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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"
Ben Lindstrom03f39322002-04-02 20:43:11 +000017RCSID("$OpenBSD: auth-rsa.c,v 1.55 2002/03/29 19:18:33 stevesk 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"
26#include "mpaux.h"
27#include "uidswap.h"
28#include "match.h"
29#include "auth-options.h"
30#include "pathnames.h"
31#include "log.h"
32#include "servconf.h"
33#include "auth.h"
Damien Miller89681212001-12-21 12:52:39 +110034#include "hostfile.h"
Ben Lindstrom7a2073c2002-03-22 02:30:41 +000035#include "monitor_wrap.h"
Ben Lindstrom03f39322002-04-02 20:43:11 +000036#include "ssh.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
53 * length of a line is 8000 characters. See the documentation for a
54 * 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 Lindstrome1f9e322002-03-27 17:38:43 +000084 error("auth_rsa_verify_response: n too small: %d bits",
85 BN_num_bits(key->rsa->n));
86 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{
Ben Lindstrombfb3a0e2001-06-05 20:25:05 +0000156 char line[8192], *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);
190 log("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 */
Damien Miller95def091999-11-25 00:26:21 +1100205 while (fgets(line, sizeof(line), f)) {
206 char *cp;
207 char *options;
208
209 linenum++;
210
Damien Miller5428f641999-11-25 11:54:57 +1100211 /* Skip leading whitespace, empty and comment lines. */
212 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
213 ;
Damien Miller95def091999-11-25 00:26:21 +1100214 if (!*cp || *cp == '\n' || *cp == '#')
215 continue;
216
Damien Miller5428f641999-11-25 11:54:57 +1100217 /*
218 * Check if there are options for this key, and if so,
219 * save their starting address and skip the option part
220 * for now. If there are no options, set the starting
221 * address to NULL.
222 */
Damien Miller95def091999-11-25 00:26:21 +1100223 if (*cp < '0' || *cp > '9') {
224 int quoted = 0;
225 options = cp;
226 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
227 if (*cp == '\\' && cp[1] == '"')
228 cp++; /* Skip both */
229 else if (*cp == '"')
230 quoted = !quoted;
231 }
232 } else
233 options = NULL;
234
235 /* Parse the key from the line. */
Damien Miller89681212001-12-21 12:52:39 +1100236 if (hostfile_read_key(&cp, &bits, key) == 0) {
Ben Lindstromf96704d2001-06-25 04:17:12 +0000237 debug("%.100s, line %lu: non ssh1 key syntax",
Ben Lindstrom226cfa02001-01-22 05:34:40 +0000238 file, linenum);
Damien Miller95def091999-11-25 00:26:21 +1100239 continue;
240 }
241 /* cp now points to the comment part. */
242
Damien Miller95def091999-11-25 00:26:21 +1100243 /* Check if the we have found the desired key (identified by its modulus). */
Damien Miller89681212001-12-21 12:52:39 +1100244 if (BN_cmp(key->rsa->n, client_n) != 0)
Damien Miller5428f641999-11-25 11:54:57 +1100245 continue;
Damien Miller95def091999-11-25 00:26:21 +1100246
Damien Milleraae6c611999-12-06 11:47:28 +1100247 /* check the real bits */
Damien Miller89681212001-12-21 12:52:39 +1100248 if (bits != BN_num_bits(key->rsa->n))
Ben Lindstrom940fb862001-08-06 21:01:49 +0000249 log("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 */
Ben Lindstrom226cfa02001-01-22 05:34:40 +0000258 if (!auth_parse_options(pw, 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
287auth_rsa(struct passwd *pw, BIGNUM *client_n)
288{
289 Key *key;
290 char *fp;
291
292 /* no user given */
293 if (pw == NULL)
294 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}