crypto/openssh/auth-rsa.c

   1 /* $OpenBSD: auth-rsa.c,v 1.80 2011/05/23 03:30:07 djm Exp $ */
   2 /*
   3  * Author: Tatu Ylonen <ylo@cs.hut.fi>
   4  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
   5  *                    All rights reserved
   6  * RSA-based authentication.  This code determines whether to admit a login
   7  * based on RSA authentication.  This file also contains functions to check
   8  * validity of the host key.
   9  *
  10  * As far as I am concerned, the code I have written for this software
  11  * can be used freely for any purpose.  Any derived versions of this
  12  * software must be clearly marked as such, and if the derived work is
  13  * incompatible with the protocol description in the RFC file, it must be
  14  * called by a name other than "ssh" or "Secure Shell".
  15  */
  16
  17 #include "includes.h"
  18
  19 #include <sys/types.h>
  20 #include <sys/stat.h>
  21
  22 #include <openssl/rsa.h>
  23 #include <openssl/md5.h>
  24
  25 #include <pwd.h>
  26 #include <stdio.h>
  27 #include <stdarg.h>
  28 #include <string.h>
  29
  30 #include "xmalloc.h"
  31 #include "rsa.h"
  32 #include "packet.h"
  33 #include "ssh1.h"
  34 #include "uidswap.h"
  35 #include "match.h"
  36 #include "buffer.h"
  37 #include "pathnames.h"
  38 #include "log.h"
  39 #include "servconf.h"
  40 #include "key.h"
  41 #include "auth-options.h"
  42 #include "hostfile.h"
  43 #include "authfile.h"
  44 #include "auth.h"
  45 #ifdef GSSAPI
  46 #include "ssh-gss.h"
  47 #endif
  48 #include "monitor_wrap.h"
  49 #include "ssh.h"
  50 #include "misc.h"
  51
  52 /* import */
  53 extern ServerOptions options;
  54
  55 /*
  56  * Session identifier that is used to bind key exchange and authentication
  57  * responses to a particular session.
  58  */
  59 extern u_char session_id[16];
  60
  61 /*
  62  * The .ssh/authorized_keys file contains public keys, one per line, in the
  63  * following format:
  64  *   options bits e n comment
  65  * where bits, e and n are decimal numbers,
  66  * and comment is any string of characters up to newline.  The maximum
  67  * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
  68  * description of the options.
  69  */
  70
  71 BIGNUM *
  72 auth_rsa_generate_challenge(Key *key)
  73 {
  74         BIGNUM *challenge;
  75         BN_CTX *ctx;
  76
  77         if ((challenge = BN_new()) == NULL)
  78                 fatal("auth_rsa_generate_challenge: BN_new() failed");
  79         /* Generate a random challenge. */
  80         if (BN_rand(challenge, 256, 0, 0) == 0)
  81                 fatal("auth_rsa_generate_challenge: BN_rand failed");
  82         if ((ctx = BN_CTX_new()) == NULL)
  83                 fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
  84         if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
  85                 fatal("auth_rsa_generate_challenge: BN_mod failed");
  86         BN_CTX_free(ctx);
  87
  88         return challenge;
  89 }
  90
  91 int
  92 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
  93 {
  94         u_char buf[32], mdbuf[16];
  95         MD5_CTX md;
  96         int len;
  97
  98         /* don't allow short keys */
  99         if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
 100                 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
 101                     BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
 102                 return (0);
 103         }
 104
 105         /* The response is MD5 of decrypted challenge plus session id. */
 106         len = BN_num_bytes(challenge);
 107         if (len <= 0 || len > 32)
 108                 fatal("auth_rsa_verify_response: bad challenge length %d", len);
 109         memset(buf, 0, 32);
 110         BN_bn2bin(challenge, buf + 32 - len);
 111         MD5_Init(&md);
 112         MD5_Update(&md, buf, 32);
 113         MD5_Update(&md, session_id, 16);
 114         MD5_Final(mdbuf, &md);
 115
 116         /* Verify that the response is the original challenge. */
 117         if (timingsafe_bcmp(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 challenge-response dialog with the client,
 127  * and returns true (non-zero) if the client gave the correct answer to
 128  * our challenge; returns zero if the client gives a wrong answer.
 129  */
 130
 131 int
 132 auth_rsa_challenge_dialog(Key *key)
 133 {
 134         BIGNUM *challenge, *encrypted_challenge;
 135         u_char response[16];
 136         int i, success;
 137
 138         if ((encrypted_challenge = BN_new()) == NULL)
 139                 fatal("auth_rsa_challenge_dialog: BN_new() failed");
 140
 141         challenge = PRIVSEP(auth_rsa_generate_challenge(key));
 142
 143         /* Encrypt the challenge with the public key. */
 144         rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
 145
 146         /* Send the encrypted challenge to the client. */
 147         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 148         packet_put_bignum(encrypted_challenge);
 149         packet_send();
 150         BN_clear_free(encrypted_challenge);
 151         packet_write_wait();
 152
 153         /* Wait for a response. */
 154         packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
 155         for (i = 0; i < 16; i++)
 156                 response[i] = (u_char)packet_get_char();
 157         packet_check_eom();
 158
 159         success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
 160         BN_clear_free(challenge);
 161         return (success);
 162 }
 163
 164 static int
 165 rsa_key_allowed_in_file(struct passwd *pw, char *file,
 166     const BIGNUM *client_n, Key **rkey)
 167 {
 168         char line[SSH_MAX_PUBKEY_BYTES];
 169         int allowed = 0;
 170         u_int bits;
 171         FILE *f;
 172         u_long linenum = 0;
 173         Key *key;
 174
 175         debug("trying public RSA key file %s", file);
 176         if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL)
 177                 return 0;
 178
 179         /*
 180          * Go though the accepted keys, looking for the current key.  If
 181          * found, perform a challenge-response dialog to verify that the
 182          * user really has the corresponding private key.
 183          */
 184         key = key_new(KEY_RSA1);
 185         while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
 186                 char *cp;
 187                 char *key_options;
 188                 int keybits;
 189                 char *fp;
 190
 191                 /* Skip leading whitespace, empty and comment lines. */
 192                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 193                         ;
 194                 if (!*cp || *cp == '\n' || *cp == '#')
 195                         continue;
 196
 197                 /*
 198                  * Check if there are options for this key, and if so,
 199                  * save their starting address and skip the option part
 200                  * for now.  If there are no options, set the starting
 201                  * address to NULL.
 202                  */
 203                 if (*cp < '0' || *cp > '9') {
 204                         int quoted = 0;
 205                         key_options = cp;
 206                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 207                                 if (*cp == '\\' && cp[1] == '"')
 208                                         cp++;   /* Skip both */
 209                                 else if (*cp == '"')
 210                                         quoted = !quoted;
 211                         }
 212                 } else
 213                         key_options = NULL;
 214
 215                 /* Parse the key from the line. */
 216                 if (hostfile_read_key(&cp, &bits, key) == 0) {
 217                         debug("%.100s, line %lu: non ssh1 key syntax",
 218                             file, linenum);
 219                         continue;
 220                 }
 221                 /* cp now points to the comment part. */
 222
 223                 /*
 224                  * Check if the we have found the desired key (identified
 225                  * by its modulus).
 226                  */
 227                 if (BN_cmp(key->rsa->n, client_n) != 0)
 228                         continue;
 229
 230                 /* check the real bits  */
 231                 keybits = BN_num_bits(key->rsa->n);
 232                 if (keybits < 0 || bits != (u_int)keybits)
 233                         logit("Warning: %s, line %lu: keysize mismatch: "
 234                             "actual %d vs. announced %d.",
 235                             file, linenum, BN_num_bits(key->rsa->n), bits);
 236
 237                 /* Never accept a revoked key */
 238                 if (auth_key_is_revoked(key))
 239                         break;
 240
 241                 if (blacklisted_key(key)) {
 242                         fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 243                         if (options.permit_blacklisted_keys)
 244                                 logit("Public key %s blacklisted (see "
 245                                     "ssh-vulnkey(1)); continuing anyway", fp);
 246                         else
 247                                 logit("Public key %s blacklisted (see "
 248                                     "ssh-vulnkey(1))", fp);
 249                         xfree(fp);
 250                         if (!options.permit_blacklisted_keys)
 251                                 continue;
 252                 }
 253
 254                 /* We have found the desired key. */
 255                 /*
 256                  * If our options do not allow this key to be used,
 257                  * do not send challenge.
 258                  */
 259                 if (!auth_parse_options(pw, key_options, file, linenum))
 260                         continue;
 261                 if (key_is_cert_authority)
 262                         continue;
 263                 /* break out, this key is allowed */
 264                 allowed = 1;
 265                 break;
 266         }
 267
 268         /* Close the file. */
 269         fclose(f);
 270
 271         /* return key if allowed */
 272         if (allowed && rkey != NULL)
 273                 *rkey = key;
 274         else
 275                 key_free(key);
 276
 277         return allowed;
 278 }
 279
 280 /*
 281  * check if there's user key matching client_n,
 282  * return key if login is allowed, NULL otherwise
 283  */
 284
 285 int
 286 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
 287 {
 288         char *file;
 289         u_int i, allowed = 0;
 290
 291         temporarily_use_uid(pw);
 292
 293         for (i = 0; !allowed && i < options.num_authkeys_files; i++) {
 294                 file = expand_authorized_keys(
 295                     options.authorized_keys_files[i], pw);
 296                 allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
 297                 xfree(file);
 298         }
 299
 300         restore_uid();
 301
 302         return allowed;
 303 }
 304
 305 /*
 306  * Performs the RSA authentication dialog with the client.  This returns
 307  * 0 if the client could not be authenticated, and 1 if authentication was
 308  * successful.  This may exit if there is a serious protocol violation.
 309  */
 310 int
 311 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
 312 {
 313         Key *key;
 314         char *fp;
 315         struct passwd *pw = authctxt->pw;
 316
 317         /* no user given */
 318         if (!authctxt->valid)
 319                 return 0;
 320
 321         if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
 322                 auth_clear_options();
 323                 return (0);
 324         }
 325
 326         /* Perform the challenge-response dialog for this key. */
 327         if (!auth_rsa_challenge_dialog(key)) {
 328                 /* Wrong response. */
 329                 verbose("Wrong response to RSA authentication challenge.");
 330                 packet_send_debug("Wrong response to RSA authentication challenge.");
 331                 /*
 332                  * Break out of the loop. Otherwise we might send
 333                  * another challenge and break the protocol.
 334                  */
 335                 key_free(key);
 336                 return (0);
 337         }
 338         /*
 339          * Correct response.  The client has been successfully
 340          * authenticated. Note that we have not yet processed the
 341          * options; this will be reset if the options cause the
 342          * authentication to be rejected.
 343          */
 344         fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 345         verbose("Found matching %s key: %s",
 346             key_type(key), fp);
 347         xfree(fp);
 348         key_free(key);
 349
 350         packet_send_debug("RSA authentication accepted.");
 351         return (1);
 352 }