auth-rsa.c

   1 /*
   2  * Author: Tatu Ylonen <ylo@cs.hut.fi>
   3  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
   4  *                    All rights reserved
   5  * RSA-based authentication.  This code determines whether to admit a login
   6  * based on RSA authentication.  This file also contains functions to check
   7  * validity of the host key.
   8  *
   9  * As far as I am concerned, the code I have written for this software
  10  * can be used freely for any purpose.  Any derived versions of this
  11  * software must be clearly marked as such, and if the derived work is
  12  * incompatible with the protocol description in the RFC file, it must be
  13  * called by a name other than "ssh" or "Secure Shell".
  14  */
  15
  16 #include "includes.h"
  17 RCSID("$OpenBSD: auth-rsa.c,v 1.62 2004/12/11 01:48:56 dtucker Exp $");
  18
  19 #include <openssl/rsa.h>
  20 #include <openssl/md5.h>
  21
  22 #include "rsa.h"
  23 #include "packet.h"
  24 #include "xmalloc.h"
  25 #include "ssh1.h"
  26 #include "uidswap.h"
  27 #include "match.h"
  28 #include "auth-options.h"
  29 #include "pathnames.h"
  30 #include "log.h"
  31 #include "servconf.h"
  32 #include "auth.h"
  33 #include "hostfile.h"
  34 #include "monitor_wrap.h"
  35 #include "ssh.h"
  36 #include "misc.h"
  37
  38 /* import */
  39 extern ServerOptions options;
  40
  41 /*
  42  * Session identifier that is used to bind key exchange and authentication
  43  * responses to a particular session.
  44  */
  45 extern u_char session_id[16];
  46
  47 /*
  48  * The .ssh/authorized_keys file contains public keys, one per line, in the
  49  * following format:
  50  *   options bits e n comment
  51  * where bits, e and n are decimal numbers,
  52  * and comment is any string of characters up to newline.  The maximum
  53  * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
  54  * description of the options.
  55  */
  56
  57 BIGNUM *
  58 auth_rsa_generate_challenge(Key *key)
  59 {
  60         BIGNUM *challenge;
  61         BN_CTX *ctx;
  62
  63         if ((challenge = BN_new()) == NULL)
  64                 fatal("auth_rsa_generate_challenge: BN_new() failed");
  65         /* Generate a random challenge. */
  66         BN_rand(challenge, 256, 0, 0);
  67         if ((ctx = BN_CTX_new()) == NULL)
  68                 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
  69         BN_mod(challenge, challenge, key->rsa->n, ctx);
  70         BN_CTX_free(ctx);
  71
  72         return challenge;
  73 }
  74
  75 int
  76 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
  77 {
  78         u_char buf[32], mdbuf[16];
  79         MD5_CTX md;
  80         int len;
  81
  82         /* don't allow short keys */
  83         if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
  84                 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
  85                     BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
  86                 return (0);
  87         }
  88
  89         /* The response is MD5 of decrypted challenge plus session id. */
  90         len = BN_num_bytes(challenge);
  91         if (len <= 0 || len > 32)
  92                 fatal("auth_rsa_verify_response: bad challenge length %d", len);
  93         memset(buf, 0, 32);
  94         BN_bn2bin(challenge, buf + 32 - len);
  95         MD5_Init(&md);
  96         MD5_Update(&md, buf, 32);
  97         MD5_Update(&md, session_id, 16);
  98         MD5_Final(mdbuf, &md);
  99
 100         /* Verify that the response is the original challenge. */
 101         if (memcmp(response, mdbuf, 16) != 0) {
 102                 /* Wrong answer. */
 103                 return (0);
 104         }
 105         /* Correct answer. */
 106         return (1);
 107 }
 108
 109 /*
 110  * Performs the RSA authentication challenge-response dialog with the client,
 111  * and returns true (non-zero) if the client gave the correct answer to
 112  * our challenge; returns zero if the client gives a wrong answer.
 113  */
 114
 115 int
 116 auth_rsa_challenge_dialog(Key *key)
 117 {
 118         BIGNUM *challenge, *encrypted_challenge;
 119         u_char response[16];
 120         int i, success;
 121
 122         if ((encrypted_challenge = BN_new()) == NULL)
 123                 fatal("auth_rsa_challenge_dialog: BN_new() failed");
 124
 125         challenge = PRIVSEP(auth_rsa_generate_challenge(key));
 126
 127         /* Encrypt the challenge with the public key. */
 128         rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
 129
 130         /* Send the encrypted challenge to the client. */
 131         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 132         packet_put_bignum(encrypted_challenge);
 133         packet_send();
 134         BN_clear_free(encrypted_challenge);
 135         packet_write_wait();
 136
 137         /* Wait for a response. */
 138         packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
 139         for (i = 0; i < 16; i++)
 140                 response[i] = packet_get_char();
 141         packet_check_eom();
 142
 143         success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
 144         BN_clear_free(challenge);
 145         return (success);
 146 }
 147
 148 /*
 149  * check if there's user key matching client_n,
 150  * return key if login is allowed, NULL otherwise
 151  */
 152
 153 int
 154 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
 155 {
 156         char line[SSH_MAX_PUBKEY_BYTES], *file;
 157         int allowed = 0;
 158         u_int bits;
 159         FILE *f;
 160         u_long linenum = 0;
 161         struct stat st;
 162         Key *key;
 163
 164         /* Temporarily use the user's uid. */
 165         temporarily_use_uid(pw);
 166
 167         /* The authorized keys. */
 168         file = authorized_keys_file(pw);
 169         debug("trying public RSA key file %s", file);
 170
 171         /* Fail quietly if file does not exist */
 172         if (stat(file, &st) < 0) {
 173                 /* Restore the privileged uid. */
 174                 restore_uid();
 175                 xfree(file);
 176                 return (0);
 177         }
 178         /* Open the file containing the authorized keys. */
 179         f = fopen(file, "r");
 180         if (!f) {
 181                 /* Restore the privileged uid. */
 182                 restore_uid();
 183                 xfree(file);
 184                 return (0);
 185         }
 186         if (options.strict_modes &&
 187             secure_filename(f, file, pw, line, sizeof(line)) != 0) {
 188                 xfree(file);
 189                 fclose(f);
 190                 logit("Authentication refused: %s", line);
 191                 restore_uid();
 192                 return (0);
 193         }
 194
 195         /* Flag indicating whether the key is allowed. */
 196         allowed = 0;
 197
 198         key = key_new(KEY_RSA1);
 199
 200         /*
 201          * Go though the accepted keys, looking for the current key.  If
 202          * found, perform a challenge-response dialog to verify that the
 203          * user really has the corresponding private key.
 204          */
 205         while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
 206                 char *cp;
 207                 char *key_options;
 208
 209                 /* Skip leading whitespace, empty and comment lines. */
 210                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 211                         ;
 212                 if (!*cp || *cp == '\n' || *cp == '#')
 213                         continue;
 214
 215                 /*
 216                  * Check if there are options for this key, and if so,
 217                  * save their starting address and skip the option part
 218                  * for now.  If there are no options, set the starting
 219                  * address to NULL.
 220                  */
 221                 if (*cp < '0' || *cp > '9') {
 222                         int quoted = 0;
 223                         key_options = cp;
 224                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 225                                 if (*cp == '\\' && cp[1] == '"')
 226                                         cp++;   /* Skip both */
 227                                 else if (*cp == '"')
 228                                         quoted = !quoted;
 229                         }
 230                 } else
 231                         key_options = NULL;
 232
 233                 /* Parse the key from the line. */
 234                 if (hostfile_read_key(&cp, &bits, key) == 0) {
 235                         debug("%.100s, line %lu: non ssh1 key syntax",
 236                             file, linenum);
 237                         continue;
 238                 }
 239                 /* cp now points to the comment part. */
 240
 241                 /* Check if the we have found the desired key (identified by its modulus). */
 242                 if (BN_cmp(key->rsa->n, client_n) != 0)
 243                         continue;
 244
 245                 /* check the real bits  */
 246                 if (bits != BN_num_bits(key->rsa->n))
 247                         logit("Warning: %s, line %lu: keysize mismatch: "
 248                             "actual %d vs. announced %d.",
 249                             file, linenum, BN_num_bits(key->rsa->n), bits);
 250
 251                 /* We have found the desired key. */
 252                 /*
 253                  * If our options do not allow this key to be used,
 254                  * do not send challenge.
 255                  */
 256                 if (!auth_parse_options(pw, key_options, file, linenum))
 257                         continue;
 258
 259                 /* break out, this key is allowed */
 260                 allowed = 1;
 261                 break;
 262         }
 263
 264         /* Restore the privileged uid. */
 265         restore_uid();
 266
 267         /* Close the file. */
 268         xfree(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         return (allowed);
 277 }
 278
 279 /*
 280  * Performs the RSA authentication dialog with the client.  This returns
 281  * 0 if the client could not be authenticated, and 1 if authentication was
 282  * successful.  This may exit if there is a serious protocol violation.
 283  */
 284 int
 285 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
 286 {
 287         Key *key;
 288         char *fp;
 289         struct passwd *pw = authctxt->pw;
 290
 291         /* no user given */
 292         if (!authctxt->valid)
 293                 return 0;
 294
 295         if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
 296                 auth_clear_options();
 297                 return (0);
 298         }
 299
 300         /* Perform the challenge-response dialog for this key. */
 301         if (!auth_rsa_challenge_dialog(key)) {
 302                 /* Wrong response. */
 303                 verbose("Wrong response to RSA authentication challenge.");
 304                 packet_send_debug("Wrong response to RSA authentication challenge.");
 305                 /*
 306                  * Break out of the loop. Otherwise we might send
 307                  * another challenge and break the protocol.
 308                  */
 309                 key_free(key);
 310                 return (0);
 311         }
 312         /*
 313          * Correct response.  The client has been successfully
 314          * authenticated. Note that we have not yet processed the
 315          * options; this will be reset if the options cause the
 316          * authentication to be rejected.
 317          */
 318         fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 319         verbose("Found matching %s key: %s",
 320             key_type(key), fp);
 321         xfree(fp);
 322         key_free(key);
 323
 324         packet_send_debug("RSA authentication accepted.");
 325         return (1);
 326 }