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Damien Millerd4a8b7e1999-10-27 13:42:43 +10001/*
Damien Miller95def091999-11-25 00:26:21 +11002 * Author: Tatu Ylonen <ylo@cs.hut.fi>
Damien Miller95def091999-11-25 00:26:21 +11003 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 * All rights reserved
Damien Miller95def091999-11-25 00:26:21 +11005 * 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.
Damien Miller4af51302000-04-16 11:18:38 +10008 *
Damien Millere4340be2000-09-16 13:29:08 +11009 * 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".
Damien Miller95def091999-11-25 00:26:21 +110014 */
Damien Millerd4a8b7e1999-10-27 13:42:43 +100015
16#include "includes.h"
Damien Millereccb9de2005-06-17 12:59:34 +100017RCSID("$OpenBSD: auth-rsa.c,v 1.63 2005/06/17 02:44:32 djm Exp $");
Damien Millerd4a8b7e1999-10-27 13:42:43 +100018
19#include <openssl/rsa.h>
20#include <openssl/md5.h>
Damien Millerd4a8b7e1999-10-27 13:42:43 +100021
Ben Lindstrom226cfa02001-01-22 05:34:40 +000022#include "rsa.h"
23#include "packet.h"
24#include "xmalloc.h"
25#include "ssh1.h"
Ben Lindstrom226cfa02001-01-22 05:34:40 +000026#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"
Damien Miller89681212001-12-21 12:52:39 +110033#include "hostfile.h"
Ben Lindstrom7a2073c2002-03-22 02:30:41 +000034#include "monitor_wrap.h"
Ben Lindstrom03f39322002-04-02 20:43:11 +000035#include "ssh.h"
Darren Tuckerf0f90982004-12-11 13:39:50 +110036#include "misc.h"
Damien Miller874d77b2000-10-14 16:23:11 +110037
38/* import */
39extern ServerOptions options;
40
Damien Miller5428f641999-11-25 11:54:57 +110041/*
42 * Session identifier that is used to bind key exchange and authentication
43 * responses to a particular session.
44 */
Ben Lindstrom46c16222000-12-22 01:43:59 +000045extern u_char session_id[16];
Damien Millerd4a8b7e1999-10-27 13:42:43 +100046
Damien Miller5428f641999-11-25 11:54:57 +110047/*
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
Darren Tucker22cc7412004-12-06 22:47:41 +110053 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
Damien Miller5428f641999-11-25 11:54:57 +110054 * description of the options.
55 */
Damien Millerd4a8b7e1999-10-27 13:42:43 +100056
Ben Lindstrom7a2073c2002-03-22 02:30:41 +000057BIGNUM *
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +000058auth_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
Ben Lindstrom7a2073c2002-03-22 02:30:41 +000075int
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +000076auth_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
Ben Lindstrome1f9e322002-03-27 17:38:43 +000082 /* don't allow short keys */
Ben Lindstrom03f39322002-04-02 20:43:11 +000083 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
Ben Lindstrom2779d282002-06-11 15:47:42 +000084 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);
Ben Lindstrome1f9e322002-03-27 17:38:43 +000086 return (0);
87 }
88
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +000089 /* 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
Damien Miller5428f641999-11-25 11:54:57 +1100109/*
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 */
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000114
115int
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000116auth_rsa_challenge_dialog(Key *key)
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000117{
Damien Miller98c7ad62000-03-09 21:27:49 +1100118 BIGNUM *challenge, *encrypted_challenge;
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000119 u_char response[16];
120 int i, success;
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000121
Damien Millerda755162002-01-22 23:09:22 +1100122 if ((encrypted_challenge = BN_new()) == NULL)
123 fatal("auth_rsa_challenge_dialog: BN_new() failed");
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000124
Ben Lindstrom7a2073c2002-03-22 02:30:41 +0000125 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000126
Damien Miller95def091999-11-25 00:26:21 +1100127 /* Encrypt the challenge with the public key. */
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000128 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000129
Damien Miller95def091999-11-25 00:26:21 +1100130 /* Send the encrypted challenge to the client. */
131 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
132 packet_put_bignum(encrypted_challenge);
133 packet_send();
Damien Miller98c7ad62000-03-09 21:27:49 +1100134 BN_clear_free(encrypted_challenge);
Damien Miller95def091999-11-25 00:26:21 +1100135 packet_write_wait();
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000136
Damien Miller98c7ad62000-03-09 21:27:49 +1100137 /* Wait for a response. */
Damien Millerdff50992002-01-22 23:16:32 +1100138 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
Damien Miller98c7ad62000-03-09 21:27:49 +1100139 for (i = 0; i < 16; i++)
140 response[i] = packet_get_char();
Damien Miller48b03fc2002-01-22 23:11:40 +1100141 packet_check_eom();
Damien Miller98c7ad62000-03-09 21:27:49 +1100142
Ben Lindstrom7a2073c2002-03-22 02:30:41 +0000143 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
Damien Miller95def091999-11-25 00:26:21 +1100144 BN_clear_free(challenge);
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000145 return (success);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000146}
147
Damien Miller5428f641999-11-25 11:54:57 +1100148/*
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000149 * check if there's user key matching client_n,
150 * return key if login is allowed, NULL otherwise
Damien Miller5428f641999-11-25 11:54:57 +1100151 */
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000152
Ben Lindstrom7a2073c2002-03-22 02:30:41 +0000153int
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000154auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000155{
Darren Tucker22cc7412004-12-06 22:47:41 +1100156 char line[SSH_MAX_PUBKEY_BYTES], *file;
Ben Lindstrom7a2073c2002-03-22 02:30:41 +0000157 int allowed = 0;
Ben Lindstrom46c16222000-12-22 01:43:59 +0000158 u_int bits;
Damien Miller95def091999-11-25 00:26:21 +1100159 FILE *f;
Ben Lindstrom46c16222000-12-22 01:43:59 +0000160 u_long linenum = 0;
Damien Miller95def091999-11-25 00:26:21 +1100161 struct stat st;
Damien Miller89681212001-12-21 12:52:39 +1100162 Key *key;
Damien Miller874d77b2000-10-14 16:23:11 +1100163
Damien Miller95def091999-11-25 00:26:21 +1100164 /* Temporarily use the user's uid. */
Ben Lindstrom3fcf1a22001-04-08 18:26:59 +0000165 temporarily_use_uid(pw);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000166
Damien Miller95def091999-11-25 00:26:21 +1100167 /* The authorized keys. */
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000168 file = authorized_keys_file(pw);
169 debug("trying public RSA key file %s", file);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000170
Damien Miller95def091999-11-25 00:26:21 +1100171 /* Fail quietly if file does not exist */
172 if (stat(file, &st) < 0) {
173 /* Restore the privileged uid. */
174 restore_uid();
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000175 xfree(file);
Ben Lindstromf6d367b2002-03-26 02:59:31 +0000176 return (0);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000177 }
Damien Miller95def091999-11-25 00:26:21 +1100178 /* Open the file containing the authorized keys. */
179 f = fopen(file, "r");
180 if (!f) {
181 /* Restore the privileged uid. */
182 restore_uid();
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000183 xfree(file);
Ben Lindstromf6d367b2002-03-26 02:59:31 +0000184 return (0);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000185 }
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000186 if (options.strict_modes &&
Ben Lindstrom248c0782001-07-04 03:40:39 +0000187 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000188 xfree(file);
189 fclose(f);
Damien Miller996acd22003-04-09 20:59:48 +1000190 logit("Authentication refused: %s", line);
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000191 restore_uid();
Ben Lindstromf6d367b2002-03-26 02:59:31 +0000192 return (0);
Damien Miller95def091999-11-25 00:26:21 +1100193 }
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000194
195 /* Flag indicating whether the key is allowed. */
196 allowed = 0;
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000197
Damien Miller89681212001-12-21 12:52:39 +1100198 key = key_new(KEY_RSA1);
Damien Miller95def091999-11-25 00:26:21 +1100199
Damien Miller5428f641999-11-25 11:54:57 +1100200 /*
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 */
Darren Tucker22cc7412004-12-06 22:47:41 +1100205 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
Damien Miller95def091999-11-25 00:26:21 +1100206 char *cp;
Darren Tucker3f9fdc72004-06-22 12:56:01 +1000207 char *key_options;
Damien Millereccb9de2005-06-17 12:59:34 +1000208 int keybits;
Damien Miller95def091999-11-25 00:26:21 +1100209
Damien Miller5428f641999-11-25 11:54:57 +1100210 /* Skip leading whitespace, empty and comment lines. */
211 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
212 ;
Damien Miller95def091999-11-25 00:26:21 +1100213 if (!*cp || *cp == '\n' || *cp == '#')
214 continue;
215
Damien Miller5428f641999-11-25 11:54:57 +1100216 /*
217 * Check if there are options for this key, and if so,
218 * save their starting address and skip the option part
219 * for now. If there are no options, set the starting
220 * address to NULL.
221 */
Damien Miller95def091999-11-25 00:26:21 +1100222 if (*cp < '0' || *cp > '9') {
223 int quoted = 0;
Darren Tucker3f9fdc72004-06-22 12:56:01 +1000224 key_options = cp;
Damien Miller95def091999-11-25 00:26:21 +1100225 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
226 if (*cp == '\\' && cp[1] == '"')
227 cp++; /* Skip both */
228 else if (*cp == '"')
229 quoted = !quoted;
230 }
231 } else
Darren Tucker3f9fdc72004-06-22 12:56:01 +1000232 key_options = NULL;
Damien Miller95def091999-11-25 00:26:21 +1100233
234 /* Parse the key from the line. */
Damien Miller89681212001-12-21 12:52:39 +1100235 if (hostfile_read_key(&cp, &bits, key) == 0) {
Ben Lindstromf96704d2001-06-25 04:17:12 +0000236 debug("%.100s, line %lu: non ssh1 key syntax",
Ben Lindstrom226cfa02001-01-22 05:34:40 +0000237 file, linenum);
Damien Miller95def091999-11-25 00:26:21 +1100238 continue;
239 }
240 /* cp now points to the comment part. */
241
Damien Miller95def091999-11-25 00:26:21 +1100242 /* Check if the we have found the desired key (identified by its modulus). */
Damien Miller89681212001-12-21 12:52:39 +1100243 if (BN_cmp(key->rsa->n, client_n) != 0)
Damien Miller5428f641999-11-25 11:54:57 +1100244 continue;
Damien Miller95def091999-11-25 00:26:21 +1100245
Damien Milleraae6c611999-12-06 11:47:28 +1100246 /* check the real bits */
Damien Millereccb9de2005-06-17 12:59:34 +1000247 keybits = BN_num_bits(key->rsa->n);
248 if (keybits < 0 || bits != (u_int)keybits)
Damien Miller996acd22003-04-09 20:59:48 +1000249 logit("Warning: %s, line %lu: keysize mismatch: "
Damien Milleraae6c611999-12-06 11:47:28 +1100250 "actual %d vs. announced %d.",
Damien Miller89681212001-12-21 12:52:39 +1100251 file, linenum, BN_num_bits(key->rsa->n), bits);
Damien Milleraae6c611999-12-06 11:47:28 +1100252
Damien Miller95def091999-11-25 00:26:21 +1100253 /* We have found the desired key. */
Ben Lindstrom14920292000-11-21 21:24:55 +0000254 /*
255 * If our options do not allow this key to be used,
256 * do not send challenge.
257 */
Darren Tucker3f9fdc72004-06-22 12:56:01 +1000258 if (!auth_parse_options(pw, key_options, file, linenum))
Ben Lindstrom14920292000-11-21 21:24:55 +0000259 continue;
Damien Miller95def091999-11-25 00:26:21 +1100260
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000261 /* break out, this key is allowed */
262 allowed = 1;
Damien Miller50a41ed2000-10-16 12:14:42 +1100263 break;
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000264 }
265
Damien Miller95def091999-11-25 00:26:21 +1100266 /* Restore the privileged uid. */
267 restore_uid();
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000268
Damien Miller95def091999-11-25 00:26:21 +1100269 /* Close the file. */
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000270 xfree(file);
Damien Miller95def091999-11-25 00:26:21 +1100271 fclose(f);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000272
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000273 /* 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 */
286int
Damien Miller3e3b5142003-11-17 21:13:40 +1100287auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000288{
289 Key *key;
290 char *fp;
Damien Miller3e3b5142003-11-17 21:13:40 +1100291 struct passwd *pw = authctxt->pw;
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000292
293 /* no user given */
Damien Miller3e3b5142003-11-17 21:13:40 +1100294 if (!authctxt->valid)
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000295 return 0;
296
Ben Lindstrom7a2073c2002-03-22 02:30:41 +0000297 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000298 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);
Damien Miller89681212001-12-21 12:52:39 +1100324 key_free(key);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000325
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000326 packet_send_debug("RSA authentication accepted.");
327 return (1);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000328}