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3c0ef626 | 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" | |
996d5e62 | 17 | RCSID("$OpenBSD: auth-rsa.c,v 1.62 2004/12/11 01:48:56 dtucker Exp $"); |
3c0ef626 | 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" | |
3c0ef626 | 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" | |
e9a17296 | 33 | #include "hostfile.h" |
700318f3 | 34 | #include "monitor_wrap.h" |
35 | #include "ssh.h" | |
996d5e62 | 36 | #include "misc.h" |
3c0ef626 | 37 | |
38 | /* import */ | |
39 | extern ServerOptions options; | |
40 | ||
41 | /* | |
42 | * Session identifier that is used to bind key exchange and authentication | |
43 | * responses to a particular session. | |
44 | */ | |
45 | extern 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 | |
996d5e62 | 53 | * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a |
3c0ef626 | 54 | * description of the options. |
55 | */ | |
56 | ||
700318f3 | 57 | BIGNUM * |
58 | auth_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 | ||
75 | int | |
76 | auth_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) { | |
f5799ae1 | 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); | |
700318f3 | 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 | ||
3c0ef626 | 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 | ||
115 | int | |
700318f3 | 116 | auth_rsa_challenge_dialog(Key *key) |
3c0ef626 | 117 | { |
118 | BIGNUM *challenge, *encrypted_challenge; | |
700318f3 | 119 | u_char response[16]; |
120 | int i, success; | |
3c0ef626 | 121 | |
e9a17296 | 122 | if ((encrypted_challenge = BN_new()) == NULL) |
123 | fatal("auth_rsa_challenge_dialog: BN_new() failed"); | |
3c0ef626 | 124 | |
700318f3 | 125 | challenge = PRIVSEP(auth_rsa_generate_challenge(key)); |
3c0ef626 | 126 | |
127 | /* Encrypt the challenge with the public key. */ | |
700318f3 | 128 | rsa_public_encrypt(encrypted_challenge, challenge, key->rsa); |
3c0ef626 | 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. */ | |
e9a17296 | 138 | packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE); |
3c0ef626 | 139 | for (i = 0; i < 16; i++) |
140 | response[i] = packet_get_char(); | |
e9a17296 | 141 | packet_check_eom(); |
3c0ef626 | 142 | |
700318f3 | 143 | success = PRIVSEP(auth_rsa_verify_response(key, challenge, response)); |
3c0ef626 | 144 | BN_clear_free(challenge); |
700318f3 | 145 | return (success); |
3c0ef626 | 146 | } |
147 | ||
148 | /* | |
700318f3 | 149 | * check if there's user key matching client_n, |
150 | * return key if login is allowed, NULL otherwise | |
3c0ef626 | 151 | */ |
152 | ||
153 | int | |
700318f3 | 154 | auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey) |
3c0ef626 | 155 | { |
996d5e62 | 156 | char line[SSH_MAX_PUBKEY_BYTES], *file; |
700318f3 | 157 | int allowed = 0; |
3c0ef626 | 158 | u_int bits; |
159 | FILE *f; | |
160 | u_long linenum = 0; | |
161 | struct stat st; | |
e9a17296 | 162 | Key *key; |
3c0ef626 | 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); | |
700318f3 | 176 | return (0); |
3c0ef626 | 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(); | |
3c0ef626 | 183 | xfree(file); |
700318f3 | 184 | return (0); |
3c0ef626 | 185 | } |
186 | if (options.strict_modes && | |
187 | secure_filename(f, file, pw, line, sizeof(line)) != 0) { | |
188 | xfree(file); | |
189 | fclose(f); | |
0fff78ff | 190 | logit("Authentication refused: %s", line); |
3c0ef626 | 191 | restore_uid(); |
700318f3 | 192 | return (0); |
3c0ef626 | 193 | } |
700318f3 | 194 | |
195 | /* Flag indicating whether the key is allowed. */ | |
196 | allowed = 0; | |
3c0ef626 | 197 | |
e9a17296 | 198 | key = key_new(KEY_RSA1); |
3c0ef626 | 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 | */ | |
996d5e62 | 205 | while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) { |
3c0ef626 | 206 | char *cp; |
c9f39d2c | 207 | char *key_options; |
3c0ef626 | 208 | |
3c0ef626 | 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; | |
c9f39d2c | 223 | key_options = cp; |
3c0ef626 | 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 | |
c9f39d2c | 231 | key_options = NULL; |
3c0ef626 | 232 | |
233 | /* Parse the key from the line. */ | |
e9a17296 | 234 | if (hostfile_read_key(&cp, &bits, key) == 0) { |
3c0ef626 | 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). */ | |
e9a17296 | 242 | if (BN_cmp(key->rsa->n, client_n) != 0) |
3c0ef626 | 243 | continue; |
244 | ||
245 | /* check the real bits */ | |
e9a17296 | 246 | if (bits != BN_num_bits(key->rsa->n)) |
0fff78ff | 247 | logit("Warning: %s, line %lu: keysize mismatch: " |
3c0ef626 | 248 | "actual %d vs. announced %d.", |
e9a17296 | 249 | file, linenum, BN_num_bits(key->rsa->n), bits); |
3c0ef626 | 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 | */ | |
c9f39d2c | 256 | if (!auth_parse_options(pw, key_options, file, linenum)) |
3c0ef626 | 257 | continue; |
258 | ||
700318f3 | 259 | /* break out, this key is allowed */ |
260 | allowed = 1; | |
3c0ef626 | 261 | break; |
262 | } | |
263 | ||
264 | /* Restore the privileged uid. */ | |
265 | restore_uid(); | |
266 | ||
267 | /* Close the file. */ | |
268 | xfree(file); | |
269 | fclose(f); | |
270 | ||
700318f3 | 271 | /* return key if allowed */ |
272 | if (allowed && rkey != NULL) | |
273 | *rkey = key; | |
3c0ef626 | 274 | else |
700318f3 | 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 | */ | |
284 | int | |
cdd66111 | 285 | auth_rsa(Authctxt *authctxt, BIGNUM *client_n) |
700318f3 | 286 | { |
287 | Key *key; | |
288 | char *fp; | |
cdd66111 | 289 | struct passwd *pw = authctxt->pw; |
700318f3 | 290 | |
291 | /* no user given */ | |
cdd66111 | 292 | if (!authctxt->valid) |
700318f3 | 293 | return 0; |
294 | ||
295 | if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) { | |
3c0ef626 | 296 | auth_clear_options(); |
700318f3 | 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); | |
3c0ef626 | 323 | |
700318f3 | 324 | packet_send_debug("RSA authentication accepted."); |
325 | return (1); | |
3c0ef626 | 326 | } |