auth-rsa.c

   1 /* $OpenBSD: auth-rsa.c,v 1.69 2006/07/22 20:48:22 stevesk 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 <string.h>
  27
  28 #include "rsa.h"
  29 #include "packet.h"
  30 #include "xmalloc.h"
  31 #include "ssh1.h"
  32 #include "uidswap.h"
  33 #include "match.h"
  34 #include "auth-options.h"
  35 #include "pathnames.h"
  36 #include "log.h"
  37 #include "servconf.h"
  38 #include "auth.h"
  39 #include "hostfile.h"
  40 #include "monitor_wrap.h"
  41 #include "ssh.h"
  42 #include "misc.h"
  43
  44 /* import */
  45 extern ServerOptions options;
  46
  47 /*
  48  * Session identifier that is used to bind key exchange and authentication
  49  * responses to a particular session.
  50  */
  51 extern u_char session_id[16];
  52
  53 /*
  54  * The .ssh/authorized_keys file contains public keys, one per line, in the
  55  * following format:
  56  *   options bits e n comment
  57  * where bits, e and n are decimal numbers,
  58  * and comment is any string of characters up to newline.  The maximum
  59  * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
  60  * description of the options.
  61  */
  62
  63 BIGNUM *
  64 auth_rsa_generate_challenge(Key *key)
  65 {
  66         BIGNUM *challenge;
  67         BN_CTX *ctx;
  68
  69         if ((challenge = BN_new()) == NULL)
  70                 fatal("auth_rsa_generate_challenge: BN_new() failed");
  71         /* Generate a random challenge. */
  72         BN_rand(challenge, 256, 0, 0);
  73         if ((ctx = BN_CTX_new()) == NULL)
  74                 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
  75         BN_mod(challenge, challenge, key->rsa->n, ctx);
  76         BN_CTX_free(ctx);
  77
  78         return challenge;
  79 }
  80
  81 int
  82 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
  83 {
  84         u_char buf[32], mdbuf[16];
  85         MD5_CTX md;
  86         int len;
  87
  88         /* don't allow short keys */
  89         if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
  90                 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
  91                     BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
  92                 return (0);
  93         }
  94
  95         /* The response is MD5 of decrypted challenge plus session id. */
  96         len = BN_num_bytes(challenge);
  97         if (len <= 0 || len > 32)
  98                 fatal("auth_rsa_verify_response: bad challenge length %d", len);
  99         memset(buf, 0, 32);
 100         BN_bn2bin(challenge, buf + 32 - len);
 101         MD5_Init(&md);
 102         MD5_Update(&md, buf, 32);
 103         MD5_Update(&md, session_id, 16);
 104         MD5_Final(mdbuf, &md);
 105
 106         /* Verify that the response is the original challenge. */
 107         if (memcmp(response, mdbuf, 16) != 0) {
 108                 /* Wrong answer. */
 109                 return (0);
 110         }
 111         /* Correct answer. */
 112         return (1);
 113 }
 114
 115 /*
 116  * Performs the RSA authentication challenge-response dialog with the client,
 117  * and returns true (non-zero) if the client gave the correct answer to
 118  * our challenge; returns zero if the client gives a wrong answer.
 119  */
 120
 121 int
 122 auth_rsa_challenge_dialog(Key *key)
 123 {
 124         BIGNUM *challenge, *encrypted_challenge;
 125         u_char response[16];
 126         int i, success;
 127
 128         if ((encrypted_challenge = BN_new()) == NULL)
 129                 fatal("auth_rsa_challenge_dialog: BN_new() failed");
 130
 131         challenge = PRIVSEP(auth_rsa_generate_challenge(key));
 132
 133         /* Encrypt the challenge with the public key. */
 134         rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
 135
 136         /* Send the encrypted challenge to the client. */
 137         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 138         packet_put_bignum(encrypted_challenge);
 139         packet_send();
 140         BN_clear_free(encrypted_challenge);
 141         packet_write_wait();
 142
 143         /* Wait for a response. */
 144         packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
 145         for (i = 0; i < 16; i++)
 146                 response[i] = (u_char)packet_get_char();
 147         packet_check_eom();
 148
 149         success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
 150         BN_clear_free(challenge);
 151         return (success);
 152 }
 153
 154 /*
 155  * check if there's user key matching client_n,
 156  * return key if login is allowed, NULL otherwise
 157  */
 158
 159 int
 160 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
 161 {
 162         char line[SSH_MAX_PUBKEY_BYTES], *file;
 163         int allowed = 0;
 164         u_int bits;
 165         FILE *f;
 166         u_long linenum = 0;
 167         struct stat st;
 168         Key *key;
 169
 170         /* Temporarily use the user's uid. */
 171         temporarily_use_uid(pw);
 172
 173         /* The authorized keys. */
 174         file = authorized_keys_file(pw);
 175         debug("trying public RSA key file %s", file);
 176
 177         /* Fail quietly if file does not exist */
 178         if (stat(file, &st) < 0) {
 179                 /* Restore the privileged uid. */
 180                 restore_uid();
 181                 xfree(file);
 182                 return (0);
 183         }
 184         /* Open the file containing the authorized keys. */
 185         f = fopen(file, "r");
 186         if (!f) {
 187                 /* Restore the privileged uid. */
 188                 restore_uid();
 189                 xfree(file);
 190                 return (0);
 191         }
 192         if (options.strict_modes &&
 193             secure_filename(f, file, pw, line, sizeof(line)) != 0) {
 194                 xfree(file);
 195                 fclose(f);
 196                 logit("Authentication refused: %s", line);
 197                 restore_uid();
 198                 return (0);
 199         }
 200
 201         /* Flag indicating whether the key is allowed. */
 202         allowed = 0;
 203
 204         key = key_new(KEY_RSA1);
 205
 206         /*
 207          * Go though the accepted keys, looking for the current key.  If
 208          * found, perform a challenge-response dialog to verify that the
 209          * user really has the corresponding private key.
 210          */
 211         while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
 212                 char *cp;
 213                 char *key_options;
 214                 int keybits;
 215
 216                 /* Skip leading whitespace, empty and comment lines. */
 217                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 218                         ;
 219                 if (!*cp || *cp == '\n' || *cp == '#')
 220                         continue;
 221
 222                 /*
 223                  * Check if there are options for this key, and if so,
 224                  * save their starting address and skip the option part
 225                  * for now.  If there are no options, set the starting
 226                  * address to NULL.
 227                  */
 228                 if (*cp < '0' || *cp > '9') {
 229                         int quoted = 0;
 230                         key_options = cp;
 231                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 232                                 if (*cp == '\\' && cp[1] == '"')
 233                                         cp++;   /* Skip both */
 234                                 else if (*cp == '"')
 235                                         quoted = !quoted;
 236                         }
 237                 } else
 238                         key_options = NULL;
 239
 240                 /* Parse the key from the line. */
 241                 if (hostfile_read_key(&cp, &bits, key) == 0) {
 242                         debug("%.100s, line %lu: non ssh1 key syntax",
 243                             file, linenum);
 244                         continue;
 245                 }
 246                 /* cp now points to the comment part. */
 247
 248                 /* Check if the we have found the desired key (identified by its modulus). */
 249                 if (BN_cmp(key->rsa->n, client_n) != 0)
 250                         continue;
 251
 252                 /* check the real bits  */
 253                 keybits = BN_num_bits(key->rsa->n);
 254                 if (keybits < 0 || bits != (u_int)keybits)
 255                         logit("Warning: %s, line %lu: keysize mismatch: "
 256                             "actual %d vs. announced %d.",
 257                             file, linenum, BN_num_bits(key->rsa->n), bits);
 258
 259                 /* We have found the desired key. */
 260                 /*
 261                  * If our options do not allow this key to be used,
 262                  * do not send challenge.
 263                  */
 264                 if (!auth_parse_options(pw, key_options, file, linenum))
 265                         continue;
 266
 267                 /* break out, this key is allowed */
 268                 allowed = 1;
 269                 break;
 270         }
 271
 272         /* Restore the privileged uid. */
 273         restore_uid();
 274
 275         /* Close the file. */
 276         xfree(file);
 277         fclose(f);
 278
 279         /* return key if allowed */
 280         if (allowed && rkey != NULL)
 281                 *rkey = key;
 282         else
 283                 key_free(key);
 284         return (allowed);
 285 }
 286
 287 /*
 288  * Performs the RSA authentication dialog with the client.  This returns
 289  * 0 if the client could not be authenticated, and 1 if authentication was
 290  * successful.  This may exit if there is a serious protocol violation.
 291  */
 292 int
 293 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
 294 {
 295         Key *key;
 296         char *fp;
 297         struct passwd *pw = authctxt->pw;
 298
 299         /* no user given */
 300         if (!authctxt->valid)
 301                 return 0;
 302
 303         if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
 304                 auth_clear_options();
 305                 return (0);
 306         }
 307
 308         /* Perform the challenge-response dialog for this key. */
 309         if (!auth_rsa_challenge_dialog(key)) {
 310                 /* Wrong response. */
 311                 verbose("Wrong response to RSA authentication challenge.");
 312                 packet_send_debug("Wrong response to RSA authentication challenge.");
 313                 /*
 314                  * Break out of the loop. Otherwise we might send
 315                  * another challenge and break the protocol.
 316                  */
 317                 key_free(key);
 318                 return (0);
 319         }
 320         /*
 321          * Correct response.  The client has been successfully
 322          * authenticated. Note that we have not yet processed the
 323          * options; this will be reset if the options cause the
 324          * authentication to be rejected.
 325          */
 326         fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 327         verbose("Found matching %s key: %s",
 328             key_type(key), fp);
 329         xfree(fp);
 330         key_free(key);
 331
 332         packet_send_debug("RSA authentication accepted.");
 333         return (1);
 334 }