1 /* $OpenBSD: auth-rsa.c,v 1.73 2008/07/02 12:03:51 dtucker 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 <sys/types.h>
18 #include <sys/stat.h>
19
20 #include <openssl/rsa.h>
21 #include <md5.h>
22
23 #include <pwd.h>
24 #include <stdio.h>
25 #include <string.h>
26
27 #include "xmalloc.h"
28 #include "rsa.h"
29 #include "packet.h"
30 #include "ssh1.h"
31 #include "uidswap.h"
32 #include "match.h"
33 #include "buffer.h"
34 #include "auth-options.h"
35 #include "pathnames.h"
36 #include "log.h"
37 #include "servconf.h"
38 #include "key.h"
39 #include "hostfile.h"
40 #include "auth.h"
41 #include "monitor_wrap.h"
42 #include "ssh.h"
43 #include "misc.h"
44
45 __RCSID("$MirOS: src/usr.bin/ssh/auth-rsa.c,v 1.10 2008/12/16 20:55:18 tg Exp $");
46
47 /* import */
48 extern ServerOptions options;
49
50 /*
51 * Session identifier that is used to bind key exchange and authentication
52 * responses to a particular session.
53 */
54 extern u_char session_id[16];
55
56 /*
57 * The .etc/ssh/authorised_keys file contains public keys, one per line, in the
58 * following format:
59 * options bits e n comment
60 * where bits, e and n are decimal numbers,
61 * and comment is any string of characters up to newline. The maximum
62 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
63 * description of the options.
64 */
65
66 BIGNUM *
auth_rsa_generate_challenge(Key * key)67 auth_rsa_generate_challenge(Key *key)
68 {
69 BIGNUM *challenge;
70 BN_CTX *ctx;
71
72 if ((challenge = BN_new()) == NULL)
73 fatal("auth_rsa_generate_challenge: BN_new() failed");
74 /* Generate a random challenge. */
75 if (BN_rand(challenge, 256, 0, 0) == 0)
76 fatal("auth_rsa_generate_challenge: BN_rand failed");
77 if ((ctx = BN_CTX_new()) == NULL)
78 fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
79 if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
80 fatal("auth_rsa_generate_challenge: BN_mod failed");
81 BN_CTX_free(ctx);
82
83 return challenge;
84 }
85
86 int
auth_rsa_verify_response(Key * key,BIGNUM * challenge,u_char response[16])87 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
88 {
89 u_char buf[32], mdbuf[16];
90 MD5_CTX md;
91 int len;
92
93 /* don't allow short keys */
94 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
95 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
96 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
97 return (0);
98 }
99
100 /* The response is MD5 of decrypted challenge plus session id. */
101 len = BN_num_bytes(challenge);
102 if (len <= 0 || len > 32)
103 fatal("auth_rsa_verify_response: bad challenge length %d", len);
104 memset(buf, 0, 32);
105 BN_bn2bin(challenge, buf + 32 - len);
106 MD5Init(&md);
107 MD5Update(&md, buf, 32);
108 MD5Update(&md, session_id, 16);
109 MD5Final(mdbuf, &md);
110
111 /* Verify that the response is the original challenge. */
112 if (memcmp(response, mdbuf, 16) != 0) {
113 /* Wrong answer. */
114 return (0);
115 }
116 /* Correct answer. */
117 return (1);
118 }
119
120 /*
121 * Performs the RSA authentication challenge-response dialog with the client,
122 * and returns true (non-zero) if the client gave the correct answer to
123 * our challenge; returns zero if the client gives a wrong answer.
124 */
125
126 int
auth_rsa_challenge_dialog(Key * key)127 auth_rsa_challenge_dialog(Key *key)
128 {
129 BIGNUM *challenge, *encrypted_challenge;
130 u_char response[16];
131 int i, success;
132
133 if ((encrypted_challenge = BN_new()) == NULL)
134 fatal("auth_rsa_challenge_dialog: BN_new() failed");
135
136 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
137
138 /* Encrypt the challenge with the public key. */
139 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
140
141 /* Send the encrypted challenge to the client. */
142 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
143 packet_put_bignum(encrypted_challenge);
144 packet_send();
145 BN_clear_free(encrypted_challenge);
146 packet_write_wait();
147
148 /* Wait for a response. */
149 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
150 for (i = 0; i < 16; i++)
151 response[i] = (u_char)packet_get_char();
152 packet_check_eom();
153
154 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
155 BN_clear_free(challenge);
156 return (success);
157 }
158
159 /*
160 * check if there's user key matching client_n,
161 * return key if login is allowed, NULL otherwise
162 */
163
164 int
auth_rsa_key_allowed(struct passwd * pw,BIGNUM * client_n,Key ** rkey)165 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
166 {
167 char line[SSH_MAX_PUBKEY_BYTES], *file;
168 int allowed = 0;
169 u_int bits;
170 FILE *f;
171 u_long linenum = 0;
172 Key *key;
173
174 /* Temporarily use the user's uid. */
175 temporarily_use_uid(pw);
176
177 /* The authorised keys. */
178 file = authorised_keys_file(pw);
179 debug("trying public RSA key file %s", file);
180 f = auth_openkeyfile(file, pw, options.strict_modes);
181 if (!f) {
182 xfree(file);
183 restore_uid();
184 return (0);
185 }
186
187 /* Flag indicating whether the key is allowed. */
188 allowed = 0;
189
190 key = key_new(KEY_RSA1);
191
192 /*
193 * Go though the accepted keys, looking for the current key. If
194 * found, perform a challenge-response dialog to verify that the
195 * user really has the corresponding private key.
196 */
197 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
198 char *cp;
199 char *key_options;
200 int keybits;
201
202 /* Skip leading whitespace, empty and comment lines. */
203 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
204 ;
205 if (!*cp || *cp == '\n' || *cp == '#')
206 continue;
207
208 /*
209 * Check if there are options for this key, and if so,
210 * save their starting address and skip the option part
211 * for now. If there are no options, set the starting
212 * address to NULL.
213 */
214 if (*cp < '0' || *cp > '9') {
215 int quoted = 0;
216 key_options = cp;
217 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
218 if (*cp == '\\' && cp[1] == '"')
219 cp++; /* Skip both */
220 else if (*cp == '"')
221 quoted = !quoted;
222 }
223 } else
224 key_options = NULL;
225
226 /* Parse the key from the line. */
227 if (hostfile_read_key(&cp, &bits, key) == 0) {
228 debug("%.100s, line %lu: non ssh1 key syntax",
229 file, linenum);
230 continue;
231 }
232 /* cp now points to the comment part. */
233
234 /* Check if the we have found the desired key (identified by its modulus). */
235 if (BN_cmp(key->rsa->n, client_n) != 0)
236 continue;
237
238 /* check the real bits */
239 keybits = BN_num_bits(key->rsa->n);
240 if (keybits < 0 || bits != (u_int)keybits)
241 logit("Warning: %s, line %lu: keysize mismatch: "
242 "actual %d vs. announced %d.",
243 file, linenum, BN_num_bits(key->rsa->n), bits);
244
245 /* We have found the desired key. */
246 /*
247 * If our options do not allow this key to be used,
248 * do not send challenge.
249 */
250 if (!auth_parse_options(pw, key_options, file, linenum))
251 continue;
252
253 /* break out, this key is allowed */
254 allowed = 1;
255 break;
256 }
257
258 /* Restore the privileged uid. */
259 restore_uid();
260
261 /* Close the file. */
262 xfree(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 return (allowed);
271 }
272
273 /*
274 * Performs the RSA authentication dialog with the client. This returns
275 * 0 if the client could not be authenticated, and 1 if authentication was
276 * successful. This may exit if there is a serious protocol violation.
277 */
278 int
auth_rsa(Authctxt * authctxt,BIGNUM * client_n)279 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
280 {
281 Key *key;
282 char *fp;
283 struct passwd *pw = authctxt->pw;
284
285 /* no user given */
286 if (!authctxt->valid)
287 return 0;
288
289 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
290 auth_clear_options();
291 return (0);
292 }
293
294 /* Perform the challenge-response dialog for this key. */
295 if (!auth_rsa_challenge_dialog(key)) {
296 /* Wrong response. */
297 verbose("Wrong response to RSA authentication challenge.");
298 packet_send_debug("Wrong response to RSA authentication challenge.");
299 /*
300 * Break out of the loop. Otherwise we might send
301 * another challenge and break the protocol.
302 */
303 key_free(key);
304 return (0);
305 }
306 /*
307 * Correct response. The client has been successfully
308 * authenticated. Note that we have not yet processed the
309 * options; this will be reset if the options cause the
310 * authentication to be rejected.
311 */
312 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
313 verbose("Found matching %s key: %s",
314 key_type(key), fp);
315 xfree(fp);
316 key_free(key);
317
318 packet_send_debug("RSA authentication accepted.");
319 return (1);
320 }
321