1 /* $OpenBSD: auth-rsa.c,v 1.80 2011/05/23 03:30:07 djm 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 "includes.h"
18
19 #include <sys/types.h>
20 #include <sys/stat.h>
21
22 #include <openssl/rsa.h>
23 #include <openssl/md5.h>
24
25 #include <pwd.h>
26 #include <stdio.h>
27 #include <stdarg.h>
28 #include <string.h>
29
30 #include "xmalloc.h"
31 #include "rsa.h"
32 #include "packet.h"
33 #include "ssh1.h"
34 #include "uidswap.h"
35 #include "match.h"
36 #include "buffer.h"
37 #include "pathnames.h"
38 #include "log.h"
39 #include "servconf.h"
40 #include "key.h"
41 #include "auth-options.h"
42 #include "hostfile.h"
43 #include "auth.h"
44 #ifdef GSSAPI
45 #include "ssh-gss.h"
46 #endif
47 #include "monitor_wrap.h"
48 #include "ssh.h"
49 #include "misc.h"
50
51 /* import */
52 extern ServerOptions options;
53
54 /*
55 * Session identifier that is used to bind key exchange and authentication
56 * responses to a particular session.
57 */
58 extern u_char session_id[16];
59
60 /*
61 * The .ssh/authorized_keys file contains public keys, one per line, in the
62 * following format:
63 * options bits e n comment
64 * where bits, e and n are decimal numbers,
65 * and comment is any string of characters up to newline. The maximum
66 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
67 * description of the options.
68 */
69
70 BIGNUM *
auth_rsa_generate_challenge(Key * key)71 auth_rsa_generate_challenge(Key *key)
72 {
73 BIGNUM *challenge;
74 BN_CTX *ctx;
75
76 if ((challenge = BN_new()) == NULL)
77 fatal("auth_rsa_generate_challenge: BN_new() failed");
78 /* Generate a random challenge. */
79 if (BN_rand(challenge, 256, 0, 0) == 0)
80 fatal("auth_rsa_generate_challenge: BN_rand failed");
81 if ((ctx = BN_CTX_new()) == NULL)
82 fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
83 if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
84 fatal("auth_rsa_generate_challenge: BN_mod failed");
85 BN_CTX_free(ctx);
86
87 return challenge;
88 }
89
90 int
auth_rsa_verify_response(Key * key,BIGNUM * challenge,u_char response[16])91 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
92 {
93 u_char buf[32], mdbuf[16];
94 MD5_CTX md;
95 int len;
96
97 /* don't allow short keys */
98 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
99 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
100 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
101 return (0);
102 }
103
104 /* The response is MD5 of decrypted challenge plus session id. */
105 len = BN_num_bytes(challenge);
106 if (len <= 0 || len > 32)
107 fatal("auth_rsa_verify_response: bad challenge length %d", len);
108 memset(buf, 0, 32);
109 BN_bn2bin(challenge, buf + 32 - len);
110 MD5_Init(&md);
111 MD5_Update(&md, buf, 32);
112 MD5_Update(&md, session_id, 16);
113 MD5_Final(mdbuf, &md);
114
115 /* Verify that the response is the original challenge. */
116 if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
117 /* Wrong answer. */
118 return (0);
119 }
120 /* Correct answer. */
121 return (1);
122 }
123
124 /*
125 * Performs the RSA authentication challenge-response dialog with the client,
126 * and returns true (non-zero) if the client gave the correct answer to
127 * our challenge; returns zero if the client gives a wrong answer.
128 */
129
130 int
auth_rsa_challenge_dialog(Key * key)131 auth_rsa_challenge_dialog(Key *key)
132 {
133 BIGNUM *challenge, *encrypted_challenge;
134 u_char response[16];
135 int i, success;
136
137 if ((encrypted_challenge = BN_new()) == NULL)
138 fatal("auth_rsa_challenge_dialog: BN_new() failed");
139
140 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
141
142 /* Encrypt the challenge with the public key. */
143 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
144
145 /* Send the encrypted challenge to the client. */
146 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
147 packet_put_bignum(encrypted_challenge);
148 packet_send();
149 BN_clear_free(encrypted_challenge);
150 packet_write_wait();
151
152 /* Wait for a response. */
153 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
154 for (i = 0; i < 16; i++)
155 response[i] = (u_char)packet_get_char();
156 packet_check_eom();
157
158 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
159 BN_clear_free(challenge);
160 return (success);
161 }
162
163 static int
rsa_key_allowed_in_file(struct passwd * pw,char * file,const BIGNUM * client_n,Key ** rkey)164 rsa_key_allowed_in_file(struct passwd *pw, char *file,
165 const BIGNUM *client_n, Key **rkey)
166 {
167 char line[SSH_MAX_PUBKEY_BYTES];
168 int allowed = 0;
169 u_int bits;
170 FILE *f;
171 u_long linenum = 0;
172 Key *key;
173
174 debug("trying public RSA key file %s", file);
175 if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL)
176 return 0;
177
178 /*
179 * Go though the accepted keys, looking for the current key. If
180 * found, perform a challenge-response dialog to verify that the
181 * user really has the corresponding private key.
182 */
183 key = key_new(KEY_RSA1);
184 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
185 char *cp;
186 char *key_options;
187 int keybits;
188
189 /* Skip leading whitespace, empty and comment lines. */
190 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
191 ;
192 if (!*cp || *cp == '\n' || *cp == '#')
193 continue;
194
195 /*
196 * Check if there are options for this key, and if so,
197 * save their starting address and skip the option part
198 * for now. If there are no options, set the starting
199 * address to NULL.
200 */
201 if (*cp < '0' || *cp > '9') {
202 int quoted = 0;
203 key_options = cp;
204 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
205 if (*cp == '\\' && cp[1] == '"')
206 cp++; /* Skip both */
207 else if (*cp == '"')
208 quoted = !quoted;
209 }
210 } else
211 key_options = NULL;
212
213 /* Parse the key from the line. */
214 if (hostfile_read_key(&cp, &bits, key) == 0) {
215 debug("%.100s, line %lu: non ssh1 key syntax",
216 file, linenum);
217 continue;
218 }
219 /* cp now points to the comment part. */
220
221 /*
222 * Check if the we have found the desired key (identified
223 * by its modulus).
224 */
225 if (BN_cmp(key->rsa->n, client_n) != 0)
226 continue;
227
228 /* check the real bits */
229 keybits = BN_num_bits(key->rsa->n);
230 if (keybits < 0 || bits != (u_int)keybits)
231 logit("Warning: %s, line %lu: keysize mismatch: "
232 "actual %d vs. announced %d.",
233 file, linenum, BN_num_bits(key->rsa->n), bits);
234
235 /* Never accept a revoked key */
236 if (auth_key_is_revoked(key))
237 break;
238
239 /* We have found the desired key. */
240 /*
241 * If our options do not allow this key to be used,
242 * do not send challenge.
243 */
244 if (!auth_parse_options(pw, key_options, file, linenum))
245 continue;
246 if (key_is_cert_authority)
247 continue;
248 /* break out, this key is allowed */
249 allowed = 1;
250 break;
251 }
252
253 /* Close the file. */
254 fclose(f);
255
256 /* return key if allowed */
257 if (allowed && rkey != NULL)
258 *rkey = key;
259 else
260 key_free(key);
261
262 return allowed;
263 }
264
265 /*
266 * check if there's user key matching client_n,
267 * return key if login is allowed, NULL otherwise
268 */
269
270 int
auth_rsa_key_allowed(struct passwd * pw,BIGNUM * client_n,Key ** rkey)271 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
272 {
273 char *file;
274 u_int i, allowed = 0;
275
276 temporarily_use_uid(pw);
277
278 for (i = 0; !allowed && i < options.num_authkeys_files; i++) {
279 file = expand_authorized_keys(
280 options.authorized_keys_files[i], pw);
281 allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
282 xfree(file);
283 }
284
285 restore_uid();
286
287 return allowed;
288 }
289
290 /*
291 * Performs the RSA authentication dialog with the client. This returns
292 * 0 if the client could not be authenticated, and 1 if authentication was
293 * successful. This may exit if there is a serious protocol violation.
294 */
295 int
auth_rsa(Authctxt * authctxt,BIGNUM * client_n)296 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
297 {
298 Key *key;
299 char *fp;
300 struct passwd *pw = authctxt->pw;
301
302 /* no user given */
303 if (!authctxt->valid)
304 return 0;
305
306 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
307 auth_clear_options();
308 return (0);
309 }
310
311 /* Perform the challenge-response dialog for this key. */
312 if (!auth_rsa_challenge_dialog(key)) {
313 /* Wrong response. */
314 verbose("Wrong response to RSA authentication challenge.");
315 packet_send_debug("Wrong response to RSA authentication challenge.");
316 /*
317 * Break out of the loop. Otherwise we might send
318 * another challenge and break the protocol.
319 */
320 key_free(key);
321 return (0);
322 }
323 /*
324 * Correct response. The client has been successfully
325 * authenticated. Note that we have not yet processed the
326 * options; this will be reset if the options cause the
327 * authentication to be rejected.
328 */
329 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
330 verbose("Found matching %s key: %s",
331 key_type(key), fp);
332 xfree(fp);
333 key_free(key);
334
335 packet_send_debug("RSA authentication accepted.");
336 return (1);
337 }
338