auth-rsa.c revision 264377 
1/* $OpenBSD: auth-rsa.c,v 1.86 2014/01/27 19:18:54 markus 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
24#include <pwd.h>
25#include <stdio.h>
26#include <stdarg.h>
27#include <string.h>
28
29#include "xmalloc.h"
30#include "rsa.h"
31#include "packet.h"
32#include "ssh1.h"
33#include "uidswap.h"
34#include "match.h"
35#include "buffer.h"
36#include "pathnames.h"
37#include "log.h"
38#include "servconf.h"
39#include "key.h"
40#include "auth-options.h"
41#include "hostfile.h"
42#include "auth.h"
43#ifdef GSSAPI
44#include "ssh-gss.h"
45#endif
46#include "monitor_wrap.h"
47#include "ssh.h"
48#include "misc.h"
49
50#include "digest.h"
51
52/* import */
53extern ServerOptions options;
54
55/*
56 * Session identifier that is used to bind key exchange and authentication
57 * responses to a particular session.
58 */
59extern u_char session_id[16];
60
61/*
62 * The .ssh/authorized_keys file contains public keys, one per line, in the
63 * following format:
64 *   options bits e n comment
65 * where bits, e and n are decimal numbers,
66 * and comment is any string of characters up to newline.  The maximum
67 * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
68 * description of the options.
69 */
70
71BIGNUM *
72auth_rsa_generate_challenge(Key *key)
73{
74	BIGNUM *challenge;
75	BN_CTX *ctx;
76
77	if ((challenge = BN_new()) == NULL)
78		fatal("auth_rsa_generate_challenge: BN_new() failed");
79	/* Generate a random challenge. */
80	if (BN_rand(challenge, 256, 0, 0) == 0)
81		fatal("auth_rsa_generate_challenge: BN_rand failed");
82	if ((ctx = BN_CTX_new()) == NULL)
83		fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
84	if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
85		fatal("auth_rsa_generate_challenge: BN_mod failed");
86	BN_CTX_free(ctx);
87
88	return challenge;
89}
90
91int
92auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
93{
94	u_char buf[32], mdbuf[16];
95	struct ssh_digest_ctx *md;
96	int len;
97
98	/* don't allow short keys */
99	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
100		error("%s: RSA modulus too small: %d < minimum %d bits",
101		    __func__,
102		    BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
103		return (0);
104	}
105
106	/* The response is MD5 of decrypted challenge plus session id. */
107	len = BN_num_bytes(challenge);
108	if (len <= 0 || len > 32)
109		fatal("%s: bad challenge length %d", __func__, len);
110	memset(buf, 0, 32);
111	BN_bn2bin(challenge, buf + 32 - len);
112	if ((md = ssh_digest_start(SSH_DIGEST_MD5)) == NULL ||
113	    ssh_digest_update(md, buf, 32) < 0 ||
114	    ssh_digest_update(md, session_id, 16) < 0 ||
115	    ssh_digest_final(md, mdbuf, sizeof(mdbuf)) < 0)
116		fatal("%s: md5 failed", __func__);
117	ssh_digest_free(md);
118
119	/* Verify that the response is the original challenge. */
120	if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
121		/* Wrong answer. */
122		return (0);
123	}
124	/* Correct answer. */
125	return (1);
126}
127
128/*
129 * Performs the RSA authentication challenge-response dialog with the client,
130 * and returns true (non-zero) if the client gave the correct answer to
131 * our challenge; returns zero if the client gives a wrong answer.
132 */
133
134int
135auth_rsa_challenge_dialog(Key *key)
136{
137	BIGNUM *challenge, *encrypted_challenge;
138	u_char response[16];
139	int i, success;
140
141	if ((encrypted_challenge = BN_new()) == NULL)
142		fatal("auth_rsa_challenge_dialog: BN_new() failed");
143
144	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
145
146	/* Encrypt the challenge with the public key. */
147	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
148
149	/* Send the encrypted challenge to the client. */
150	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
151	packet_put_bignum(encrypted_challenge);
152	packet_send();
153	BN_clear_free(encrypted_challenge);
154	packet_write_wait();
155
156	/* Wait for a response. */
157	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
158	for (i = 0; i < 16; i++)
159		response[i] = (u_char)packet_get_char();
160	packet_check_eom();
161
162	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
163	BN_clear_free(challenge);
164	return (success);
165}
166
167static int
168rsa_key_allowed_in_file(struct passwd *pw, char *file,
169    const BIGNUM *client_n, Key **rkey)
170{
171	char *fp, line[SSH_MAX_PUBKEY_BYTES];
172	int allowed = 0, bits;
173	FILE *f;
174	u_long linenum = 0;
175	Key *key;
176
177	debug("trying public RSA key file %s", file);
178	if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL)
179		return 0;
180
181	/*
182	 * Go though the accepted keys, looking for the current key.  If
183	 * found, perform a challenge-response dialog to verify that the
184	 * user really has the corresponding private key.
185	 */
186	key = key_new(KEY_RSA1);
187	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
188		char *cp;
189		char *key_options;
190		int keybits;
191
192		/* Skip leading whitespace, empty and comment lines. */
193		for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
194			;
195		if (!*cp || *cp == '\n' || *cp == '#')
196			continue;
197
198		/*
199		 * Check if there are options for this key, and if so,
200		 * save their starting address and skip the option part
201		 * for now.  If there are no options, set the starting
202		 * address to NULL.
203		 */
204		if (*cp < '0' || *cp > '9') {
205			int quoted = 0;
206			key_options = cp;
207			for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
208				if (*cp == '\\' && cp[1] == '"')
209					cp++;	/* Skip both */
210				else if (*cp == '"')
211					quoted = !quoted;
212			}
213		} else
214			key_options = NULL;
215
216		/* Parse the key from the line. */
217		if (hostfile_read_key(&cp, &bits, key) == 0) {
218			debug("%.100s, line %lu: non ssh1 key syntax",
219			    file, linenum);
220			continue;
221		}
222		/* cp now points to the comment part. */
223
224		/*
225		 * Check if the we have found the desired key (identified
226		 * by its modulus).
227		 */
228		if (BN_cmp(key->rsa->n, client_n) != 0)
229			continue;
230
231		/* check the real bits  */
232		keybits = BN_num_bits(key->rsa->n);
233		if (keybits < 0 || bits != keybits)
234			logit("Warning: %s, line %lu: keysize mismatch: "
235			    "actual %d vs. announced %d.",
236			    file, linenum, BN_num_bits(key->rsa->n), bits);
237
238		fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
239		debug("matching key found: file %s, line %lu %s %s",
240		    file, linenum, key_type(key), fp);
241		free(fp);
242
243		/* Never accept a revoked key */
244		if (auth_key_is_revoked(key))
245			break;
246
247		/* We have found the desired key. */
248		/*
249		 * If our options do not allow this key to be used,
250		 * do not send challenge.
251		 */
252		if (!auth_parse_options(pw, key_options, file, linenum))
253			continue;
254		if (key_is_cert_authority)
255			continue;
256		/* break out, this key is allowed */
257		allowed = 1;
258		break;
259	}
260
261	/* Close the file. */
262	fclose(f);
263
264	/* return key if allowed */
265	if (allowed && rkey != NULL)
266		*rkey = key;
267	else
268		key_free(key);
269
270	return allowed;
271}
272
273/*
274 * check if there's user key matching client_n,
275 * return key if login is allowed, NULL otherwise
276 */
277
278int
279auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
280{
281	char *file;
282	u_int i, allowed = 0;
283
284	temporarily_use_uid(pw);
285
286	for (i = 0; !allowed && i < options.num_authkeys_files; i++) {
287		if (strcasecmp(options.authorized_keys_files[i], "none") == 0)
288			continue;
289		file = expand_authorized_keys(
290		    options.authorized_keys_files[i], pw);
291		allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
292		free(file);
293	}
294
295	restore_uid();
296
297	return allowed;
298}
299
300/*
301 * Performs the RSA authentication dialog with the client.  This returns
302 * 0 if the client could not be authenticated, and 1 if authentication was
303 * successful.  This may exit if there is a serious protocol violation.
304 */
305int
306auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
307{
308	Key *key;
309	struct passwd *pw = authctxt->pw;
310
311	/* no user given */
312	if (!authctxt->valid)
313		return 0;
314
315	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
316		auth_clear_options();
317		return (0);
318	}
319
320	/* Perform the challenge-response dialog for this key. */
321	if (!auth_rsa_challenge_dialog(key)) {
322		/* Wrong response. */
323		verbose("Wrong response to RSA authentication challenge.");
324		packet_send_debug("Wrong response to RSA authentication challenge.");
325		/*
326		 * Break out of the loop. Otherwise we might send
327		 * another challenge and break the protocol.
328		 */
329		key_free(key);
330		return (0);
331	}
332	/*
333	 * Correct response.  The client has been successfully
334	 * authenticated. Note that we have not yet processed the
335	 * options; this will be reset if the options cause the
336	 * authentication to be rejected.
337	 */
338	pubkey_auth_info(authctxt, key, NULL);
339
340	packet_send_debug("RSA authentication accepted.");
341	return (1);
342}
343