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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"
Darren Tuckere14e0052004-05-13 16:30:44 +100017RCSID("$OpenBSD: auth-rsa.c,v 1.59 2004/05/09 01:19:27 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"
Damien Miller874d77b2000-10-14 16:23:11 +110036
37/* import */
38extern ServerOptions options;
39
Damien Miller5428f641999-11-25 11:54:57 +110040/*
41 * Session identifier that is used to bind key exchange and authentication
42 * responses to a particular session.
43 */
Ben Lindstrom46c16222000-12-22 01:43:59 +000044extern u_char session_id[16];
Damien Millerd4a8b7e1999-10-27 13:42:43 +100045
Damien Miller5428f641999-11-25 11:54:57 +110046/*
47 * The .ssh/authorized_keys file contains public keys, one per line, in the
48 * following format:
49 * options bits e n comment
50 * where bits, e and n are decimal numbers,
51 * and comment is any string of characters up to newline. The maximum
52 * length of a line is 8000 characters. See the documentation for a
53 * description of the options.
54 */
Damien Millerd4a8b7e1999-10-27 13:42:43 +100055
Ben Lindstrom7a2073c2002-03-22 02:30:41 +000056BIGNUM *
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +000057auth_rsa_generate_challenge(Key *key)
58{
59 BIGNUM *challenge;
60 BN_CTX *ctx;
61
62 if ((challenge = BN_new()) == NULL)
63 fatal("auth_rsa_generate_challenge: BN_new() failed");
64 /* Generate a random challenge. */
65 BN_rand(challenge, 256, 0, 0);
66 if ((ctx = BN_CTX_new()) == NULL)
67 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
68 BN_mod(challenge, challenge, key->rsa->n, ctx);
69 BN_CTX_free(ctx);
70
71 return challenge;
72}
73
Ben Lindstrom7a2073c2002-03-22 02:30:41 +000074int
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +000075auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
76{
77 u_char buf[32], mdbuf[16];
78 MD5_CTX md;
79 int len;
80
Ben Lindstrome1f9e322002-03-27 17:38:43 +000081 /* don't allow short keys */
Ben Lindstrom03f39322002-04-02 20:43:11 +000082 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
Ben Lindstrom2779d282002-06-11 15:47:42 +000083 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
84 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
Ben Lindstrome1f9e322002-03-27 17:38:43 +000085 return (0);
86 }
87
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +000088 /* The response is MD5 of decrypted challenge plus session id. */
89 len = BN_num_bytes(challenge);
90 if (len <= 0 || len > 32)
91 fatal("auth_rsa_verify_response: bad challenge length %d", len);
92 memset(buf, 0, 32);
93 BN_bn2bin(challenge, buf + 32 - len);
94 MD5_Init(&md);
95 MD5_Update(&md, buf, 32);
96 MD5_Update(&md, session_id, 16);
97 MD5_Final(mdbuf, &md);
98
99 /* Verify that the response is the original challenge. */
100 if (memcmp(response, mdbuf, 16) != 0) {
101 /* Wrong answer. */
102 return (0);
103 }
104 /* Correct answer. */
105 return (1);
106}
107
Damien Miller5428f641999-11-25 11:54:57 +1100108/*
109 * Performs the RSA authentication challenge-response dialog with the client,
110 * and returns true (non-zero) if the client gave the correct answer to
111 * our challenge; returns zero if the client gives a wrong answer.
112 */
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000113
114int
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000115auth_rsa_challenge_dialog(Key *key)
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000116{
Damien Miller98c7ad62000-03-09 21:27:49 +1100117 BIGNUM *challenge, *encrypted_challenge;
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000118 u_char response[16];
119 int i, success;
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000120
Damien Millerda755162002-01-22 23:09:22 +1100121 if ((encrypted_challenge = BN_new()) == NULL)
122 fatal("auth_rsa_challenge_dialog: BN_new() failed");
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000123
Ben Lindstrom7a2073c2002-03-22 02:30:41 +0000124 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000125
Damien Miller95def091999-11-25 00:26:21 +1100126 /* Encrypt the challenge with the public key. */
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000127 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000128
Damien Miller95def091999-11-25 00:26:21 +1100129 /* Send the encrypted challenge to the client. */
130 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
131 packet_put_bignum(encrypted_challenge);
132 packet_send();
Damien Miller98c7ad62000-03-09 21:27:49 +1100133 BN_clear_free(encrypted_challenge);
Damien Miller95def091999-11-25 00:26:21 +1100134 packet_write_wait();
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000135
Damien Miller98c7ad62000-03-09 21:27:49 +1100136 /* Wait for a response. */
Damien Millerdff50992002-01-22 23:16:32 +1100137 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
Damien Miller98c7ad62000-03-09 21:27:49 +1100138 for (i = 0; i < 16; i++)
139 response[i] = packet_get_char();
Damien Miller48b03fc2002-01-22 23:11:40 +1100140 packet_check_eom();
Damien Miller98c7ad62000-03-09 21:27:49 +1100141
Ben Lindstrom7a2073c2002-03-22 02:30:41 +0000142 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
Damien Miller95def091999-11-25 00:26:21 +1100143 BN_clear_free(challenge);
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000144 return (success);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000145}
146
Damien Miller5428f641999-11-25 11:54:57 +1100147/*
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000148 * check if there's user key matching client_n,
149 * return key if login is allowed, NULL otherwise
Damien Miller5428f641999-11-25 11:54:57 +1100150 */
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000151
Ben Lindstrom7a2073c2002-03-22 02:30:41 +0000152int
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000153auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000154{
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000155 char line[8192], *file;
Ben Lindstrom7a2073c2002-03-22 02:30:41 +0000156 int allowed = 0;
Ben Lindstrom46c16222000-12-22 01:43:59 +0000157 u_int bits;
Damien Miller95def091999-11-25 00:26:21 +1100158 FILE *f;
Ben Lindstrom46c16222000-12-22 01:43:59 +0000159 u_long linenum = 0;
Damien Miller95def091999-11-25 00:26:21 +1100160 struct stat st;
Damien Miller89681212001-12-21 12:52:39 +1100161 Key *key;
Damien Miller874d77b2000-10-14 16:23:11 +1100162
Damien Miller95def091999-11-25 00:26:21 +1100163 /* Temporarily use the user's uid. */
Ben Lindstrom3fcf1a22001-04-08 18:26:59 +0000164 temporarily_use_uid(pw);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000165
Damien Miller95def091999-11-25 00:26:21 +1100166 /* The authorized keys. */
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000167 file = authorized_keys_file(pw);
168 debug("trying public RSA key file %s", file);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000169
Damien Miller95def091999-11-25 00:26:21 +1100170 /* Fail quietly if file does not exist */
171 if (stat(file, &st) < 0) {
172 /* Restore the privileged uid. */
173 restore_uid();
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000174 xfree(file);
Ben Lindstromf6d367b2002-03-26 02:59:31 +0000175 return (0);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000176 }
Damien Miller95def091999-11-25 00:26:21 +1100177 /* Open the file containing the authorized keys. */
178 f = fopen(file, "r");
179 if (!f) {
180 /* Restore the privileged uid. */
181 restore_uid();
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000182 xfree(file);
Ben Lindstromf6d367b2002-03-26 02:59:31 +0000183 return (0);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000184 }
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000185 if (options.strict_modes &&
Ben Lindstrom248c0782001-07-04 03:40:39 +0000186 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000187 xfree(file);
188 fclose(f);
Damien Miller996acd22003-04-09 20:59:48 +1000189 logit("Authentication refused: %s", line);
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000190 restore_uid();
Ben Lindstromf6d367b2002-03-26 02:59:31 +0000191 return (0);
Damien Miller95def091999-11-25 00:26:21 +1100192 }
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000193
194 /* Flag indicating whether the key is allowed. */
195 allowed = 0;
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000196
Damien Miller89681212001-12-21 12:52:39 +1100197 key = key_new(KEY_RSA1);
Damien Miller95def091999-11-25 00:26:21 +1100198
Damien Miller5428f641999-11-25 11:54:57 +1100199 /*
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 */
Damien Miller95def091999-11-25 00:26:21 +1100204 while (fgets(line, sizeof(line), f)) {
205 char *cp;
206 char *options;
207
208 linenum++;
209
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;
224 options = cp;
225 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
232 options = NULL;
233
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 Miller89681212001-12-21 12:52:39 +1100247 if (bits != BN_num_bits(key->rsa->n))
Damien Miller996acd22003-04-09 20:59:48 +1000248 logit("Warning: %s, line %lu: keysize mismatch: "
Damien Milleraae6c611999-12-06 11:47:28 +1100249 "actual %d vs. announced %d.",
Damien Miller89681212001-12-21 12:52:39 +1100250 file, linenum, BN_num_bits(key->rsa->n), bits);
Damien Milleraae6c611999-12-06 11:47:28 +1100251
Damien Miller95def091999-11-25 00:26:21 +1100252 /* We have found the desired key. */
Ben Lindstrom14920292000-11-21 21:24:55 +0000253 /*
254 * If our options do not allow this key to be used,
255 * do not send challenge.
256 */
Ben Lindstrom226cfa02001-01-22 05:34:40 +0000257 if (!auth_parse_options(pw, options, file, linenum))
Ben Lindstrom14920292000-11-21 21:24:55 +0000258 continue;
Damien Miller95def091999-11-25 00:26:21 +1100259
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000260 /* break out, this key is allowed */
261 allowed = 1;
Damien Miller50a41ed2000-10-16 12:14:42 +1100262 break;
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000263 }
264
Damien Miller95def091999-11-25 00:26:21 +1100265 /* Restore the privileged uid. */
266 restore_uid();
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000267
Damien Miller95def091999-11-25 00:26:21 +1100268 /* Close the file. */
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000269 xfree(file);
Damien Miller95def091999-11-25 00:26:21 +1100270 fclose(f);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000271
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000272 /* return key if allowed */
273 if (allowed && rkey != NULL)
274 *rkey = key;
275 else
276 key_free(key);
277 return (allowed);
278}
279
280/*
281 * Performs the RSA authentication dialog with the client. This returns
282 * 0 if the client could not be authenticated, and 1 if authentication was
283 * successful. This may exit if there is a serious protocol violation.
284 */
285int
Damien Miller3e3b5142003-11-17 21:13:40 +1100286auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000287{
288 Key *key;
289 char *fp;
Damien Miller3e3b5142003-11-17 21:13:40 +1100290 struct passwd *pw = authctxt->pw;
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000291
292 /* no user given */
Damien Miller3e3b5142003-11-17 21:13:40 +1100293 if (!authctxt->valid)
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000294 return 0;
295
Ben Lindstrom7a2073c2002-03-22 02:30:41 +0000296 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000297 auth_clear_options();
298 return (0);
299 }
300
301 /* Perform the challenge-response dialog for this key. */
302 if (!auth_rsa_challenge_dialog(key)) {
303 /* Wrong response. */
304 verbose("Wrong response to RSA authentication challenge.");
305 packet_send_debug("Wrong response to RSA authentication challenge.");
306 /*
307 * Break out of the loop. Otherwise we might send
308 * another challenge and break the protocol.
309 */
310 key_free(key);
311 return (0);
312 }
313 /*
314 * Correct response. The client has been successfully
315 * authenticated. Note that we have not yet processed the
316 * options; this will be reset if the options cause the
317 * authentication to be rejected.
318 */
319 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
320 verbose("Found matching %s key: %s",
321 key_type(key), fp);
322 xfree(fp);
Damien Miller89681212001-12-21 12:52:39 +1100323 key_free(key);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000324
Ben Lindstrom9c8aefe2002-03-22 01:12:58 +0000325 packet_send_debug("RSA authentication accepted.");
326 return (1);
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000327}