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 <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 "auth-options.h"
  38 #include "pathnames.h"
  39 #include "log.h"
  40 #include "servconf.h"
  41 #include "key.h"
  42 #include "hostfile.h"
  43 #include "auth.h"
  44 #ifdef GSSAPI
  45 #include "ssh-gss.h"
  46 #endif
  47 #include "monitor_wrap.h"
  48 #include "ssh.h"
  49 #include "misc.h"
  50
  51 /* import */
  52 extern ServerOptions options;
  53
  54 /*
  55  * Session identifier that is used to bind key exchange and authentication
  56  * responses to a particular session.
  57  */
  58 extern u_char session_id[16];
  59
  60 /*
  61  * The .ssh/authorized_keys file contains public keys, one per line, in the
  62  * following format:
  63  *   options bits e n comment
  64  * where bits, e and n are decimal numbers,
  65  * and comment is any string of characters up to newline.  The maximum
  66  * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
  67  * description of the options.
  68  */
  69
  70 BIGNUM *
  71 auth_rsa_generate_challenge(Key *key)
  72 {
  73         BIGNUM *challenge;
  74         BN_CTX *ctx;
  75
  76         if ((challenge = BN_new()) == NULL)
  77                 fatal("auth_rsa_generate_challenge: BN_new() failed");
  78         /* Generate a random challenge. */
  79         BN_rand(challenge, 256, 0, 0);
  80         if ((ctx = BN_CTX_new()) == NULL)
  81                 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
  82         BN_mod(challenge, challenge, key->rsa->n, ctx);
  83         BN_CTX_free(ctx);
  84
  85         return challenge;
  86 }
  87
  88 int
  89 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
  90 {
  91         u_char buf[32], mdbuf[16];
  92         MD5_CTX md;
  93         int len;
  94
  95         /* don't allow short keys */
  96         if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
  97                 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
  98                     BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
  99                 return (0);
 100         }
 101
 102         /* The response is MD5 of decrypted challenge plus session id. */
 103         len = BN_num_bytes(challenge);
 104         if (len <= 0 || len > 32)
 105                 fatal("auth_rsa_verify_response: bad challenge length %d", len);
 106         memset(buf, 0, 32);
 107         BN_bn2bin(challenge, buf + 32 - len);
 108         MD5_Init(&md);
 109         MD5_Update(&md, buf, 32);
 110         MD5_Update(&md, session_id, 16);
 111         MD5_Final(mdbuf, &md);
 112
 113         /* Verify that the response is the original challenge. */
 114         if (memcmp(response, mdbuf, 16) != 0) {
 115                 /* Wrong answer. */
 116                 return (0);
 117         }
 118         /* Correct answer. */
 119         return (1);
 120 }
 121
 122 /*
 123  * Performs the RSA authentication challenge-response dialog with the client,
 124  * and returns true (non-zero) if the client gave the correct answer to
 125  * our challenge; returns zero if the client gives a wrong answer.
 126  */
 127
 128 int
 129 auth_rsa_challenge_dialog(Key *key)
 130 {
 131         BIGNUM *challenge, *encrypted_challenge;
 132         u_char response[16];
 133         int i, success;
 134
 135         if ((encrypted_challenge = BN_new()) == NULL)
 136                 fatal("auth_rsa_challenge_dialog: BN_new() failed");
 137
 138         challenge = PRIVSEP(auth_rsa_generate_challenge(key));
 139
 140         /* Encrypt the challenge with the public key. */
 141         rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
 142
 143         /* Send the encrypted challenge to the client. */
 144         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 145         packet_put_bignum(encrypted_challenge);
 146         packet_send();
 147         BN_clear_free(encrypted_challenge);
 148         packet_write_wait();
 149
 150         /* Wait for a response. */
 151         packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
 152         for (i = 0; i < 16; i++)
 153                 response[i] = (u_char)packet_get_char();
 154         packet_check_eom();
 155
 156         success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
 157         BN_clear_free(challenge);
 158         return (success);
 159 }
 160
 161 /*
 162  * check if there's user key matching client_n,
 163  * return key if login is allowed, NULL otherwise
 164  */
 165
 166 int
 167 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
 168 {
 169         char line[SSH_MAX_PUBKEY_BYTES], *file;
 170         int allowed = 0;
 171         u_int bits;
 172         FILE *f;
 173         u_long linenum = 0;
 174         struct stat st;
 175         Key *key;
 176
 177         /* Temporarily use the user's uid. */
 178         temporarily_use_uid(pw);
 179
 180         /* The authorized keys. */
 181         file = authorized_keys_file(pw);
 182         debug("trying public RSA key file %s", file);
 183
 184         /* Fail quietly if file does not exist */
 185         if (stat(file, &st) < 0) {
 186                 /* Restore the privileged uid. */
 187                 restore_uid();
 188                 xfree(file);
 189                 return (0);
 190         }
 191         /* Open the file containing the authorized keys. */
 192         f = fopen(file, "r");
 193         if (!f) {
 194                 /* Restore the privileged uid. */
 195                 restore_uid();
 196                 xfree(file);
 197                 return (0);
 198         }
 199         if (options.strict_modes &&
 200             secure_filename(f, file, pw, line, sizeof(line)) != 0) {
 201                 xfree(file);
 202                 fclose(f);
 203                 logit("Authentication refused: %s", line);
 204                 restore_uid();
 205                 return (0);
 206         }
 207
 208         /* Flag indicating whether the key is allowed. */
 209         allowed = 0;
 210
 211         key = key_new(KEY_RSA1);
 212
 213         /*
 214          * Go though the accepted keys, looking for the current key.  If
 215          * found, perform a challenge-response dialog to verify that the
 216          * user really has the corresponding private key.
 217          */
 218         while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
 219                 char *cp;
 220                 char *key_options;
 221                 int keybits;
 222
 223                 /* Skip leading whitespace, empty and comment lines. */
 224                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 225                         ;
 226                 if (!*cp || *cp == '\n' || *cp == '#')
 227                         continue;
 228
 229                 /*
 230                  * Check if there are options for this key, and if so,
 231                  * save their starting address and skip the option part
 232                  * for now.  If there are no options, set the starting
 233                  * address to NULL.
 234                  */
 235                 if (*cp < '0' || *cp > '9') {
 236                         int quoted = 0;
 237                         key_options = cp;
 238                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 239                                 if (*cp == '\\' && cp[1] == '"')
 240                                         cp++;   /* Skip both */
 241                                 else if (*cp == '"')
 242                                         quoted = !quoted;
 243                         }
 244                 } else
 245                         key_options = NULL;
 246
 247                 /* Parse the key from the line. */
 248                 if (hostfile_read_key(&cp, &bits, key) == 0) {
 249                         debug("%.100s, line %lu: non ssh1 key syntax",
 250                             file, linenum);
 251                         continue;
 252                 }
 253                 /* cp now points to the comment part. */
 254
 255                 /* Check if the we have found the desired key (identified by its modulus). */
 256                 if (BN_cmp(key->rsa->n, client_n) != 0)
 257                         continue;
 258
 259                 /* check the real bits  */
 260                 keybits = BN_num_bits(key->rsa->n);
 261                 if (keybits < 0 || bits != (u_int)keybits)
 262                         logit("Warning: %s, line %lu: keysize mismatch: "
 263                             "actual %d vs. announced %d.",
 264                             file, linenum, BN_num_bits(key->rsa->n), bits);
 265
 266                 /* We have found the desired key. */
 267                 /*
 268                  * If our options do not allow this key to be used,
 269                  * do not send challenge.
 270                  */
 271                 if (!auth_parse_options(pw, key_options, file, linenum))
 272                         continue;
 273
 274                 /* break out, this key is allowed */
 275                 allowed = 1;
 276                 break;
 277         }
 278
 279         /* Restore the privileged uid. */
 280         restore_uid();
 281
 282         /* Close the file. */
 283         xfree(file);
 284         fclose(f);
 285
 286         /* return key if allowed */
 287         if (allowed && rkey != NULL)
 288                 *rkey = key;
 289         else
 290                 key_free(key);
 291         return (allowed);
 292 }
 293
 294 /*
 295  * Performs the RSA authentication dialog with the client.  This returns
 296  * 0 if the client could not be authenticated, and 1 if authentication was
 297  * successful.  This may exit if there is a serious protocol violation.
 298  */
 299 int
 300 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
 301 {
 302         Key *key;
 303         char *fp;
 304         struct passwd *pw = authctxt->pw;
 305
 306         /* no user given */
 307         if (!authctxt->valid)
 308                 return 0;
 309
 310         if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
 311                 auth_clear_options();
 312                 return (0);
 313         }
 314
 315         /* Perform the challenge-response dialog for this key. */
 316         if (!auth_rsa_challenge_dialog(key)) {
 317                 /* Wrong response. */
 318                 verbose("Wrong response to RSA authentication challenge.");
 319                 packet_send_debug("Wrong response to RSA authentication challenge.");
 320                 /*
 321                  * Break out of the loop. Otherwise we might send
 322                  * another challenge and break the protocol.
 323                  */
 324                 key_free(key);
 325                 return (0);
 326         }
 327         /*
 328          * Correct response.  The client has been successfully
 329          * authenticated. Note that we have not yet processed the
 330          * options; this will be reset if the options cause the
 331          * authentication to be rejected.
 332          */
 333         fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 334         verbose("Found matching %s key: %s",
 335             key_type(key), fp);
 336         xfree(fp);
 337         key_free(key);
 338
 339         packet_send_debug("RSA authentication accepted.");
 340         return (1);
 341 }