1/*
2 * Author: Tatu Ylonen <ylo@cs.hut.fi>
3 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 * All rights reserved
5 * RSA-based authentication. This code determines whether to admit a login
6 * based on RSA authentication. This file also contains functions to check
7 * validity of the host key.
8 *
9 * As far as I am concerned, the code I have written for this software
10 * can be used freely for any purpose. Any derived versions of this
11 * software must be clearly marked as such, and if the derived work is
12 * incompatible with the protocol description in the RFC file, it must be
13 * called by a name other than "ssh" or "Secure Shell".
14 */
15
16#include "includes.h"
17RCSID("$OpenBSD: auth-rsa.c,v 1.62 2004/12/11 01:48:56 dtucker Exp $");
18
19#include <openssl/rsa.h>
20#include <openssl/md5.h>
21
22#include "rsa.h"
23#include "packet.h"
24#include "xmalloc.h"
25#include "ssh1.h"
26#include "uidswap.h"
27#include "match.h"
28#include "auth-options.h"
29#include "pathnames.h"
30#include "log.h"
31#include "servconf.h"
32#include "auth.h"
33#include "hostfile.h"
34#include "monitor_wrap.h"
35#include "ssh.h"
36#include "misc.h"
37
38/* import */
39extern ServerOptions options;
40
41/*
42 * Session identifier that is used to bind key exchange and authentication
43 * responses to a particular session.
44 */
45extern u_char session_id[16];
46
47/*
48 * The .ssh/authorized_keys file contains public keys, one per line, in the
49 * following format:
50 * options bits e n comment
51 * where bits, e and n are decimal numbers,
52 * and comment is any string of characters up to newline. The maximum
53 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
54 * description of the options.
55 */
56
57BIGNUM *
58auth_rsa_generate_challenge(Key *key)
59{
60 BIGNUM *challenge;
61 BN_CTX *ctx;
62
63 if ((challenge = BN_new()) == NULL)
64 fatal("auth_rsa_generate_challenge: BN_new() failed");
65 /* Generate a random challenge. */
66 BN_rand(challenge, 256, 0, 0);
67 if ((ctx = BN_CTX_new()) == NULL)
68 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
69 BN_mod(challenge, challenge, key->rsa->n, ctx);
70 BN_CTX_free(ctx);
71
72 return challenge;
73}
74
75int
76auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
77{
78 u_char buf[32], mdbuf[16];
79 MD5_CTX md;
80 int len;
81
82 /* don't allow short keys */
83 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
84 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
85 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
86 return (0);
87 }
88
89 /* The response is MD5 of decrypted challenge plus session id. */
90 len = BN_num_bytes(challenge);
91 if (len <= 0 || len > 32)
92 fatal("auth_rsa_verify_response: bad challenge length %d", len);
93 memset(buf, 0, 32);
94 BN_bn2bin(challenge, buf + 32 - len);
95 MD5_Init(&md);
96 MD5_Update(&md, buf, 32);
97 MD5_Update(&md, session_id, 16);
98 MD5_Final(mdbuf, &md);
99
100 /* Verify that the response is the original challenge. */
101 if (memcmp(response, mdbuf, 16) != 0) {
102 /* Wrong answer. */
103 return (0);
104 }
105 /* Correct answer. */
106 return (1);
107}
108
109/*
110 * Performs the RSA authentication challenge-response dialog with the client,
111 * and returns true (non-zero) if the client gave the correct answer to
112 * our challenge; returns zero if the client gives a wrong answer.
113 */
114
115int
116auth_rsa_challenge_dialog(Key *key)
117{
118 BIGNUM *challenge, *encrypted_challenge;
119 u_char response[16];
120 int i, success;
121
122 if ((encrypted_challenge = BN_new()) == NULL)
123 fatal("auth_rsa_challenge_dialog: BN_new() failed");
124
125 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
126
127 /* Encrypt the challenge with the public key. */
128 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
129
130 /* Send the encrypted challenge to the client. */
131 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
132 packet_put_bignum(encrypted_challenge);
133 packet_send();
134 BN_clear_free(encrypted_challenge);
135 packet_write_wait();
136
137 /* Wait for a response. */
138 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
139 for (i = 0; i < 16; i++)
140 response[i] = packet_get_char();
141 packet_check_eom();
142
143 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
144 BN_clear_free(challenge);
145 return (success);
146}
147
148/*
149 * check if there's user key matching client_n,
150 * return key if login is allowed, NULL otherwise
151 */
152
153int
154auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
155{
156 char line[SSH_MAX_PUBKEY_BYTES], *file;
157 int allowed = 0;
158 u_int bits;
159 FILE *f;
160 u_long linenum = 0;
161 struct stat st;
162 Key *key;
163
164 /* Temporarily use the user's uid. */
165 temporarily_use_uid(pw);
166
167 /* The authorized keys. */
168 file = authorized_keys_file(pw);
169 debug("trying public RSA key file %s", file);
170
171 /* Fail quietly if file does not exist */
172 if (stat(file, &st) < 0) {
173 /* Restore the privileged uid. */
174 restore_uid();
175 xfree(file);
176 return (0);
177 }
178 /* Open the file containing the authorized keys. */
179 f = fopen(file, "r");
180 if (!f) {
181 /* Restore the privileged uid. */
182 restore_uid();
183 xfree(file);
184 return (0);
185 }
186 if (options.strict_modes &&
187 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
188 xfree(file);
189 fclose(f);
190 logit("Authentication refused: %s", line);
191 restore_uid();
192 return (0);
193 }
194
195 /* Flag indicating whether the key is allowed. */
196 allowed = 0;
197
198 key = key_new(KEY_RSA1);
199
200 /*
201 * Go though the accepted keys, looking for the current key. If
202 * found, perform a challenge-response dialog to verify that the
203 * user really has the corresponding private key.
204 */
205 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
206 char *cp;
207 char *key_options;
208
209 /* Skip leading whitespace, empty and comment lines. */
210 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
211 ;
212 if (!*cp || *cp == '\n' || *cp == '#')
213 continue;
214
215 /*
216 * Check if there are options for this key, and if so,
217 * save their starting address and skip the option part
218 * for now. If there are no options, set the starting
219 * address to NULL.
220 */
221 if (*cp < '0' || *cp > '9') {
222 int quoted = 0;
223 key_options = cp;
224 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
225 if (*cp == '\\' && cp[1] == '"')
226 cp++; /* Skip both */
227 else if (*cp == '"')
228 quoted = !quoted;
229 }
230 } else
231 key_options = NULL;
232
233 /* Parse the key from the line. */
234 if (hostfile_read_key(&cp, &bits, key) == 0) {
235 debug("%.100s, line %lu: non ssh1 key syntax",
236 file, linenum);
237 continue;
238 }
239 /* cp now points to the comment part. */
240
241 /* Check if the we have found the desired key (identified by its modulus). */
242 if (BN_cmp(key->rsa->n, client_n) != 0)
243 continue;
244
245 /* check the real bits */
246 if (bits != BN_num_bits(key->rsa->n))
247 logit("Warning: %s, line %lu: keysize mismatch: "
248 "actual %d vs. announced %d.",
249 file, linenum, BN_num_bits(key->rsa->n), bits);
250
251 /* We have found the desired key. */
252 /*
253 * If our options do not allow this key to be used,
254 * do not send challenge.
255 */
256 if (!auth_parse_options(pw, key_options, file, linenum))
257 continue;
258
259 /* break out, this key is allowed */
260 allowed = 1;
261 break;
262 }
263
264 /* Restore the privileged uid. */
265 restore_uid();
266
267 /* Close the file. */
268 xfree(file);
269 fclose(f);
270
271 /* return key if allowed */
272 if (allowed && rkey != NULL)
273 *rkey = key;
274 else
275 key_free(key);
276 return (allowed);
277}
278
279/*
280 * Performs the RSA authentication dialog with the client. This returns
281 * 0 if the client could not be authenticated, and 1 if authentication was
282 * successful. This may exit if there is a serious protocol violation.
283 */
284int
285auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
286{
287 Key *key;
288 char *fp;
289 struct passwd *pw = authctxt->pw;
290
291 /* no user given */
292 if (!authctxt->valid)
293 return 0;
294
295 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
296 auth_clear_options();
297 return (0);
298 }
299
300 /* Perform the challenge-response dialog for this key. */
301 if (!auth_rsa_challenge_dialog(key)) {
302 /* Wrong response. */
303 verbose("Wrong response to RSA authentication challenge.");
304 packet_send_debug("Wrong response to RSA authentication challenge.");
305 /*
306 * Break out of the loop. Otherwise we might send
307 * another challenge and break the protocol.
308 */
309 key_free(key);
310 return (0);
311 }
312 /*
313 * Correct response. The client has been successfully
314 * authenticated. Note that we have not yet processed the
315 * options; this will be reset if the options cause the
316 * authentication to be rejected.
317 */
318 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
319 verbose("Found matching %s key: %s",
320 key_type(key), fp);
321 xfree(fp);
322 key_free(key);
323
324 packet_send_debug("RSA authentication accepted.");
325 return (1);
326}
2 * Author: Tatu Ylonen <ylo@cs.hut.fi>
3 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 * All rights reserved
5 * RSA-based authentication. This code determines whether to admit a login
6 * based on RSA authentication. This file also contains functions to check
7 * validity of the host key.
8 *
9 * As far as I am concerned, the code I have written for this software
10 * can be used freely for any purpose. Any derived versions of this
11 * software must be clearly marked as such, and if the derived work is
12 * incompatible with the protocol description in the RFC file, it must be
13 * called by a name other than "ssh" or "Secure Shell".
14 */
15
16#include "includes.h"
17RCSID("$OpenBSD: auth-rsa.c,v 1.62 2004/12/11 01:48:56 dtucker Exp $");
18
19#include <openssl/rsa.h>
20#include <openssl/md5.h>
21
22#include "rsa.h"
23#include "packet.h"
24#include "xmalloc.h"
25#include "ssh1.h"
26#include "uidswap.h"
27#include "match.h"
28#include "auth-options.h"
29#include "pathnames.h"
30#include "log.h"
31#include "servconf.h"
32#include "auth.h"
33#include "hostfile.h"
34#include "monitor_wrap.h"
35#include "ssh.h"
36#include "misc.h"
37
38/* import */
39extern ServerOptions options;
40
41/*
42 * Session identifier that is used to bind key exchange and authentication
43 * responses to a particular session.
44 */
45extern u_char session_id[16];
46
47/*
48 * The .ssh/authorized_keys file contains public keys, one per line, in the
49 * following format:
50 * options bits e n comment
51 * where bits, e and n are decimal numbers,
52 * and comment is any string of characters up to newline. The maximum
53 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
54 * description of the options.
55 */
56
57BIGNUM *
58auth_rsa_generate_challenge(Key *key)
59{
60 BIGNUM *challenge;
61 BN_CTX *ctx;
62
63 if ((challenge = BN_new()) == NULL)
64 fatal("auth_rsa_generate_challenge: BN_new() failed");
65 /* Generate a random challenge. */
66 BN_rand(challenge, 256, 0, 0);
67 if ((ctx = BN_CTX_new()) == NULL)
68 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
69 BN_mod(challenge, challenge, key->rsa->n, ctx);
70 BN_CTX_free(ctx);
71
72 return challenge;
73}
74
75int
76auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
77{
78 u_char buf[32], mdbuf[16];
79 MD5_CTX md;
80 int len;
81
82 /* don't allow short keys */
83 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
84 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
85 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
86 return (0);
87 }
88
89 /* The response is MD5 of decrypted challenge plus session id. */
90 len = BN_num_bytes(challenge);
91 if (len <= 0 || len > 32)
92 fatal("auth_rsa_verify_response: bad challenge length %d", len);
93 memset(buf, 0, 32);
94 BN_bn2bin(challenge, buf + 32 - len);
95 MD5_Init(&md);
96 MD5_Update(&md, buf, 32);
97 MD5_Update(&md, session_id, 16);
98 MD5_Final(mdbuf, &md);
99
100 /* Verify that the response is the original challenge. */
101 if (memcmp(response, mdbuf, 16) != 0) {
102 /* Wrong answer. */
103 return (0);
104 }
105 /* Correct answer. */
106 return (1);
107}
108
109/*
110 * Performs the RSA authentication challenge-response dialog with the client,
111 * and returns true (non-zero) if the client gave the correct answer to
112 * our challenge; returns zero if the client gives a wrong answer.
113 */
114
115int
116auth_rsa_challenge_dialog(Key *key)
117{
118 BIGNUM *challenge, *encrypted_challenge;
119 u_char response[16];
120 int i, success;
121
122 if ((encrypted_challenge = BN_new()) == NULL)
123 fatal("auth_rsa_challenge_dialog: BN_new() failed");
124
125 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
126
127 /* Encrypt the challenge with the public key. */
128 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
129
130 /* Send the encrypted challenge to the client. */
131 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
132 packet_put_bignum(encrypted_challenge);
133 packet_send();
134 BN_clear_free(encrypted_challenge);
135 packet_write_wait();
136
137 /* Wait for a response. */
138 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
139 for (i = 0; i < 16; i++)
140 response[i] = packet_get_char();
141 packet_check_eom();
142
143 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
144 BN_clear_free(challenge);
145 return (success);
146}
147
148/*
149 * check if there's user key matching client_n,
150 * return key if login is allowed, NULL otherwise
151 */
152
153int
154auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
155{
156 char line[SSH_MAX_PUBKEY_BYTES], *file;
157 int allowed = 0;
158 u_int bits;
159 FILE *f;
160 u_long linenum = 0;
161 struct stat st;
162 Key *key;
163
164 /* Temporarily use the user's uid. */
165 temporarily_use_uid(pw);
166
167 /* The authorized keys. */
168 file = authorized_keys_file(pw);
169 debug("trying public RSA key file %s", file);
170
171 /* Fail quietly if file does not exist */
172 if (stat(file, &st) < 0) {
173 /* Restore the privileged uid. */
174 restore_uid();
175 xfree(file);
176 return (0);
177 }
178 /* Open the file containing the authorized keys. */
179 f = fopen(file, "r");
180 if (!f) {
181 /* Restore the privileged uid. */
182 restore_uid();
183 xfree(file);
184 return (0);
185 }
186 if (options.strict_modes &&
187 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
188 xfree(file);
189 fclose(f);
190 logit("Authentication refused: %s", line);
191 restore_uid();
192 return (0);
193 }
194
195 /* Flag indicating whether the key is allowed. */
196 allowed = 0;
197
198 key = key_new(KEY_RSA1);
199
200 /*
201 * Go though the accepted keys, looking for the current key. If
202 * found, perform a challenge-response dialog to verify that the
203 * user really has the corresponding private key.
204 */
205 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
206 char *cp;
207 char *key_options;
208
209 /* Skip leading whitespace, empty and comment lines. */
210 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
211 ;
212 if (!*cp || *cp == '\n' || *cp == '#')
213 continue;
214
215 /*
216 * Check if there are options for this key, and if so,
217 * save their starting address and skip the option part
218 * for now. If there are no options, set the starting
219 * address to NULL.
220 */
221 if (*cp < '0' || *cp > '9') {
222 int quoted = 0;
223 key_options = cp;
224 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
225 if (*cp == '\\' && cp[1] == '"')
226 cp++; /* Skip both */
227 else if (*cp == '"')
228 quoted = !quoted;
229 }
230 } else
231 key_options = NULL;
232
233 /* Parse the key from the line. */
234 if (hostfile_read_key(&cp, &bits, key) == 0) {
235 debug("%.100s, line %lu: non ssh1 key syntax",
236 file, linenum);
237 continue;
238 }
239 /* cp now points to the comment part. */
240
241 /* Check if the we have found the desired key (identified by its modulus). */
242 if (BN_cmp(key->rsa->n, client_n) != 0)
243 continue;
244
245 /* check the real bits */
246 if (bits != BN_num_bits(key->rsa->n))
247 logit("Warning: %s, line %lu: keysize mismatch: "
248 "actual %d vs. announced %d.",
249 file, linenum, BN_num_bits(key->rsa->n), bits);
250
251 /* We have found the desired key. */
252 /*
253 * If our options do not allow this key to be used,
254 * do not send challenge.
255 */
256 if (!auth_parse_options(pw, key_options, file, linenum))
257 continue;
258
259 /* break out, this key is allowed */
260 allowed = 1;
261 break;
262 }
263
264 /* Restore the privileged uid. */
265 restore_uid();
266
267 /* Close the file. */
268 xfree(file);
269 fclose(f);
270
271 /* return key if allowed */
272 if (allowed && rkey != NULL)
273 *rkey = key;
274 else
275 key_free(key);
276 return (allowed);
277}
278
279/*
280 * Performs the RSA authentication dialog with the client. This returns
281 * 0 if the client could not be authenticated, and 1 if authentication was
282 * successful. This may exit if there is a serious protocol violation.
283 */
284int
285auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
286{
287 Key *key;
288 char *fp;
289 struct passwd *pw = authctxt->pw;
290
291 /* no user given */
292 if (!authctxt->valid)
293 return 0;
294
295 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
296 auth_clear_options();
297 return (0);
298 }
299
300 /* Perform the challenge-response dialog for this key. */
301 if (!auth_rsa_challenge_dialog(key)) {
302 /* Wrong response. */
303 verbose("Wrong response to RSA authentication challenge.");
304 packet_send_debug("Wrong response to RSA authentication challenge.");
305 /*
306 * Break out of the loop. Otherwise we might send
307 * another challenge and break the protocol.
308 */
309 key_free(key);
310 return (0);
311 }
312 /*
313 * Correct response. The client has been successfully
314 * authenticated. Note that we have not yet processed the
315 * options; this will be reset if the options cause the
316 * authentication to be rejected.
317 */
318 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
319 verbose("Found matching %s key: %s",
320 key_type(key), fp);
321 xfree(fp);
322 key_free(key);
323
324 packet_send_debug("RSA authentication accepted.");
325 return (1);
326}