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8efc0c15 | 1 | /* |
5260325f | 2 | * Author: Tatu Ylonen <ylo@cs.hut.fi> |
5260325f | 3 | * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
4 | * All rights reserved | |
5260325f | 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. | |
6ae2364d | 8 | * |
bcbf86ec | 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". | |
5260325f | 14 | */ |
8efc0c15 | 15 | |
16 | #include "includes.h" | |
bcbf86ec | 17 | RCSID("$OpenBSD: auth-rsa.c,v 1.29 2000/09/07 21:13:36 markus Exp $"); |
8efc0c15 | 18 | |
19 | #include "rsa.h" | |
20 | #include "packet.h" | |
21 | #include "xmalloc.h" | |
22 | #include "ssh.h" | |
23 | #include "mpaux.h" | |
24 | #include "uidswap.h" | |
4fe2af09 | 25 | #include "match.h" |
6fa724bc | 26 | #include "servconf.h" |
38c295d6 | 27 | #include "auth-options.h" |
8efc0c15 | 28 | |
29 | #include <openssl/rsa.h> | |
30 | #include <openssl/md5.h> | |
31 | ||
aa3378df | 32 | /* |
33 | * Session identifier that is used to bind key exchange and authentication | |
34 | * responses to a particular session. | |
35 | */ | |
8efc0c15 | 36 | extern unsigned char session_id[16]; |
37 | ||
aa3378df | 38 | /* |
39 | * The .ssh/authorized_keys file contains public keys, one per line, in the | |
40 | * following format: | |
41 | * options bits e n comment | |
42 | * where bits, e and n are decimal numbers, | |
43 | * and comment is any string of characters up to newline. The maximum | |
44 | * length of a line is 8000 characters. See the documentation for a | |
45 | * description of the options. | |
46 | */ | |
8efc0c15 | 47 | |
aa3378df | 48 | /* |
49 | * Performs the RSA authentication challenge-response dialog with the client, | |
50 | * and returns true (non-zero) if the client gave the correct answer to | |
51 | * our challenge; returns zero if the client gives a wrong answer. | |
52 | */ | |
8efc0c15 | 53 | |
54 | int | |
4fe2af09 | 55 | auth_rsa_challenge_dialog(RSA *pk) |
8efc0c15 | 56 | { |
c8d54615 | 57 | BIGNUM *challenge, *encrypted_challenge; |
c8d54615 | 58 | BN_CTX *ctx; |
5260325f | 59 | unsigned char buf[32], mdbuf[16], response[16]; |
60 | MD5_CTX md; | |
61 | unsigned int i; | |
62 | int plen, len; | |
63 | ||
64 | encrypted_challenge = BN_new(); | |
65 | challenge = BN_new(); | |
5260325f | 66 | |
67 | /* Generate a random challenge. */ | |
68 | BN_rand(challenge, 256, 0, 0); | |
c8d54615 | 69 | ctx = BN_CTX_new(); |
4fe2af09 | 70 | BN_mod(challenge, challenge, pk->n, ctx); |
c8d54615 | 71 | BN_CTX_free(ctx); |
5260325f | 72 | |
5260325f | 73 | /* Encrypt the challenge with the public key. */ |
74 | rsa_public_encrypt(encrypted_challenge, challenge, pk); | |
5260325f | 75 | |
76 | /* Send the encrypted challenge to the client. */ | |
77 | packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE); | |
78 | packet_put_bignum(encrypted_challenge); | |
79 | packet_send(); | |
c8d54615 | 80 | BN_clear_free(encrypted_challenge); |
5260325f | 81 | packet_write_wait(); |
82 | ||
c8d54615 | 83 | /* Wait for a response. */ |
84 | packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE); | |
85 | packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE); | |
86 | for (i = 0; i < 16; i++) | |
87 | response[i] = packet_get_char(); | |
88 | ||
5260325f | 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_challenge_dialog: 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); | |
5260325f | 99 | BN_clear_free(challenge); |
5260325f | 100 | |
101 | /* Verify that the response is the original challenge. */ | |
102 | if (memcmp(response, mdbuf, 16) != 0) { | |
103 | /* Wrong answer. */ | |
104 | return 0; | |
105 | } | |
106 | /* Correct answer. */ | |
107 | return 1; | |
8efc0c15 | 108 | } |
109 | ||
aa3378df | 110 | /* |
111 | * Performs the RSA authentication dialog with the client. This returns | |
112 | * 0 if the client could not be authenticated, and 1 if authentication was | |
113 | * successful. This may exit if there is a serious protocol violation. | |
114 | */ | |
8efc0c15 | 115 | |
116 | int | |
6fa724bc | 117 | auth_rsa(struct passwd *pw, BIGNUM *client_n) |
8efc0c15 | 118 | { |
5260325f | 119 | extern ServerOptions options; |
120 | char line[8192], file[1024]; | |
121 | int authenticated; | |
122 | unsigned int bits; | |
123 | FILE *f; | |
124 | unsigned long linenum = 0; | |
125 | struct stat st; | |
4fe2af09 | 126 | RSA *pk; |
5260325f | 127 | |
128 | /* Temporarily use the user's uid. */ | |
129 | temporarily_use_uid(pw->pw_uid); | |
130 | ||
131 | /* The authorized keys. */ | |
132 | snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir, | |
133 | SSH_USER_PERMITTED_KEYS); | |
134 | ||
135 | /* Fail quietly if file does not exist */ | |
136 | if (stat(file, &st) < 0) { | |
137 | /* Restore the privileged uid. */ | |
138 | restore_uid(); | |
139 | return 0; | |
8efc0c15 | 140 | } |
5260325f | 141 | /* Open the file containing the authorized keys. */ |
142 | f = fopen(file, "r"); | |
143 | if (!f) { | |
144 | /* Restore the privileged uid. */ | |
145 | restore_uid(); | |
146 | packet_send_debug("Could not open %.900s for reading.", file); | |
147 | packet_send_debug("If your home is on an NFS volume, it may need to be world-readable."); | |
148 | return 0; | |
8efc0c15 | 149 | } |
5260325f | 150 | if (options.strict_modes) { |
151 | int fail = 0; | |
152 | char buf[1024]; | |
153 | /* Check open file in order to avoid open/stat races */ | |
154 | if (fstat(fileno(f), &st) < 0 || | |
155 | (st.st_uid != 0 && st.st_uid != pw->pw_uid) || | |
156 | (st.st_mode & 022) != 0) { | |
157 | snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: " | |
158 | "bad ownership or modes for '%s'.", pw->pw_name, file); | |
159 | fail = 1; | |
160 | } else { | |
161 | /* Check path to SSH_USER_PERMITTED_KEYS */ | |
162 | int i; | |
163 | static const char *check[] = { | |
164 | "", SSH_USER_DIR, NULL | |
165 | }; | |
166 | for (i = 0; check[i]; i++) { | |
167 | snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]); | |
168 | if (stat(line, &st) < 0 || | |
169 | (st.st_uid != 0 && st.st_uid != pw->pw_uid) || | |
170 | (st.st_mode & 022) != 0) { | |
171 | snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: " | |
172 | "bad ownership or modes for '%s'.", pw->pw_name, line); | |
173 | fail = 1; | |
174 | break; | |
175 | } | |
176 | } | |
8efc0c15 | 177 | } |
5260325f | 178 | if (fail) { |
a306f2dd | 179 | fclose(f); |
089fbbd2 | 180 | log("%s",buf); |
181 | packet_send_debug("%s",buf); | |
5260325f | 182 | restore_uid(); |
183 | return 0; | |
8efc0c15 | 184 | } |
5260325f | 185 | } |
186 | /* Flag indicating whether authentication has succeeded. */ | |
187 | authenticated = 0; | |
188 | ||
4fe2af09 | 189 | pk = RSA_new(); |
190 | pk->e = BN_new(); | |
191 | pk->n = BN_new(); | |
5260325f | 192 | |
aa3378df | 193 | /* |
194 | * Go though the accepted keys, looking for the current key. If | |
195 | * found, perform a challenge-response dialog to verify that the | |
196 | * user really has the corresponding private key. | |
197 | */ | |
5260325f | 198 | while (fgets(line, sizeof(line), f)) { |
199 | char *cp; | |
200 | char *options; | |
201 | ||
202 | linenum++; | |
203 | ||
aa3378df | 204 | /* Skip leading whitespace, empty and comment lines. */ |
205 | for (cp = line; *cp == ' ' || *cp == '\t'; cp++) | |
206 | ; | |
5260325f | 207 | if (!*cp || *cp == '\n' || *cp == '#') |
208 | continue; | |
209 | ||
aa3378df | 210 | /* |
211 | * Check if there are options for this key, and if so, | |
212 | * save their starting address and skip the option part | |
213 | * for now. If there are no options, set the starting | |
214 | * address to NULL. | |
215 | */ | |
5260325f | 216 | if (*cp < '0' || *cp > '9') { |
217 | int quoted = 0; | |
218 | options = cp; | |
219 | for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) { | |
220 | if (*cp == '\\' && cp[1] == '"') | |
221 | cp++; /* Skip both */ | |
222 | else if (*cp == '"') | |
223 | quoted = !quoted; | |
8efc0c15 | 224 | } |
5260325f | 225 | } else |
226 | options = NULL; | |
227 | ||
228 | /* Parse the key from the line. */ | |
4fe2af09 | 229 | if (!auth_rsa_read_key(&cp, &bits, pk->e, pk->n)) { |
5260325f | 230 | debug("%.100s, line %lu: bad key syntax", |
231 | SSH_USER_PERMITTED_KEYS, linenum); | |
232 | packet_send_debug("%.100s, line %lu: bad key syntax", | |
6ae2364d | 233 | SSH_USER_PERMITTED_KEYS, linenum); |
5260325f | 234 | continue; |
8efc0c15 | 235 | } |
5260325f | 236 | /* cp now points to the comment part. */ |
237 | ||
5260325f | 238 | /* Check if the we have found the desired key (identified by its modulus). */ |
4fe2af09 | 239 | if (BN_cmp(pk->n, client_n) != 0) |
aa3378df | 240 | continue; |
5260325f | 241 | |
57112b5a | 242 | /* check the real bits */ |
4fe2af09 | 243 | if (bits != BN_num_bits(pk->n)) |
57112b5a | 244 | log("Warning: %s, line %ld: keysize mismatch: " |
245 | "actual %d vs. announced %d.", | |
4fe2af09 | 246 | file, linenum, BN_num_bits(pk->n), bits); |
57112b5a | 247 | |
5260325f | 248 | /* We have found the desired key. */ |
249 | ||
250 | /* Perform the challenge-response dialog for this key. */ | |
4fe2af09 | 251 | if (!auth_rsa_challenge_dialog(pk)) { |
5260325f | 252 | /* Wrong response. */ |
253 | verbose("Wrong response to RSA authentication challenge."); | |
254 | packet_send_debug("Wrong response to RSA authentication challenge."); | |
255 | continue; | |
8efc0c15 | 256 | } |
aa3378df | 257 | /* |
258 | * Correct response. The client has been successfully | |
259 | * authenticated. Note that we have not yet processed the | |
260 | * options; this will be reset if the options cause the | |
261 | * authentication to be rejected. | |
aa3378df | 262 | * Break out of the loop if authentication was successful; |
263 | * otherwise continue searching. | |
264 | */ | |
38c295d6 | 265 | authenticated = auth_parse_options(pw, options, linenum); |
5260325f | 266 | if (authenticated) |
267 | break; | |
8efc0c15 | 268 | } |
269 | ||
5260325f | 270 | /* Restore the privileged uid. */ |
271 | restore_uid(); | |
8efc0c15 | 272 | |
5260325f | 273 | /* Close the file. */ |
274 | fclose(f); | |
8efc0c15 | 275 | |
4fe2af09 | 276 | RSA_free(pk); |
8efc0c15 | 277 | |
5260325f | 278 | if (authenticated) |
279 | packet_send_debug("RSA authentication accepted."); | |
8efc0c15 | 280 | |
5260325f | 281 | /* Return authentication result. */ |
282 | return authenticated; | |
8efc0c15 | 283 | } |