auth-rsa.c revision 98684 
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 1.56 2002/06/10 16:53:06 stevesk Exp $");
18RCSID("$FreeBSD: head/crypto/openssh/auth-rsa.c 98684 2002-06-23 16:09:08Z des $");
19
20#include <openssl/rsa.h>
21#include <openssl/md5.h>
22
23#include "rsa.h"
24#include "packet.h"
25#include "xmalloc.h"
26#include "ssh1.h"
27#include "mpaux.h"
28#include "uidswap.h"
29#include "match.h"
30#include "auth-options.h"
31#include "pathnames.h"
32#include "log.h"
33#include "servconf.h"
34#include "auth.h"
35#include "hostfile.h"
36#include "monitor_wrap.h"
37#include "ssh.h"
38
39/* import */
40extern ServerOptions options;
41
42/*
43 * Session identifier that is used to bind key exchange and authentication
44 * responses to a particular session.
45 */
46extern u_char session_id[16];
47
48/*
49 * The .ssh/authorized_keys file contains public keys, one per line, in the
50 * following format:
51 *   options bits e n comment
52 * where bits, e and n are decimal numbers,
53 * and comment is any string of characters up to newline.  The maximum
54 * length of a line is 8000 characters.  See the documentation for a
55 * description of the options.
56 */
57
58BIGNUM *
59auth_rsa_generate_challenge(Key *key)
60{
61	BIGNUM *challenge;
62	BN_CTX *ctx;
63
64	if ((challenge = BN_new()) == NULL)
65		fatal("auth_rsa_generate_challenge: BN_new() failed");
66	/* Generate a random challenge. */
67	BN_rand(challenge, 256, 0, 0);
68	if ((ctx = BN_CTX_new()) == NULL)
69		fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
70	BN_mod(challenge, challenge, key->rsa->n, ctx);
71	BN_CTX_free(ctx);
72
73	return challenge;
74}
75
76int
77auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
78{
79	u_char buf[32], mdbuf[16];
80	MD5_CTX md;
81	int len;
82
83	/* don't allow short keys */
84	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
85		error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
86		    BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
87		return (0);
88	}
89
90	/* The response is MD5 of decrypted challenge plus session id. */
91	len = BN_num_bytes(challenge);
92	if (len <= 0 || len > 32)
93		fatal("auth_rsa_verify_response: bad challenge length %d", len);
94	memset(buf, 0, 32);
95	BN_bn2bin(challenge, buf + 32 - len);
96	MD5_Init(&md);
97	MD5_Update(&md, buf, 32);
98	MD5_Update(&md, session_id, 16);
99	MD5_Final(mdbuf, &md);
100
101	/* Verify that the response is the original challenge. */
102	if (memcmp(response, mdbuf, 16) != 0) {
103		/* Wrong answer. */
104		return (0);
105	}
106	/* Correct answer. */
107	return (1);
108}
109
110/*
111 * Performs the RSA authentication challenge-response dialog with the client,
112 * and returns true (non-zero) if the client gave the correct answer to
113 * our challenge; returns zero if the client gives a wrong answer.
114 */
115
116int
117auth_rsa_challenge_dialog(Key *key)
118{
119	BIGNUM *challenge, *encrypted_challenge;
120	u_char response[16];
121	int i, success;
122
123	if ((encrypted_challenge = BN_new()) == NULL)
124		fatal("auth_rsa_challenge_dialog: BN_new() failed");
125
126	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
127
128	/* Encrypt the challenge with the public key. */
129	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
130
131	/* Send the encrypted challenge to the client. */
132	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
133	packet_put_bignum(encrypted_challenge);
134	packet_send();
135	BN_clear_free(encrypted_challenge);
136	packet_write_wait();
137
138	/* Wait for a response. */
139	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
140	for (i = 0; i < 16; i++)
141		response[i] = packet_get_char();
142	packet_check_eom();
143
144	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
145	BN_clear_free(challenge);
146	return (success);
147}
148
149/*
150 * check if there's user key matching client_n,
151 * return key if login is allowed, NULL otherwise
152 */
153
154int
155auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
156{
157	char line[8192], *file;
158	int allowed = 0;
159	u_int bits;
160	FILE *f;
161	u_long linenum = 0;
162	struct stat st;
163	Key *key;
164
165	/* Temporarily use the user's uid. */
166	temporarily_use_uid(pw);
167
168	/* The authorized keys. */
169	file = authorized_keys_file(pw);
170	debug("trying public RSA key file %s", file);
171
172	/* Fail quietly if file does not exist */
173	if (stat(file, &st) < 0) {
174		/* Restore the privileged uid. */
175		restore_uid();
176		xfree(file);
177		return (0);
178	}
179	/* Open the file containing the authorized keys. */
180	f = fopen(file, "r");
181	if (!f) {
182		/* Restore the privileged uid. */
183		restore_uid();
184		xfree(file);
185		return (0);
186	}
187	if (options.strict_modes &&
188	    secure_filename(f, file, pw, line, sizeof(line)) != 0) {
189		xfree(file);
190		fclose(f);
191		log("Authentication refused: %s", line);
192		restore_uid();
193		return (0);
194	}
195
196	/* Flag indicating whether the key is allowed. */
197	allowed = 0;
198
199	key = key_new(KEY_RSA1);
200
201	/*
202	 * Go though the accepted keys, looking for the current key.  If
203	 * found, perform a challenge-response dialog to verify that the
204	 * user really has the corresponding private key.
205	 */
206	while (fgets(line, sizeof(line), f)) {
207		char *cp;
208		char *options;
209
210		linenum++;
211
212		/* Skip leading whitespace, empty and comment lines. */
213		for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
214			;
215		if (!*cp || *cp == '\n' || *cp == '#')
216			continue;
217
218		/*
219		 * Check if there are options for this key, and if so,
220		 * save their starting address and skip the option part
221		 * for now.  If there are no options, set the starting
222		 * address to NULL.
223		 */
224		if (*cp < '0' || *cp > '9') {
225			int quoted = 0;
226			options = cp;
227			for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
228				if (*cp == '\\' && cp[1] == '"')
229					cp++;	/* Skip both */
230				else if (*cp == '"')
231					quoted = !quoted;
232			}
233		} else
234			options = NULL;
235
236		/* Parse the key from the line. */
237		if (hostfile_read_key(&cp, &bits, key) == 0) {
238			debug("%.100s, line %lu: non ssh1 key syntax",
239			    file, linenum);
240			continue;
241		}
242		/* cp now points to the comment part. */
243
244		/* Check if the we have found the desired key (identified by its modulus). */
245		if (BN_cmp(key->rsa->n, client_n) != 0)
246			continue;
247
248		/* check the real bits  */
249		if (bits != BN_num_bits(key->rsa->n))
250			log("Warning: %s, line %lu: keysize mismatch: "
251			    "actual %d vs. announced %d.",
252			    file, linenum, BN_num_bits(key->rsa->n), bits);
253
254		/* We have found the desired key. */
255		/*
256		 * If our options do not allow this key to be used,
257		 * do not send challenge.
258		 */
259		if (!auth_parse_options(pw, options, file, linenum))
260			continue;
261
262		/* break out, this key is allowed */
263		allowed = 1;
264		break;
265	}
266
267	/* Restore the privileged uid. */
268	restore_uid();
269
270	/* Close the file. */
271	xfree(file);
272	fclose(f);
273
274	/* return key if allowed */
275	if (allowed && rkey != NULL)
276		*rkey = key;
277	else
278		key_free(key);
279	return (allowed);
280}
281
282/*
283 * Performs the RSA authentication dialog with the client.  This returns
284 * 0 if the client could not be authenticated, and 1 if authentication was
285 * successful.  This may exit if there is a serious protocol violation.
286 */
287int
288auth_rsa(struct passwd *pw, BIGNUM *client_n)
289{
290	Key *key;
291	char *fp;
292
293	/* no user given */
294	if (pw == NULL)
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