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#ifdef __APPLE_CRYPTO__
23#include "ossl-rsa.h"
24#include "ossl-md5.h"
25#else
26#include <openssl/rsa.h>
27#include <openssl/md5.h>
28#endif
29
30#include <pwd.h>
31#include <stdio.h>
32#include <stdarg.h>
33#include <string.h>
34
35#include "xmalloc.h"
36#include "rsa.h"
37#include "packet.h"
38#include "ssh1.h"
39#include "uidswap.h"
40#include "match.h"
41#include "buffer.h"
42#include "pathnames.h"
43#include "log.h"
44#include "servconf.h"
45#include "key.h"
46#include "auth-options.h"
47#include "hostfile.h"
48#include "auth.h"
49#ifdef GSSAPI
50#include "ssh-gss.h"
51#endif
52#include "monitor_wrap.h"
53#include "ssh.h"
54#include "misc.h"
55
56/* import */
57extern ServerOptions options;
58
59/*
60 * Session identifier that is used to bind key exchange and authentication
61 * responses to a particular session.
62 */
63extern u_char session_id[16];
64
65/*
66 * The .ssh/authorized_keys file contains public keys, one per line, in the
67 * following format:
68 *   options bits e n comment
69 * where bits, e and n are decimal numbers,
70 * and comment is any string of characters up to newline.  The maximum
71 * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
72 * description of the options.
73 */
74
75BIGNUM *
76auth_rsa_generate_challenge(Key *key)
77{
78	BIGNUM *challenge;
79	BN_CTX *ctx;
80
81	if ((challenge = BN_new()) == NULL)
82		fatal("auth_rsa_generate_challenge: BN_new() failed");
83	/* Generate a random challenge. */
84	if (BN_rand(challenge, 256, 0, 0) == 0)
85		fatal("auth_rsa_generate_challenge: BN_rand failed");
86	if ((ctx = BN_CTX_new()) == NULL)
87		fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
88	if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
89		fatal("auth_rsa_generate_challenge: BN_mod failed");
90	BN_CTX_free(ctx);
91
92	return challenge;
93}
94
95int
96auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
97{
98	u_char buf[32], mdbuf[16];
99	MD5_CTX md;
100	int len;
101
102	/* don't allow short keys */
103	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
104		error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
105		    BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
106		return (0);
107	}
108
109	/* The response is MD5 of decrypted challenge plus session id. */
110	len = BN_num_bytes(challenge);
111	if (len <= 0 || len > 32)
112		fatal("auth_rsa_verify_response: bad challenge length %d", len);
113	memset(buf, 0, 32);
114	BN_bn2bin(challenge, buf + 32 - len);
115	MD5_Init(&md);
116	MD5_Update(&md, buf, 32);
117	MD5_Update(&md, session_id, 16);
118	MD5_Final(mdbuf, &md);
119
120	/* Verify that the response is the original challenge. */
121	if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
122		/* Wrong answer. */
123		return (0);
124	}
125	/* Correct answer. */
126	return (1);
127}
128
129/*
130 * Performs the RSA authentication challenge-response dialog with the client,
131 * and returns true (non-zero) if the client gave the correct answer to
132 * our challenge; returns zero if the client gives a wrong answer.
133 */
134
135int
136auth_rsa_challenge_dialog(Key *key)
137{
138	BIGNUM *challenge, *encrypted_challenge;
139	u_char response[16];
140	int i, success;
141
142	if ((encrypted_challenge = BN_new()) == NULL)
143		fatal("auth_rsa_challenge_dialog: BN_new() failed");
144
145	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
146
147	/* Encrypt the challenge with the public key. */
148	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
149
150	/* Send the encrypted challenge to the client. */
151	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
152	packet_put_bignum(encrypted_challenge);
153	packet_send();
154	BN_clear_free(encrypted_challenge);
155	packet_write_wait();
156
157	/* Wait for a response. */
158	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
159	for (i = 0; i < 16; i++)
160		response[i] = (u_char)packet_get_char();
161	packet_check_eom();
162
163	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
164	BN_clear_free(challenge);
165	return (success);
166}
167
168static int
169rsa_key_allowed_in_file(struct passwd *pw, char *file,
170    const BIGNUM *client_n, Key **rkey)
171{
172	char line[SSH_MAX_PUBKEY_BYTES];
173	int allowed = 0;
174	u_int bits;
175	FILE *f;
176	u_long linenum = 0;
177	Key *key;
178
179	debug("trying public RSA key file %s", file);
180	if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL)
181		return 0;
182
183	/*
184	 * Go though the accepted keys, looking for the current key.  If
185	 * found, perform a challenge-response dialog to verify that the
186	 * user really has the corresponding private key.
187	 */
188	key = key_new(KEY_RSA1);
189	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
190		char *cp;
191		char *key_options;
192		int keybits;
193
194		/* Skip leading whitespace, empty and comment lines. */
195		for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
196			;
197		if (!*cp || *cp == '\n' || *cp == '#')
198			continue;
199
200		/*
201		 * Check if there are options for this key, and if so,
202		 * save their starting address and skip the option part
203		 * for now.  If there are no options, set the starting
204		 * address to NULL.
205		 */
206		if (*cp < '0' || *cp > '9') {
207			int quoted = 0;
208			key_options = cp;
209			for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
210				if (*cp == '\\' && cp[1] == '"')
211					cp++;	/* Skip both */
212				else if (*cp == '"')
213					quoted = !quoted;
214			}
215		} else
216			key_options = NULL;
217
218		/* Parse the key from the line. */
219		if (hostfile_read_key(&cp, &bits, key) == 0) {
220			debug("%.100s, line %lu: non ssh1 key syntax",
221			    file, linenum);
222			continue;
223		}
224		/* cp now points to the comment part. */
225
226		/*
227		 * Check if the we have found the desired key (identified
228		 * by its modulus).
229		 */
230		if (BN_cmp(key->rsa->n, client_n) != 0)
231			continue;
232
233		/* check the real bits  */
234		keybits = BN_num_bits(key->rsa->n);
235		if (keybits < 0 || bits != (u_int)keybits)
236			logit("Warning: %s, line %lu: keysize mismatch: "
237			    "actual %d vs. announced %d.",
238			    file, linenum, BN_num_bits(key->rsa->n), bits);
239
240		/* Never accept a revoked key */
241		if (auth_key_is_revoked(key))
242			break;
243
244		/* We have found the desired key. */
245		/*
246		 * If our options do not allow this key to be used,
247		 * do not send challenge.
248		 */
249		if (!auth_parse_options(pw, key_options, file, linenum))
250			continue;
251		if (key_is_cert_authority)
252			continue;
253		/* break out, this key is allowed */
254		allowed = 1;
255		break;
256	}
257
258	/* Close the file. */
259	fclose(f);
260
261	/* return key if allowed */
262	if (allowed && rkey != NULL)
263		*rkey = key;
264	else
265		key_free(key);
266
267	return allowed;
268}
269
270/*
271 * check if there's user key matching client_n,
272 * return key if login is allowed, NULL otherwise
273 */
274
275int
276auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
277{
278	char *file;
279	u_int i, allowed = 0;
280
281	temporarily_use_uid(pw);
282
283	for (i = 0; !allowed && i < options.num_authkeys_files; i++) {
284		if (strcasecmp(options.authorized_keys_files[i], "none") == 0)
285			continue;
286		file = expand_authorized_keys(
287		    options.authorized_keys_files[i], pw);
288		allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
289		xfree(file);
290	}
291
292	restore_uid();
293
294	return allowed;
295}
296
297/*
298 * Performs the RSA authentication dialog with the client.  This returns
299 * 0 if the client could not be authenticated, and 1 if authentication was
300 * successful.  This may exit if there is a serious protocol violation.
301 */
302int
303auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
304{
305	Key *key;
306	char *fp;
307	struct passwd *pw = authctxt->pw;
308
309	/* no user given */
310	if (!authctxt->valid)
311		return 0;
312
313	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
314		auth_clear_options();
315		return (0);
316	}
317
318	/* Perform the challenge-response dialog for this key. */
319	if (!auth_rsa_challenge_dialog(key)) {
320		/* Wrong response. */
321		verbose("Wrong response to RSA authentication challenge.");
322		packet_send_debug("Wrong response to RSA authentication challenge.");
323		/*
324		 * Break out of the loop. Otherwise we might send
325		 * another challenge and break the protocol.
326		 */
327		key_free(key);
328		return (0);
329	}
330	/*
331	 * Correct response.  The client has been successfully
332	 * authenticated. Note that we have not yet processed the
333	 * options; this will be reset if the options cause the
334	 * authentication to be rejected.
335	 */
336	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
337	verbose("Found matching %s key: %s",
338	    key_type(key), fp);
339	xfree(fp);
340	key_free(key);
341
342	packet_send_debug("RSA authentication accepted.");
343	return (1);
344}
345