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1/* $OpenBSD: auth-rsa.c,v 1.90 2015/01/28 22:36:00 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#ifdef WITH_SSH1
20
21#include <sys/types.h>
22#include <sys/stat.h>
23
24#include <openssl/rsa.h>
25
26#include <pwd.h>
27#include <stdio.h>
28#include <stdarg.h>
29#include <string.h>
30
31#include "xmalloc.h"
32#include "rsa.h"
33#include "packet.h"
34#include "ssh1.h"
35#include "uidswap.h"
36#include "match.h"
37#include "buffer.h"
38#include "pathnames.h"
39#include "log.h"
40#include "misc.h"
41#include "servconf.h"
42#include "key.h"
43#include "auth-options.h"
44#include "hostfile.h"
45#include "auth.h"
46#ifdef GSSAPI
47#include "ssh-gss.h"
48#endif
49#include "monitor_wrap.h"
50#include "ssh.h"
51
52#include "digest.h"
53
54/* import */
55extern ServerOptions options;
56
57/*
58 * Session identifier that is used to bind key exchange and authentication
59 * responses to a particular session.
60 */
61extern u_char session_id[16];
62
63/*
64 * The .ssh/authorized_keys file contains public keys, one per line, in the
65 * following format:
66 * options bits e n comment
67 * where bits, e and n are decimal numbers,
68 * and comment is any string of characters up to newline. The maximum
69 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
70 * description of the options.
71 */
72
73BIGNUM *
74auth_rsa_generate_challenge(Key *key)
75{
76 BIGNUM *challenge;
77 BN_CTX *ctx;
78
79 if ((challenge = BN_new()) == NULL)
80 fatal("auth_rsa_generate_challenge: BN_new() failed");
81 /* Generate a random challenge. */
82 if (BN_rand(challenge, 256, 0, 0) == 0)
83 fatal("auth_rsa_generate_challenge: BN_rand failed");
84 if ((ctx = BN_CTX_new()) == NULL)
85 fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
86 if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
87 fatal("auth_rsa_generate_challenge: BN_mod failed");
88 BN_CTX_free(ctx);
89
90 return challenge;
91}
92
93int
94auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
95{
96 u_char buf[32], mdbuf[16];
97 struct ssh_digest_ctx *md;
98 int len;
99
100 /* don't allow short keys */
101 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
102 error("%s: RSA modulus too small: %d < minimum %d bits",
103 __func__,
104 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
105 return (0);
106 }
107
108 /* The response is MD5 of decrypted challenge plus session id. */
109 len = BN_num_bytes(challenge);
110 if (len <= 0 || len > 32)
111 fatal("%s: bad challenge length %d", __func__, len);
112 memset(buf, 0, 32);
113 BN_bn2bin(challenge, buf + 32 - len);
114 if ((md = ssh_digest_start(SSH_DIGEST_MD5)) == NULL ||
115 ssh_digest_update(md, buf, 32) < 0 ||
116 ssh_digest_update(md, session_id, 16) < 0 ||
117 ssh_digest_final(md, mdbuf, sizeof(mdbuf)) < 0)
118 fatal("%s: md5 failed", __func__);
119 ssh_digest_free(md);
120
121 /* Verify that the response is the original challenge. */
122 if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
123 /* Wrong answer. */
124 return (0);
125 }
126 /* Correct answer. */
127 return (1);
128}
129
130/*
131 * Performs the RSA authentication challenge-response dialog with the client,
132 * and returns true (non-zero) if the client gave the correct answer to
133 * our challenge; returns zero if the client gives a wrong answer.
134 */
135
136int
137auth_rsa_challenge_dialog(Key *key)
138{
139 BIGNUM *challenge, *encrypted_challenge;
140 u_char response[16];
141 int i, success;
142
143 if ((encrypted_challenge = BN_new()) == NULL)
144 fatal("auth_rsa_challenge_dialog: BN_new() failed");
145
146 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
147
148 /* Encrypt the challenge with the public key. */
149 if (rsa_public_encrypt(encrypted_challenge, challenge, key->rsa) != 0)
150 fatal("%s: rsa_public_encrypt failed", __func__);
151
152 /* Send the encrypted challenge to the client. */
153 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
154 packet_put_bignum(encrypted_challenge);
155 packet_send();
156 BN_clear_free(encrypted_challenge);
157 packet_write_wait();
158
159 /* Wait for a response. */
160 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
161 for (i = 0; i < 16; i++)
162 response[i] = (u_char)packet_get_char();
163 packet_check_eom();
164
165 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
166 BN_clear_free(challenge);
167 return (success);
168}
169
170static int
171rsa_key_allowed_in_file(struct passwd *pw, char *file,
172 const BIGNUM *client_n, Key **rkey)
173{
174 char *fp, line[SSH_MAX_PUBKEY_BYTES];
175 int allowed = 0, bits;
176 FILE *f;
177 u_long linenum = 0;
178 Key *key;
179
180 debug("trying public RSA key file %s", file);
181 if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL)
182 return 0;
183
184 /*
185 * Go though the accepted keys, looking for the current key. If
186 * found, perform a challenge-response dialog to verify that the
187 * user really has the corresponding private key.
188 */
189 key = key_new(KEY_RSA1);
190 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
191 char *cp;
192 char *key_options;
193 int keybits;
194
195 /* Skip leading whitespace, empty and comment lines. */
196 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
197 ;
198 if (!*cp || *cp == '\n' || *cp == '#')
199 continue;
200
201 /*
202 * Check if there are options for this key, and if so,
203 * save their starting address and skip the option part
204 * for now. If there are no options, set the starting
205 * address to NULL.
206 */
207 if (*cp < '0' || *cp > '9') {
208 int quoted = 0;
209 key_options = cp;
210 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
211 if (*cp == '\\' && cp[1] == '"')
212 cp++; /* Skip both */
213 else if (*cp == '"')
214 quoted = !quoted;
215 }
216 } else
217 key_options = NULL;
218
219 /* Parse the key from the line. */
220 if (hostfile_read_key(&cp, &bits, key) == 0) {
221 debug("%.100s, line %lu: non ssh1 key syntax",
222 file, linenum);
223 continue;
224 }
225 /* cp now points to the comment part. */
226
227 /*
228 * Check if the we have found the desired key (identified
229 * by its modulus).
230 */
231 if (BN_cmp(key->rsa->n, client_n) != 0)
232 continue;
233
234 /* check the real bits */
235 keybits = BN_num_bits(key->rsa->n);
236 if (keybits < 0 || bits != keybits)
237 logit("Warning: %s, line %lu: keysize mismatch: "
238 "actual %d vs. announced %d.",
239 file, linenum, BN_num_bits(key->rsa->n), bits);
240
241 if ((fp = sshkey_fingerprint(key, options.fingerprint_hash,
242 SSH_FP_DEFAULT)) == NULL)
243 continue;
244 debug("matching key found: file %s, line %lu %s %s",
245 file, linenum, key_type(key), fp);
246 free(fp);
247
248 /* Never accept a revoked key */
249 if (auth_key_is_revoked(key))
250 break;
251
252 /* We have found the desired key. */
253 /*
254 * If our options do not allow this key to be used,
255 * do not send challenge.
256 */
257 if (!auth_parse_options(pw, key_options, file, linenum))
258 continue;
259 if (key_is_cert_authority)
260 continue;
261 /* break out, this key is allowed */
262 allowed = 1;
263 break;
264 }
265
266 /* Close the file. */
267 fclose(f);
268
269 /* return key if allowed */
270 if (allowed && rkey != NULL)
271 *rkey = key;
272 else
273 key_free(key);
274
275 return allowed;
276}
277
278/*
279 * check if there's user key matching client_n,
280 * return key if login is allowed, NULL otherwise
281 */
282
283int
284auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
285{
286 char *file;
287 u_int i, allowed = 0;
288
289 temporarily_use_uid(pw);
290
291 for (i = 0; !allowed && i < options.num_authkeys_files; i++) {
292 if (strcasecmp(options.authorized_keys_files[i], "none") == 0)
293 continue;
294 file = expand_authorized_keys(
295 options.authorized_keys_files[i], pw);
296 allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
297 free(file);
298 }
299
300 restore_uid();
301
302 return allowed;
303}
304
305/*
306 * Performs the RSA authentication dialog with the client. This returns
307 * 0 if the client could not be authenticated, and 1 if authentication was
308 * successful. This may exit if there is a serious protocol violation.
309 */
310int
311auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
312{
313 Key *key;
314 struct passwd *pw = authctxt->pw;
315
316 /* no user given */
317 if (!authctxt->valid)
318 return 0;
319
320 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
321 auth_clear_options();
322 return (0);
323 }
324
325 /* Perform the challenge-response dialog for this key. */
326 if (!auth_rsa_challenge_dialog(key)) {
327 /* Wrong response. */
328 verbose("Wrong response to RSA authentication challenge.");
329 packet_send_debug("Wrong response to RSA authentication challenge.");
330 /*
331 * Break out of the loop. Otherwise we might send
332 * another challenge and break the protocol.
333 */
334 key_free(key);
335 return (0);
336 }
337 /*
338 * Correct response. The client has been successfully
339 * authenticated. Note that we have not yet processed the
340 * options; this will be reset if the options cause the
341 * authentication to be rejected.
342 */
343 pubkey_auth_info(authctxt, key, NULL);
344
345 packet_send_debug("RSA authentication accepted.");
346 return (1);
347}
348
349#endif /* WITH_SSH1 */
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