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 */
55 extern ServerOptions options;
56 
57 /*
58  * Session identifier that is used to bind key exchange and authentication
59  * responses to a particular session.
60  */
61 extern 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 
73 BIGNUM *
auth_rsa_generate_challenge(Key * key)74 auth_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 
93 int
auth_rsa_verify_response(Key * key,BIGNUM * challenge,u_char response[16])94 auth_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 
136 int
auth_rsa_challenge_dialog(Key * key)137 auth_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 
170 static int
rsa_key_allowed_in_file(struct passwd * pw,char * file,const BIGNUM * client_n,Key ** rkey)171 rsa_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 
283 int
auth_rsa_key_allowed(struct passwd * pw,BIGNUM * client_n,Key ** rkey)284 auth_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  */
310 int
auth_rsa(Authctxt * authctxt,BIGNUM * client_n)311 auth_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