1 /*
2 * Author: Tatu Ylonen <ylo@cs.hut.fi>
3 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 * All rights reserved
5 * 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.
8 *
9 * 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".
14 */
15
16 #include "includes.h"
17 RCSID("$OpenBSD: auth-rsa.c,v 1.56 2002/06/10 16:53:06 stevesk Exp $");
18
19 #pragma ident "%Z%%M% %I% %E% SMI"
20
21 #include <openssl/rsa.h>
22 #include <openssl/md5.h>
23
24 #include "rsa.h"
25 #include "packet.h"
26 #include "xmalloc.h"
27 #include "ssh1.h"
28 #include "mpaux.h"
29 #include "uidswap.h"
30 #include "match.h"
31 #include "auth-options.h"
32 #include "pathnames.h"
33 #include "log.h"
34 #include "servconf.h"
35 #include "auth.h"
36 #include "hostfile.h"
37 #include "ssh.h"
38
39 /* import */
40 extern ServerOptions options;
41
42 /*
43 * Session identifier that is used to bind key exchange and authentication
44 * responses to a particular session.
45 */
46 extern u_char session_id[16];
47
48 /*
49 * The .ssh/authorized_keys file contains public keys, one per line, in the
50 * following format:
51 * options bits e n comment
52 * where bits, e and n are decimal numbers,
53 * and comment is any string of characters up to newline. The maximum
54 * length of a line is 8000 characters. See the documentation for a
55 * description of the options.
56 */
57
58 BIGNUM *
59 auth_rsa_generate_challenge(Key *key)
60 {
61 BIGNUM *challenge;
62 BN_CTX *ctx;
63
64 if ((challenge = BN_new()) == NULL)
65 fatal("auth_rsa_generate_challenge: BN_new() failed");
66 /* Generate a random challenge. */
67 BN_rand(challenge, 256, 0, 0);
68 if ((ctx = BN_CTX_new()) == NULL)
69 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
70 BN_mod(challenge, challenge, key->rsa->n, ctx);
71 BN_CTX_free(ctx);
72
73 return challenge;
74 }
75
76 int
77 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
78 {
79 u_char buf[32], mdbuf[16];
80 MD5_CTX md;
81 int len;
82
83 /* don't allow short keys */
84 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
85 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
86 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
87 return (0);
88 }
89
90 /* The response is MD5 of decrypted challenge plus session id. */
91 len = BN_num_bytes(challenge);
92 if (len <= 0 || len > 32)
93 fatal("auth_rsa_verify_response: bad challenge length %d", len);
94 memset(buf, 0, 32);
95 BN_bn2bin(challenge, buf + 32 - len);
96 MD5_Init(&md);
97 MD5_Update(&md, buf, 32);
98 MD5_Update(&md, session_id, 16);
99 MD5_Final(mdbuf, &md);
100
101 /* Verify that the response is the original challenge. */
102 if (memcmp(response, mdbuf, 16) != 0) {
103 /* Wrong answer. */
104 return (0);
105 }
106 /* Correct answer. */
107 return (1);
108 }
109
110 /*
111 * Performs the RSA authentication challenge-response dialog with the client,
112 * and returns true (non-zero) if the client gave the correct answer to
113 * our challenge; returns zero if the client gives a wrong answer.
114 */
115
116 int
117 auth_rsa_challenge_dialog(Key *key)
118 {
119 BIGNUM *challenge, *encrypted_challenge;
120 u_char response[16];
121 int i, success;
122
123 if ((encrypted_challenge = BN_new()) == NULL)
124 fatal("auth_rsa_challenge_dialog: BN_new() failed");
125
126 challenge = auth_rsa_generate_challenge(key);
127
128 /* Encrypt the challenge with the public key. */
129 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
130
131 /* Send the encrypted challenge to the client. */
132 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
133 packet_put_bignum(encrypted_challenge);
134 packet_send();
135 BN_clear_free(encrypted_challenge);
136 packet_write_wait();
137
138 /* Wait for a response. */
139 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
140 for (i = 0; i < 16; i++)
141 response[i] = packet_get_char();
142 packet_check_eom();
143
144 success = auth_rsa_verify_response(key, challenge, response);
145 BN_clear_free(challenge);
146 return (success);
147 }
148
149 /*
150 * check if there's user key matching client_n,
151 * return key if login is allowed, NULL otherwise
152 */
153
154 int
155 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
156 {
157 char line[8192], *file;
158 int allowed = 0;
159 u_int bits;
160 FILE *f;
161 u_long linenum = 0;
162 struct stat st;
163 Key *key;
164
165 /* Temporarily use the user's uid. */
166 temporarily_use_uid(pw);
167
168 /* The authorized keys. */
169 file = authorized_keys_file(pw);
170 debug("trying public RSA key file %s", file);
171
172 /* Fail quietly if file does not exist */
173 if (stat(file, &st) < 0) {
174 /* Restore the privileged uid. */
175 restore_uid();
176 xfree(file);
177 return (0);
178 }
179 /* Open the file containing the authorized keys. */
180 f = fopen(file, "r");
181 if (!f) {
182 /* Restore the privileged uid. */
183 restore_uid();
184 xfree(file);
185 return (0);
186 }
187 if (options.strict_modes &&
188 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
189 xfree(file);
190 fclose(f);
191 log("Authentication refused: %s", line);
192 restore_uid();
193 return (0);
194 }
195
196 /* Flag indicating whether the key is allowed. */
197 allowed = 0;
198
199 key = key_new(KEY_RSA1);
200
201 /*
202 * Go though the accepted keys, looking for the current key. If
203 * found, perform a challenge-response dialog to verify that the
204 * user really has the corresponding private key.
205 */
206 while (fgets(line, sizeof(line), f)) {
207 char *cp;
208 char *options;
209
210 linenum++;
211
212 /* Skip leading whitespace, empty and comment lines. */
213 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
214 ;
215 if (!*cp || *cp == '\n' || *cp == '#')
216 continue;
217
218 /*
219 * Check if there are options for this key, and if so,
220 * save their starting address and skip the option part
221 * for now. If there are no options, set the starting
222 * address to NULL.
223 */
224 if (*cp < '0' || *cp > '9') {
225 int quoted = 0;
226 options = cp;
227 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
228 if (*cp == '\\' && cp[1] == '"')
229 cp++; /* Skip both */
230 else if (*cp == '"')
231 quoted = !quoted;
232 }
233 } else
234 options = NULL;
235
236 /* Parse the key from the line. */
237 if (hostfile_read_key(&cp, &bits, key) == 0) {
238 debug("%.100s, line %lu: non ssh1 key syntax",
239 file, linenum);
240 continue;
241 }
242 /* cp now points to the comment part. */
243
244 /* Check if the we have found the desired key (identified by its modulus). */
245 if (BN_cmp(key->rsa->n, client_n) != 0)
246 continue;
247
248 /* check the real bits */
249 if (bits != BN_num_bits(key->rsa->n))
250 log("Warning: %s, line %lu: keysize mismatch: "
251 "actual %d vs. announced %d.",
252 file, linenum, BN_num_bits(key->rsa->n), bits);
253
254 /* We have found the desired key. */
255 /*
256 * If our options do not allow this key to be used,
257 * do not send challenge.
258 */
259 if (!auth_parse_options(pw, options, file, linenum))
260 continue;
261
262 /* break out, this key is allowed */
263 allowed = 1;
264 break;
265 }
266
267 /* Restore the privileged uid. */
268 restore_uid();
269
270 /* Close the file. */
271 xfree(file);
272 fclose(f);
273
274 /* return key if allowed */
275 if (allowed && rkey != NULL)
276 *rkey = key;
277 else
278 key_free(key);
279 return (allowed);
280 }
281
282 /*
283 * Performs the RSA authentication dialog with the client. This returns
284 * 0 if the client could not be authenticated, and 1 if authentication was
285 * successful. This may exit if there is a serious protocol violation.
286 */
287 int
288 auth_rsa(struct passwd *pw, BIGNUM *client_n)
289 {
290 Key *key;
291 char *fp;
292
293 /* no user given */
294 if (pw == NULL)
295 return 0;
296
297 if (!auth_rsa_key_allowed(pw, client_n, &key)) {
298 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);
324 key_free(key);
325
326 packet_send_debug("RSA authentication accepted.");
327 return (1);
328 }