1 /* $OpenBSD: auth-rsa.c,v 1.86 2014/01/27 19:18:54 markus 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
24 #include <pwd.h>
25 #include <stdio.h>
26 #include <stdarg.h>
27 #include <string.h>
28
29 #include "xmalloc.h"
30 #include "rsa.h"
31 #include "packet.h"
32 #include "ssh1.h"
33 #include "uidswap.h"
34 #include "match.h"
35 #include "buffer.h"
36 #include "pathnames.h"
37 #include "log.h"
38 #include "servconf.h"
39 #include "key.h"
40 #include "auth-options.h"
41 #include "hostfile.h"
42 #include "auth.h"
43 #ifdef GSSAPI
44 #include "ssh-gss.h"
45 #endif
46 #include "monitor_wrap.h"
47 #include "ssh.h"
48 #include "misc.h"
49
50 #include "digest.h"
51
52 /* import */
53 extern ServerOptions options;
54
55 /*
56 * Session identifier that is used to bind key exchange and authentication
57 * responses to a particular session.
58 */
59 extern u_char session_id[16];
60
61 /*
62 * The .ssh/authorized_keys file contains public keys, one per line, in the
63 * following format:
64 * options bits e n comment
65 * where bits, e and n are decimal numbers,
66 * and comment is any string of characters up to newline. The maximum
67 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
68 * description of the options.
69 */
70
71 BIGNUM *
auth_rsa_generate_challenge(Key * key)72 auth_rsa_generate_challenge(Key *key)
73 {
74 BIGNUM *challenge;
75 BN_CTX *ctx;
76
77 if ((challenge = BN_new()) == NULL)
78 fatal("auth_rsa_generate_challenge: BN_new() failed");
79 /* Generate a random challenge. */
80 if (BN_rand(challenge, 256, 0, 0) == 0)
81 fatal("auth_rsa_generate_challenge: BN_rand failed");
82 if ((ctx = BN_CTX_new()) == NULL)
83 fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
84 if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
85 fatal("auth_rsa_generate_challenge: BN_mod failed");
86 BN_CTX_free(ctx);
87
88 return challenge;
89 }
90
91 int
auth_rsa_verify_response(Key * key,BIGNUM * challenge,u_char response[16])92 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
93 {
94 u_char buf[32], mdbuf[16];
95 struct ssh_digest_ctx *md;
96 int len;
97
98 /* don't allow short keys */
99 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
100 error("%s: RSA modulus too small: %d < minimum %d bits",
101 __func__,
102 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
103 return (0);
104 }
105
106 /* The response is MD5 of decrypted challenge plus session id. */
107 len = BN_num_bytes(challenge);
108 if (len <= 0 || len > 32)
109 fatal("%s: bad challenge length %d", __func__, len);
110 memset(buf, 0, 32);
111 BN_bn2bin(challenge, buf + 32 - len);
112 if ((md = ssh_digest_start(SSH_DIGEST_MD5)) == NULL ||
113 ssh_digest_update(md, buf, 32) < 0 ||
114 ssh_digest_update(md, session_id, 16) < 0 ||
115 ssh_digest_final(md, mdbuf, sizeof(mdbuf)) < 0)
116 fatal("%s: md5 failed", __func__);
117 ssh_digest_free(md);
118
119 /* Verify that the response is the original challenge. */
120 if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
121 /* Wrong answer. */
122 return (0);
123 }
124 /* Correct answer. */
125 return (1);
126 }
127
128 /*
129 * Performs the RSA authentication challenge-response dialog with the client,
130 * and returns true (non-zero) if the client gave the correct answer to
131 * our challenge; returns zero if the client gives a wrong answer.
132 */
133
134 int
auth_rsa_challenge_dialog(Key * key)135 auth_rsa_challenge_dialog(Key *key)
136 {
137 BIGNUM *challenge, *encrypted_challenge;
138 u_char response[16];
139 int i, success;
140
141 if ((encrypted_challenge = BN_new()) == NULL)
142 fatal("auth_rsa_challenge_dialog: BN_new() failed");
143
144 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
145
146 /* Encrypt the challenge with the public key. */
147 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
148
149 /* Send the encrypted challenge to the client. */
150 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
151 packet_put_bignum(encrypted_challenge);
152 packet_send();
153 BN_clear_free(encrypted_challenge);
154 packet_write_wait();
155
156 /* Wait for a response. */
157 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
158 for (i = 0; i < 16; i++)
159 response[i] = (u_char)packet_get_char();
160 packet_check_eom();
161
162 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
163 BN_clear_free(challenge);
164 return (success);
165 }
166
167 static int
rsa_key_allowed_in_file(struct passwd * pw,char * file,const BIGNUM * client_n,Key ** rkey)168 rsa_key_allowed_in_file(struct passwd *pw, char *file,
169 const BIGNUM *client_n, Key **rkey)
170 {
171 char *fp, line[SSH_MAX_PUBKEY_BYTES];
172 int allowed = 0, bits;
173 FILE *f;
174 u_long linenum = 0;
175 Key *key;
176
177 debug("trying public RSA key file %s", file);
178 if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL)
179 return 0;
180
181 /*
182 * Go though the accepted keys, looking for the current key. If
183 * found, perform a challenge-response dialog to verify that the
184 * user really has the corresponding private key.
185 */
186 key = key_new(KEY_RSA1);
187 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
188 char *cp;
189 char *key_options;
190 int keybits;
191
192 /* Skip leading whitespace, empty and comment lines. */
193 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
194 ;
195 if (!*cp || *cp == '\n' || *cp == '#')
196 continue;
197
198 /*
199 * Check if there are options for this key, and if so,
200 * save their starting address and skip the option part
201 * for now. If there are no options, set the starting
202 * address to NULL.
203 */
204 if (*cp < '0' || *cp > '9') {
205 int quoted = 0;
206 key_options = cp;
207 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
208 if (*cp == '\\' && cp[1] == '"')
209 cp++; /* Skip both */
210 else if (*cp == '"')
211 quoted = !quoted;
212 }
213 } else
214 key_options = NULL;
215
216 /* Parse the key from the line. */
217 if (hostfile_read_key(&cp, &bits, key) == 0) {
218 debug("%.100s, line %lu: non ssh1 key syntax",
219 file, linenum);
220 continue;
221 }
222 /* cp now points to the comment part. */
223
224 /*
225 * Check if the we have found the desired key (identified
226 * by its modulus).
227 */
228 if (BN_cmp(key->rsa->n, client_n) != 0)
229 continue;
230
231 /* check the real bits */
232 keybits = BN_num_bits(key->rsa->n);
233 if (keybits < 0 || bits != keybits)
234 logit("Warning: %s, line %lu: keysize mismatch: "
235 "actual %d vs. announced %d.",
236 file, linenum, BN_num_bits(key->rsa->n), bits);
237
238 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
239 debug("matching key found: file %s, line %lu %s %s",
240 file, linenum, key_type(key), fp);
241 free(fp);
242
243 /* Never accept a revoked key */
244 if (auth_key_is_revoked(key))
245 break;
246
247 /* We have found the desired key. */
248 /*
249 * If our options do not allow this key to be used,
250 * do not send challenge.
251 */
252 if (!auth_parse_options(pw, key_options, file, linenum))
253 continue;
254 if (key_is_cert_authority)
255 continue;
256 /* break out, this key is allowed */
257 allowed = 1;
258 break;
259 }
260
261 /* Close the file. */
262 fclose(f);
263
264 /* return key if allowed */
265 if (allowed && rkey != NULL)
266 *rkey = key;
267 else
268 key_free(key);
269
270 return allowed;
271 }
272
273 /*
274 * check if there's user key matching client_n,
275 * return key if login is allowed, NULL otherwise
276 */
277
278 int
auth_rsa_key_allowed(struct passwd * pw,BIGNUM * client_n,Key ** rkey)279 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
280 {
281 char *file;
282 u_int i, allowed = 0;
283
284 temporarily_use_uid(pw);
285
286 for (i = 0; !allowed && i < options.num_authkeys_files; i++) {
287 if (strcasecmp(options.authorized_keys_files[i], "none") == 0)
288 continue;
289 file = expand_authorized_keys(
290 options.authorized_keys_files[i], pw);
291 allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
292 free(file);
293 }
294
295 restore_uid();
296
297 return allowed;
298 }
299
300 /*
301 * Performs the RSA authentication dialog with the client. This returns
302 * 0 if the client could not be authenticated, and 1 if authentication was
303 * successful. This may exit if there is a serious protocol violation.
304 */
305 int
auth_rsa(Authctxt * authctxt,BIGNUM * client_n)306 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
307 {
308 Key *key;
309 struct passwd *pw = authctxt->pw;
310
311 /* no user given */
312 if (!authctxt->valid)
313 return 0;
314
315 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
316 auth_clear_options();
317 return (0);
318 }
319
320 /* Perform the challenge-response dialog for this key. */
321 if (!auth_rsa_challenge_dialog(key)) {
322 /* Wrong response. */
323 verbose("Wrong response to RSA authentication challenge.");
324 packet_send_debug("Wrong response to RSA authentication challenge.");
325 /*
326 * Break out of the loop. Otherwise we might send
327 * another challenge and break the protocol.
328 */
329 key_free(key);
330 return (0);
331 }
332 /*
333 * Correct response. The client has been successfully
334 * authenticated. Note that we have not yet processed the
335 * options; this will be reset if the options cause the
336 * authentication to be rejected.
337 */
338 pubkey_auth_info(authctxt, key, NULL);
339
340 packet_send_debug("RSA authentication accepted.");
341 return (1);
342 }
343