auth-rsa.c revision 248619 
1/* $OpenBSD: auth-rsa.c,v 1.81 2012/10/30 21:29:54 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 */
52extern ServerOptions options;
53
54/*
55 * Session identifier that is used to bind key exchange and authentication
56 * responses to a particular session.
57 */
58extern 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
70BIGNUM *
71auth_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
90int
91auth_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
130int
131auth_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
163static int
164rsa_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
270int
271auth_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		if (strcasecmp(options.authorized_keys_files[i], "none") == 0)
280			continue;
281		file = expand_authorized_keys(
282		    options.authorized_keys_files[i], pw);
283		allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
284		xfree(file);
285	}
286
287	restore_uid();
288
289	return allowed;
290}
291
292/*
293 * Performs the RSA authentication dialog with the client.  This returns
294 * 0 if the client could not be authenticated, and 1 if authentication was
295 * successful.  This may exit if there is a serious protocol violation.
296 */
297int
298auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
299{
300	Key *key;
301	char *fp;
302	struct passwd *pw = authctxt->pw;
303
304	/* no user given */
305	if (!authctxt->valid)
306		return 0;
307
308	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
309		auth_clear_options();
310		return (0);
311	}
312
313	/* Perform the challenge-response dialog for this key. */
314	if (!auth_rsa_challenge_dialog(key)) {
315		/* Wrong response. */
316		verbose("Wrong response to RSA authentication challenge.");
317		packet_send_debug("Wrong response to RSA authentication challenge.");
318		/*
319		 * Break out of the loop. Otherwise we might send
320		 * another challenge and break the protocol.
321		 */
322		key_free(key);
323		return (0);
324	}
325	/*
326	 * Correct response.  The client has been successfully
327	 * authenticated. Note that we have not yet processed the
328	 * options; this will be reset if the options cause the
329	 * authentication to be rejected.
330	 */
331	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
332	verbose("Found matching %s key: %s",
333	    key_type(key), fp);
334	xfree(fp);
335	key_free(key);
336
337	packet_send_debug("RSA authentication accepted.");
338	return (1);
339}
340