auth-rsa.c

   1 /* $OpenBSD: auth-rsa.c,v 1.79 2010/12/03 23:55:27 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         /* don't allow short keys */
  98         if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
  99                 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
 100                     BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
 101                 return (0);
 102         }
 103
 104         /* The response is MD5 of decrypted challenge plus session id. */
 105         len = BN_num_bytes(challenge);
 106         if (len <= 0 || len > 32)
 107                 fatal("auth_rsa_verify_response: bad challenge length %d", len);
 108         memset(buf, 0, 32);
 109         BN_bn2bin(challenge, buf + 32 - len);
 110         MD5_Init(&md);
 111         MD5_Update(&md, buf, 32);
 112         MD5_Update(&md, session_id, 16);
 113         MD5_Final(mdbuf, &md);
 114
 115         /* Verify that the response is the original challenge. */
 116         if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
 117                 /* Wrong answer. */
 118                 return (0);
 119         }
 120         /* Correct answer. */
 121         return (1);
 122 }
 123
 124 /*
 125  * Performs the RSA authentication challenge-response dialog with the client,
 126  * and returns true (non-zero) if the client gave the correct answer to
 127  * our challenge; returns zero if the client gives a wrong answer.
 128  */
 129
 130 int
 131 auth_rsa_challenge_dialog(Key *key)
 132 {
 133         BIGNUM *challenge, *encrypted_challenge;
 134         u_char response[16];
 135         int i, success;
 136
 137         if ((encrypted_challenge = BN_new()) == NULL)
 138                 fatal("auth_rsa_challenge_dialog: BN_new() failed");
 139
 140         challenge = PRIVSEP(auth_rsa_generate_challenge(key));
 141
 142         /* Encrypt the challenge with the public key. */
 143         rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
 144
 145         /* Send the encrypted challenge to the client. */
 146         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 147         packet_put_bignum(encrypted_challenge);
 148         packet_send();
 149         BN_clear_free(encrypted_challenge);
 150         packet_write_wait();
 151
 152         /* Wait for a response. */
 153         packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
 154         for (i = 0; i < 16; i++)
 155                 response[i] = (u_char)packet_get_char();
 156         packet_check_eom();
 157
 158         success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
 159         BN_clear_free(challenge);
 160         return (success);
 161 }
 162
 163 /*
 164  * check if there's user key matching client_n,
 165  * return key if login is allowed, NULL otherwise
 166  */
 167
 168 int
 169 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
 170 {
 171         char line[SSH_MAX_PUBKEY_BYTES], *file;
 172         int allowed = 0;
 173         u_int bits;
 174         FILE *f;
 175         u_long linenum = 0;
 176         Key *key;
 177
 178         /* Temporarily use the user's uid. */
 179         temporarily_use_uid(pw);
 180
 181         /* The authorized keys. */
 182         file = authorized_keys_file(pw);
 183         debug("trying public RSA key file %s", file);
 184         f = auth_openkeyfile(file, pw, options.strict_modes);
 185         if (!f) {
 186                 xfree(file);
 187                 restore_uid();
 188                 return (0);
 189         }
 190
 191         /* Flag indicating whether the key is allowed. */
 192         allowed = 0;
 193
 194         key = key_new(KEY_RSA1);
 195
 196         /*
 197          * Go though the accepted keys, looking for the current key.  If
 198          * found, perform a challenge-response dialog to verify that the
 199          * user really has the corresponding private key.
 200          */
 201         while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
 202                 char *cp;
 203                 char *key_options;
 204                 int keybits;
 205
 206                 /* Skip leading whitespace, empty and comment lines. */
 207                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 208                         ;
 209                 if (!*cp || *cp == '\n' || *cp == '#')
 210                         continue;
 211
 212                 /*
 213                  * Check if there are options for this key, and if so,
 214                  * save their starting address and skip the option part
 215                  * for now.  If there are no options, set the starting
 216                  * address to NULL.
 217                  */
 218                 if (*cp < '0' || *cp > '9') {
 219                         int quoted = 0;
 220                         key_options = cp;
 221                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 222                                 if (*cp == '\\' && cp[1] == '"')
 223                                         cp++;   /* Skip both */
 224                                 else if (*cp == '"')
 225                                         quoted = !quoted;
 226                         }
 227                 } else
 228                         key_options = NULL;
 229
 230                 /* Parse the key from the line. */
 231                 if (hostfile_read_key(&cp, &bits, key) == 0) {
 232                         debug("%.100s, line %lu: non ssh1 key syntax",
 233                             file, linenum);
 234                         continue;
 235                 }
 236                 /* cp now points to the comment part. */
 237
 238                 /* Check if the we have found the desired key (identified by its modulus). */
 239                 if (BN_cmp(key->rsa->n, client_n) != 0)
 240                         continue;
 241
 242                 /* check the real bits  */
 243                 keybits = BN_num_bits(key->rsa->n);
 244                 if (keybits < 0 || bits != (u_int)keybits)
 245                         logit("Warning: %s, line %lu: keysize mismatch: "
 246                             "actual %d vs. announced %d.",
 247                             file, linenum, BN_num_bits(key->rsa->n), bits);
 248
 249                 /* Never accept a revoked key */
 250                 if (auth_key_is_revoked(key))
 251                         break;
 252
 253                 /* We have found the desired key. */
 254                 /*
 255                  * If our options do not allow this key to be used,
 256                  * do not send challenge.
 257                  */
 258                 if (!auth_parse_options(pw, key_options, file, linenum))
 259                         continue;
 260                 if (key_is_cert_authority)
 261                         continue;
 262                 /* break out, this key is allowed */
 263                 allowed = 1;
 264                 break;
 265         }
 266
 267         /* Restore the privileged uid. */
 268         restore_uid();
 269
 270         /* Close the file. */
 271         xfree(file);
 272         fclose(f);
 273
 274         /* return key if allowed */
 275         if (allowed && rkey != NULL)
 276                 *rkey = key;
 277         else
 278                 key_free(key);
 279         return (allowed);
 280 }
 281
 282 /*
 283  * Performs the RSA authentication dialog with the client.  This returns
 284  * 0 if the client could not be authenticated, and 1 if authentication was
 285  * successful.  This may exit if there is a serious protocol violation.
 286  */
 287 int
 288 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
 289 {
 290         Key *key;
 291         char *fp;
 292         struct passwd *pw = authctxt->pw;
 293
 294         /* no user given */
 295         if (!authctxt->valid)
 296                 return 0;
 297
 298         if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
 299                 auth_clear_options();
 300                 return (0);
 301         }
 302
 303         /* Perform the challenge-response dialog for this key. */
 304         if (!auth_rsa_challenge_dialog(key)) {
 305                 /* Wrong response. */
 306                 verbose("Wrong response to RSA authentication challenge.");
 307                 packet_send_debug("Wrong response to RSA authentication challenge.");
 308                 /*
 309                  * Break out of the loop. Otherwise we might send
 310                  * another challenge and break the protocol.
 311                  */
 312                 key_free(key);
 313                 return (0);
 314         }
 315         /*
 316          * Correct response.  The client has been successfully
 317          * authenticated. Note that we have not yet processed the
 318          * options; this will be reset if the options cause the
 319          * authentication to be rejected.
 320          */
 321         fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 322         verbose("Found matching %s key: %s",
 323             key_type(key), fp);
 324         xfree(fp);
 325         key_free(key);
 326
 327         packet_send_debug("RSA authentication accepted.");
 328         return (1);
 329 }