crypto/openssh/auth-rsa.c

   1 /* $OpenBSD: auth-rsa.c,v 1.88 2014/07/15 15:54:14 millert 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
  24 #include <pwd.h>
  25 #include <stdio.h>
  26 #include <stdarg.h>
  27 #include <string.h>
  28
  29 #include "xmalloc.h"
  30 #include "rsa.h"
  31 #include "packet.h"
  32 #include "ssh1.h"
  33 #include "uidswap.h"
  34 #include "match.h"
  35 #include "buffer.h"
  36 #include "pathnames.h"
  37 #include "log.h"
  38 #include "misc.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
  50 #include "digest.h"
  51
  52 /* import */
  53 extern ServerOptions options;
  54
  55 /*
  56  * Session identifier that is used to bind key exchange and authentication
  57  * responses to a particular session.
  58  */
  59 extern u_char session_id[16];
  60
  61 /*
  62  * The .ssh/authorized_keys file contains public keys, one per line, in the
  63  * following format:
  64  *   options bits e n comment
  65  * where bits, e and n are decimal numbers,
  66  * and comment is any string of characters up to newline.  The maximum
  67  * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
  68  * description of the options.
  69  */
  70
  71 BIGNUM *
  72 auth_rsa_generate_challenge(Key *key)
  73 {
  74         BIGNUM *challenge;
  75         BN_CTX *ctx;
  76
  77         if ((challenge = BN_new()) == NULL)
  78                 fatal("auth_rsa_generate_challenge: BN_new() failed");
  79         /* Generate a random challenge. */
  80         if (BN_rand(challenge, 256, 0, 0) == 0)
  81                 fatal("auth_rsa_generate_challenge: BN_rand failed");
  82         if ((ctx = BN_CTX_new()) == NULL)
  83                 fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
  84         if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
  85                 fatal("auth_rsa_generate_challenge: BN_mod failed");
  86         BN_CTX_free(ctx);
  87
  88         return challenge;
  89 }
  90
  91 int
  92 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
  93 {
  94         u_char buf[32], mdbuf[16];
  95         struct ssh_digest_ctx *md;
  96         int len;
  97
  98         /* don't allow short keys */
  99         if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
 100                 error("%s: RSA modulus too small: %d < minimum %d bits",
 101                     __func__,
 102                     BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
 103                 return (0);
 104         }
 105
 106         /* The response is MD5 of decrypted challenge plus session id. */
 107         len = BN_num_bytes(challenge);
 108         if (len <= 0 || len > 32)
 109                 fatal("%s: bad challenge length %d", __func__, len);
 110         memset(buf, 0, 32);
 111         BN_bn2bin(challenge, buf + 32 - len);
 112         if ((md = ssh_digest_start(SSH_DIGEST_MD5)) == NULL ||
 113             ssh_digest_update(md, buf, 32) < 0 ||
 114             ssh_digest_update(md, session_id, 16) < 0 ||
 115             ssh_digest_final(md, mdbuf, sizeof(mdbuf)) < 0)
 116                 fatal("%s: md5 failed", __func__);
 117         ssh_digest_free(md);
 118
 119         /* Verify that the response is the original challenge. */
 120         if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
 121                 /* Wrong answer. */
 122                 return (0);
 123         }
 124         /* Correct answer. */
 125         return (1);
 126 }
 127
 128 /*
 129  * Performs the RSA authentication challenge-response dialog with the client,
 130  * and returns true (non-zero) if the client gave the correct answer to
 131  * our challenge; returns zero if the client gives a wrong answer.
 132  */
 133
 134 int
 135 auth_rsa_challenge_dialog(Key *key)
 136 {
 137         BIGNUM *challenge, *encrypted_challenge;
 138         u_char response[16];
 139         int i, success;
 140
 141         if ((encrypted_challenge = BN_new()) == NULL)
 142                 fatal("auth_rsa_challenge_dialog: BN_new() failed");
 143
 144         challenge = PRIVSEP(auth_rsa_generate_challenge(key));
 145
 146         /* Encrypt the challenge with the public key. */
 147         if (rsa_public_encrypt(encrypted_challenge, challenge, key->rsa) != 0)
 148                 fatal("%s: rsa_public_encrypt failed", __func__);
 149
 150         /* Send the encrypted challenge to the client. */
 151         packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
 152         packet_put_bignum(encrypted_challenge);
 153         packet_send();
 154         BN_clear_free(encrypted_challenge);
 155         packet_write_wait();
 156
 157         /* Wait for a response. */
 158         packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
 159         for (i = 0; i < 16; i++)
 160                 response[i] = (u_char)packet_get_char();
 161         packet_check_eom();
 162
 163         success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
 164         BN_clear_free(challenge);
 165         return (success);
 166 }
 167
 168 static int
 169 rsa_key_allowed_in_file(struct passwd *pw, char *file,
 170     const BIGNUM *client_n, Key **rkey)
 171 {
 172         char *fp, line[SSH_MAX_PUBKEY_BYTES];
 173         int allowed = 0, bits;
 174         FILE *f;
 175         u_long linenum = 0;
 176         Key *key;
 177
 178         debug("trying public RSA key file %s", file);
 179         if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL)
 180                 return 0;
 181
 182         /*
 183          * Go though the accepted keys, looking for the current key.  If
 184          * found, perform a challenge-response dialog to verify that the
 185          * user really has the corresponding private key.
 186          */
 187         key = key_new(KEY_RSA1);
 188         while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
 189                 char *cp;
 190                 char *key_options;
 191                 int keybits;
 192
 193                 /* Skip leading whitespace, empty and comment lines. */
 194                 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
 195                         ;
 196                 if (!*cp || *cp == '\n' || *cp == '#')
 197                         continue;
 198
 199                 /*
 200                  * Check if there are options for this key, and if so,
 201                  * save their starting address and skip the option part
 202                  * for now.  If there are no options, set the starting
 203                  * address to NULL.
 204                  */
 205                 if (*cp < '0' || *cp > '9') {
 206                         int quoted = 0;
 207                         key_options = cp;
 208                         for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
 209                                 if (*cp == '\\' && cp[1] == '"')
 210                                         cp++;   /* Skip both */
 211                                 else if (*cp == '"')
 212                                         quoted = !quoted;
 213                         }
 214                 } else
 215                         key_options = NULL;
 216
 217                 /* Parse the key from the line. */
 218                 if (hostfile_read_key(&cp, &bits, key) == 0) {
 219                         debug("%.100s, line %lu: non ssh1 key syntax",
 220                             file, linenum);
 221                         continue;
 222                 }
 223                 /* cp now points to the comment part. */
 224
 225                 /*
 226                  * Check if the we have found the desired key (identified
 227                  * by its modulus).
 228                  */
 229                 if (BN_cmp(key->rsa->n, client_n) != 0)
 230                         continue;
 231
 232                 /* check the real bits  */
 233                 keybits = BN_num_bits(key->rsa->n);
 234                 if (keybits < 0 || bits != keybits)
 235                         logit("Warning: %s, line %lu: keysize mismatch: "
 236                             "actual %d vs. announced %d.",
 237                             file, linenum, BN_num_bits(key->rsa->n), bits);
 238
 239                 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 240                 debug("matching key found: file %s, line %lu %s %s",
 241                     file, linenum, key_type(key), fp);
 242                 free(fp);
 243
 244                 /* Never accept a revoked key */
 245                 if (auth_key_is_revoked(key))
 246                         break;
 247
 248                 /* We have found the desired key. */
 249                 /*
 250                  * If our options do not allow this key to be used,
 251                  * do not send challenge.
 252                  */
 253                 if (!auth_parse_options(pw, key_options, file, linenum))
 254                         continue;
 255                 if (key_is_cert_authority)
 256                         continue;
 257                 /* break out, this key is allowed */
 258                 allowed = 1;
 259                 break;
 260         }
 261
 262         /* Close the file. */
 263         fclose(f);
 264
 265         /* return key if allowed */
 266         if (allowed && rkey != NULL)
 267                 *rkey = key;
 268         else
 269                 key_free(key);
 270
 271         return allowed;
 272 }
 273
 274 /*
 275  * check if there's user key matching client_n,
 276  * return key if login is allowed, NULL otherwise
 277  */
 278
 279 int
 280 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
 281 {
 282         char *file;
 283         u_int i, allowed = 0;
 284
 285         temporarily_use_uid(pw);
 286
 287         for (i = 0; !allowed && i < options.num_authkeys_files; i++) {
 288                 if (strcasecmp(options.authorized_keys_files[i], "none") == 0)
 289                         continue;
 290                 file = expand_authorized_keys(
 291                     options.authorized_keys_files[i], pw);
 292                 allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
 293                 free(file);
 294         }
 295
 296         restore_uid();
 297
 298         return allowed;
 299 }
 300
 301 /*
 302  * Performs the RSA authentication dialog with the client.  This returns
 303  * 0 if the client could not be authenticated, and 1 if authentication was
 304  * successful.  This may exit if there is a serious protocol violation.
 305  */
 306 int
 307 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
 308 {
 309         Key *key;
 310         struct passwd *pw = authctxt->pw;
 311
 312         /* no user given */
 313         if (!authctxt->valid)
 314                 return 0;
 315
 316         if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
 317                 auth_clear_options();
 318                 return (0);
 319         }
 320
 321         /* Perform the challenge-response dialog for this key. */
 322         if (!auth_rsa_challenge_dialog(key)) {
 323                 /* Wrong response. */
 324                 verbose("Wrong response to RSA authentication challenge.");
 325                 packet_send_debug("Wrong response to RSA authentication challenge.");
 326                 /*
 327                  * Break out of the loop. Otherwise we might send
 328                  * another challenge and break the protocol.
 329                  */
 330                 key_free(key);
 331                 return (0);
 332         }
 333         /*
 334          * Correct response.  The client has been successfully
 335          * authenticated. Note that we have not yet processed the
 336          * options; this will be reset if the options cause the
 337          * authentication to be rejected.
 338          */
 339         pubkey_auth_info(authctxt, key, NULL);
 340
 341         packet_send_debug("RSA authentication accepted.");
 342         return (1);
 343 }