auth-rsa.c revision 149753 
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"
17RCSID("$OpenBSD: auth-rsa.c,v 1.63 2005/06/17 02:44:32 djm Exp $");
18
19#include <openssl/rsa.h>
20#include <openssl/md5.h>
21
22#include "rsa.h"
23#include "packet.h"
24#include "xmalloc.h"
25#include "ssh1.h"
26#include "uidswap.h"
27#include "match.h"
28#include "auth-options.h"
29#include "pathnames.h"
30#include "log.h"
31#include "servconf.h"
32#include "auth.h"
33#include "hostfile.h"
34#include "monitor_wrap.h"
35#include "ssh.h"
36#include "misc.h"
37
38/* import */
39extern ServerOptions options;
40
41/*
42 * Session identifier that is used to bind key exchange and authentication
43 * responses to a particular session.
44 */
45extern u_char session_id[16];
46
47/*
48 * The .ssh/authorized_keys file contains public keys, one per line, in the
49 * following format:
50 *   options bits e n comment
51 * where bits, e and n are decimal numbers,
52 * and comment is any string of characters up to newline.  The maximum
53 * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
54 * description of the options.
55 */
56
57BIGNUM *
58auth_rsa_generate_challenge(Key *key)
59{
60	BIGNUM *challenge;
61	BN_CTX *ctx;
62
63	if ((challenge = BN_new()) == NULL)
64		fatal("auth_rsa_generate_challenge: BN_new() failed");
65	/* Generate a random challenge. */
66	BN_rand(challenge, 256, 0, 0);
67	if ((ctx = BN_CTX_new()) == NULL)
68		fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
69	BN_mod(challenge, challenge, key->rsa->n, ctx);
70	BN_CTX_free(ctx);
71
72	return challenge;
73}
74
75int
76auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
77{
78	u_char buf[32], mdbuf[16];
79	MD5_CTX md;
80	int len;
81
82	/* don't allow short keys */
83	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
84		error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
85		    BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
86		return (0);
87	}
88
89	/* The response is MD5 of decrypted challenge plus session id. */
90	len = BN_num_bytes(challenge);
91	if (len <= 0 || len > 32)
92		fatal("auth_rsa_verify_response: bad challenge length %d", len);
93	memset(buf, 0, 32);
94	BN_bn2bin(challenge, buf + 32 - len);
95	MD5_Init(&md);
96	MD5_Update(&md, buf, 32);
97	MD5_Update(&md, session_id, 16);
98	MD5_Final(mdbuf, &md);
99
100	/* Verify that the response is the original challenge. */
101	if (memcmp(response, mdbuf, 16) != 0) {
102		/* Wrong answer. */
103		return (0);
104	}
105	/* Correct answer. */
106	return (1);
107}
108
109/*
110 * Performs the RSA authentication challenge-response dialog with the client,
111 * and returns true (non-zero) if the client gave the correct answer to
112 * our challenge; returns zero if the client gives a wrong answer.
113 */
114
115int
116auth_rsa_challenge_dialog(Key *key)
117{
118	BIGNUM *challenge, *encrypted_challenge;
119	u_char response[16];
120	int i, success;
121
122	if ((encrypted_challenge = BN_new()) == NULL)
123		fatal("auth_rsa_challenge_dialog: BN_new() failed");
124
125	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
126
127	/* Encrypt the challenge with the public key. */
128	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
129
130	/* Send the encrypted challenge to the client. */
131	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
132	packet_put_bignum(encrypted_challenge);
133	packet_send();
134	BN_clear_free(encrypted_challenge);
135	packet_write_wait();
136
137	/* Wait for a response. */
138	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
139	for (i = 0; i < 16; i++)
140		response[i] = packet_get_char();
141	packet_check_eom();
142
143	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
144	BN_clear_free(challenge);
145	return (success);
146}
147
148/*
149 * check if there's user key matching client_n,
150 * return key if login is allowed, NULL otherwise
151 */
152
153int
154auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
155{
156	char line[SSH_MAX_PUBKEY_BYTES], *file;
157	int allowed = 0;
158	u_int bits;
159	FILE *f;
160	u_long linenum = 0;
161	struct stat st;
162	Key *key;
163
164	/* Temporarily use the user's uid. */
165	temporarily_use_uid(pw);
166
167	/* The authorized keys. */
168	file = authorized_keys_file(pw);
169	debug("trying public RSA key file %s", file);
170
171	/* Fail quietly if file does not exist */
172	if (stat(file, &st) < 0) {
173		/* Restore the privileged uid. */
174		restore_uid();
175		xfree(file);
176		return (0);
177	}
178	/* Open the file containing the authorized keys. */
179	f = fopen(file, "r");
180	if (!f) {
181		/* Restore the privileged uid. */
182		restore_uid();
183		xfree(file);
184		return (0);
185	}
186	if (options.strict_modes &&
187	    secure_filename(f, file, pw, line, sizeof(line)) != 0) {
188		xfree(file);
189		fclose(f);
190		logit("Authentication refused: %s", line);
191		restore_uid();
192		return (0);
193	}
194
195	/* Flag indicating whether the key is allowed. */
196	allowed = 0;
197
198	key = key_new(KEY_RSA1);
199
200	/*
201	 * Go though the accepted keys, looking for the current key.  If
202	 * found, perform a challenge-response dialog to verify that the
203	 * user really has the corresponding private key.
204	 */
205	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
206		char *cp;
207		char *key_options;
208		int keybits;
209
210		/* Skip leading whitespace, empty and comment lines. */
211		for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
212			;
213		if (!*cp || *cp == '\n' || *cp == '#')
214			continue;
215
216		/*
217		 * Check if there are options for this key, and if so,
218		 * save their starting address and skip the option part
219		 * for now.  If there are no options, set the starting
220		 * address to NULL.
221		 */
222		if (*cp < '0' || *cp > '9') {
223			int quoted = 0;
224			key_options = cp;
225			for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
226				if (*cp == '\\' && cp[1] == '"')
227					cp++;	/* Skip both */
228				else if (*cp == '"')
229					quoted = !quoted;
230			}
231		} else
232			key_options = NULL;
233
234		/* Parse the key from the line. */
235		if (hostfile_read_key(&cp, &bits, key) == 0) {
236			debug("%.100s, line %lu: non ssh1 key syntax",
237			    file, linenum);
238			continue;
239		}
240		/* cp now points to the comment part. */
241
242		/* Check if the we have found the desired key (identified by its modulus). */
243		if (BN_cmp(key->rsa->n, client_n) != 0)
244			continue;
245
246		/* check the real bits  */
247		keybits = BN_num_bits(key->rsa->n);
248		if (keybits < 0 || bits != (u_int)keybits)
249			logit("Warning: %s, line %lu: keysize mismatch: "
250			    "actual %d vs. announced %d.",
251			    file, linenum, BN_num_bits(key->rsa->n), bits);
252
253		/* We have found the desired key. */
254		/*
255		 * If our options do not allow this key to be used,
256		 * do not send challenge.
257		 */
258		if (!auth_parse_options(pw, key_options, file, linenum))
259			continue;
260
261		/* break out, this key is allowed */
262		allowed = 1;
263		break;
264	}
265
266	/* Restore the privileged uid. */
267	restore_uid();
268
269	/* Close the file. */
270	xfree(file);
271	fclose(f);
272
273	/* return key if allowed */
274	if (allowed && rkey != NULL)
275		*rkey = key;
276	else
277		key_free(key);
278	return (allowed);
279}
280
281/*
282 * Performs the RSA authentication dialog with the client.  This returns
283 * 0 if the client could not be authenticated, and 1 if authentication was
284 * successful.  This may exit if there is a serious protocol violation.
285 */
286int
287auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
288{
289	Key *key;
290	char *fp;
291	struct passwd *pw = authctxt->pw;
292
293	/* no user given */
294	if (!authctxt->valid)
295		return 0;
296
297	if (!PRIVSEP(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}
329