->load_host_key(kex->hostkey_type);
	if (server_host_key == NULL)
	fatal("Unsupported hostkey type %d", kex->hostkey_type);

	type = packet_read(&plen);
	switch(type){
	case SSH2_MSG_KEX_DH_GEX_REQUEST:
	debug("SSH2_MSG_KEX_DH_GEX_REQUEST received");
	min = packet_get_int();
	nbits = packet_get_int();
	max = packet_get_int();
	min = MAX(DH_GRP_MIN, min);
	max = MIN(DH_GRP_MAX, max);
	break;
	case SSH2_MSG_KEX_DH_GEX_REQUEST_OLD:
	debug("SSH2_MSG_KEX_DH_GEX_REQUEST_OLD received");
	nbits = packet_get_int();
	min = DH_GRP_MIN;
	max = DH_GRP_MAX;
	/* unused for old GEX */
	break;
	default:
	fatal("protocol error during kex, no DH_GEX_REQUEST: %d", type);
	}
	packet_done();

	if (max < min \|\| nbits < min \|\| max < nbits)
	fatal("DH_GEX_REQUEST, bad parameters: %d !< %d !< %d",
	min, nbits, max);

	dh = choose_dh(min, nbits, max);
	if (dh == NULL)
	packet_disconnect("Protocol error: no matching DH grp found");

	debug("SSH2_MSG_KEX_DH_GEX_GROUP sent");
	packet_start(SSH2_MSG_KEX_DH_GEX_GROUP);
	packet_put_bignum2(dh->p);
	packet_put_bignum2(dh->g);
	packet_send();

	/* flush */
	packet_write_wait();

	/* Compute our exchange value in parallel with the client */
	dh_gen_key(dh, kex->we_need * 8);

	debug("expecting SSH2_MSG_KEX_DH_GEX_INIT");
	packet_read_expect(&plen, SSH2_MSG_KEX_DH_GEX_INIT);

	/* key, cert */
	dh_client_pub = BN_new();
	if (dh_client_pub == NULL)
	fatal("dh_client_pub == NULL");
	packet_get_bignum2(dh_client_pub, &dlen);

	#ifdef DEBUG_KEXDH
	fprintf(stderr, "dh_client_pub= ");
	BN_print_fp(stderr, dh_client_pub);
	fprintf(stderr, "\n");
	debug("bits %d", BN_num_bits(dh_client_pub));
	#endif

	#ifdef DEBUG_KEXDH
	DHparams_print_fp(stderr, dh);
	fprintf(stderr, "pub= ");
	BN_print_fp(stderr, dh->pub_key);
	fprintf(stderr, "\n");
	#endif
	if (!dh_pub_is_valid(dh, dh_client_pub))
	packet_disconnect("bad client public DH value");

	klen = DH_size(dh);
	kbuf = xmalloc(klen);
	kout = DH_compute_key(kbuf, dh_client_pub, dh);
	#ifdef DEBUG_KEXDH
	dump_digest("shared secret", kbuf, kout);
	#endif
	shared_secret = BN_new();
	BN_bin2bn(kbuf, kout, shared_secret);
	memset(kbuf, 0, klen);
	xfree(kbuf);

	key_to_blob(server_host_key, &server_host_key_blob, &sbloblen);

	if (type == SSH2_MSG_KEX_DH_GEX_REQUEST_OLD)
	min = max = -1;

	/* calc H / / XXX depends on 'kex' */
	hash = kexgex_hash(
	kex->client_version_string,
	kex->server_version_string,
	buffer_ptr(&kex->peer), buffer_len(&kex->peer),
	buffer_ptr(&kex->my), buffer_len(&kex->my),
	(char *)server_host_key_blob, sbloblen,
	min, nbits, max,
	dh->p, dh->g,
	dh_client_pub,
	dh->pub_key,
	shared_secret
	);
	BN_free(dh_client_pub);

	/* save session id := H */
	/* XXX hashlen depends on KEX */
	if (kex->session_id == NULL) {
	kex->session_id_len = 20;
	kex->session_id = xmalloc(kex->session_id_len);
	memcpy(kex->session_id, hash, kex->session_id_len);
	}

	/* sign H */
	/* XXX hashlen depends on KEX */
	key_sign(server_host_key, &signature, &slen, hash, 20);

	/* destroy_sensitive_data(); */

	/* send server hostkey, DH pubkey 'f' and singed H */
	debug("SSH2_MSG_KEX_DH_GEX_REPLY sent");
	packet_start(SSH2_MSG_KEX_DH_GEX_REPLY);
	packet_put_string((char *)server_host_key_blob, sbloblen);
	packet_put_bignum2(dh->pub_key); /* f */
	packet_put_string((char *)signature, slen);
	packet_send();
	xfree(signature);
	xfree(server_host_key_blob);
	/* have keys, free DH */
	DH_free(dh);

	kex_derive_keys(kex, hash, shared_secret);
	BN_clear_free(shared_secret);

	kex_finish(kex);
	}

	void
	kexgex(Kex *kex)
	{
	if (kex->server)
	kexgex_server(kex);
	else
	kexgex_client(kex);
	}