Damien Miller | d4a8b7e | 1999-10-27 13:42:43 +1000 | [diff] [blame^] | 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: auth-rsa.c,v 1.1 1999/10/27 03:42:43 damien Exp $"); |
| 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 | |
| 28 | #include <openssl/rsa.h> |
| 29 | #include <openssl/md5.h> |
| 30 | |
| 31 | /* Flags that may be set in authorized_keys options. */ |
| 32 | extern int no_port_forwarding_flag; |
| 33 | extern int no_agent_forwarding_flag; |
| 34 | extern int no_x11_forwarding_flag; |
| 35 | extern int no_pty_flag; |
| 36 | extern char *forced_command; |
| 37 | extern struct envstring *custom_environment; |
| 38 | |
| 39 | /* Session identifier that is used to bind key exchange and authentication |
| 40 | responses to a particular session. */ |
| 41 | extern unsigned char session_id[16]; |
| 42 | |
| 43 | /* The .ssh/authorized_keys file contains public keys, one per line, in the |
| 44 | following format: |
| 45 | options bits e n comment |
| 46 | where bits, e and n are decimal numbers, |
| 47 | and comment is any string of characters up to newline. The maximum |
| 48 | length of a line is 8000 characters. See the documentation for a |
| 49 | description of the options. |
| 50 | */ |
| 51 | |
| 52 | /* Performs the RSA authentication challenge-response dialog with the client, |
| 53 | and returns true (non-zero) if the client gave the correct answer to |
| 54 | our challenge; returns zero if the client gives a wrong answer. */ |
| 55 | |
| 56 | int |
| 57 | auth_rsa_challenge_dialog(unsigned int bits, BIGNUM *e, BIGNUM *n) |
| 58 | { |
| 59 | BIGNUM *challenge, *encrypted_challenge, *aux; |
| 60 | RSA *pk; |
| 61 | BN_CTX *ctx = BN_CTX_new(); |
| 62 | unsigned char buf[32], mdbuf[16], response[16]; |
| 63 | MD5_CTX md; |
| 64 | unsigned int i; |
| 65 | int plen, len; |
| 66 | |
| 67 | encrypted_challenge = BN_new(); |
| 68 | challenge = BN_new(); |
| 69 | aux = BN_new(); |
| 70 | |
| 71 | /* Generate a random challenge. */ |
| 72 | BN_rand(challenge, 256, 0, 0); |
| 73 | BN_mod(challenge, challenge, n, ctx); |
| 74 | |
| 75 | /* Create the public key data structure. */ |
| 76 | pk = RSA_new(); |
| 77 | pk->e = BN_new(); |
| 78 | BN_copy(pk->e, e); |
| 79 | pk->n = BN_new(); |
| 80 | BN_copy(pk->n, n); |
| 81 | |
| 82 | /* Encrypt the challenge with the public key. */ |
| 83 | rsa_public_encrypt(encrypted_challenge, challenge, pk); |
| 84 | RSA_free(pk); |
| 85 | |
| 86 | /* Send the encrypted challenge to the client. */ |
| 87 | packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE); |
| 88 | packet_put_bignum(encrypted_challenge); |
| 89 | packet_send(); |
| 90 | packet_write_wait(); |
| 91 | |
| 92 | /* The response is MD5 of decrypted challenge plus session id. */ |
| 93 | len = BN_num_bytes(challenge); |
| 94 | assert(len <= 32 && len); |
| 95 | memset(buf, 0, 32); |
| 96 | BN_bn2bin(challenge, buf + 32 - len); |
| 97 | MD5_Init(&md); |
| 98 | MD5_Update(&md, buf, 32); |
| 99 | MD5_Update(&md, session_id, 16); |
| 100 | MD5_Final(mdbuf, &md); |
| 101 | |
| 102 | /* We will no longer need these. */ |
| 103 | BN_clear_free(encrypted_challenge); |
| 104 | BN_clear_free(challenge); |
| 105 | BN_clear_free(aux); |
| 106 | BN_CTX_free(ctx); |
| 107 | |
| 108 | /* Wait for a response. */ |
| 109 | packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE); |
| 110 | packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE); |
| 111 | for (i = 0; i < 16; i++) |
| 112 | response[i] = packet_get_char(); |
| 113 | |
| 114 | /* Verify that the response is the original challenge. */ |
| 115 | if (memcmp(response, mdbuf, 16) != 0) |
| 116 | { |
| 117 | /* Wrong answer. */ |
| 118 | return 0; |
| 119 | } |
| 120 | |
| 121 | /* Correct answer. */ |
| 122 | return 1; |
| 123 | } |
| 124 | |
| 125 | /* Performs the RSA authentication dialog with the client. This returns |
| 126 | 0 if the client could not be authenticated, and 1 if authentication was |
| 127 | successful. This may exit if there is a serious protocol violation. */ |
| 128 | |
| 129 | int |
| 130 | auth_rsa(struct passwd *pw, BIGNUM *client_n, int strict_modes) |
| 131 | { |
| 132 | char line[8192]; |
| 133 | int authenticated; |
| 134 | unsigned int bits; |
| 135 | FILE *f; |
| 136 | unsigned long linenum = 0; |
| 137 | struct stat st; |
| 138 | BIGNUM *e, *n; |
| 139 | |
| 140 | /* Temporarily use the user's uid. */ |
| 141 | temporarily_use_uid(pw->pw_uid); |
| 142 | |
| 143 | /* The authorized keys. */ |
| 144 | snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, |
| 145 | SSH_USER_PERMITTED_KEYS); |
| 146 | |
| 147 | /* Fail quietly if file does not exist */ |
| 148 | if (stat(line, &st) < 0) |
| 149 | { |
| 150 | /* Restore the privileged uid. */ |
| 151 | restore_uid(); |
| 152 | return 0; |
| 153 | } |
| 154 | |
| 155 | /* Open the file containing the authorized keys. */ |
| 156 | f = fopen(line, "r"); |
| 157 | if (!f) |
| 158 | { |
| 159 | /* Restore the privileged uid. */ |
| 160 | restore_uid(); |
| 161 | packet_send_debug("Could not open %.900s for reading.", line); |
| 162 | packet_send_debug("If your home is on an NFS volume, it may need to be world-readable."); |
| 163 | return 0; |
| 164 | } |
| 165 | |
| 166 | if (strict_modes) { |
| 167 | int fail=0; |
| 168 | char buf[1024]; |
| 169 | /* Check open file in order to avoid open/stat races */ |
| 170 | if (fstat(fileno(f), &st) < 0 || |
| 171 | (st.st_uid != 0 && st.st_uid != pw->pw_uid) || |
| 172 | (st.st_mode & 022) != 0) { |
| 173 | snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: " |
| 174 | "bad ownership or modes for '%s'.", pw->pw_name, line); |
| 175 | fail=1; |
| 176 | }else{ |
| 177 | /* Check path to SSH_USER_PERMITTED_KEYS */ |
| 178 | int i; |
| 179 | static const char *check[] = { |
| 180 | "", SSH_USER_DIR, NULL |
| 181 | }; |
| 182 | for (i=0; check[i]; i++) { |
| 183 | snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]); |
| 184 | if (stat(line, &st) < 0 || |
| 185 | (st.st_uid != 0 && st.st_uid != pw->pw_uid) || |
| 186 | (st.st_mode & 022) != 0) { |
| 187 | snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: " |
| 188 | "bad ownership or modes for '%s'.", pw->pw_name, line); |
| 189 | fail=1; |
| 190 | break; |
| 191 | } |
| 192 | } |
| 193 | } |
| 194 | if (fail) { |
| 195 | log(buf); |
| 196 | packet_send_debug(buf); |
| 197 | restore_uid(); |
| 198 | return 0; |
| 199 | } |
| 200 | } |
| 201 | |
| 202 | /* Flag indicating whether authentication has succeeded. */ |
| 203 | authenticated = 0; |
| 204 | |
| 205 | /* Initialize mp-int variables. */ |
| 206 | e = BN_new(); |
| 207 | n = BN_new(); |
| 208 | |
| 209 | /* Go though the accepted keys, looking for the current key. If found, |
| 210 | perform a challenge-response dialog to verify that the user really has |
| 211 | the corresponding private key. */ |
| 212 | while (fgets(line, sizeof(line), f)) |
| 213 | { |
| 214 | char *cp; |
| 215 | char *options; |
| 216 | |
| 217 | linenum++; |
| 218 | |
| 219 | /* Skip leading whitespace. */ |
| 220 | for (cp = line; *cp == ' ' || *cp == '\t'; cp++) |
| 221 | ; |
| 222 | |
| 223 | /* Skip empty and comment lines. */ |
| 224 | if (!*cp || *cp == '\n' || *cp == '#') |
| 225 | continue; |
| 226 | |
| 227 | /* Check if there are options for this key, and if so, save their |
| 228 | starting address and skip the option part for now. If there are no |
| 229 | options, set the starting address to NULL. */ |
| 230 | if (*cp < '0' || *cp > '9') |
| 231 | { |
| 232 | int quoted = 0; |
| 233 | options = cp; |
| 234 | for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) |
| 235 | { |
| 236 | if (*cp == '\\' && cp[1] == '"') |
| 237 | cp++; /* Skip both */ |
| 238 | else |
| 239 | if (*cp == '"') |
| 240 | quoted = !quoted; |
| 241 | } |
| 242 | } |
| 243 | else |
| 244 | options = NULL; |
| 245 | |
| 246 | /* Parse the key from the line. */ |
| 247 | if (!auth_rsa_read_key(&cp, &bits, e, n)) |
| 248 | { |
| 249 | debug("%.100s, line %lu: bad key syntax", |
| 250 | SSH_USER_PERMITTED_KEYS, linenum); |
| 251 | packet_send_debug("%.100s, line %lu: bad key syntax", |
| 252 | SSH_USER_PERMITTED_KEYS, linenum); |
| 253 | continue; |
| 254 | } |
| 255 | /* cp now points to the comment part. */ |
| 256 | |
| 257 | /* Check if the we have found the desired key (identified by its |
| 258 | modulus). */ |
| 259 | if (BN_cmp(n, client_n) != 0) |
| 260 | continue; /* Wrong key. */ |
| 261 | |
| 262 | /* We have found the desired key. */ |
| 263 | |
| 264 | /* Perform the challenge-response dialog for this key. */ |
| 265 | if (!auth_rsa_challenge_dialog(bits, e, n)) |
| 266 | { |
| 267 | /* Wrong response. */ |
| 268 | log("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 authenticated. |
| 274 | Note that we have not yet processed the options; this will be reset |
| 275 | if the options cause the authentication to be rejected. */ |
| 276 | authenticated = 1; |
| 277 | |
| 278 | /* RSA part of authentication was accepted. Now process the options. */ |
| 279 | if (options) |
| 280 | { |
| 281 | while (*options && *options != ' ' && *options != '\t') |
| 282 | { |
| 283 | cp = "no-port-forwarding"; |
| 284 | if (strncmp(options, cp, strlen(cp)) == 0) |
| 285 | { |
| 286 | packet_send_debug("Port forwarding disabled."); |
| 287 | no_port_forwarding_flag = 1; |
| 288 | options += strlen(cp); |
| 289 | goto next_option; |
| 290 | } |
| 291 | cp = "no-agent-forwarding"; |
| 292 | if (strncmp(options, cp, strlen(cp)) == 0) |
| 293 | { |
| 294 | packet_send_debug("Agent forwarding disabled."); |
| 295 | no_agent_forwarding_flag = 1; |
| 296 | options += strlen(cp); |
| 297 | goto next_option; |
| 298 | } |
| 299 | cp = "no-X11-forwarding"; |
| 300 | if (strncmp(options, cp, strlen(cp)) == 0) |
| 301 | { |
| 302 | packet_send_debug("X11 forwarding disabled."); |
| 303 | no_x11_forwarding_flag = 1; |
| 304 | options += strlen(cp); |
| 305 | goto next_option; |
| 306 | } |
| 307 | cp = "no-pty"; |
| 308 | if (strncmp(options, cp, strlen(cp)) == 0) |
| 309 | { |
| 310 | packet_send_debug("Pty allocation disabled."); |
| 311 | no_pty_flag = 1; |
| 312 | options += strlen(cp); |
| 313 | goto next_option; |
| 314 | } |
| 315 | cp = "command=\""; |
| 316 | if (strncmp(options, cp, strlen(cp)) == 0) |
| 317 | { |
| 318 | int i; |
| 319 | options += strlen(cp); |
| 320 | forced_command = xmalloc(strlen(options) + 1); |
| 321 | i = 0; |
| 322 | while (*options) |
| 323 | { |
| 324 | if (*options == '"') |
| 325 | break; |
| 326 | if (*options == '\\' && options[1] == '"') |
| 327 | { |
| 328 | options += 2; |
| 329 | forced_command[i++] = '"'; |
| 330 | continue; |
| 331 | } |
| 332 | forced_command[i++] = *options++; |
| 333 | } |
| 334 | if (!*options) |
| 335 | { |
| 336 | debug("%.100s, line %lu: missing end quote", |
| 337 | SSH_USER_PERMITTED_KEYS, linenum); |
| 338 | packet_send_debug("%.100s, line %lu: missing end quote", |
| 339 | SSH_USER_PERMITTED_KEYS, linenum); |
| 340 | continue; |
| 341 | } |
| 342 | forced_command[i] = 0; |
| 343 | packet_send_debug("Forced command: %.900s", forced_command); |
| 344 | options++; |
| 345 | goto next_option; |
| 346 | } |
| 347 | cp = "environment=\""; |
| 348 | if (strncmp(options, cp, strlen(cp)) == 0) |
| 349 | { |
| 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 | { |
| 358 | if (*options == '"') |
| 359 | break; |
| 360 | if (*options == '\\' && options[1] == '"') |
| 361 | { |
| 362 | options += 2; |
| 363 | s[i++] = '"'; |
| 364 | continue; |
| 365 | } |
| 366 | s[i++] = *options++; |
| 367 | } |
| 368 | if (!*options) |
| 369 | { |
| 370 | debug("%.100s, line %lu: missing end quote", |
| 371 | SSH_USER_PERMITTED_KEYS, linenum); |
| 372 | packet_send_debug("%.100s, line %lu: missing end quote", |
| 373 | SSH_USER_PERMITTED_KEYS, linenum); |
| 374 | continue; |
| 375 | } |
| 376 | s[i] = 0; |
| 377 | packet_send_debug("Adding to environment: %.900s", s); |
| 378 | debug("Adding to environment: %.900s", s); |
| 379 | options++; |
| 380 | new_envstring = xmalloc(sizeof(struct envstring)); |
| 381 | new_envstring->s = s; |
| 382 | new_envstring->next = custom_environment; |
| 383 | custom_environment = new_envstring; |
| 384 | goto next_option; |
| 385 | } |
| 386 | cp = "from=\""; |
| 387 | if (strncmp(options, cp, strlen(cp)) == 0) |
| 388 | { |
| 389 | char *patterns = xmalloc(strlen(options) + 1); |
| 390 | int i; |
| 391 | options += strlen(cp); |
| 392 | i = 0; |
| 393 | while (*options) |
| 394 | { |
| 395 | if (*options == '"') |
| 396 | break; |
| 397 | if (*options == '\\' && options[1] == '"') |
| 398 | { |
| 399 | options += 2; |
| 400 | patterns[i++] = '"'; |
| 401 | continue; |
| 402 | } |
| 403 | patterns[i++] = *options++; |
| 404 | } |
| 405 | if (!*options) |
| 406 | { |
| 407 | debug("%.100s, line %lu: missing end quote", |
| 408 | SSH_USER_PERMITTED_KEYS, linenum); |
| 409 | packet_send_debug("%.100s, line %lu: missing end quote", |
| 410 | SSH_USER_PERMITTED_KEYS, linenum); |
| 411 | continue; |
| 412 | } |
| 413 | patterns[i] = 0; |
| 414 | options++; |
| 415 | if (!match_hostname(get_canonical_hostname(), patterns, |
| 416 | strlen(patterns)) && |
| 417 | !match_hostname(get_remote_ipaddr(), patterns, |
| 418 | strlen(patterns))) |
| 419 | { |
| 420 | log("RSA authentication tried for %.100s with correct key but not from a permitted host (host=%.200s, ip=%.200s).", |
| 421 | pw->pw_name, get_canonical_hostname(), |
| 422 | get_remote_ipaddr()); |
| 423 | packet_send_debug("Your host '%.200s' is not permitted to use this key for login.", |
| 424 | get_canonical_hostname()); |
| 425 | xfree(patterns); |
| 426 | authenticated = 0; |
| 427 | break; |
| 428 | } |
| 429 | xfree(patterns); |
| 430 | /* Host name matches. */ |
| 431 | goto next_option; |
| 432 | } |
| 433 | bad_option: |
| 434 | /* Unknown option. */ |
| 435 | log("Bad options in %.100s file, line %lu: %.50s", |
| 436 | SSH_USER_PERMITTED_KEYS, linenum, options); |
| 437 | packet_send_debug("Bad options in %.100s file, line %lu: %.50s", |
| 438 | SSH_USER_PERMITTED_KEYS, linenum, options); |
| 439 | authenticated = 0; |
| 440 | break; |
| 441 | |
| 442 | next_option: |
| 443 | /* Skip the comma, and move to the next option (or break out |
| 444 | if there are no more). */ |
| 445 | if (!*options) |
| 446 | fatal("Bugs in auth-rsa.c option processing."); |
| 447 | if (*options == ' ' || *options == '\t') |
| 448 | break; /* End of options. */ |
| 449 | if (*options != ',') |
| 450 | goto bad_option; |
| 451 | options++; |
| 452 | /* Process the next option. */ |
| 453 | continue; |
| 454 | } |
| 455 | } |
| 456 | |
| 457 | /* Break out of the loop if authentication was successful; otherwise |
| 458 | continue searching. */ |
| 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 | /* Clear any mp-int variables. */ |
| 470 | BN_clear_free(n); |
| 471 | BN_clear_free(e); |
| 472 | |
| 473 | if (authenticated) |
| 474 | packet_send_debug("RSA authentication accepted."); |
| 475 | |
| 476 | /* Return authentication result. */ |
| 477 | return authenticated; |
| 478 | } |