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1/*
2 *
3 * auth-rsa.c
4 *
5 * Author: Tatu Ylonen <ylo@cs.hut.fi>
6 *
7 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
8 * All rights reserved
9 *
10 * Created: Mon Mar 27 01:46:52 1995 ylo
11 *
12 * RSA-based authentication. This code determines whether to admit a login
13 * based on RSA authentication. This file also contains functions to check
14 * validity of the host key.
15 *
16 */
17
18#include "includes.h"
19RCSID("$OpenBSD: auth-rsa.c,v 1.27 2000/07/07 03:55:03 todd Exp $");
20
21#include "rsa.h"
22#include "packet.h"
23#include "xmalloc.h"
24#include "ssh.h"
25#include "mpaux.h"
26#include "uidswap.h"
27#include "match.h"
28#include "servconf.h"
29#include "auth-options.h"
30
31#include <openssl/rsa.h>
32#include <openssl/md5.h>
33
34/*
35 * Session identifier that is used to bind key exchange and authentication
36 * responses to a particular session.
37 */
38extern unsigned char session_id[16];
39
40/*
41 * The .ssh/authorized_keys file contains public keys, one per line, in the
42 * following format:
43 * options bits e n comment
44 * where bits, e and n are decimal numbers,
45 * and comment is any string of characters up to newline. The maximum
46 * length of a line is 8000 characters. See the documentation for a
47 * description of the options.
48 */
49
50/*
51 * Performs the RSA authentication challenge-response dialog with the client,
52 * and returns true (non-zero) if the client gave the correct answer to
53 * our challenge; returns zero if the client gives a wrong answer.
54 */
55
56int
57auth_rsa_challenge_dialog(RSA *pk)
58{
59 BIGNUM *challenge, *encrypted_challenge;
60 BN_CTX *ctx;
61 unsigned char buf[32], mdbuf[16], response[16];
62 MD5_CTX md;
63 unsigned int i;
64 int plen, len;
65
66 encrypted_challenge = BN_new();
67 challenge = BN_new();
68
69 /* Generate a random challenge. */
70 BN_rand(challenge, 256, 0, 0);
71 ctx = BN_CTX_new();
72 BN_mod(challenge, challenge, pk->n, ctx);
73 BN_CTX_free(ctx);
74
75 /* Encrypt the challenge with the public key. */
76 rsa_public_encrypt(encrypted_challenge, challenge, pk);
77
78 /* Send the encrypted challenge to the client. */
79 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
80 packet_put_bignum(encrypted_challenge);
81 packet_send();
82 BN_clear_free(encrypted_challenge);
83 packet_write_wait();
84
85 /* Wait for a response. */
86 packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE);
87 packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE);
88 for (i = 0; i < 16; i++)
89 response[i] = packet_get_char();
90
91 /* The response is MD5 of decrypted challenge plus session id. */
92 len = BN_num_bytes(challenge);
93 if (len <= 0 || len > 32)
94 fatal("auth_rsa_challenge_dialog: bad challenge length %d", len);
95 memset(buf, 0, 32);
96 BN_bn2bin(challenge, buf + 32 - len);
97 MD5_Init(&md);
98 MD5_Update(&md, buf, 32);
99 MD5_Update(&md, session_id, 16);
100 MD5_Final(mdbuf, &md);
101 BN_clear_free(challenge);
102
103 /* Verify that the response is the original challenge. */
104 if (memcmp(response, mdbuf, 16) != 0) {
105 /* Wrong answer. */
106 return 0;
107 }
108 /* Correct answer. */
109 return 1;
110}
111
112/*
113 * Performs the RSA authentication dialog with the client. This returns
114 * 0 if the client could not be authenticated, and 1 if authentication was
115 * successful. This may exit if there is a serious protocol violation.
116 */
117
118int
119auth_rsa(struct passwd *pw, BIGNUM *client_n)
120{
121 extern ServerOptions options;
122 char line[8192], file[1024];
123 int authenticated;
124 unsigned int bits;
125 FILE *f;
126 unsigned long linenum = 0;
127 struct stat st;
128 RSA *pk;
129
130 /* Temporarily use the user's uid. */
131 temporarily_use_uid(pw->pw_uid);
132
133 /* The authorized keys. */
134 snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir,
135 SSH_USER_PERMITTED_KEYS);
136
137 /* Fail quietly if file does not exist */
138 if (stat(file, &st) < 0) {
139 /* Restore the privileged uid. */
140 restore_uid();
141 return 0;
142 }
143 /* Open the file containing the authorized keys. */
144 f = fopen(file, "r");
145 if (!f) {
146 /* Restore the privileged uid. */
147 restore_uid();
148 packet_send_debug("Could not open %.900s for reading.", file);
149 packet_send_debug("If your home is on an NFS volume, it may need to be world-readable.");
150 return 0;
151 }
152 if (options.strict_modes) {
153 int fail = 0;
154 char buf[1024];
155 /* Check open file in order to avoid open/stat races */
156 if (fstat(fileno(f), &st) < 0 ||
157 (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
158 (st.st_mode & 022) != 0) {
159 snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
160 "bad ownership or modes for '%s'.", pw->pw_name, file);
161 fail = 1;
162 } else {
163 /* Check path to SSH_USER_PERMITTED_KEYS */
164 int i;
165 static const char *check[] = {
166 "", SSH_USER_DIR, NULL
167 };
168 for (i = 0; check[i]; i++) {
169 snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]);
170 if (stat(line, &st) < 0 ||
171 (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
172 (st.st_mode & 022) != 0) {
173 snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
174 "bad ownership or modes for '%s'.", pw->pw_name, line);
175 fail = 1;
176 break;
177 }
178 }
179 }
180 if (fail) {
181 fclose(f);
182 log("%s",buf);
183 packet_send_debug("%s",buf);
184 restore_uid();
185 return 0;
186 }
187 }
188 /* Flag indicating whether authentication has succeeded. */
189 authenticated = 0;
190
191 pk = RSA_new();
192 pk->e = BN_new();
193 pk->n = BN_new();
194
195 /*
196 * Go though the accepted keys, looking for the current key. If
197 * found, perform a challenge-response dialog to verify that the
198 * user really has the corresponding private key.
199 */
200 while (fgets(line, sizeof(line), f)) {
201 char *cp;
202 char *options;
203
204 linenum++;
205
206 /* Skip leading whitespace, empty and comment lines. */
207 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
208 ;
209 if (!*cp || *cp == '\n' || *cp == '#')
210 continue;
211
212 /*
213 * Check if there are options for this key, and if so,
214 * save their starting address and skip the option part
215 * for now. If there are no options, set the starting
216 * address to NULL.
217 */
218 if (*cp < '0' || *cp > '9') {
219 int quoted = 0;
220 options = cp;
221 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
222 if (*cp == '\\' && cp[1] == '"')
223 cp++; /* Skip both */
224 else if (*cp == '"')
225 quoted = !quoted;
226 }
227 } else
228 options = NULL;
229
230 /* Parse the key from the line. */
231 if (!auth_rsa_read_key(&cp, &bits, pk->e, pk->n)) {
232 debug("%.100s, line %lu: bad key syntax",
233 SSH_USER_PERMITTED_KEYS, linenum);
234 packet_send_debug("%.100s, line %lu: bad key syntax",
235 SSH_USER_PERMITTED_KEYS, linenum);
236 continue;
237 }
238 /* cp now points to the comment part. */
239
240 /* Check if the we have found the desired key (identified by its modulus). */
241 if (BN_cmp(pk->n, client_n) != 0)
242 continue;
243
244 /* check the real bits */
245 if (bits != BN_num_bits(pk->n))
246 log("Warning: %s, line %ld: keysize mismatch: "
247 "actual %d vs. announced %d.",
248 file, linenum, BN_num_bits(pk->n), bits);
249
250 /* We have found the desired key. */
251
252 /* Perform the challenge-response dialog for this key. */
253 if (!auth_rsa_challenge_dialog(pk)) {
254 /* Wrong response. */
255 verbose("Wrong response to RSA authentication challenge.");
256 packet_send_debug("Wrong response to RSA authentication challenge.");
257 continue;
258 }
259 /*
260 * Correct response. The client has been successfully
261 * authenticated. Note that we have not yet processed the
262 * options; this will be reset if the options cause the
263 * authentication to be rejected.
264 * Break out of the loop if authentication was successful;
265 * otherwise continue searching.
266 */
267 authenticated = auth_parse_options(pw, options, linenum);
268 if (authenticated)
269 break;
270 }
271
272 /* Restore the privileged uid. */
273 restore_uid();
274
275 /* Close the file. */
276 fclose(f);
277
278 RSA_free(pk);
279
280 if (authenticated)
281 packet_send_debug("RSA authentication accepted.");
282
283 /* Return authentication result. */
284 return authenticated;
285}