auth-rsa.c

   1 /* $OpenBSD: auth-rsa.c,v 1.78 2010/07/13 23:13:16 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 "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         if (BN_rand(challenge, 256, 0, 0) == 0)
  80                 fatal("auth_rsa_generate_challenge: BN_rand failed");
  81         if ((ctx = BN_CTX_new()) == NULL)
  82                 fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
  83         if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
  84                 fatal("auth_rsa_generate_challenge: BN_mod failed");
  85         BN_CTX_free(ctx);
  86
  87         return challenge;
  88 }
  89
  90 int
  91 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
  92 {
  93         u_char buf[32], mdbuf[16];
  94         MD5_CTX md;
  95         int len;
  96
  97         if (auth_key_is_revoked(key))
  98                 return 0;
  99
 100         /* don't allow short keys */
 101         if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
 102                 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
 103                     BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
 104                 return (0);
 105         }
 106
 107         /* The response is MD5 of decrypted challenge plus session id. */
 108         len = BN_num_bytes(challenge);
 109         if (len <= 0 || len > 32)
 110                 fatal("auth_rsa_verify_response: bad challenge length %d", len);
 111         memset(buf, 0, 32);
 112         BN_bn2bin(challenge, buf + 32 - len);
 113         MD5_Init(&md);
 114         MD5_Update(&md, buf, 32);
 115         MD5_Update(&md, session_id, 16);
 116         MD5_Final(mdbuf, &md);
 117
 118         /* Verify that the response is the original challenge. */
 119         if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
 120                 /* Wrong answer. */
 121                 return (0);
 122         }
 123         /* Correct answer. */
 124         return (1);
 125 }
 126
 127 /*
 128  * Performs the RSA authentication challenge-response dialog with the client,
 129  * and returns true (non-zero) if the client gave the correct answer to
 130  * our challenge; returns zero if the client gives a wrong answer.
 131  */
 132
 133 int
 134 auth_rsa_challenge_dialog(Key *key)
 135 {
 136         BIGNUM *challenge, *encrypted_challenge;
 137         u_char response[16];
 138         int i, success;
 139
 140         if ((encrypted_challenge = BN_new()) == NULL)
 141                 fatal("auth_rsa_challenge_dialog: BN_new() failed");
 142
 143         challenge = PRIVSEP(auth_rsa_generate_challenge(key));
 144
 145         /* Encrypt the challenge with the public key. */
 146         rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
 147
 148         /* Send the encrypted challenge to the client. */
 149         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 150         packet_put_bignum(encrypted_challenge);
 151         packet_send();
 152         BN_clear_free(encrypted_challenge);
 153         packet_write_wait();
 154
 155         /* Wait for a response. */
 156         packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
 157         for (i = 0; i < 16; i++)
 158                 response[i] = (u_char)packet_get_char();
 159         packet_check_eom();
 160
 161         success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
 162         BN_clear_free(challenge);
 163         return (success);
 164 }
 165
 166 /*
 167  * check if there's user key matching client_n,
 168  * return key if login is allowed, NULL otherwise
 169  */
 170
 171 int
 172 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
 173 {
 174         char line[SSH_MAX_PUBKEY_BYTES], *file;
 175         int allowed = 0;
 176         u_int bits;
 177         FILE *f;
 178         u_long linenum = 0;
 179         Key *key;
 180
 181         /* Temporarily use the user's uid. */
 182         temporarily_use_uid(pw);
 183
 184         /* The authorized keys. */
 185         file = authorized_keys_file(pw);
 186         debug("trying public RSA key file %s", file);
 187         f = auth_openkeyfile(file, pw, options.strict_modes);
 188         if (!f) {
 189                 xfree(file);
 190                 restore_uid();
 191                 return (0);
 192         }
 193
 194         /* Flag indicating whether the key is allowed. */
 195         allowed = 0;
 196
 197         key = key_new(KEY_RSA1);
 198
 199         /*
 200          * Go though the accepted keys, looking for the current key.  If
 201          * found, perform a challenge-response dialog to verify that the
 202          * user really has the corresponding private key.
 203          */
 204         while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
 205                 char *cp;
 206                 char *key_options;
 207                 int keybits;
 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                 keybits = BN_num_bits(key->rsa->n);
 247                 if (keybits < 0 || bits != (u_int)keybits)
 248                         logit("Warning: %s, line %lu: keysize mismatch: "
 249                             "actual %d vs. announced %d.",
 250                             file, linenum, BN_num_bits(key->rsa->n), bits);
 251
 252                 /* We have found the desired key. */
 253                 /*
 254                  * If our options do not allow this key to be used,
 255                  * do not send challenge.
 256                  */
 257                 if (!auth_parse_options(pw, key_options, file, linenum))
 258                         continue;
 259                 if (key_is_cert_authority)
 260                         continue;
 261                 /* break out, this key is allowed */
 262                 allowed = 1;
 263                 break;
 264         }
 265
 266         /* Restore the privileged uid. */
 267         restore_uid();
 268
 269         /* Close the file. */
 270         xfree(file);
 271         fclose(f);
 272
 273         /* return key if allowed */
 274         if (allowed && rkey != NULL)
 275                 *rkey = key;
 276         else
 277                 key_free(key);
 278         return (allowed);
 279 }
 280
 281 /*
 282  * Performs the RSA authentication dialog with the client.  This returns
 283  * 0 if the client could not be authenticated, and 1 if authentication was
 284  * successful.  This may exit if there is a serious protocol violation.
 285  */
 286 int
 287 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
 288 {
 289         Key *key;
 290         char *fp;
 291         struct passwd *pw = authctxt->pw;
 292
 293         /* no user given */
 294         if (!authctxt->valid)
 295                 return 0;
 296
 297         if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
 298                 auth_clear_options();
 299                 return (0);
 300         }
 301
 302         /* Perform the challenge-response dialog for this key. */
 303         if (!auth_rsa_challenge_dialog(key)) {
 304                 /* Wrong response. */
 305                 verbose("Wrong response to RSA authentication challenge.");
 306                 packet_send_debug("Wrong response to RSA authentication challenge.");
 307                 /*
 308                  * Break out of the loop. Otherwise we might send
 309                  * another challenge and break the protocol.
 310                  */
 311                 key_free(key);
 312                 return (0);
 313         }
 314         /*
 315          * Correct response.  The client has been successfully
 316          * authenticated. Note that we have not yet processed the
 317          * options; this will be reset if the options cause the
 318          * authentication to be rejected.
 319          */
 320         fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 321         verbose("Found matching %s key: %s",
 322             key_type(key), fp);
 323         xfree(fp);
 324         key_free(key);
 325
 326         packet_send_debug("RSA authentication accepted.");
 327         return (1);
 328 }