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31652869 | 1 | /* $OpenBSD: auth-rsa.c,v 1.71 2006/08/03 03:34:41 deraadt Exp $ */ |
8efc0c15 | 2 | /* |
5260325f | 3 | * Author: Tatu Ylonen <ylo@cs.hut.fi> |
5260325f | 4 | * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
5 | * All rights reserved | |
5260325f | 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. | |
6ae2364d | 9 | * |
bcbf86ec | 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". | |
5260325f | 15 | */ |
8efc0c15 | 16 | |
17 | #include "includes.h" | |
4095f623 | 18 | |
19 | #include <sys/types.h> | |
20 | #include <sys/stat.h> | |
42f11eb2 | 21 | |
22 | #include <openssl/rsa.h> | |
23 | #include <openssl/md5.h> | |
8efc0c15 | 24 | |
b1842393 | 25 | #include <pwd.h> |
cf851879 | 26 | #include <stdio.h> |
00146caa | 27 | #include <string.h> |
b1842393 | 28 | |
31652869 | 29 | #include "xmalloc.h" |
8efc0c15 | 30 | #include "rsa.h" |
31 | #include "packet.h" | |
42f11eb2 | 32 | #include "ssh1.h" |
8efc0c15 | 33 | #include "uidswap.h" |
4fe2af09 | 34 | #include "match.h" |
31652869 | 35 | #include "buffer.h" |
38c295d6 | 36 | #include "auth-options.h" |
42f11eb2 | 37 | #include "pathnames.h" |
38 | #include "log.h" | |
39 | #include "servconf.h" | |
31652869 | 40 | #include "key.h" |
46df736f | 41 | #include "hostfile.h" |
31652869 | 42 | #include "auth.h" |
43 | #ifdef GSSAPI | |
44 | #include "ssh-gss.h" | |
45 | #endif | |
1853d1ef | 46 | #include "monitor_wrap.h" |
82b00264 | 47 | #include "ssh.h" |
41feb690 | 48 | #include "misc.h" |
94ec8c6b | 49 | |
50 | /* import */ | |
51 | extern ServerOptions options; | |
52 | ||
aa3378df | 53 | /* |
54 | * Session identifier that is used to bind key exchange and authentication | |
55 | * responses to a particular session. | |
56 | */ | |
1e3b8b07 | 57 | extern u_char session_id[16]; |
8efc0c15 | 58 | |
aa3378df | 59 | /* |
60 | * The .ssh/authorized_keys file contains public keys, one per line, in the | |
61 | * following format: | |
62 | * options bits e n comment | |
63 | * where bits, e and n are decimal numbers, | |
64 | * and comment is any string of characters up to newline. The maximum | |
ea067773 | 65 | * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a |
aa3378df | 66 | * description of the options. |
67 | */ | |
8efc0c15 | 68 | |
1853d1ef | 69 | BIGNUM * |
46f1eece | 70 | auth_rsa_generate_challenge(Key *key) |
71 | { | |
72 | BIGNUM *challenge; | |
73 | BN_CTX *ctx; | |
74 | ||
75 | if ((challenge = BN_new()) == NULL) | |
76 | fatal("auth_rsa_generate_challenge: BN_new() failed"); | |
77 | /* Generate a random challenge. */ | |
78 | BN_rand(challenge, 256, 0, 0); | |
79 | if ((ctx = BN_CTX_new()) == NULL) | |
80 | fatal("auth_rsa_generate_challenge: BN_CTX_new() failed"); | |
81 | BN_mod(challenge, challenge, key->rsa->n, ctx); | |
82 | BN_CTX_free(ctx); | |
83 | ||
84 | return challenge; | |
85 | } | |
86 | ||
1853d1ef | 87 | int |
46f1eece | 88 | auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16]) |
89 | { | |
90 | u_char buf[32], mdbuf[16]; | |
91 | MD5_CTX md; | |
92 | int len; | |
93 | ||
52103b10 | 94 | /* don't allow short keys */ |
82b00264 | 95 | if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) { |
b7ced7d1 | 96 | error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits", |
97 | BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE); | |
52103b10 | 98 | return (0); |
99 | } | |
100 | ||
46f1eece | 101 | /* The response is MD5 of decrypted challenge plus session id. */ |
102 | len = BN_num_bytes(challenge); | |
103 | if (len <= 0 || len > 32) | |
104 | fatal("auth_rsa_verify_response: bad challenge length %d", len); | |
105 | memset(buf, 0, 32); | |
106 | BN_bn2bin(challenge, buf + 32 - len); | |
107 | MD5_Init(&md); | |
108 | MD5_Update(&md, buf, 32); | |
109 | MD5_Update(&md, session_id, 16); | |
110 | MD5_Final(mdbuf, &md); | |
111 | ||
112 | /* Verify that the response is the original challenge. */ | |
113 | if (memcmp(response, mdbuf, 16) != 0) { | |
114 | /* Wrong answer. */ | |
115 | return (0); | |
116 | } | |
117 | /* Correct answer. */ | |
118 | return (1); | |
119 | } | |
120 | ||
aa3378df | 121 | /* |
122 | * Performs the RSA authentication challenge-response dialog with the client, | |
123 | * and returns true (non-zero) if the client gave the correct answer to | |
124 | * our challenge; returns zero if the client gives a wrong answer. | |
125 | */ | |
8efc0c15 | 126 | |
127 | int | |
46f1eece | 128 | auth_rsa_challenge_dialog(Key *key) |
8efc0c15 | 129 | { |
c8d54615 | 130 | BIGNUM *challenge, *encrypted_challenge; |
46f1eece | 131 | u_char response[16]; |
132 | int i, success; | |
5260325f | 133 | |
b775c6f2 | 134 | if ((encrypted_challenge = BN_new()) == NULL) |
135 | fatal("auth_rsa_challenge_dialog: BN_new() failed"); | |
5260325f | 136 | |
1853d1ef | 137 | challenge = PRIVSEP(auth_rsa_generate_challenge(key)); |
5260325f | 138 | |
5260325f | 139 | /* Encrypt the challenge with the public key. */ |
46f1eece | 140 | rsa_public_encrypt(encrypted_challenge, challenge, key->rsa); |
5260325f | 141 | |
142 | /* Send the encrypted challenge to the client. */ | |
143 | packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE); | |
144 | packet_put_bignum(encrypted_challenge); | |
145 | packet_send(); | |
c8d54615 | 146 | BN_clear_free(encrypted_challenge); |
5260325f | 147 | packet_write_wait(); |
148 | ||
c8d54615 | 149 | /* Wait for a response. */ |
54a5250f | 150 | packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE); |
c8d54615 | 151 | for (i = 0; i < 16; i++) |
febd6f21 | 152 | response[i] = (u_char)packet_get_char(); |
95500969 | 153 | packet_check_eom(); |
c8d54615 | 154 | |
1853d1ef | 155 | success = PRIVSEP(auth_rsa_verify_response(key, challenge, response)); |
5260325f | 156 | BN_clear_free(challenge); |
46f1eece | 157 | return (success); |
8efc0c15 | 158 | } |
159 | ||
aa3378df | 160 | /* |
46f1eece | 161 | * check if there's user key matching client_n, |
162 | * return key if login is allowed, NULL otherwise | |
aa3378df | 163 | */ |
8efc0c15 | 164 | |
1853d1ef | 165 | int |
46f1eece | 166 | auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey) |
8efc0c15 | 167 | { |
ea067773 | 168 | char line[SSH_MAX_PUBKEY_BYTES], *file; |
1853d1ef | 169 | int allowed = 0; |
1e3b8b07 | 170 | u_int bits; |
5260325f | 171 | FILE *f; |
1e3b8b07 | 172 | u_long linenum = 0; |
5260325f | 173 | struct stat st; |
46df736f | 174 | Key *key; |
94ec8c6b | 175 | |
5260325f | 176 | /* Temporarily use the user's uid. */ |
63bd8c36 | 177 | temporarily_use_uid(pw); |
5260325f | 178 | |
179 | /* The authorized keys. */ | |
c8445989 | 180 | file = authorized_keys_file(pw); |
181 | debug("trying public RSA key file %s", file); | |
5260325f | 182 | |
183 | /* Fail quietly if file does not exist */ | |
184 | if (stat(file, &st) < 0) { | |
185 | /* Restore the privileged uid. */ | |
186 | restore_uid(); | |
c8445989 | 187 | xfree(file); |
159897f3 | 188 | return (0); |
8efc0c15 | 189 | } |
5260325f | 190 | /* Open the file containing the authorized keys. */ |
191 | f = fopen(file, "r"); | |
192 | if (!f) { | |
193 | /* Restore the privileged uid. */ | |
194 | restore_uid(); | |
c8445989 | 195 | xfree(file); |
159897f3 | 196 | return (0); |
8efc0c15 | 197 | } |
c8445989 | 198 | if (options.strict_modes && |
6978866a | 199 | secure_filename(f, file, pw, line, sizeof(line)) != 0) { |
c8445989 | 200 | xfree(file); |
201 | fclose(f); | |
bbe88b6d | 202 | logit("Authentication refused: %s", line); |
c8445989 | 203 | restore_uid(); |
159897f3 | 204 | return (0); |
5260325f | 205 | } |
46f1eece | 206 | |
207 | /* Flag indicating whether the key is allowed. */ | |
208 | allowed = 0; | |
5260325f | 209 | |
46df736f | 210 | key = key_new(KEY_RSA1); |
5260325f | 211 | |
aa3378df | 212 | /* |
213 | * Go though the accepted keys, looking for the current key. If | |
214 | * found, perform a challenge-response dialog to verify that the | |
215 | * user really has the corresponding private key. | |
216 | */ | |
ea067773 | 217 | while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) { |
5260325f | 218 | char *cp; |
ca75d7de | 219 | char *key_options; |
2ceb8101 | 220 | int keybits; |
5260325f | 221 | |
aa3378df | 222 | /* Skip leading whitespace, empty and comment lines. */ |
223 | for (cp = line; *cp == ' ' || *cp == '\t'; cp++) | |
224 | ; | |
5260325f | 225 | if (!*cp || *cp == '\n' || *cp == '#') |
226 | continue; | |
227 | ||
aa3378df | 228 | /* |
229 | * Check if there are options for this key, and if so, | |
230 | * save their starting address and skip the option part | |
231 | * for now. If there are no options, set the starting | |
232 | * address to NULL. | |
233 | */ | |
5260325f | 234 | if (*cp < '0' || *cp > '9') { |
235 | int quoted = 0; | |
ca75d7de | 236 | key_options = cp; |
5260325f | 237 | for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) { |
238 | if (*cp == '\\' && cp[1] == '"') | |
239 | cp++; /* Skip both */ | |
240 | else if (*cp == '"') | |
241 | quoted = !quoted; | |
8efc0c15 | 242 | } |
5260325f | 243 | } else |
ca75d7de | 244 | key_options = NULL; |
5260325f | 245 | |
246 | /* Parse the key from the line. */ | |
46df736f | 247 | if (hostfile_read_key(&cp, &bits, key) == 0) { |
96a7b0cc | 248 | debug("%.100s, line %lu: non ssh1 key syntax", |
42f11eb2 | 249 | file, linenum); |
5260325f | 250 | continue; |
8efc0c15 | 251 | } |
5260325f | 252 | /* cp now points to the comment part. */ |
253 | ||
5260325f | 254 | /* Check if the we have found the desired key (identified by its modulus). */ |
46df736f | 255 | if (BN_cmp(key->rsa->n, client_n) != 0) |
aa3378df | 256 | continue; |
5260325f | 257 | |
57112b5a | 258 | /* check the real bits */ |
2ceb8101 | 259 | keybits = BN_num_bits(key->rsa->n); |
260 | if (keybits < 0 || bits != (u_int)keybits) | |
bbe88b6d | 261 | logit("Warning: %s, line %lu: keysize mismatch: " |
57112b5a | 262 | "actual %d vs. announced %d.", |
46df736f | 263 | file, linenum, BN_num_bits(key->rsa->n), bits); |
57112b5a | 264 | |
5260325f | 265 | /* We have found the desired key. */ |
d343d900 | 266 | /* |
267 | * If our options do not allow this key to be used, | |
268 | * do not send challenge. | |
269 | */ | |
ca75d7de | 270 | if (!auth_parse_options(pw, key_options, file, linenum)) |
d343d900 | 271 | continue; |
5260325f | 272 | |
46f1eece | 273 | /* break out, this key is allowed */ |
274 | allowed = 1; | |
33de75a3 | 275 | break; |
8efc0c15 | 276 | } |
277 | ||
5260325f | 278 | /* Restore the privileged uid. */ |
279 | restore_uid(); | |
8efc0c15 | 280 | |
5260325f | 281 | /* Close the file. */ |
c8445989 | 282 | xfree(file); |
5260325f | 283 | fclose(f); |
8efc0c15 | 284 | |
46f1eece | 285 | /* return key if allowed */ |
286 | if (allowed && rkey != NULL) | |
287 | *rkey = key; | |
94ec8c6b | 288 | else |
46f1eece | 289 | key_free(key); |
290 | return (allowed); | |
291 | } | |
292 | ||
293 | /* | |
294 | * Performs the RSA authentication dialog with the client. This returns | |
295 | * 0 if the client could not be authenticated, and 1 if authentication was | |
296 | * successful. This may exit if there is a serious protocol violation. | |
297 | */ | |
298 | int | |
fdaef11e | 299 | auth_rsa(Authctxt *authctxt, BIGNUM *client_n) |
46f1eece | 300 | { |
301 | Key *key; | |
302 | char *fp; | |
fdaef11e | 303 | struct passwd *pw = authctxt->pw; |
46f1eece | 304 | |
305 | /* no user given */ | |
fdaef11e | 306 | if (!authctxt->valid) |
46f1eece | 307 | return 0; |
308 | ||
1853d1ef | 309 | if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) { |
94ec8c6b | 310 | auth_clear_options(); |
46f1eece | 311 | return (0); |
312 | } | |
313 | ||
314 | /* Perform the challenge-response dialog for this key. */ | |
315 | if (!auth_rsa_challenge_dialog(key)) { | |
316 | /* Wrong response. */ | |
317 | verbose("Wrong response to RSA authentication challenge."); | |
318 | packet_send_debug("Wrong response to RSA authentication challenge."); | |
319 | /* | |
320 | * Break out of the loop. Otherwise we might send | |
321 | * another challenge and break the protocol. | |
322 | */ | |
323 | key_free(key); | |
324 | return (0); | |
325 | } | |
326 | /* | |
327 | * Correct response. The client has been successfully | |
328 | * authenticated. Note that we have not yet processed the | |
329 | * options; this will be reset if the options cause the | |
330 | * authentication to be rejected. | |
331 | */ | |
332 | fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX); | |
333 | verbose("Found matching %s key: %s", | |
334 | key_type(key), fp); | |
335 | xfree(fp); | |
336 | key_free(key); | |
8efc0c15 | 337 | |
46f1eece | 338 | packet_send_debug("RSA authentication accepted."); |
339 | return (1); | |
8efc0c15 | 340 | } |