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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" | |
416fd2a8 | 17 | RCSID("$OpenBSD: auth-rsa.c,v 1.58 2003/11/04 08:54:09 djm 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" | |
26 | #include "mpaux.h" | |
27 | #include "uidswap.h" | |
28 | #include "match.h" | |
29 | #include "auth-options.h" | |
30 | #include "pathnames.h" | |
31 | #include "log.h" | |
32 | #include "servconf.h" | |
33 | #include "auth.h" | |
e9a17296 | 34 | #include "hostfile.h" |
2980ea68 | 35 | #include "monitor_wrap.h" |
36 | #include "ssh.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 | |
53 | * length of a line is 8000 characters. See the documentation for a | |
54 | * description of the options. | |
55 | */ | |
56 | ||
2980ea68 | 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) { | |
ff2d7a98 | 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); | |
2980ea68 | 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 | |
2980ea68 | 116 | auth_rsa_challenge_dialog(Key *key) |
3c0ef626 | 117 | { |
118 | BIGNUM *challenge, *encrypted_challenge; | |
2980ea68 | 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 | |
2980ea68 | 125 | challenge = PRIVSEP(auth_rsa_generate_challenge(key)); |
3c0ef626 | 126 | |
127 | /* Encrypt the challenge with the public key. */ | |
2980ea68 | 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 | |
2980ea68 | 143 | success = PRIVSEP(auth_rsa_verify_response(key, challenge, response)); |
3c0ef626 | 144 | BN_clear_free(challenge); |
2980ea68 | 145 | return (success); |
3c0ef626 | 146 | } |
147 | ||
148 | /* | |
2980ea68 | 149 | * check if there's user key matching client_n, |
150 | * return key if login is allowed, NULL otherwise | |
3c0ef626 | 151 | */ |
152 | ||
153 | int | |
2980ea68 | 154 | auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey) |
3c0ef626 | 155 | { |
156 | char line[8192], *file; | |
2980ea68 | 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); | |
2980ea68 | 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); |
2980ea68 | 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); | |
70791e56 | 190 | logit("Authentication refused: %s", line); |
3c0ef626 | 191 | restore_uid(); |
2980ea68 | 192 | return (0); |
3c0ef626 | 193 | } |
2980ea68 | 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 | */ | |
205 | while (fgets(line, sizeof(line), f)) { | |
206 | char *cp; | |
207 | char *options; | |
208 | ||
209 | linenum++; | |
210 | ||
211 | /* Skip leading whitespace, empty and comment lines. */ | |
212 | for (cp = line; *cp == ' ' || *cp == '\t'; cp++) | |
213 | ; | |
214 | if (!*cp || *cp == '\n' || *cp == '#') | |
215 | continue; | |
216 | ||
217 | /* | |
218 | * Check if there are options for this key, and if so, | |
219 | * save their starting address and skip the option part | |
220 | * for now. If there are no options, set the starting | |
221 | * address to NULL. | |
222 | */ | |
223 | if (*cp < '0' || *cp > '9') { | |
224 | int quoted = 0; | |
225 | options = cp; | |
226 | for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) { | |
227 | if (*cp == '\\' && cp[1] == '"') | |
228 | cp++; /* Skip both */ | |
229 | else if (*cp == '"') | |
230 | quoted = !quoted; | |
231 | } | |
232 | } else | |
233 | options = NULL; | |
234 | ||
235 | /* Parse the key from the line. */ | |
e9a17296 | 236 | if (hostfile_read_key(&cp, &bits, key) == 0) { |
3c0ef626 | 237 | debug("%.100s, line %lu: non ssh1 key syntax", |
238 | file, linenum); | |
239 | continue; | |
240 | } | |
241 | /* cp now points to the comment part. */ | |
242 | ||
243 | /* Check if the we have found the desired key (identified by its modulus). */ | |
e9a17296 | 244 | if (BN_cmp(key->rsa->n, client_n) != 0) |
3c0ef626 | 245 | continue; |
246 | ||
247 | /* check the real bits */ | |
e9a17296 | 248 | if (bits != BN_num_bits(key->rsa->n)) |
70791e56 | 249 | logit("Warning: %s, line %lu: keysize mismatch: " |
3c0ef626 | 250 | "actual %d vs. announced %d.", |
e9a17296 | 251 | file, linenum, BN_num_bits(key->rsa->n), bits); |
3c0ef626 | 252 | |
253 | /* We have found the desired key. */ | |
254 | /* | |
255 | * If our options do not allow this key to be used, | |
256 | * do not send challenge. | |
257 | */ | |
258 | if (!auth_parse_options(pw, options, file, linenum)) | |
259 | continue; | |
260 | ||
2980ea68 | 261 | /* break out, this key is allowed */ |
262 | allowed = 1; | |
3c0ef626 | 263 | break; |
264 | } | |
265 | ||
266 | /* Restore the privileged uid. */ | |
267 | restore_uid(); | |
268 | ||
269 | /* Close the file. */ | |
270 | xfree(file); | |
271 | fclose(f); | |
272 | ||
2980ea68 | 273 | /* return key if allowed */ |
274 | if (allowed && rkey != NULL) | |
275 | *rkey = key; | |
3c0ef626 | 276 | else |
2980ea68 | 277 | key_free(key); |
278 | return (allowed); | |
279 | } | |
280 | ||
281 | /* | |
282 | * Performs the RSA authentication dialog with the client. This returns | |
283 | * 0 if the client could not be authenticated, and 1 if authentication was | |
284 | * successful. This may exit if there is a serious protocol violation. | |
285 | */ | |
286 | int | |
416fd2a8 | 287 | auth_rsa(Authctxt *authctxt, BIGNUM *client_n) |
2980ea68 | 288 | { |
289 | Key *key; | |
290 | char *fp; | |
416fd2a8 | 291 | struct passwd *pw = authctxt->pw; |
2980ea68 | 292 | |
293 | /* no user given */ | |
416fd2a8 | 294 | if (!authctxt->valid) |
2980ea68 | 295 | return 0; |
296 | ||
297 | if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) { | |
3c0ef626 | 298 | auth_clear_options(); |
2980ea68 | 299 | return (0); |
300 | } | |
301 | ||
302 | /* Perform the challenge-response dialog for this key. */ | |
303 | if (!auth_rsa_challenge_dialog(key)) { | |
304 | /* Wrong response. */ | |
305 | verbose("Wrong response to RSA authentication challenge."); | |
306 | packet_send_debug("Wrong response to RSA authentication challenge."); | |
307 | /* | |
308 | * Break out of the loop. Otherwise we might send | |
309 | * another challenge and break the protocol. | |
310 | */ | |
311 | key_free(key); | |
312 | return (0); | |
313 | } | |
314 | /* | |
315 | * Correct response. The client has been successfully | |
316 | * authenticated. Note that we have not yet processed the | |
317 | * options; this will be reset if the options cause the | |
318 | * authentication to be rejected. | |
319 | */ | |
320 | fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX); | |
321 | verbose("Found matching %s key: %s", | |
322 | key_type(key), fp); | |
323 | xfree(fp); | |
324 | key_free(key); | |
3c0ef626 | 325 | |
2980ea68 | 326 | packet_send_debug("RSA authentication accepted."); |
327 | return (1); | |
3c0ef626 | 328 | } |