auth-rsa.c

   1 /*
   2  *
   3  * auth-rsa.c
   4  *
   5  * Author: Tatu Ylonen <ylo@cs.hut.fi>
   6  *
   7  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
   8  *                    All rights reserved
   9  *
  10  * Created: Mon Mar 27 01:46:52 1995 ylo
  11  *
  12  * RSA-based authentication.  This code determines whether to admit a login
  13  * based on RSA authentication.  This file also contains functions to check
  14  * validity of the host key.
  15  *
  16  */
  17
  18 #include "includes.h"
  19 RCSID("$Id$");
  20
  21 #include "rsa.h"
  22 #include "packet.h"
  23 #include "xmalloc.h"
  24 #include "ssh.h"
  25 #include "mpaux.h"
  26 #include "uidswap.h"
  27 #include "servconf.h"
  28
  29 #ifdef HAVE_OPENSSL
  30 #include <openssl/rsa.h>
  31 #include <openssl/md5.h>
  32 #endif
  33 #ifdef HAVE_SSL
  34 #include <ssl/rsa.h>
  35 #include <ssl/md5.h>
  36 #endif
  37
  38 /* Flags that may be set in authorized_keys options. */
  39 extern int no_port_forwarding_flag;
  40 extern int no_agent_forwarding_flag;
  41 extern int no_x11_forwarding_flag;
  42 extern int no_pty_flag;
  43 extern char *forced_command;
  44 extern struct envstring *custom_environment;
  45
  46 /*
  47  * Session identifier that is used to bind key exchange and authentication
  48  * responses to a particular session.
  49  */
  50 extern unsigned char session_id[16];
  51
  52 /*
  53  * The .ssh/authorized_keys file contains public keys, one per line, in the
  54  * following format:
  55  *   options bits e n comment
  56  * where bits, e and n are decimal numbers,
  57  * and comment is any string of characters up to newline.  The maximum
  58  * length of a line is 8000 characters.  See the documentation for a
  59  * description of the options.
  60  */
  61
  62 /*
  63  * Performs the RSA authentication challenge-response dialog with the client,
  64  * and returns true (non-zero) if the client gave the correct answer to
  65  * our challenge; returns zero if the client gives a wrong answer.
  66  */
  67
  68 int
  69 auth_rsa_challenge_dialog(BIGNUM *e, BIGNUM *n)
  70 {
  71         BIGNUM *challenge, *encrypted_challenge;
  72         RSA *pk;
  73         BN_CTX *ctx;
  74         unsigned char buf[32], mdbuf[16], response[16];
  75         MD5_CTX md;
  76         unsigned int i;
  77         int plen, len;
  78
  79         encrypted_challenge = BN_new();
  80         challenge = BN_new();
  81
  82         /* Generate a random challenge. */
  83         BN_rand(challenge, 256, 0, 0);
  84         ctx = BN_CTX_new();
  85         BN_mod(challenge, challenge, n, ctx);
  86         BN_CTX_free(ctx);
  87
  88         /* Create the public key data structure. */
  89         pk = RSA_new();
  90         pk->e = BN_new();
  91         BN_copy(pk->e, e);
  92         pk->n = BN_new();
  93         BN_copy(pk->n, n);
  94
  95         /* Encrypt the challenge with the public key. */
  96         rsa_public_encrypt(encrypted_challenge, challenge, pk);
  97         RSA_free(pk);
  98
  99         /* Send the encrypted challenge to the client. */
 100         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 101         packet_put_bignum(encrypted_challenge);
 102         packet_send();
 103         BN_clear_free(encrypted_challenge);
 104         packet_write_wait();
 105
 106         /* Wait for a response. */
 107         packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE);
 108         packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE);
 109         for (i = 0; i < 16; i++)
 110                 response[i] = packet_get_char();
 111
 112         /* The response is MD5 of decrypted challenge plus session id. */
 113         len = BN_num_bytes(challenge);
 114         if (len <= 0 || len > 32)
 115                 fatal("auth_rsa_challenge_dialog: bad challenge length %d", len);
 116         memset(buf, 0, 32);
 117         BN_bn2bin(challenge, buf + 32 - len);
 118         MD5_Init(&md);
 119         MD5_Update(&md, buf, 32);
 120         MD5_Update(&md, session_id, 16);
 121         MD5_Final(mdbuf, &md);
 122         BN_clear_free(challenge);
 123
 124         /* Verify that the response is the original challenge. */
 125         if (memcmp(response, mdbuf, 16) != 0) {
 126                 /* Wrong answer. */
 127                 return 0;
 128         }
 129         /* Correct answer. */
 130         return 1;
 131 }
 132
 133 /*
 134  * Performs the RSA authentication dialog with the client.  This returns
 135  * 0 if the client could not be authenticated, and 1 if authentication was
 136  * successful.  This may exit if there is a serious protocol violation.
 137  */
 138
 139 int
 140 auth_rsa(struct passwd *pw, BIGNUM *client_n)
 141 {
 142         extern ServerOptions options;
 143         char line[8192], file[1024];
 144         int authenticated;
 145         unsigned int bits;
 146         FILE *f;
 147         unsigned long linenum = 0;
 148         struct stat st;
 149         BIGNUM *e, *n;
 150
 151         /* Temporarily use the user's uid. */
 152         temporarily_use_uid(pw->pw_uid);
 153
 154         /* The authorized keys. */
 155         snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir,
 156                  SSH_USER_PERMITTED_KEYS);
 157
 158         /* Fail quietly if file does not exist */
 159         if (stat(file, &st) < 0) {
 160                 /* Restore the privileged uid. */
 161                 restore_uid();
 162                 return 0;
 163         }
 164         /* Open the file containing the authorized keys. */
 165         f = fopen(file, "r");
 166         if (!f) {
 167                 /* Restore the privileged uid. */
 168                 restore_uid();
 169                 packet_send_debug("Could not open %.900s for reading.", file);
 170                 packet_send_debug("If your home is on an NFS volume, it may need to be world-readable.");
 171                 return 0;
 172         }
 173         if (options.strict_modes) {
 174                 int fail = 0;
 175                 char buf[1024];
 176                 /* Check open file in order to avoid open/stat races */
 177                 if (fstat(fileno(f), &st) < 0 ||
 178                     (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
 179                     (st.st_mode & 022) != 0) {
 180                         snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
 181                                  "bad ownership or modes for '%s'.", pw->pw_name, file);
 182                         fail = 1;
 183                 } else {
 184                         /* Check path to SSH_USER_PERMITTED_KEYS */
 185                         int i;
 186                         static const char *check[] = {
 187                                 "", SSH_USER_DIR, NULL
 188                         };
 189                         for (i = 0; check[i]; i++) {
 190                                 snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]);
 191                                 if (stat(line, &st) < 0 ||
 192                                     (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
 193                                     (st.st_mode & 022) != 0) {
 194                                         snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
 195                                                  "bad ownership or modes for '%s'.", pw->pw_name, line);
 196                                         fail = 1;
 197                                         break;
 198                                 }
 199                         }
 200                 }
 201                 if (fail) {
 202                         log(buf);
 203                         packet_send_debug(buf);
 204                         restore_uid();
 205                         return 0;
 206                 }
 207         }
 208         /* Flag indicating whether authentication has succeeded. */
 209         authenticated = 0;
 210
 211         e = BN_new();
 212         n = BN_new();
 213
 214         /*
 215          * Go though the accepted keys, looking for the current key.  If
 216          * found, perform a challenge-response dialog to verify that the
 217          * user really has the corresponding private key.
 218          */
 219         while (fgets(line, sizeof(line), f)) {
 220                 char *cp;
 221                 char *options;
 222
 223                 linenum++;
 224
 225                 /* Skip leading whitespace, empty and comment lines. */
 226                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 227                         ;
 228                 if (!*cp || *cp == '\n' || *cp == '#')
 229                         continue;
 230
 231                 /*
 232                  * Check if there are options for this key, and if so,
 233                  * save their starting address and skip the option part
 234                  * for now.  If there are no options, set the starting
 235                  * address to NULL.
 236                  */
 237                 if (*cp < '0' || *cp > '9') {
 238                         int quoted = 0;
 239                         options = cp;
 240                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 241                                 if (*cp == '\\' && cp[1] == '"')
 242                                         cp++;   /* Skip both */
 243                                 else if (*cp == '"')
 244                                         quoted = !quoted;
 245                         }
 246                 } else
 247                         options = NULL;
 248
 249                 /* Parse the key from the line. */
 250                 if (!auth_rsa_read_key(&cp, &bits, e, n)) {
 251                         debug("%.100s, line %lu: bad key syntax",
 252                               SSH_USER_PERMITTED_KEYS, linenum);
 253                         packet_send_debug("%.100s, line %lu: bad key syntax",
 254                                           SSH_USER_PERMITTED_KEYS, linenum);
 255                         continue;
 256                 }
 257                 /* cp now points to the comment part. */
 258
 259                 /* Check if the we have found the desired key (identified by its modulus). */
 260                 if (BN_cmp(n, client_n) != 0)
 261                         continue;
 262
 263                 /* check the real bits  */
 264                 if (bits != BN_num_bits(n))
 265                         log("Warning: %s, line %ld: keysize mismatch: "
 266                             "actual %d vs. announced %d.",
 267                             file, linenum, BN_num_bits(n), bits);
 268
 269                 /* We have found the desired key. */
 270
 271                 /* Perform the challenge-response dialog for this key. */
 272                 if (!auth_rsa_challenge_dialog(e, n)) {
 273                         /* Wrong response. */
 274                         verbose("Wrong response to RSA authentication challenge.");
 275                         packet_send_debug("Wrong response to RSA authentication challenge.");
 276                         continue;
 277                 }
 278                 /*
 279                  * Correct response.  The client has been successfully
 280                  * authenticated. Note that we have not yet processed the
 281                  * options; this will be reset if the options cause the
 282                  * authentication to be rejected.
 283                  */
 284                 authenticated = 1;
 285
 286                 /* RSA part of authentication was accepted.  Now process the options. */
 287                 if (options) {
 288                         while (*options && *options != ' ' && *options != '\t') {
 289                                 cp = "no-port-forwarding";
 290                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 291                                         packet_send_debug("Port forwarding disabled.");
 292                                         no_port_forwarding_flag = 1;
 293                                         options += strlen(cp);
 294                                         goto next_option;
 295                                 }
 296                                 cp = "no-agent-forwarding";
 297                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 298                                         packet_send_debug("Agent forwarding disabled.");
 299                                         no_agent_forwarding_flag = 1;
 300                                         options += strlen(cp);
 301                                         goto next_option;
 302                                 }
 303                                 cp = "no-X11-forwarding";
 304                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 305                                         packet_send_debug("X11 forwarding disabled.");
 306                                         no_x11_forwarding_flag = 1;
 307                                         options += strlen(cp);
 308                                         goto next_option;
 309                                 }
 310                                 cp = "no-pty";
 311                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 312                                         packet_send_debug("Pty allocation disabled.");
 313                                         no_pty_flag = 1;
 314                                         options += strlen(cp);
 315                                         goto next_option;
 316                                 }
 317                                 cp = "command=\"";
 318                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 319                                         int i;
 320                                         options += strlen(cp);
 321                                         forced_command = xmalloc(strlen(options) + 1);
 322                                         i = 0;
 323                                         while (*options) {
 324                                                 if (*options == '"')
 325                                                         break;
 326                                                 if (*options == '\\' && options[1] == '"') {
 327                                                         options += 2;
 328                                                         forced_command[i++] = '"';
 329                                                         continue;
 330                                                 }
 331                                                 forced_command[i++] = *options++;
 332                                         }
 333                                         if (!*options) {
 334                                                 debug("%.100s, line %lu: missing end quote",
 335                                                       SSH_USER_PERMITTED_KEYS, linenum);
 336                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 337                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 338                                                 continue;
 339                                         }
 340                                         forced_command[i] = 0;
 341                                         packet_send_debug("Forced command: %.900s", forced_command);
 342                                         options++;
 343                                         goto next_option;
 344                                 }
 345                                 cp = "environment=\"";
 346                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 347                                         int i;
 348                                         char *s;
 349                                         struct envstring *new_envstring;
 350                                         options += strlen(cp);
 351                                         s = xmalloc(strlen(options) + 1);
 352                                         i = 0;
 353                                         while (*options) {
 354                                                 if (*options == '"')
 355                                                         break;
 356                                                 if (*options == '\\' && options[1] == '"') {
 357                                                         options += 2;
 358                                                         s[i++] = '"';
 359                                                         continue;
 360                                                 }
 361                                                 s[i++] = *options++;
 362                                         }
 363                                         if (!*options) {
 364                                                 debug("%.100s, line %lu: missing end quote",
 365                                                       SSH_USER_PERMITTED_KEYS, linenum);
 366                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 367                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 368                                                 continue;
 369                                         }
 370                                         s[i] = 0;
 371                                         packet_send_debug("Adding to environment: %.900s", s);
 372                                         debug("Adding to environment: %.900s", s);
 373                                         options++;
 374                                         new_envstring = xmalloc(sizeof(struct envstring));
 375                                         new_envstring->s = s;
 376                                         new_envstring->next = custom_environment;
 377                                         custom_environment = new_envstring;
 378                                         goto next_option;
 379                                 }
 380                                 cp = "from=\"";
 381                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 382                                         char *patterns = xmalloc(strlen(options) + 1);
 383                                         int i;
 384                                         options += strlen(cp);
 385                                         i = 0;
 386                                         while (*options) {
 387                                                 if (*options == '"')
 388                                                         break;
 389                                                 if (*options == '\\' && options[1] == '"') {
 390                                                         options += 2;
 391                                                         patterns[i++] = '"';
 392                                                         continue;
 393                                                 }
 394                                                 patterns[i++] = *options++;
 395                                         }
 396                                         if (!*options) {
 397                                                 debug("%.100s, line %lu: missing end quote",
 398                                                       SSH_USER_PERMITTED_KEYS, linenum);
 399                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 400                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 401                                                 continue;
 402                                         }
 403                                         patterns[i] = 0;
 404                                         options++;
 405                                         if (!match_hostname(get_canonical_hostname(), patterns,
 406                                                      strlen(patterns)) &&
 407                                             !match_hostname(get_remote_ipaddr(), patterns,
 408                                                      strlen(patterns))) {
 409                                                 log("RSA authentication tried for %.100s with correct key but not from a permitted host (host=%.200s, ip=%.200s).",
 410                                                     pw->pw_name, get_canonical_hostname(),
 411                                                     get_remote_ipaddr());
 412                                                 packet_send_debug("Your host '%.200s' is not permitted to use this key for login.",
 413                                                 get_canonical_hostname());
 414                                                 xfree(patterns);
 415                                                 /* key invalid for this host, reset flags */
 416                                                 authenticated = 0;
 417                                                 no_agent_forwarding_flag = 0;
 418                                                 no_port_forwarding_flag = 0;
 419                                                 no_pty_flag = 0;
 420                                                 no_x11_forwarding_flag = 0;
 421                                                 while (custom_environment) {
 422                                                         struct envstring *ce = custom_environment;
 423                                                         custom_environment = ce->next;
 424                                                         xfree(ce->s);
 425                                                         xfree(ce);
 426                                                 }
 427                                                 if (forced_command) {
 428                                                         xfree(forced_command);
 429                                                         forced_command = NULL;
 430                                                 }
 431                                                 break;
 432                                         }
 433                                         xfree(patterns);
 434                                         /* Host name matches. */
 435                                         goto next_option;
 436                                 }
 437                 bad_option:
 438                                 log("Bad options in %.100s file, line %lu: %.50s",
 439                                     SSH_USER_PERMITTED_KEYS, linenum, options);
 440                                 packet_send_debug("Bad options in %.100s file, line %lu: %.50s",
 441                                                   SSH_USER_PERMITTED_KEYS, linenum, options);
 442                                 authenticated = 0;
 443                                 break;
 444
 445                 next_option:
 446                                 /*
 447                                  * Skip the comma, and move to the next option
 448                                  * (or break out if there are no more).
 449                                  */
 450                                 if (!*options)
 451                                         fatal("Bugs in auth-rsa.c option processing.");
 452                                 if (*options == ' ' || *options == '\t')
 453                                         break;          /* End of options. */
 454                                 if (*options != ',')
 455                                         goto bad_option;
 456                                 options++;
 457                                 /* Process the next option. */
 458                                 continue;
 459                         }
 460                 }
 461                 /*
 462                  * Break out of the loop if authentication was successful;
 463                  * otherwise continue searching.
 464                  */
 465                 if (authenticated)
 466                         break;
 467         }
 468
 469         /* Restore the privileged uid. */
 470         restore_uid();
 471
 472         /* Close the file. */
 473         fclose(f);
 474
 475         BN_clear_free(n);
 476         BN_clear_free(e);
 477
 478         if (authenticated)
 479                 packet_send_debug("RSA authentication accepted.");
 480
 481         /* Return authentication result. */
 482         return authenticated;
 483 }