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, *aux;
  72         RSA *pk;
  73         BN_CTX *ctx = BN_CTX_new();
  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         aux = BN_new();
  82
  83         /* Generate a random challenge. */
  84         BN_rand(challenge, 256, 0, 0);
  85         BN_mod(challenge, challenge, n, ctx);
  86
  87         /* Create the public key data structure. */
  88         pk = RSA_new();
  89         pk->e = BN_new();
  90         BN_copy(pk->e, e);
  91         pk->n = BN_new();
  92         BN_copy(pk->n, n);
  93
  94         /* Encrypt the challenge with the public key. */
  95         rsa_public_encrypt(encrypted_challenge, challenge, pk);
  96         RSA_free(pk);
  97
  98         /* Send the encrypted challenge to the client. */
  99         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 100         packet_put_bignum(encrypted_challenge);
 101         packet_send();
 102         packet_write_wait();
 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
 115         /* We will no longer need these. */
 116         BN_clear_free(encrypted_challenge);
 117         BN_clear_free(challenge);
 118         BN_clear_free(aux);
 119         BN_CTX_free(ctx);
 120
 121         /* Wait for a response. */
 122         packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE);
 123         packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE);
 124         for (i = 0; i < 16; i++)
 125                 response[i] = packet_get_char();
 126
 127         /* Verify that the response is the original challenge. */
 128         if (memcmp(response, mdbuf, 16) != 0) {
 129                 /* Wrong answer. */
 130                 return 0;
 131         }
 132         /* Correct answer. */
 133         return 1;
 134 }
 135
 136 /*
 137  * Performs the RSA authentication dialog with the client.  This returns
 138  * 0 if the client could not be authenticated, and 1 if authentication was
 139  * successful.  This may exit if there is a serious protocol violation.
 140  */
 141
 142 int
 143 auth_rsa(struct passwd *pw, BIGNUM *client_n)
 144 {
 145         extern ServerOptions options;
 146         char line[8192], file[1024];
 147         int authenticated;
 148         unsigned int bits;
 149         FILE *f;
 150         unsigned long linenum = 0;
 151         struct stat st;
 152         BIGNUM *e, *n;
 153
 154         /* Temporarily use the user's uid. */
 155         temporarily_use_uid(pw->pw_uid);
 156
 157         /* The authorized keys. */
 158         snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir,
 159                  SSH_USER_PERMITTED_KEYS);
 160
 161         /* Fail quietly if file does not exist */
 162         if (stat(file, &st) < 0) {
 163                 /* Restore the privileged uid. */
 164                 restore_uid();
 165                 return 0;
 166         }
 167         /* Open the file containing the authorized keys. */
 168         f = fopen(file, "r");
 169         if (!f) {
 170                 /* Restore the privileged uid. */
 171                 restore_uid();
 172                 packet_send_debug("Could not open %.900s for reading.", file);
 173                 packet_send_debug("If your home is on an NFS volume, it may need to be world-readable.");
 174                 return 0;
 175         }
 176         if (options.strict_modes) {
 177                 int fail = 0;
 178                 char buf[1024];
 179                 /* Check open file in order to avoid open/stat races */
 180                 if (fstat(fileno(f), &st) < 0 ||
 181                     (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
 182                     (st.st_mode & 022) != 0) {
 183                         snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
 184                                  "bad ownership or modes for '%s'.", pw->pw_name, file);
 185                         fail = 1;
 186                 } else {
 187                         /* Check path to SSH_USER_PERMITTED_KEYS */
 188                         int i;
 189                         static const char *check[] = {
 190                                 "", SSH_USER_DIR, NULL
 191                         };
 192                         for (i = 0; check[i]; i++) {
 193                                 snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]);
 194                                 if (stat(line, &st) < 0 ||
 195                                     (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
 196                                     (st.st_mode & 022) != 0) {
 197                                         snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
 198                                                  "bad ownership or modes for '%s'.", pw->pw_name, line);
 199                                         fail = 1;
 200                                         break;
 201                                 }
 202                         }
 203                 }
 204                 if (fail) {
 205                         log(buf);
 206                         packet_send_debug(buf);
 207                         restore_uid();
 208                         return 0;
 209                 }
 210         }
 211         /* Flag indicating whether authentication has succeeded. */
 212         authenticated = 0;
 213
 214         e = BN_new();
 215         n = BN_new();
 216
 217         /*
 218          * Go though the accepted keys, looking for the current key.  If
 219          * found, perform a challenge-response dialog to verify that the
 220          * user really has the corresponding private key.
 221          */
 222         while (fgets(line, sizeof(line), f)) {
 223                 char *cp;
 224                 char *options;
 225
 226                 linenum++;
 227
 228                 /* Skip leading whitespace, empty and comment lines. */
 229                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 230                         ;
 231                 if (!*cp || *cp == '\n' || *cp == '#')
 232                         continue;
 233
 234                 /*
 235                  * Check if there are options for this key, and if so,
 236                  * save their starting address and skip the option part
 237                  * for now.  If there are no options, set the starting
 238                  * address to NULL.
 239                  */
 240                 if (*cp < '0' || *cp > '9') {
 241                         int quoted = 0;
 242                         options = cp;
 243                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 244                                 if (*cp == '\\' && cp[1] == '"')
 245                                         cp++;   /* Skip both */
 246                                 else if (*cp == '"')
 247                                         quoted = !quoted;
 248                         }
 249                 } else
 250                         options = NULL;
 251
 252                 /* Parse the key from the line. */
 253                 if (!auth_rsa_read_key(&cp, &bits, e, n)) {
 254                         debug("%.100s, line %lu: bad key syntax",
 255                               SSH_USER_PERMITTED_KEYS, linenum);
 256                         packet_send_debug("%.100s, line %lu: bad key syntax",
 257                                           SSH_USER_PERMITTED_KEYS, linenum);
 258                         continue;
 259                 }
 260                 /* cp now points to the comment part. */
 261
 262                 /* check the real bits  */
 263                 if (bits != BN_num_bits(n))
 264                         error("Warning: error in %s, line %ld: keysize mismatch: "
 265                               "actual size %d vs. announced %d.",
 266                               file, linenum, BN_num_bits(n), bits);
 267
 268                 /* Check if the we have found the desired key (identified by its modulus). */
 269                 if (BN_cmp(n, client_n) != 0)
 270                         continue;
 271
 272                 /* We have found the desired key. */
 273
 274                 /* Perform the challenge-response dialog for this key. */
 275                 if (!auth_rsa_challenge_dialog(e, n)) {
 276                         /* Wrong response. */
 277                         verbose("Wrong response to RSA authentication challenge.");
 278                         packet_send_debug("Wrong response to RSA authentication challenge.");
 279                         continue;
 280                 }
 281                 /*
 282                  * Correct response.  The client has been successfully
 283                  * authenticated. Note that we have not yet processed the
 284                  * options; this will be reset if the options cause the
 285                  * authentication to be rejected.
 286                  */
 287                 authenticated = 1;
 288
 289                 /* RSA part of authentication was accepted.  Now process the options. */
 290                 if (options) {
 291                         while (*options && *options != ' ' && *options != '\t') {
 292                                 cp = "no-port-forwarding";
 293                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 294                                         packet_send_debug("Port forwarding disabled.");
 295                                         no_port_forwarding_flag = 1;
 296                                         options += strlen(cp);
 297                                         goto next_option;
 298                                 }
 299                                 cp = "no-agent-forwarding";
 300                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 301                                         packet_send_debug("Agent forwarding disabled.");
 302                                         no_agent_forwarding_flag = 1;
 303                                         options += strlen(cp);
 304                                         goto next_option;
 305                                 }
 306                                 cp = "no-X11-forwarding";
 307                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 308                                         packet_send_debug("X11 forwarding disabled.");
 309                                         no_x11_forwarding_flag = 1;
 310                                         options += strlen(cp);
 311                                         goto next_option;
 312                                 }
 313                                 cp = "no-pty";
 314                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 315                                         packet_send_debug("Pty allocation disabled.");
 316                                         no_pty_flag = 1;
 317                                         options += strlen(cp);
 318                                         goto next_option;
 319                                 }
 320                                 cp = "command=\"";
 321                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 322                                         int i;
 323                                         options += strlen(cp);
 324                                         forced_command = xmalloc(strlen(options) + 1);
 325                                         i = 0;
 326                                         while (*options) {
 327                                                 if (*options == '"')
 328                                                         break;
 329                                                 if (*options == '\\' && options[1] == '"') {
 330                                                         options += 2;
 331                                                         forced_command[i++] = '"';
 332                                                         continue;
 333                                                 }
 334                                                 forced_command[i++] = *options++;
 335                                         }
 336                                         if (!*options) {
 337                                                 debug("%.100s, line %lu: missing end quote",
 338                                                       SSH_USER_PERMITTED_KEYS, linenum);
 339                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 340                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 341                                                 continue;
 342                                         }
 343                                         forced_command[i] = 0;
 344                                         packet_send_debug("Forced command: %.900s", forced_command);
 345                                         options++;
 346                                         goto next_option;
 347                                 }
 348                                 cp = "environment=\"";
 349                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 350                                         int i;
 351                                         char *s;
 352                                         struct envstring *new_envstring;
 353                                         options += strlen(cp);
 354                                         s = xmalloc(strlen(options) + 1);
 355                                         i = 0;
 356                                         while (*options) {
 357                                                 if (*options == '"')
 358                                                         break;
 359                                                 if (*options == '\\' && options[1] == '"') {
 360                                                         options += 2;
 361                                                         s[i++] = '"';
 362                                                         continue;
 363                                                 }
 364                                                 s[i++] = *options++;
 365                                         }
 366                                         if (!*options) {
 367                                                 debug("%.100s, line %lu: missing end quote",
 368                                                       SSH_USER_PERMITTED_KEYS, linenum);
 369                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 370                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 371                                                 continue;
 372                                         }
 373                                         s[i] = 0;
 374                                         packet_send_debug("Adding to environment: %.900s", s);
 375                                         debug("Adding to environment: %.900s", s);
 376                                         options++;
 377                                         new_envstring = xmalloc(sizeof(struct envstring));
 378                                         new_envstring->s = s;
 379                                         new_envstring->next = custom_environment;
 380                                         custom_environment = new_envstring;
 381                                         goto next_option;
 382                                 }
 383                                 cp = "from=\"";
 384                                 if (strncmp(options, cp, strlen(cp)) == 0) {
 385                                         char *patterns = xmalloc(strlen(options) + 1);
 386                                         int i;
 387                                         options += strlen(cp);
 388                                         i = 0;
 389                                         while (*options) {
 390                                                 if (*options == '"')
 391                                                         break;
 392                                                 if (*options == '\\' && options[1] == '"') {
 393                                                         options += 2;
 394                                                         patterns[i++] = '"';
 395                                                         continue;
 396                                                 }
 397                                                 patterns[i++] = *options++;
 398                                         }
 399                                         if (!*options) {
 400                                                 debug("%.100s, line %lu: missing end quote",
 401                                                       SSH_USER_PERMITTED_KEYS, linenum);
 402                                                 packet_send_debug("%.100s, line %lu: missing end quote",
 403                                                                   SSH_USER_PERMITTED_KEYS, linenum);
 404                                                 continue;
 405                                         }
 406                                         patterns[i] = 0;
 407                                         options++;
 408                                         if (!match_hostname(get_canonical_hostname(), patterns,
 409                                                      strlen(patterns)) &&
 410                                             !match_hostname(get_remote_ipaddr(), patterns,
 411                                                      strlen(patterns))) {
 412                                                 log("RSA authentication tried for %.100s with correct key but not from a permitted host (host=%.200s, ip=%.200s).",
 413                                                     pw->pw_name, get_canonical_hostname(),
 414                                                     get_remote_ipaddr());
 415                                                 packet_send_debug("Your host '%.200s' is not permitted to use this key for login.",
 416                                                 get_canonical_hostname());
 417                                                 xfree(patterns);
 418                                                 authenticated = 0;
 419                                                 break;
 420                                         }
 421                                         xfree(patterns);
 422                                         /* Host name matches. */
 423                                         goto next_option;
 424                                 }
 425                 bad_option:
 426                                 log("Bad options in %.100s file, line %lu: %.50s",
 427                                     SSH_USER_PERMITTED_KEYS, linenum, options);
 428                                 packet_send_debug("Bad options in %.100s file, line %lu: %.50s",
 429                                                   SSH_USER_PERMITTED_KEYS, linenum, options);
 430                                 authenticated = 0;
 431                                 break;
 432
 433                 next_option:
 434                                 /*
 435                                  * Skip the comma, and move to the next option
 436                                  * (or break out if there are no more).
 437                                  */
 438                                 if (!*options)
 439                                         fatal("Bugs in auth-rsa.c option processing.");
 440                                 if (*options == ' ' || *options == '\t')
 441                                         break;          /* End of options. */
 442                                 if (*options != ',')
 443                                         goto bad_option;
 444                                 options++;
 445                                 /* Process the next option. */
 446                                 continue;
 447                         }
 448                 }
 449                 /*
 450                  * Break out of the loop if authentication was successful;
 451                  * otherwise continue searching.
 452                  */
 453                 if (authenticated)
 454                         break;
 455         }
 456
 457         /* Restore the privileged uid. */
 458         restore_uid();
 459
 460         /* Close the file. */
 461         fclose(f);
 462
 463         BN_clear_free(n);
 464         BN_clear_free(e);
 465
 466         if (authenticated)
 467                 packet_send_debug("RSA authentication accepted.");
 468
 469         /* Return authentication result. */
 470         return authenticated;
 471 }