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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" | |
17 | RCSID("$OpenBSD: auth-rsa.c,v 1.44 2001/07/23 18:14:58 stevesk Exp $"); | |
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" | |
34 | ||
35 | /* import */ | |
36 | extern ServerOptions options; | |
37 | ||
38 | /* | |
39 | * Session identifier that is used to bind key exchange and authentication | |
40 | * responses to a particular session. | |
41 | */ | |
42 | extern u_char session_id[16]; | |
43 | ||
44 | /* | |
45 | * The .ssh/authorized_keys file contains public keys, one per line, in the | |
46 | * following format: | |
47 | * options bits e n comment | |
48 | * where bits, e and n are decimal numbers, | |
49 | * and comment is any string of characters up to newline. The maximum | |
50 | * length of a line is 8000 characters. See the documentation for a | |
51 | * description of the options. | |
52 | */ | |
53 | ||
54 | /* | |
55 | * Performs the RSA authentication challenge-response dialog with the client, | |
56 | * and returns true (non-zero) if the client gave the correct answer to | |
57 | * our challenge; returns zero if the client gives a wrong answer. | |
58 | */ | |
59 | ||
60 | int | |
61 | auth_rsa_challenge_dialog(RSA *pk) | |
62 | { | |
63 | BIGNUM *challenge, *encrypted_challenge; | |
64 | BN_CTX *ctx; | |
65 | u_char buf[32], mdbuf[16], response[16]; | |
66 | MD5_CTX md; | |
67 | u_int i; | |
68 | int plen, len; | |
69 | ||
70 | encrypted_challenge = BN_new(); | |
71 | challenge = BN_new(); | |
72 | ||
73 | /* Generate a random challenge. */ | |
74 | BN_rand(challenge, 256, 0, 0); | |
75 | ctx = BN_CTX_new(); | |
76 | BN_mod(challenge, challenge, pk->n, ctx); | |
77 | BN_CTX_free(ctx); | |
78 | ||
79 | /* Encrypt the challenge with the public key. */ | |
80 | rsa_public_encrypt(encrypted_challenge, challenge, pk); | |
81 | ||
82 | /* Send the encrypted challenge to the client. */ | |
83 | packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE); | |
84 | packet_put_bignum(encrypted_challenge); | |
85 | packet_send(); | |
86 | BN_clear_free(encrypted_challenge); | |
87 | packet_write_wait(); | |
88 | ||
89 | /* Wait for a response. */ | |
90 | packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE); | |
91 | packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE); | |
92 | for (i = 0; i < 16; i++) | |
93 | response[i] = packet_get_char(); | |
94 | ||
95 | /* The response is MD5 of decrypted challenge plus session id. */ | |
96 | len = BN_num_bytes(challenge); | |
97 | if (len <= 0 || len > 32) | |
98 | fatal("auth_rsa_challenge_dialog: bad challenge length %d", len); | |
99 | memset(buf, 0, 32); | |
100 | BN_bn2bin(challenge, buf + 32 - len); | |
101 | MD5_Init(&md); | |
102 | MD5_Update(&md, buf, 32); | |
103 | MD5_Update(&md, session_id, 16); | |
104 | MD5_Final(mdbuf, &md); | |
105 | BN_clear_free(challenge); | |
106 | ||
107 | /* Verify that the response is the original challenge. */ | |
108 | if (memcmp(response, mdbuf, 16) != 0) { | |
109 | /* Wrong answer. */ | |
110 | return 0; | |
111 | } | |
112 | /* Correct answer. */ | |
113 | return 1; | |
114 | } | |
115 | ||
116 | /* | |
117 | * Performs the RSA authentication dialog with the client. This returns | |
118 | * 0 if the client could not be authenticated, and 1 if authentication was | |
119 | * successful. This may exit if there is a serious protocol violation. | |
120 | */ | |
121 | ||
122 | int | |
123 | auth_rsa(struct passwd *pw, BIGNUM *client_n) | |
124 | { | |
125 | char line[8192], *file; | |
126 | int authenticated; | |
127 | u_int bits; | |
128 | FILE *f; | |
129 | u_long linenum = 0; | |
130 | struct stat st; | |
131 | RSA *pk; | |
132 | ||
133 | /* no user given */ | |
134 | if (pw == NULL) | |
135 | return 0; | |
136 | ||
137 | /* Temporarily use the user's uid. */ | |
138 | temporarily_use_uid(pw); | |
139 | ||
140 | /* The authorized keys. */ | |
141 | file = authorized_keys_file(pw); | |
142 | debug("trying public RSA key file %s", file); | |
143 | ||
144 | /* Fail quietly if file does not exist */ | |
145 | if (stat(file, &st) < 0) { | |
146 | /* Restore the privileged uid. */ | |
147 | restore_uid(); | |
148 | xfree(file); | |
149 | return 0; | |
150 | } | |
151 | /* Open the file containing the authorized keys. */ | |
152 | f = fopen(file, "r"); | |
153 | if (!f) { | |
154 | /* Restore the privileged uid. */ | |
155 | restore_uid(); | |
156 | packet_send_debug("Could not open %.900s for reading.", file); | |
157 | packet_send_debug("If your home is on an NFS volume, it may need to be world-readable."); | |
158 | xfree(file); | |
159 | return 0; | |
160 | } | |
161 | if (options.strict_modes && | |
162 | secure_filename(f, file, pw, line, sizeof(line)) != 0) { | |
163 | xfree(file); | |
164 | fclose(f); | |
165 | log("Authentication refused: %s", line); | |
166 | packet_send_debug("Authentication refused: %s", line); | |
167 | restore_uid(); | |
168 | return 0; | |
169 | } | |
170 | /* Flag indicating whether authentication has succeeded. */ | |
171 | authenticated = 0; | |
172 | ||
173 | pk = RSA_new(); | |
174 | pk->e = BN_new(); | |
175 | pk->n = BN_new(); | |
176 | ||
177 | /* | |
178 | * Go though the accepted keys, looking for the current key. If | |
179 | * found, perform a challenge-response dialog to verify that the | |
180 | * user really has the corresponding private key. | |
181 | */ | |
182 | while (fgets(line, sizeof(line), f)) { | |
183 | char *cp; | |
184 | char *options; | |
185 | ||
186 | linenum++; | |
187 | ||
188 | /* Skip leading whitespace, empty and comment lines. */ | |
189 | for (cp = line; *cp == ' ' || *cp == '\t'; cp++) | |
190 | ; | |
191 | if (!*cp || *cp == '\n' || *cp == '#') | |
192 | continue; | |
193 | ||
194 | /* | |
195 | * Check if there are options for this key, and if so, | |
196 | * save their starting address and skip the option part | |
197 | * for now. If there are no options, set the starting | |
198 | * address to NULL. | |
199 | */ | |
200 | if (*cp < '0' || *cp > '9') { | |
201 | int quoted = 0; | |
202 | options = cp; | |
203 | for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) { | |
204 | if (*cp == '\\' && cp[1] == '"') | |
205 | cp++; /* Skip both */ | |
206 | else if (*cp == '"') | |
207 | quoted = !quoted; | |
208 | } | |
209 | } else | |
210 | options = NULL; | |
211 | ||
212 | /* Parse the key from the line. */ | |
213 | if (!auth_rsa_read_key(&cp, &bits, pk->e, pk->n)) { | |
214 | debug("%.100s, line %lu: non ssh1 key syntax", | |
215 | file, linenum); | |
216 | continue; | |
217 | } | |
218 | /* cp now points to the comment part. */ | |
219 | ||
220 | /* Check if the we have found the desired key (identified by its modulus). */ | |
221 | if (BN_cmp(pk->n, client_n) != 0) | |
222 | continue; | |
223 | ||
224 | /* check the real bits */ | |
225 | if (bits != BN_num_bits(pk->n)) | |
226 | log("Warning: %s, line %lu: keysize mismatch: " | |
227 | "actual %d vs. announced %d.", | |
228 | file, linenum, BN_num_bits(pk->n), bits); | |
229 | ||
230 | /* We have found the desired key. */ | |
231 | /* | |
232 | * If our options do not allow this key to be used, | |
233 | * do not send challenge. | |
234 | */ | |
235 | if (!auth_parse_options(pw, options, file, linenum)) | |
236 | continue; | |
237 | ||
238 | /* Perform the challenge-response dialog for this key. */ | |
239 | if (!auth_rsa_challenge_dialog(pk)) { | |
240 | /* Wrong response. */ | |
241 | verbose("Wrong response to RSA authentication challenge."); | |
242 | packet_send_debug("Wrong response to RSA authentication challenge."); | |
243 | continue; | |
244 | } | |
245 | /* | |
246 | * Correct response. The client has been successfully | |
247 | * authenticated. Note that we have not yet processed the | |
248 | * options; this will be reset if the options cause the | |
249 | * authentication to be rejected. | |
250 | * Break out of the loop if authentication was successful; | |
251 | * otherwise continue searching. | |
252 | */ | |
253 | authenticated = 1; | |
254 | break; | |
255 | } | |
256 | ||
257 | /* Restore the privileged uid. */ | |
258 | restore_uid(); | |
259 | ||
260 | /* Close the file. */ | |
261 | xfree(file); | |
262 | fclose(f); | |
263 | ||
264 | RSA_free(pk); | |
265 | ||
266 | if (authenticated) | |
267 | packet_send_debug("RSA authentication accepted."); | |
268 | else | |
269 | auth_clear_options(); | |
270 | ||
271 | /* Return authentication result. */ | |
272 | return authenticated; | |
273 | } |