auth-rsa.c revision 295367 
1/* $OpenBSD: auth-rsa.c,v 1.90 2015/01/28 22:36:00 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#ifdef WITH_SSH1
20
21#include <sys/types.h>
22#include <sys/stat.h>
23
24#include <openssl/rsa.h>
25
26#include <pwd.h>
27#include <stdio.h>
28#include <stdarg.h>
29#include <string.h>
30
31#include "xmalloc.h"
32#include "rsa.h"
33#include "packet.h"
34#include "ssh1.h"
35#include "uidswap.h"
36#include "match.h"
37#include "buffer.h"
38#include "pathnames.h"
39#include "log.h"
40#include "misc.h"
41#include "servconf.h"
42#include "key.h"
43#include "auth-options.h"
44#include "hostfile.h"
45#include "auth.h"
46#ifdef GSSAPI
47#include "ssh-gss.h"
48#endif
49#include "monitor_wrap.h"
50#include "ssh.h"
51
52#include "digest.h"
53
54/* import */
55extern ServerOptions options;
56
57/*
58 * Session identifier that is used to bind key exchange and authentication
59 * responses to a particular session.
60 */
61extern u_char session_id[16];
62
63/*
64 * The .ssh/authorized_keys file contains public keys, one per line, in the
65 * following format:
66 *   options bits e n comment
67 * where bits, e and n are decimal numbers,
68 * and comment is any string of characters up to newline.  The maximum
69 * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
70 * description of the options.
71 */
72
73BIGNUM *
74auth_rsa_generate_challenge(Key *key)
75{
76	BIGNUM *challenge;
77	BN_CTX *ctx;
78
79	if ((challenge = BN_new()) == NULL)
80		fatal("auth_rsa_generate_challenge: BN_new() failed");
81	/* Generate a random challenge. */
82	if (BN_rand(challenge, 256, 0, 0) == 0)
83		fatal("auth_rsa_generate_challenge: BN_rand failed");
84	if ((ctx = BN_CTX_new()) == NULL)
85		fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
86	if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
87		fatal("auth_rsa_generate_challenge: BN_mod failed");
88	BN_CTX_free(ctx);
89
90	return challenge;
91}
92
93int
94auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
95{
96	u_char buf[32], mdbuf[16];
97	struct ssh_digest_ctx *md;
98	int len;
99
100	/* don't allow short keys */
101	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
102		error("%s: RSA modulus too small: %d < minimum %d bits",
103		    __func__,
104		    BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
105		return (0);
106	}
107
108	/* The response is MD5 of decrypted challenge plus session id. */
109	len = BN_num_bytes(challenge);
110	if (len <= 0 || len > 32)
111		fatal("%s: bad challenge length %d", __func__, len);
112	memset(buf, 0, 32);
113	BN_bn2bin(challenge, buf + 32 - len);
114	if ((md = ssh_digest_start(SSH_DIGEST_MD5)) == NULL ||
115	    ssh_digest_update(md, buf, 32) < 0 ||
116	    ssh_digest_update(md, session_id, 16) < 0 ||
117	    ssh_digest_final(md, mdbuf, sizeof(mdbuf)) < 0)
118		fatal("%s: md5 failed", __func__);
119	ssh_digest_free(md);
120
121	/* Verify that the response is the original challenge. */
122	if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
123		/* Wrong answer. */
124		return (0);
125	}
126	/* Correct answer. */
127	return (1);
128}
129
130/*
131 * Performs the RSA authentication challenge-response dialog with the client,
132 * and returns true (non-zero) if the client gave the correct answer to
133 * our challenge; returns zero if the client gives a wrong answer.
134 */
135
136int
137auth_rsa_challenge_dialog(Key *key)
138{
139	BIGNUM *challenge, *encrypted_challenge;
140	u_char response[16];
141	int i, success;
142
143	if ((encrypted_challenge = BN_new()) == NULL)
144		fatal("auth_rsa_challenge_dialog: BN_new() failed");
145
146	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
147
148	/* Encrypt the challenge with the public key. */
149	if (rsa_public_encrypt(encrypted_challenge, challenge, key->rsa) != 0)
150		fatal("%s: rsa_public_encrypt failed", __func__);
151
152	/* Send the encrypted challenge to the client. */
153	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
154	packet_put_bignum(encrypted_challenge);
155	packet_send();
156	BN_clear_free(encrypted_challenge);
157	packet_write_wait();
158
159	/* Wait for a response. */
160	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
161	for (i = 0; i < 16; i++)
162		response[i] = (u_char)packet_get_char();
163	packet_check_eom();
164
165	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
166	BN_clear_free(challenge);
167	return (success);
168}
169
170static int
171rsa_key_allowed_in_file(struct passwd *pw, char *file,
172    const BIGNUM *client_n, Key **rkey)
173{
174	char *fp, line[SSH_MAX_PUBKEY_BYTES];
175	int allowed = 0, bits;
176	FILE *f;
177	u_long linenum = 0;
178	Key *key;
179
180	debug("trying public RSA key file %s", file);
181	if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL)
182		return 0;
183
184	/*
185	 * Go though the accepted keys, looking for the current key.  If
186	 * found, perform a challenge-response dialog to verify that the
187	 * user really has the corresponding private key.
188	 */
189	key = key_new(KEY_RSA1);
190	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
191		char *cp;
192		char *key_options;
193		int keybits;
194
195		/* Skip leading whitespace, empty and comment lines. */
196		for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
197			;
198		if (!*cp || *cp == '\n' || *cp == '#')
199			continue;
200
201		/*
202		 * Check if there are options for this key, and if so,
203		 * save their starting address and skip the option part
204		 * for now.  If there are no options, set the starting
205		 * address to NULL.
206		 */
207		if (*cp < '0' || *cp > '9') {
208			int quoted = 0;
209			key_options = cp;
210			for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
211				if (*cp == '\\' && cp[1] == '"')
212					cp++;	/* Skip both */
213				else if (*cp == '"')
214					quoted = !quoted;
215			}
216		} else
217			key_options = NULL;
218
219		/* Parse the key from the line. */
220		if (hostfile_read_key(&cp, &bits, key) == 0) {
221			debug("%.100s, line %lu: non ssh1 key syntax",
222			    file, linenum);
223			continue;
224		}
225		/* cp now points to the comment part. */
226
227		/*
228		 * Check if the we have found the desired key (identified
229		 * by its modulus).
230		 */
231		if (BN_cmp(key->rsa->n, client_n) != 0)
232			continue;
233
234		/* check the real bits  */
235		keybits = BN_num_bits(key->rsa->n);
236		if (keybits < 0 || bits != keybits)
237			logit("Warning: %s, line %lu: keysize mismatch: "
238			    "actual %d vs. announced %d.",
239			    file, linenum, BN_num_bits(key->rsa->n), bits);
240
241		if ((fp = sshkey_fingerprint(key, options.fingerprint_hash,
242		    SSH_FP_DEFAULT)) == NULL)
243			continue;
244		debug("matching key found: file %s, line %lu %s %s",
245		    file, linenum, key_type(key), fp);
246		free(fp);
247
248		/* Never accept a revoked key */
249		if (auth_key_is_revoked(key))
250			break;
251
252		/* We have found the desired key. */
253		/*
254		 * If our options do not allow this key to be used,
255		 * do not send challenge.
256		 */
257		if (!auth_parse_options(pw, key_options, file, linenum))
258			continue;
259		if (key_is_cert_authority)
260			continue;
261		/* break out, this key is allowed */
262		allowed = 1;
263		break;
264	}
265
266	/* Close the file. */
267	fclose(f);
268
269	/* return key if allowed */
270	if (allowed && rkey != NULL)
271		*rkey = key;
272	else
273		key_free(key);
274
275	return allowed;
276}
277
278/*
279 * check if there's user key matching client_n,
280 * return key if login is allowed, NULL otherwise
281 */
282
283int
284auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
285{
286	char *file;
287	u_int i, allowed = 0;
288
289	temporarily_use_uid(pw);
290
291	for (i = 0; !allowed && i < options.num_authkeys_files; i++) {
292		if (strcasecmp(options.authorized_keys_files[i], "none") == 0)
293			continue;
294		file = expand_authorized_keys(
295		    options.authorized_keys_files[i], pw);
296		allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
297		free(file);
298	}
299
300	restore_uid();
301
302	return allowed;
303}
304
305/*
306 * Performs the RSA authentication dialog with the client.  This returns
307 * 0 if the client could not be authenticated, and 1 if authentication was
308 * successful.  This may exit if there is a serious protocol violation.
309 */
310int
311auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
312{
313	Key *key;
314	struct passwd *pw = authctxt->pw;
315
316	/* no user given */
317	if (!authctxt->valid)
318		return 0;
319
320	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
321		auth_clear_options();
322		return (0);
323	}
324
325	/* Perform the challenge-response dialog for this key. */
326	if (!auth_rsa_challenge_dialog(key)) {
327		/* Wrong response. */
328		verbose("Wrong response to RSA authentication challenge.");
329		packet_send_debug("Wrong response to RSA authentication challenge.");
330		/*
331		 * Break out of the loop. Otherwise we might send
332		 * another challenge and break the protocol.
333		 */
334		key_free(key);
335		return (0);
336	}
337	/*
338	 * Correct response.  The client has been successfully
339	 * authenticated. Note that we have not yet processed the
340	 * options; this will be reset if the options cause the
341	 * authentication to be rejected.
342	 */
343	pubkey_auth_info(authctxt, key, NULL);
344
345	packet_send_debug("RSA authentication accepted.");
346	return (1);
347}
348
349#endif /* WITH_SSH1 */
350