kex.c

   1 /*
   2  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions
   6  * are met:
   7  * 1. Redistributions of source code must retain the above copyright
   8  *    notice, this list of conditions and the following disclaimer.
   9  * 2. Redistributions in binary form must reproduce the above copyright
  10  *    notice, this list of conditions and the following disclaimer in the
  11  *    documentation and/or other materials provided with the distribution.
  12  *
  13  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  14  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  15  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  16  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  17  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  18  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  19  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  20  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  21  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  22  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  23  */
  24
  25 #include "includes.h"
  26 RCSID("$OpenBSD: kex.c,v 1.61 2005/06/17 02:44:32 djm Exp $");
  27
  28 #include <openssl/crypto.h>
  29
  30 #include "ssh2.h"
  31 #include "xmalloc.h"
  32 #include "buffer.h"
  33 #include "bufaux.h"
  34 #include "packet.h"
  35 #include "compat.h"
  36 #include "cipher.h"
  37 #include "kex.h"
  38 #include "key.h"
  39 #include "log.h"
  40 #include "mac.h"
  41 #include "match.h"
  42 #include "dispatch.h"
  43 #include "monitor.h"
  44
  45 #define KEX_COOKIE_LEN  16
  46
  47 /* prototype */
  48 static void kex_kexinit_finish(Kex *);
  49 static void kex_choose_conf(Kex *);
  50
  51 /* put algorithm proposal into buffer */
  52 static void
  53 kex_prop2buf(Buffer *b, char *proposal[PROPOSAL_MAX])
  54 {
  55         u_int i;
  56
  57         buffer_clear(b);
  58         /*
  59          * add a dummy cookie, the cookie will be overwritten by
  60          * kex_send_kexinit(), each time a kexinit is set
  61          */
  62         for (i = 0; i < KEX_COOKIE_LEN; i++)
  63                 buffer_put_char(b, 0);
  64         for (i = 0; i < PROPOSAL_MAX; i++)
  65                 buffer_put_cstring(b, proposal[i]);
  66         buffer_put_char(b, 0);                  /* first_kex_packet_follows */
  67         buffer_put_int(b, 0);                   /* uint32 reserved */
  68 }
  69
  70 /* parse buffer and return algorithm proposal */
  71 static char **
  72 kex_buf2prop(Buffer *raw, int *first_kex_follows)
  73 {
  74         Buffer b;
  75         int i;
  76         char **proposal;
  77
  78         proposal = xmalloc(PROPOSAL_MAX * sizeof(char *));
  79
  80         buffer_init(&b);
  81         buffer_append(&b, buffer_ptr(raw), buffer_len(raw));
  82         /* skip cookie */
  83         for (i = 0; i < KEX_COOKIE_LEN; i++)
  84                 buffer_get_char(&b);
  85         /* extract kex init proposal strings */
  86         for (i = 0; i < PROPOSAL_MAX; i++) {
  87                 proposal[i] = buffer_get_string(&b,NULL);
  88                 debug2("kex_parse_kexinit: %s", proposal[i]);
  89         }
  90         /* first kex follows / reserved */
  91         i = buffer_get_char(&b);
  92         if (first_kex_follows != NULL)
  93                 *first_kex_follows = i;
  94         debug2("kex_parse_kexinit: first_kex_follows %d ", i);
  95         i = buffer_get_int(&b);
  96         debug2("kex_parse_kexinit: reserved %d ", i);
  97         buffer_free(&b);
  98         return proposal;
  99 }
 100
 101 static void
 102 kex_prop_free(char **proposal)
 103 {
 104         u_int i;
 105
 106         for (i = 0; i < PROPOSAL_MAX; i++)
 107                 xfree(proposal[i]);
 108         xfree(proposal);
 109 }
 110
 111 static void
 112 kex_protocol_error(int type, u_int32_t seq, void *ctxt)
 113 {
 114         error("Hm, kex protocol error: type %d seq %u", type, seq);
 115 }
 116
 117 static void
 118 kex_reset_dispatch(void)
 119 {
 120         dispatch_range(SSH2_MSG_TRANSPORT_MIN,
 121             SSH2_MSG_TRANSPORT_MAX, &kex_protocol_error);
 122         dispatch_set(SSH2_MSG_KEXINIT, &kex_input_kexinit);
 123 }
 124
 125 void
 126 kex_finish(Kex *kex)
 127 {
 128         kex_reset_dispatch();
 129
 130         packet_start(SSH2_MSG_NEWKEYS);
 131         packet_send();
 132         /* packet_write_wait(); */
 133         debug("SSH2_MSG_NEWKEYS sent");
 134
 135         debug("expecting SSH2_MSG_NEWKEYS");
 136         packet_read_expect(SSH2_MSG_NEWKEYS);
 137         packet_check_eom();
 138         debug("SSH2_MSG_NEWKEYS received");
 139
 140         kex->done = 1;
 141         buffer_clear(&kex->peer);
 142         /* buffer_clear(&kex->my); */
 143         kex->flags &= ~KEX_INIT_SENT;
 144         xfree(kex->name);
 145         kex->name = NULL;
 146 }
 147
 148 void
 149 kex_send_kexinit(Kex *kex)
 150 {
 151         u_int32_t rnd = 0;
 152         u_char *cookie;
 153         u_int i;
 154
 155         if (kex == NULL) {
 156                 error("kex_send_kexinit: no kex, cannot rekey");
 157                 return;
 158         }
 159         if (kex->flags & KEX_INIT_SENT) {
 160                 debug("KEX_INIT_SENT");
 161                 return;
 162         }
 163         kex->done = 0;
 164
 165         /* generate a random cookie */
 166         if (buffer_len(&kex->my) < KEX_COOKIE_LEN)
 167                 fatal("kex_send_kexinit: kex proposal too short");
 168         cookie = buffer_ptr(&kex->my);
 169         for (i = 0; i < KEX_COOKIE_LEN; i++) {
 170                 if (i % 4 == 0)
 171                         rnd = arc4random();
 172                 cookie[i] = rnd;
 173                 rnd >>= 8;
 174         }
 175         packet_start(SSH2_MSG_KEXINIT);
 176         packet_put_raw(buffer_ptr(&kex->my), buffer_len(&kex->my));
 177         packet_send();
 178         debug("SSH2_MSG_KEXINIT sent");
 179         kex->flags |= KEX_INIT_SENT;
 180 }
 181
 182 void
 183 kex_input_kexinit(int type, u_int32_t seq, void *ctxt)
 184 {
 185         char *ptr;
 186         u_int i, dlen;
 187         Kex *kex = (Kex *)ctxt;
 188
 189         debug("SSH2_MSG_KEXINIT received");
 190         if (kex == NULL)
 191                 fatal("kex_input_kexinit: no kex, cannot rekey");
 192
 193         ptr = packet_get_raw(&dlen);
 194         buffer_append(&kex->peer, ptr, dlen);
 195
 196         /* discard packet */
 197         for (i = 0; i < KEX_COOKIE_LEN; i++)
 198                 packet_get_char();
 199         for (i = 0; i < PROPOSAL_MAX; i++)
 200                 xfree(packet_get_string(NULL));
 201         (void) packet_get_char();
 202         (void) packet_get_int();
 203         packet_check_eom();
 204
 205         kex_kexinit_finish(kex);
 206 }
 207
 208 Kex *
 209 kex_setup(char *proposal[PROPOSAL_MAX])
 210 {
 211         Kex *kex;
 212
 213         kex = xmalloc(sizeof(*kex));
 214         memset(kex, 0, sizeof(*kex));
 215         buffer_init(&kex->peer);
 216         buffer_init(&kex->my);
 217         kex_prop2buf(&kex->my, proposal);
 218         kex->done = 0;
 219
 220         kex_send_kexinit(kex);                                  /* we start */
 221         kex_reset_dispatch();
 222
 223         return kex;
 224 }
 225
 226 static void
 227 kex_kexinit_finish(Kex *kex)
 228 {
 229         if (!(kex->flags & KEX_INIT_SENT))
 230                 kex_send_kexinit(kex);
 231
 232         kex_choose_conf(kex);
 233
 234         if (kex->kex_type >= 0 && kex->kex_type < KEX_MAX &&
 235             kex->kex[kex->kex_type] != NULL) {
 236                 (kex->kex[kex->kex_type])(kex);
 237         } else {
 238                 fatal("Unsupported key exchange %d", kex->kex_type);
 239         }
 240 }
 241
 242 static void
 243 choose_enc(Enc *enc, char *client, char *server)
 244 {
 245         char *name = match_list(client, server, NULL);
 246         if (name == NULL)
 247                 fatal("no matching cipher found: client %s server %s", client, server);
 248         if ((enc->cipher = cipher_by_name(name)) == NULL)
 249                 fatal("matching cipher is not supported: %s", name);
 250         enc->name = name;
 251         enc->enabled = 0;
 252         enc->iv = NULL;
 253         enc->key = NULL;
 254         enc->key_len = cipher_keylen(enc->cipher);
 255         enc->block_size = cipher_blocksize(enc->cipher);
 256 }
 257 static void
 258 choose_mac(Mac *mac, char *client, char *server)
 259 {
 260         char *name = match_list(client, server, NULL);
 261         if (name == NULL)
 262                 fatal("no matching mac found: client %s server %s", client, server);
 263         if (mac_init(mac, name) < 0)
 264                 fatal("unsupported mac %s", name);
 265         /* truncate the key */
 266         if (datafellows & SSH_BUG_HMAC)
 267                 mac->key_len = 16;
 268         mac->name = name;
 269         mac->key = NULL;
 270         mac->enabled = 0;
 271 }
 272 static void
 273 choose_comp(Comp *comp, char *client, char *server)
 274 {
 275         char *name = match_list(client, server, NULL);
 276         if (name == NULL)
 277                 fatal("no matching comp found: client %s server %s", client, server);
 278         if (strcmp(name, "zlib") == 0) {
 279                 comp->type = 1;
 280         } else if (strcmp(name, "none") == 0) {
 281                 comp->type = 0;
 282         } else {
 283                 fatal("unsupported comp %s", name);
 284         }
 285         comp->name = name;
 286 }
 287 static void
 288 choose_kex(Kex *k, char *client, char *server)
 289 {
 290         k->name = match_list(client, server, NULL);
 291         if (k->name == NULL)
 292                 fatal("no kex alg");
 293         if (strcmp(k->name, KEX_DH1) == 0) {
 294                 k->kex_type = KEX_DH_GRP1_SHA1;
 295         } else if (strcmp(k->name, KEX_DH14) == 0) {
 296                 k->kex_type = KEX_DH_GRP14_SHA1;
 297         } else if (strcmp(k->name, KEX_DHGEX) == 0) {
 298                 k->kex_type = KEX_DH_GEX_SHA1;
 299         } else
 300                 fatal("bad kex alg %s", k->name);
 301 }
 302 static void
 303 choose_hostkeyalg(Kex *k, char *client, char *server)
 304 {
 305         char *hostkeyalg = match_list(client, server, NULL);
 306         if (hostkeyalg == NULL)
 307                 fatal("no hostkey alg");
 308         k->hostkey_type = key_type_from_name(hostkeyalg);
 309         if (k->hostkey_type == KEY_UNSPEC)
 310                 fatal("bad hostkey alg '%s'", hostkeyalg);
 311         xfree(hostkeyalg);
 312 }
 313
 314 static int
 315 proposals_match(char *my[PROPOSAL_MAX], char *peer[PROPOSAL_MAX])
 316 {
 317         static int check[] = {
 318                 PROPOSAL_KEX_ALGS, PROPOSAL_SERVER_HOST_KEY_ALGS, -1
 319         };
 320         int *idx;
 321         char *p;
 322
 323         for (idx = &check[0]; *idx != -1; idx++) {
 324                 if ((p = strchr(my[*idx], ',')) != NULL)
 325                         *p = '\0';
 326                 if ((p = strchr(peer[*idx], ',')) != NULL)
 327                         *p = '\0';
 328                 if (strcmp(my[*idx], peer[*idx]) != 0) {
 329                         debug2("proposal mismatch: my %s peer %s",
 330                             my[*idx], peer[*idx]);
 331                         return (0);
 332                 }
 333         }
 334         debug2("proposals match");
 335         return (1);
 336 }
 337
 338 static void
 339 kex_choose_conf(Kex *kex)
 340 {
 341         Newkeys *newkeys;
 342         char **my, **peer;
 343         char **cprop, **sprop;
 344         int nenc, nmac, ncomp;
 345         u_int mode, ctos, need;
 346         int first_kex_follows, type;
 347
 348         my   = kex_buf2prop(&kex->my, NULL);
 349         peer = kex_buf2prop(&kex->peer, &first_kex_follows);
 350
 351         if (kex->server) {
 352                 cprop=peer;
 353                 sprop=my;
 354         } else {
 355                 cprop=my;
 356                 sprop=peer;
 357         }
 358
 359         /* Algorithm Negotiation */
 360         for (mode = 0; mode < MODE_MAX; mode++) {
 361                 newkeys = xmalloc(sizeof(*newkeys));
 362                 memset(newkeys, 0, sizeof(*newkeys));
 363                 kex->newkeys[mode] = newkeys;
 364                 ctos = (!kex->server && mode == MODE_OUT) || (kex->server && mode == MODE_IN);
 365                 nenc  = ctos ? PROPOSAL_ENC_ALGS_CTOS  : PROPOSAL_ENC_ALGS_STOC;
 366                 nmac  = ctos ? PROPOSAL_MAC_ALGS_CTOS  : PROPOSAL_MAC_ALGS_STOC;
 367                 ncomp = ctos ? PROPOSAL_COMP_ALGS_CTOS : PROPOSAL_COMP_ALGS_STOC;
 368                 choose_enc (&newkeys->enc,  cprop[nenc],  sprop[nenc]);
 369                 choose_mac (&newkeys->mac,  cprop[nmac],  sprop[nmac]);
 370                 choose_comp(&newkeys->comp, cprop[ncomp], sprop[ncomp]);
 371                 debug("kex: %s %s %s %s",
 372                     ctos ? "client->server" : "server->client",
 373                     newkeys->enc.name,
 374                     newkeys->mac.name,
 375                     newkeys->comp.name);
 376         }
 377         choose_kex(kex, cprop[PROPOSAL_KEX_ALGS], sprop[PROPOSAL_KEX_ALGS]);
 378         choose_hostkeyalg(kex, cprop[PROPOSAL_SERVER_HOST_KEY_ALGS],
 379             sprop[PROPOSAL_SERVER_HOST_KEY_ALGS]);
 380         need = 0;
 381         for (mode = 0; mode < MODE_MAX; mode++) {
 382                 newkeys = kex->newkeys[mode];
 383                 if (need < newkeys->enc.key_len)
 384                         need = newkeys->enc.key_len;
 385                 if (need < newkeys->enc.block_size)
 386                         need = newkeys->enc.block_size;
 387                 if (need < newkeys->mac.key_len)
 388                         need = newkeys->mac.key_len;
 389         }
 390         /* XXX need runden? */
 391         kex->we_need = need;
 392
 393         /* ignore the next message if the proposals do not match */
 394         if (first_kex_follows && !proposals_match(my, peer) &&
 395            !(datafellows & SSH_BUG_FIRSTKEX)) {
 396                 type = packet_read();
 397                 debug2("skipping next packet (type %u)", type);
 398         }
 399
 400         kex_prop_free(my);
 401         kex_prop_free(peer);
 402 }
 403
 404 static u_char *
 405 derive_key(Kex *kex, int id, u_int need, u_char *hash, BIGNUM *shared_secret)
 406 {
 407         Buffer b;
 408         const EVP_MD *evp_md = EVP_sha1();
 409         EVP_MD_CTX md;
 410         char c = id;
 411         u_int have;
 412         int mdsz = EVP_MD_size(evp_md);
 413         u_char *digest;
 414
 415         if (mdsz < 0)
 416                 fatal("derive_key: mdsz < 0");
 417         digest = xmalloc(roundup(need, mdsz));
 418
 419         buffer_init(&b);
 420         buffer_put_bignum2(&b, shared_secret);
 421
 422         /* K1 = HASH(K || H || "A" || session_id) */
 423         EVP_DigestInit(&md, evp_md);
 424         if (!(datafellows & SSH_BUG_DERIVEKEY))
 425                 EVP_DigestUpdate(&md, buffer_ptr(&b), buffer_len(&b));
 426         EVP_DigestUpdate(&md, hash, mdsz);
 427         EVP_DigestUpdate(&md, &c, 1);
 428         EVP_DigestUpdate(&md, kex->session_id, kex->session_id_len);
 429         EVP_DigestFinal(&md, digest, NULL);
 430
 431         /*
 432          * expand key:
 433          * Kn = HASH(K || H || K1 || K2 || ... || Kn-1)
 434          * Key = K1 || K2 || ... || Kn
 435          */
 436         for (have = mdsz; need > have; have += mdsz) {
 437                 EVP_DigestInit(&md, evp_md);
 438                 if (!(datafellows & SSH_BUG_DERIVEKEY))
 439                         EVP_DigestUpdate(&md, buffer_ptr(&b), buffer_len(&b));
 440                 EVP_DigestUpdate(&md, hash, mdsz);
 441                 EVP_DigestUpdate(&md, digest, have);
 442                 EVP_DigestFinal(&md, digest + have, NULL);
 443         }
 444         buffer_free(&b);
 445 #ifdef DEBUG_KEX
 446         fprintf(stderr, "key '%c'== ", c);
 447         dump_digest("key", digest, need);
 448 #endif
 449         return digest;
 450 }
 451
 452 Newkeys *current_keys[MODE_MAX];
 453
 454 #define NKEYS   6
 455 void
 456 kex_derive_keys(Kex *kex, u_char *hash, BIGNUM *shared_secret)
 457 {
 458         u_char *keys[NKEYS];
 459         u_int i, mode, ctos;
 460
 461         for (i = 0; i < NKEYS; i++)
 462                 keys[i] = derive_key(kex, 'A'+i, kex->we_need, hash, shared_secret);
 463
 464         debug2("kex_derive_keys");
 465         for (mode = 0; mode < MODE_MAX; mode++) {
 466                 current_keys[mode] = kex->newkeys[mode];
 467                 kex->newkeys[mode] = NULL;
 468                 ctos = (!kex->server && mode == MODE_OUT) || (kex->server && mode == MODE_IN);
 469                 current_keys[mode]->enc.iv  = keys[ctos ? 0 : 1];
 470                 current_keys[mode]->enc.key = keys[ctos ? 2 : 3];
 471                 current_keys[mode]->mac.key = keys[ctos ? 4 : 5];
 472         }
 473 }
 474
 475 Newkeys *
 476 kex_get_newkeys(int mode)
 477 {
 478         Newkeys *ret;
 479
 480         ret = current_keys[mode];
 481         current_keys[mode] = NULL;
 482         return ret;
 483 }
 484
 485 void
 486 derive_ssh1_session_id(BIGNUM *host_modulus, BIGNUM *server_modulus,
 487     u_int8_t cookie[8], u_int8_t id[16])
 488 {
 489         const EVP_MD *evp_md = EVP_md5();
 490         EVP_MD_CTX md;
 491         u_int8_t nbuf[2048], obuf[EVP_MAX_MD_SIZE];
 492         int len;
 493
 494         EVP_DigestInit(&md, evp_md);
 495
 496         len = BN_num_bytes(host_modulus);
 497         if (len < (512 / 8) || (u_int)len > sizeof(nbuf))
 498                 fatal("%s: bad host modulus (len %d)", __func__, len);
 499         BN_bn2bin(host_modulus, nbuf);
 500         EVP_DigestUpdate(&md, nbuf, len);
 501
 502         len = BN_num_bytes(server_modulus);
 503         if (len < (512 / 8) || (u_int)len > sizeof(nbuf))
 504                 fatal("%s: bad server modulus (len %d)", __func__, len);
 505         BN_bn2bin(server_modulus, nbuf);
 506         EVP_DigestUpdate(&md, nbuf, len);
 507
 508         EVP_DigestUpdate(&md, cookie, 8);
 509
 510         EVP_DigestFinal(&md, obuf, NULL);
 511         memcpy(id, obuf, 16);
 512
 513         memset(nbuf, 0, sizeof(nbuf));
 514         memset(obuf, 0, sizeof(obuf));
 515         memset(&md, 0, sizeof(md));
 516 }
 517
 518 #if defined(DEBUG_KEX) || defined(DEBUG_KEXDH)
 519 void
 520 dump_digest(char *msg, u_char *digest, int len)
 521 {
 522         u_int i;
 523
 524         fprintf(stderr, "%s\n", msg);
 525         for (i = 0; i< len; i++) {
 526                 fprintf(stderr, "%02x", digest[i]);
 527                 if (i%32 == 31)
 528                         fprintf(stderr, "\n");
 529                 else if (i%8 == 7)
 530                         fprintf(stderr, " ");
 531         }
 532         fprintf(stderr, "\n");
 533 }
 534 #endif