auth-rsa.c

   1 /* $OpenBSD: auth-rsa.c,v 1.71 2006/08/03 03:34:41 deraadt 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 <string.h>
  28
  29 #include "xmalloc.h"
  30 #include "rsa.h"
  31 #include "packet.h"
  32 #include "ssh1.h"
  33 #include "uidswap.h"
  34 #include "match.h"
  35 #include "buffer.h"
  36 #include "auth-options.h"
  37 #include "pathnames.h"
  38 #include "log.h"
  39 #include "servconf.h"
  40 #include "key.h"
  41 #include "hostfile.h"
  42 #include "auth.h"
  43 #ifdef GSSAPI
  44 #include "ssh-gss.h"
  45 #endif
  46 #include "monitor_wrap.h"
  47 #include "ssh.h"
  48 #include "misc.h"
  49
  50 /* import */
  51 extern ServerOptions options;
  52
  53 /*
  54  * Session identifier that is used to bind key exchange and authentication
  55  * responses to a particular session.
  56  */
  57 extern u_char session_id[16];
  58
  59 /*
  60  * The .ssh/authorized_keys file contains public keys, one per line, in the
  61  * following format:
  62  *   options bits e n comment
  63  * where bits, e and n are decimal numbers,
  64  * and comment is any string of characters up to newline.  The maximum
  65  * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
  66  * description of the options.
  67  */
  68
  69 BIGNUM *
  70 auth_rsa_generate_challenge(Key *key)
  71 {
  72         BIGNUM *challenge;
  73         BN_CTX *ctx;
  74
  75         if ((challenge = BN_new()) == NULL)
  76                 fatal("auth_rsa_generate_challenge: BN_new() failed");
  77         /* Generate a random challenge. */
  78         BN_rand(challenge, 256, 0, 0);
  79         if ((ctx = BN_CTX_new()) == NULL)
  80                 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
  81         BN_mod(challenge, challenge, key->rsa->n, ctx);
  82         BN_CTX_free(ctx);
  83
  84         return challenge;
  85 }
  86
  87 int
  88 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
  89 {
  90         u_char buf[32], mdbuf[16];
  91         MD5_CTX md;
  92         int len;
  93
  94         /* don't allow short keys */
  95         if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
  96                 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
  97                     BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
  98                 return (0);
  99         }
 100
 101         /* The response is MD5 of decrypted challenge plus session id. */
 102         len = BN_num_bytes(challenge);
 103         if (len <= 0 || len > 32)
 104                 fatal("auth_rsa_verify_response: bad challenge length %d", len);
 105         memset(buf, 0, 32);
 106         BN_bn2bin(challenge, buf + 32 - len);
 107         MD5_Init(&md);
 108         MD5_Update(&md, buf, 32);
 109         MD5_Update(&md, session_id, 16);
 110         MD5_Final(mdbuf, &md);
 111
 112         /* Verify that the response is the original challenge. */
 113         if (memcmp(response, mdbuf, 16) != 0) {
 114                 /* Wrong answer. */
 115                 return (0);
 116         }
 117         /* Correct answer. */
 118         return (1);
 119 }
 120
 121 /*
 122  * Performs the RSA authentication challenge-response dialog with the client,
 123  * and returns true (non-zero) if the client gave the correct answer to
 124  * our challenge; returns zero if the client gives a wrong answer.
 125  */
 126
 127 int
 128 auth_rsa_challenge_dialog(Key *key)
 129 {
 130         BIGNUM *challenge, *encrypted_challenge;
 131         u_char response[16];
 132         int i, success;
 133
 134         if ((encrypted_challenge = BN_new()) == NULL)
 135                 fatal("auth_rsa_challenge_dialog: BN_new() failed");
 136
 137         challenge = PRIVSEP(auth_rsa_generate_challenge(key));
 138
 139         /* Encrypt the challenge with the public key. */
 140         rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
 141
 142         /* Send the encrypted challenge to the client. */
 143         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 144         packet_put_bignum(encrypted_challenge);
 145         packet_send();
 146         BN_clear_free(encrypted_challenge);
 147         packet_write_wait();
 148
 149         /* Wait for a response. */
 150         packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
 151         for (i = 0; i < 16; i++)
 152                 response[i] = (u_char)packet_get_char();
 153         packet_check_eom();
 154
 155         success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
 156         BN_clear_free(challenge);
 157         return (success);
 158 }
 159
 160 /*
 161  * check if there's user key matching client_n,
 162  * return key if login is allowed, NULL otherwise
 163  */
 164
 165 int
 166 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
 167 {
 168         char line[SSH_MAX_PUBKEY_BYTES], *file;
 169         int allowed = 0;
 170         u_int bits;
 171         FILE *f;
 172         u_long linenum = 0;
 173         struct stat st;
 174         Key *key;
 175
 176         /* Temporarily use the user's uid. */
 177         temporarily_use_uid(pw);
 178
 179         /* The authorized keys. */
 180         file = authorized_keys_file(pw);
 181         debug("trying public RSA key file %s", file);
 182
 183         /* Fail quietly if file does not exist */
 184         if (stat(file, &st) < 0) {
 185                 /* Restore the privileged uid. */
 186                 restore_uid();
 187                 xfree(file);
 188                 return (0);
 189         }
 190         /* Open the file containing the authorized keys. */
 191         f = fopen(file, "r");
 192         if (!f) {
 193                 /* Restore the privileged uid. */
 194                 restore_uid();
 195                 xfree(file);
 196                 return (0);
 197         }
 198         if (options.strict_modes &&
 199             secure_filename(f, file, pw, line, sizeof(line)) != 0) {
 200                 xfree(file);
 201                 fclose(f);
 202                 logit("Authentication refused: %s", line);
 203                 restore_uid();
 204                 return (0);
 205         }
 206
 207         /* Flag indicating whether the key is allowed. */
 208         allowed = 0;
 209
 210         key = key_new(KEY_RSA1);
 211
 212         /*
 213          * Go though the accepted keys, looking for the current key.  If
 214          * found, perform a challenge-response dialog to verify that the
 215          * user really has the corresponding private key.
 216          */
 217         while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
 218                 char *cp;
 219                 char *key_options;
 220                 int keybits;
 221
 222                 /* Skip leading whitespace, empty and comment lines. */
 223                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 224                         ;
 225                 if (!*cp || *cp == '\n' || *cp == '#')
 226                         continue;
 227
 228                 /*
 229                  * Check if there are options for this key, and if so,
 230                  * save their starting address and skip the option part
 231                  * for now.  If there are no options, set the starting
 232                  * address to NULL.
 233                  */
 234                 if (*cp < '0' || *cp > '9') {
 235                         int quoted = 0;
 236                         key_options = cp;
 237                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 238                                 if (*cp == '\\' && cp[1] == '"')
 239                                         cp++;   /* Skip both */
 240                                 else if (*cp == '"')
 241                                         quoted = !quoted;
 242                         }
 243                 } else
 244                         key_options = NULL;
 245
 246                 /* Parse the key from the line. */
 247                 if (hostfile_read_key(&cp, &bits, key) == 0) {
 248                         debug("%.100s, line %lu: non ssh1 key syntax",
 249                             file, linenum);
 250                         continue;
 251                 }
 252                 /* cp now points to the comment part. */
 253
 254                 /* Check if the we have found the desired key (identified by its modulus). */
 255                 if (BN_cmp(key->rsa->n, client_n) != 0)
 256                         continue;
 257
 258                 /* check the real bits  */
 259                 keybits = BN_num_bits(key->rsa->n);
 260                 if (keybits < 0 || bits != (u_int)keybits)
 261                         logit("Warning: %s, line %lu: keysize mismatch: "
 262                             "actual %d vs. announced %d.",
 263                             file, linenum, BN_num_bits(key->rsa->n), bits);
 264
 265                 /* We have found the desired key. */
 266                 /*
 267                  * If our options do not allow this key to be used,
 268                  * do not send challenge.
 269                  */
 270                 if (!auth_parse_options(pw, key_options, file, linenum))
 271                         continue;
 272
 273                 /* break out, this key is allowed */
 274                 allowed = 1;
 275                 break;
 276         }
 277
 278         /* Restore the privileged uid. */
 279         restore_uid();
 280
 281         /* Close the file. */
 282         xfree(file);
 283         fclose(f);
 284
 285         /* return key if allowed */
 286         if (allowed && rkey != NULL)
 287                 *rkey = key;
 288         else
 289                 key_free(key);
 290         return (allowed);
 291 }
 292
 293 /*
 294  * Performs the RSA authentication dialog with the client.  This returns
 295  * 0 if the client could not be authenticated, and 1 if authentication was
 296  * successful.  This may exit if there is a serious protocol violation.
 297  */
 298 int
 299 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
 300 {
 301         Key *key;
 302         char *fp;
 303         struct passwd *pw = authctxt->pw;
 304
 305         /* no user given */
 306         if (!authctxt->valid)
 307                 return 0;
 308
 309         if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
 310                 auth_clear_options();
 311                 return (0);
 312         }
 313
 314         /* Perform the challenge-response dialog for this key. */
 315         if (!auth_rsa_challenge_dialog(key)) {
 316                 /* Wrong response. */
 317                 verbose("Wrong response to RSA authentication challenge.");
 318                 packet_send_debug("Wrong response to RSA authentication challenge.");
 319                 /*
 320                  * Break out of the loop. Otherwise we might send
 321                  * another challenge and break the protocol.
 322                  */
 323                 key_free(key);
 324                 return (0);
 325         }
 326         /*
 327          * Correct response.  The client has been successfully
 328          * authenticated. Note that we have not yet processed the
 329          * options; this will be reset if the options cause the
 330          * authentication to be rejected.
 331          */
 332         fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 333         verbose("Found matching %s key: %s",
 334             key_type(key), fp);
 335         xfree(fp);
 336         key_free(key);
 337
 338         packet_send_debug("RSA authentication accepted.");
 339         return (1);
 340 }