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2e437378 | 1 | /* $OpenBSD: auth-rsa.c,v 1.72 2006/11/06 21:25:27 markus Exp $ */ |
3c0ef626 | 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" | |
2e437378 | 18 | |
19 | #include <sys/types.h> | |
20 | #include <sys/stat.h> | |
3c0ef626 | 21 | |
22 | #include <openssl/rsa.h> | |
23 | #include <openssl/md5.h> | |
24 | ||
2e437378 | 25 | #include <pwd.h> |
26 | #include <stdio.h> | |
27 | #include <stdarg.h> | |
28 | #include <string.h> | |
29 | ||
30 | #include "xmalloc.h" | |
3c0ef626 | 31 | #include "rsa.h" |
32 | #include "packet.h" | |
3c0ef626 | 33 | #include "ssh1.h" |
3c0ef626 | 34 | #include "uidswap.h" |
35 | #include "match.h" | |
2e437378 | 36 | #include "buffer.h" |
3c0ef626 | 37 | #include "auth-options.h" |
38 | #include "pathnames.h" | |
39 | #include "log.h" | |
40 | #include "servconf.h" | |
2e437378 | 41 | #include "key.h" |
e9a17296 | 42 | #include "hostfile.h" |
2e437378 | 43 | #include "auth.h" |
44 | #ifdef GSSAPI | |
45 | #include "ssh-gss.h" | |
46 | #endif | |
2980ea68 | 47 | #include "monitor_wrap.h" |
48 | #include "ssh.h" | |
34fee935 | 49 | #include "misc.h" |
3c0ef626 | 50 | |
51 | /* import */ | |
52 | extern ServerOptions options; | |
53 | ||
54 | /* | |
55 | * Session identifier that is used to bind key exchange and authentication | |
56 | * responses to a particular session. | |
57 | */ | |
58 | extern u_char session_id[16]; | |
59 | ||
60 | /* | |
61 | * The .ssh/authorized_keys file contains public keys, one per line, in the | |
62 | * following format: | |
63 | * options bits e n comment | |
64 | * where bits, e and n are decimal numbers, | |
65 | * and comment is any string of characters up to newline. The maximum | |
34fee935 | 66 | * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a |
3c0ef626 | 67 | * description of the options. |
68 | */ | |
69 | ||
2980ea68 | 70 | BIGNUM * |
71 | auth_rsa_generate_challenge(Key *key) | |
72 | { | |
73 | BIGNUM *challenge; | |
74 | BN_CTX *ctx; | |
75 | ||
76 | if ((challenge = BN_new()) == NULL) | |
77 | fatal("auth_rsa_generate_challenge: BN_new() failed"); | |
78 | /* Generate a random challenge. */ | |
2e437378 | 79 | if (BN_rand(challenge, 256, 0, 0) == 0) |
80 | fatal("auth_rsa_generate_challenge: BN_rand failed"); | |
2980ea68 | 81 | if ((ctx = BN_CTX_new()) == NULL) |
2e437378 | 82 | fatal("auth_rsa_generate_challenge: BN_CTX_new failed"); |
83 | if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0) | |
84 | fatal("auth_rsa_generate_challenge: BN_mod failed"); | |
2980ea68 | 85 | BN_CTX_free(ctx); |
86 | ||
87 | return challenge; | |
88 | } | |
89 | ||
90 | int | |
91 | auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16]) | |
92 | { | |
93 | u_char buf[32], mdbuf[16]; | |
94 | MD5_CTX md; | |
95 | int len; | |
96 | ||
97 | /* don't allow short keys */ | |
98 | if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) { | |
ff2d7a98 | 99 | error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits", |
100 | BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE); | |
2980ea68 | 101 | return (0); |
102 | } | |
103 | ||
104 | /* The response is MD5 of decrypted challenge plus session id. */ | |
105 | len = BN_num_bytes(challenge); | |
106 | if (len <= 0 || len > 32) | |
107 | fatal("auth_rsa_verify_response: bad challenge length %d", len); | |
108 | memset(buf, 0, 32); | |
109 | BN_bn2bin(challenge, buf + 32 - len); | |
110 | MD5_Init(&md); | |
111 | MD5_Update(&md, buf, 32); | |
112 | MD5_Update(&md, session_id, 16); | |
113 | MD5_Final(mdbuf, &md); | |
114 | ||
115 | /* Verify that the response is the original challenge. */ | |
116 | if (memcmp(response, mdbuf, 16) != 0) { | |
117 | /* Wrong answer. */ | |
118 | return (0); | |
119 | } | |
120 | /* Correct answer. */ | |
121 | return (1); | |
122 | } | |
123 | ||
3c0ef626 | 124 | /* |
125 | * Performs the RSA authentication challenge-response dialog with the client, | |
126 | * and returns true (non-zero) if the client gave the correct answer to | |
127 | * our challenge; returns zero if the client gives a wrong answer. | |
128 | */ | |
129 | ||
130 | int | |
2980ea68 | 131 | auth_rsa_challenge_dialog(Key *key) |
3c0ef626 | 132 | { |
133 | BIGNUM *challenge, *encrypted_challenge; | |
2980ea68 | 134 | u_char response[16]; |
135 | int i, success; | |
3c0ef626 | 136 | |
e9a17296 | 137 | if ((encrypted_challenge = BN_new()) == NULL) |
138 | fatal("auth_rsa_challenge_dialog: BN_new() failed"); | |
3c0ef626 | 139 | |
2980ea68 | 140 | challenge = PRIVSEP(auth_rsa_generate_challenge(key)); |
3c0ef626 | 141 | |
142 | /* Encrypt the challenge with the public key. */ | |
2980ea68 | 143 | rsa_public_encrypt(encrypted_challenge, challenge, key->rsa); |
3c0ef626 | 144 | |
145 | /* Send the encrypted challenge to the client. */ | |
146 | packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE); | |
147 | packet_put_bignum(encrypted_challenge); | |
148 | packet_send(); | |
149 | BN_clear_free(encrypted_challenge); | |
150 | packet_write_wait(); | |
151 | ||
152 | /* Wait for a response. */ | |
e9a17296 | 153 | packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE); |
3c0ef626 | 154 | for (i = 0; i < 16; i++) |
2e437378 | 155 | response[i] = (u_char)packet_get_char(); |
e9a17296 | 156 | packet_check_eom(); |
3c0ef626 | 157 | |
2980ea68 | 158 | success = PRIVSEP(auth_rsa_verify_response(key, challenge, response)); |
3c0ef626 | 159 | BN_clear_free(challenge); |
2980ea68 | 160 | return (success); |
3c0ef626 | 161 | } |
162 | ||
163 | /* | |
2980ea68 | 164 | * check if there's user key matching client_n, |
165 | * return key if login is allowed, NULL otherwise | |
3c0ef626 | 166 | */ |
167 | ||
168 | int | |
2980ea68 | 169 | auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey) |
3c0ef626 | 170 | { |
34fee935 | 171 | char line[SSH_MAX_PUBKEY_BYTES], *file; |
2980ea68 | 172 | int allowed = 0; |
3c0ef626 | 173 | u_int bits; |
174 | FILE *f; | |
175 | u_long linenum = 0; | |
176 | struct stat st; | |
e9a17296 | 177 | Key *key; |
3c0ef626 | 178 | |
179 | /* Temporarily use the user's uid. */ | |
180 | temporarily_use_uid(pw); | |
181 | ||
182 | /* The authorized keys. */ | |
183 | file = authorized_keys_file(pw); | |
184 | debug("trying public RSA key file %s", file); | |
185 | ||
186 | /* Fail quietly if file does not exist */ | |
187 | if (stat(file, &st) < 0) { | |
188 | /* Restore the privileged uid. */ | |
189 | restore_uid(); | |
190 | xfree(file); | |
2980ea68 | 191 | return (0); |
3c0ef626 | 192 | } |
193 | /* Open the file containing the authorized keys. */ | |
194 | f = fopen(file, "r"); | |
195 | if (!f) { | |
196 | /* Restore the privileged uid. */ | |
197 | restore_uid(); | |
3c0ef626 | 198 | xfree(file); |
2980ea68 | 199 | return (0); |
3c0ef626 | 200 | } |
201 | if (options.strict_modes && | |
202 | secure_filename(f, file, pw, line, sizeof(line)) != 0) { | |
203 | xfree(file); | |
204 | fclose(f); | |
70791e56 | 205 | logit("Authentication refused: %s", line); |
3c0ef626 | 206 | restore_uid(); |
2980ea68 | 207 | return (0); |
3c0ef626 | 208 | } |
2980ea68 | 209 | |
210 | /* Flag indicating whether the key is allowed. */ | |
211 | allowed = 0; | |
3c0ef626 | 212 | |
e9a17296 | 213 | key = key_new(KEY_RSA1); |
3c0ef626 | 214 | |
215 | /* | |
216 | * Go though the accepted keys, looking for the current key. If | |
217 | * found, perform a challenge-response dialog to verify that the | |
218 | * user really has the corresponding private key. | |
219 | */ | |
34fee935 | 220 | while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) { |
3c0ef626 | 221 | char *cp; |
1b56ff3d | 222 | char *key_options; |
34fee935 | 223 | int keybits; |
3c0ef626 | 224 | |
225 | /* Skip leading whitespace, empty and comment lines. */ | |
226 | for (cp = line; *cp == ' ' || *cp == '\t'; cp++) | |
227 | ; | |
228 | if (!*cp || *cp == '\n' || *cp == '#') | |
229 | continue; | |
230 | ||
231 | /* | |
232 | * Check if there are options for this key, and if so, | |
233 | * save their starting address and skip the option part | |
234 | * for now. If there are no options, set the starting | |
235 | * address to NULL. | |
236 | */ | |
237 | if (*cp < '0' || *cp > '9') { | |
238 | int quoted = 0; | |
1b56ff3d | 239 | key_options = cp; |
3c0ef626 | 240 | for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) { |
241 | if (*cp == '\\' && cp[1] == '"') | |
242 | cp++; /* Skip both */ | |
243 | else if (*cp == '"') | |
244 | quoted = !quoted; | |
245 | } | |
246 | } else | |
1b56ff3d | 247 | key_options = NULL; |
3c0ef626 | 248 | |
249 | /* Parse the key from the line. */ | |
e9a17296 | 250 | if (hostfile_read_key(&cp, &bits, key) == 0) { |
3c0ef626 | 251 | debug("%.100s, line %lu: non ssh1 key syntax", |
252 | file, linenum); | |
253 | continue; | |
254 | } | |
255 | /* cp now points to the comment part. */ | |
256 | ||
257 | /* Check if the we have found the desired key (identified by its modulus). */ | |
e9a17296 | 258 | if (BN_cmp(key->rsa->n, client_n) != 0) |
3c0ef626 | 259 | continue; |
260 | ||
261 | /* check the real bits */ | |
34fee935 | 262 | keybits = BN_num_bits(key->rsa->n); |
263 | if (keybits < 0 || bits != (u_int)keybits) | |
70791e56 | 264 | logit("Warning: %s, line %lu: keysize mismatch: " |
3c0ef626 | 265 | "actual %d vs. announced %d.", |
e9a17296 | 266 | file, linenum, BN_num_bits(key->rsa->n), bits); |
3c0ef626 | 267 | |
268 | /* We have found the desired key. */ | |
269 | /* | |
270 | * If our options do not allow this key to be used, | |
271 | * do not send challenge. | |
272 | */ | |
1b56ff3d | 273 | if (!auth_parse_options(pw, key_options, file, linenum)) |
3c0ef626 | 274 | continue; |
275 | ||
2980ea68 | 276 | /* break out, this key is allowed */ |
277 | allowed = 1; | |
3c0ef626 | 278 | break; |
279 | } | |
280 | ||
281 | /* Restore the privileged uid. */ | |
282 | restore_uid(); | |
283 | ||
284 | /* Close the file. */ | |
285 | xfree(file); | |
286 | fclose(f); | |
287 | ||
2980ea68 | 288 | /* return key if allowed */ |
289 | if (allowed && rkey != NULL) | |
290 | *rkey = key; | |
3c0ef626 | 291 | else |
2980ea68 | 292 | key_free(key); |
293 | return (allowed); | |
294 | } | |
295 | ||
296 | /* | |
297 | * Performs the RSA authentication dialog with the client. This returns | |
298 | * 0 if the client could not be authenticated, and 1 if authentication was | |
299 | * successful. This may exit if there is a serious protocol violation. | |
300 | */ | |
301 | int | |
416fd2a8 | 302 | auth_rsa(Authctxt *authctxt, BIGNUM *client_n) |
2980ea68 | 303 | { |
304 | Key *key; | |
305 | char *fp; | |
416fd2a8 | 306 | struct passwd *pw = authctxt->pw; |
2980ea68 | 307 | |
308 | /* no user given */ | |
416fd2a8 | 309 | if (!authctxt->valid) |
2980ea68 | 310 | return 0; |
311 | ||
312 | if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) { | |
3c0ef626 | 313 | auth_clear_options(); |
2980ea68 | 314 | return (0); |
315 | } | |
316 | ||
317 | /* Perform the challenge-response dialog for this key. */ | |
318 | if (!auth_rsa_challenge_dialog(key)) { | |
319 | /* Wrong response. */ | |
320 | verbose("Wrong response to RSA authentication challenge."); | |
321 | packet_send_debug("Wrong response to RSA authentication challenge."); | |
322 | /* | |
323 | * Break out of the loop. Otherwise we might send | |
324 | * another challenge and break the protocol. | |
325 | */ | |
326 | key_free(key); | |
327 | return (0); | |
328 | } | |
329 | /* | |
330 | * Correct response. The client has been successfully | |
331 | * authenticated. Note that we have not yet processed the | |
332 | * options; this will be reset if the options cause the | |
333 | * authentication to be rejected. | |
334 | */ | |
335 | fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX); | |
336 | verbose("Found matching %s key: %s", | |
337 | key_type(key), fp); | |
338 | xfree(fp); | |
339 | key_free(key); | |
3c0ef626 | 340 | |
2980ea68 | 341 | packet_send_debug("RSA authentication accepted."); |
342 | return (1); | |
3c0ef626 | 343 | } |