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