auth-rsa.c

   1 /* $OpenBSD: auth-rsa.c,v 1.68 2006/07/06 16:03:53 stevesk 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
  27 #include "rsa.h"
  28 #include "packet.h"
  29 #include "xmalloc.h"
  30 #include "ssh1.h"
  31 #include "uidswap.h"
  32 #include "match.h"
  33 #include "auth-options.h"
  34 #include "pathnames.h"
  35 #include "log.h"
  36 #include "servconf.h"
  37 #include "auth.h"
  38 #include "hostfile.h"
  39 #include "monitor_wrap.h"
  40 #include "ssh.h"
  41 #include "misc.h"
  42
  43 /* import */
  44 extern ServerOptions options;
  45
  46 /*
  47  * Session identifier that is used to bind key exchange and authentication
  48  * responses to a particular session.
  49  */
  50 extern u_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 SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
  59  * description of the options.
  60  */
  61
  62 BIGNUM *
  63 auth_rsa_generate_challenge(Key *key)
  64 {
  65         BIGNUM *challenge;
  66         BN_CTX *ctx;
  67
  68         if ((challenge = BN_new()) == NULL)
  69                 fatal("auth_rsa_generate_challenge: BN_new() failed");
  70         /* Generate a random challenge. */
  71         BN_rand(challenge, 256, 0, 0);
  72         if ((ctx = BN_CTX_new()) == NULL)
  73                 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
  74         BN_mod(challenge, challenge, key->rsa->n, ctx);
  75         BN_CTX_free(ctx);
  76
  77         return challenge;
  78 }
  79
  80 int
  81 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
  82 {
  83         u_char buf[32], mdbuf[16];
  84         MD5_CTX md;
  85         int len;
  86
  87         /* don't allow short keys */
  88         if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
  89                 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
  90                     BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
  91                 return (0);
  92         }
  93
  94         /* The response is MD5 of decrypted challenge plus session id. */
  95         len = BN_num_bytes(challenge);
  96         if (len <= 0 || len > 32)
  97                 fatal("auth_rsa_verify_response: bad challenge length %d", len);
  98         memset(buf, 0, 32);
  99         BN_bn2bin(challenge, buf + 32 - len);
 100         MD5_Init(&md);
 101         MD5_Update(&md, buf, 32);
 102         MD5_Update(&md, session_id, 16);
 103         MD5_Final(mdbuf, &md);
 104
 105         /* Verify that the response is the original challenge. */
 106         if (memcmp(response, mdbuf, 16) != 0) {
 107                 /* Wrong answer. */
 108                 return (0);
 109         }
 110         /* Correct answer. */
 111         return (1);
 112 }
 113
 114 /*
 115  * Performs the RSA authentication challenge-response dialog with the client,
 116  * and returns true (non-zero) if the client gave the correct answer to
 117  * our challenge; returns zero if the client gives a wrong answer.
 118  */
 119
 120 int
 121 auth_rsa_challenge_dialog(Key *key)
 122 {
 123         BIGNUM *challenge, *encrypted_challenge;
 124         u_char response[16];
 125         int i, success;
 126
 127         if ((encrypted_challenge = BN_new()) == NULL)
 128                 fatal("auth_rsa_challenge_dialog: BN_new() failed");
 129
 130         challenge = PRIVSEP(auth_rsa_generate_challenge(key));
 131
 132         /* Encrypt the challenge with the public key. */
 133         rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
 134
 135         /* Send the encrypted challenge to the client. */
 136         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 137         packet_put_bignum(encrypted_challenge);
 138         packet_send();
 139         BN_clear_free(encrypted_challenge);
 140         packet_write_wait();
 141
 142         /* Wait for a response. */
 143         packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
 144         for (i = 0; i < 16; i++)
 145                 response[i] = (u_char)packet_get_char();
 146         packet_check_eom();
 147
 148         success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
 149         BN_clear_free(challenge);
 150         return (success);
 151 }
 152
 153 /*
 154  * check if there's user key matching client_n,
 155  * return key if login is allowed, NULL otherwise
 156  */
 157
 158 int
 159 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
 160 {
 161         char line[SSH_MAX_PUBKEY_BYTES], *file;
 162         int allowed = 0;
 163         u_int bits;
 164         FILE *f;
 165         u_long linenum = 0;
 166         struct stat st;
 167         Key *key;
 168
 169         /* Temporarily use the user's uid. */
 170         temporarily_use_uid(pw);
 171
 172         /* The authorized keys. */
 173         file = authorized_keys_file(pw);
 174         debug("trying public RSA key file %s", file);
 175
 176         /* Fail quietly if file does not exist */
 177         if (stat(file, &st) < 0) {
 178                 /* Restore the privileged uid. */
 179                 restore_uid();
 180                 xfree(file);
 181                 return (0);
 182         }
 183         /* Open the file containing the authorized keys. */
 184         f = fopen(file, "r");
 185         if (!f) {
 186                 /* Restore the privileged uid. */
 187                 restore_uid();
 188                 xfree(file);
 189                 return (0);
 190         }
 191         if (options.strict_modes &&
 192             secure_filename(f, file, pw, line, sizeof(line)) != 0) {
 193                 xfree(file);
 194                 fclose(f);
 195                 logit("Authentication refused: %s", line);
 196                 restore_uid();
 197                 return (0);
 198         }
 199
 200         /* Flag indicating whether the key is allowed. */
 201         allowed = 0;
 202
 203         key = key_new(KEY_RSA1);
 204
 205         /*
 206          * Go though the accepted keys, looking for the current key.  If
 207          * found, perform a challenge-response dialog to verify that the
 208          * user really has the corresponding private key.
 209          */
 210         while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
 211                 char *cp;
 212                 char *key_options;
 213                 int keybits;
 214
 215                 /* Skip leading whitespace, empty and comment lines. */
 216                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 217                         ;
 218                 if (!*cp || *cp == '\n' || *cp == '#')
 219                         continue;
 220
 221                 /*
 222                  * Check if there are options for this key, and if so,
 223                  * save their starting address and skip the option part
 224                  * for now.  If there are no options, set the starting
 225                  * address to NULL.
 226                  */
 227                 if (*cp < '0' || *cp > '9') {
 228                         int quoted = 0;
 229                         key_options = cp;
 230                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 231                                 if (*cp == '\\' && cp[1] == '"')
 232                                         cp++;   /* Skip both */
 233                                 else if (*cp == '"')
 234                                         quoted = !quoted;
 235                         }
 236                 } else
 237                         key_options = NULL;
 238
 239                 /* Parse the key from the line. */
 240                 if (hostfile_read_key(&cp, &bits, key) == 0) {
 241                         debug("%.100s, line %lu: non ssh1 key syntax",
 242                             file, linenum);
 243                         continue;
 244                 }
 245                 /* cp now points to the comment part. */
 246
 247                 /* Check if the we have found the desired key (identified by its modulus). */
 248                 if (BN_cmp(key->rsa->n, client_n) != 0)
 249                         continue;
 250
 251                 /* check the real bits  */
 252                 keybits = BN_num_bits(key->rsa->n);
 253                 if (keybits < 0 || bits != (u_int)keybits)
 254                         logit("Warning: %s, line %lu: keysize mismatch: "
 255                             "actual %d vs. announced %d.",
 256                             file, linenum, BN_num_bits(key->rsa->n), bits);
 257
 258                 /* We have found the desired key. */
 259                 /*
 260                  * If our options do not allow this key to be used,
 261                  * do not send challenge.
 262                  */
 263                 if (!auth_parse_options(pw, key_options, file, linenum))
 264                         continue;
 265
 266                 /* break out, this key is allowed */
 267                 allowed = 1;
 268                 break;
 269         }
 270
 271         /* Restore the privileged uid. */
 272         restore_uid();
 273
 274         /* Close the file. */
 275         xfree(file);
 276         fclose(f);
 277
 278         /* return key if allowed */
 279         if (allowed && rkey != NULL)
 280                 *rkey = key;
 281         else
 282                 key_free(key);
 283         return (allowed);
 284 }
 285
 286 /*
 287  * Performs the RSA authentication dialog with the client.  This returns
 288  * 0 if the client could not be authenticated, and 1 if authentication was
 289  * successful.  This may exit if there is a serious protocol violation.
 290  */
 291 int
 292 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
 293 {
 294         Key *key;
 295         char *fp;
 296         struct passwd *pw = authctxt->pw;
 297
 298         /* no user given */
 299         if (!authctxt->valid)
 300                 return 0;
 301
 302         if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
 303                 auth_clear_options();
 304                 return (0);
 305         }
 306
 307         /* Perform the challenge-response dialog for this key. */
 308         if (!auth_rsa_challenge_dialog(key)) {
 309                 /* Wrong response. */
 310                 verbose("Wrong response to RSA authentication challenge.");
 311                 packet_send_debug("Wrong response to RSA authentication challenge.");
 312                 /*
 313                  * Break out of the loop. Otherwise we might send
 314                  * another challenge and break the protocol.
 315                  */
 316                 key_free(key);
 317                 return (0);
 318         }
 319         /*
 320          * Correct response.  The client has been successfully
 321          * authenticated. Note that we have not yet processed the
 322          * options; this will be reset if the options cause the
 323          * authentication to be rejected.
 324          */
 325         fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 326         verbose("Found matching %s key: %s",
 327             key_type(key), fp);
 328         xfree(fp);
 329         key_free(key);
 330
 331         packet_send_debug("RSA authentication accepted.");
 332         return (1);
 333 }