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8efc0c15 | 1 | /* |
6ae2364d | 2 | * |
5260325f | 3 | * auth-rsa.c |
6ae2364d | 4 | * |
5260325f | 5 | * Author: Tatu Ylonen <ylo@cs.hut.fi> |
6ae2364d | 6 | * |
5260325f | 7 | * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
8 | * All rights reserved | |
6ae2364d | 9 | * |
5260325f | 10 | * Created: Mon Mar 27 01:46:52 1995 ylo |
6ae2364d | 11 | * |
5260325f | 12 | * RSA-based authentication. This code determines whether to admit a login |
13 | * based on RSA authentication. This file also contains functions to check | |
14 | * validity of the host key. | |
6ae2364d | 15 | * |
5260325f | 16 | */ |
8efc0c15 | 17 | |
18 | #include "includes.h" | |
19 | RCSID("$Id$"); | |
20 | ||
21 | #include "rsa.h" | |
22 | #include "packet.h" | |
23 | #include "xmalloc.h" | |
24 | #include "ssh.h" | |
25 | #include "mpaux.h" | |
26 | #include "uidswap.h" | |
4fe2af09 | 27 | #include "match.h" |
6fa724bc | 28 | #include "servconf.h" |
38c295d6 | 29 | #include "auth-options.h" |
8efc0c15 | 30 | |
31 | #include <openssl/rsa.h> | |
32 | #include <openssl/md5.h> | |
33 | ||
aa3378df | 34 | /* |
35 | * Session identifier that is used to bind key exchange and authentication | |
36 | * responses to a particular session. | |
37 | */ | |
8efc0c15 | 38 | extern unsigned char session_id[16]; |
39 | ||
aa3378df | 40 | /* |
41 | * The .ssh/authorized_keys file contains public keys, one per line, in the | |
42 | * following format: | |
43 | * options bits e n comment | |
44 | * where bits, e and n are decimal numbers, | |
45 | * and comment is any string of characters up to newline. The maximum | |
46 | * length of a line is 8000 characters. See the documentation for a | |
47 | * description of the options. | |
48 | */ | |
8efc0c15 | 49 | |
aa3378df | 50 | /* |
51 | * Performs the RSA authentication challenge-response dialog with the client, | |
52 | * and returns true (non-zero) if the client gave the correct answer to | |
53 | * our challenge; returns zero if the client gives a wrong answer. | |
54 | */ | |
8efc0c15 | 55 | |
56 | int | |
4fe2af09 | 57 | auth_rsa_challenge_dialog(RSA *pk) |
8efc0c15 | 58 | { |
c8d54615 | 59 | BIGNUM *challenge, *encrypted_challenge; |
c8d54615 | 60 | BN_CTX *ctx; |
5260325f | 61 | unsigned char buf[32], mdbuf[16], response[16]; |
62 | MD5_CTX md; | |
63 | unsigned int i; | |
64 | int plen, len; | |
65 | ||
66 | encrypted_challenge = BN_new(); | |
67 | challenge = BN_new(); | |
5260325f | 68 | |
69 | /* Generate a random challenge. */ | |
70 | BN_rand(challenge, 256, 0, 0); | |
c8d54615 | 71 | ctx = BN_CTX_new(); |
4fe2af09 | 72 | BN_mod(challenge, challenge, pk->n, ctx); |
c8d54615 | 73 | BN_CTX_free(ctx); |
5260325f | 74 | |
5260325f | 75 | /* Encrypt the challenge with the public key. */ |
76 | rsa_public_encrypt(encrypted_challenge, challenge, pk); | |
5260325f | 77 | |
78 | /* Send the encrypted challenge to the client. */ | |
79 | packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE); | |
80 | packet_put_bignum(encrypted_challenge); | |
81 | packet_send(); | |
c8d54615 | 82 | BN_clear_free(encrypted_challenge); |
5260325f | 83 | packet_write_wait(); |
84 | ||
c8d54615 | 85 | /* Wait for a response. */ |
86 | packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE); | |
87 | packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE); | |
88 | for (i = 0; i < 16; i++) | |
89 | response[i] = packet_get_char(); | |
90 | ||
5260325f | 91 | /* The response is MD5 of decrypted challenge plus session id. */ |
92 | len = BN_num_bytes(challenge); | |
93 | if (len <= 0 || len > 32) | |
94 | fatal("auth_rsa_challenge_dialog: bad challenge length %d", len); | |
95 | memset(buf, 0, 32); | |
96 | BN_bn2bin(challenge, buf + 32 - len); | |
97 | MD5_Init(&md); | |
98 | MD5_Update(&md, buf, 32); | |
99 | MD5_Update(&md, session_id, 16); | |
100 | MD5_Final(mdbuf, &md); | |
5260325f | 101 | BN_clear_free(challenge); |
5260325f | 102 | |
103 | /* Verify that the response is the original challenge. */ | |
104 | if (memcmp(response, mdbuf, 16) != 0) { | |
105 | /* Wrong answer. */ | |
106 | return 0; | |
107 | } | |
108 | /* Correct answer. */ | |
109 | return 1; | |
8efc0c15 | 110 | } |
111 | ||
aa3378df | 112 | /* |
113 | * Performs the RSA authentication dialog with the client. This returns | |
114 | * 0 if the client could not be authenticated, and 1 if authentication was | |
115 | * successful. This may exit if there is a serious protocol violation. | |
116 | */ | |
8efc0c15 | 117 | |
118 | int | |
6fa724bc | 119 | auth_rsa(struct passwd *pw, BIGNUM *client_n) |
8efc0c15 | 120 | { |
5260325f | 121 | extern ServerOptions options; |
122 | char line[8192], file[1024]; | |
123 | int authenticated; | |
124 | unsigned int bits; | |
125 | FILE *f; | |
126 | unsigned long linenum = 0; | |
127 | struct stat st; | |
4fe2af09 | 128 | RSA *pk; |
5260325f | 129 | |
130 | /* Temporarily use the user's uid. */ | |
131 | temporarily_use_uid(pw->pw_uid); | |
132 | ||
133 | /* The authorized keys. */ | |
134 | snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir, | |
135 | SSH_USER_PERMITTED_KEYS); | |
136 | ||
137 | /* Fail quietly if file does not exist */ | |
138 | if (stat(file, &st) < 0) { | |
139 | /* Restore the privileged uid. */ | |
140 | restore_uid(); | |
141 | return 0; | |
8efc0c15 | 142 | } |
5260325f | 143 | /* Open the file containing the authorized keys. */ |
144 | f = fopen(file, "r"); | |
145 | if (!f) { | |
146 | /* Restore the privileged uid. */ | |
147 | restore_uid(); | |
148 | packet_send_debug("Could not open %.900s for reading.", file); | |
149 | packet_send_debug("If your home is on an NFS volume, it may need to be world-readable."); | |
150 | return 0; | |
8efc0c15 | 151 | } |
5260325f | 152 | if (options.strict_modes) { |
153 | int fail = 0; | |
154 | char buf[1024]; | |
155 | /* Check open file in order to avoid open/stat races */ | |
156 | if (fstat(fileno(f), &st) < 0 || | |
157 | (st.st_uid != 0 && st.st_uid != pw->pw_uid) || | |
158 | (st.st_mode & 022) != 0) { | |
159 | snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: " | |
160 | "bad ownership or modes for '%s'.", pw->pw_name, file); | |
161 | fail = 1; | |
162 | } else { | |
163 | /* Check path to SSH_USER_PERMITTED_KEYS */ | |
164 | int i; | |
165 | static const char *check[] = { | |
166 | "", SSH_USER_DIR, NULL | |
167 | }; | |
168 | for (i = 0; check[i]; i++) { | |
169 | snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]); | |
170 | if (stat(line, &st) < 0 || | |
171 | (st.st_uid != 0 && st.st_uid != pw->pw_uid) || | |
172 | (st.st_mode & 022) != 0) { | |
173 | snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: " | |
174 | "bad ownership or modes for '%s'.", pw->pw_name, line); | |
175 | fail = 1; | |
176 | break; | |
177 | } | |
178 | } | |
8efc0c15 | 179 | } |
5260325f | 180 | if (fail) { |
a306f2dd | 181 | fclose(f); |
5260325f | 182 | log(buf); |
183 | packet_send_debug(buf); | |
184 | restore_uid(); | |
185 | return 0; | |
8efc0c15 | 186 | } |
5260325f | 187 | } |
188 | /* Flag indicating whether authentication has succeeded. */ | |
189 | authenticated = 0; | |
190 | ||
4fe2af09 | 191 | pk = RSA_new(); |
192 | pk->e = BN_new(); | |
193 | pk->n = BN_new(); | |
5260325f | 194 | |
aa3378df | 195 | /* |
196 | * Go though the accepted keys, looking for the current key. If | |
197 | * found, perform a challenge-response dialog to verify that the | |
198 | * user really has the corresponding private key. | |
199 | */ | |
5260325f | 200 | while (fgets(line, sizeof(line), f)) { |
201 | char *cp; | |
202 | char *options; | |
203 | ||
204 | linenum++; | |
205 | ||
aa3378df | 206 | /* Skip leading whitespace, empty and comment lines. */ |
207 | for (cp = line; *cp == ' ' || *cp == '\t'; cp++) | |
208 | ; | |
5260325f | 209 | if (!*cp || *cp == '\n' || *cp == '#') |
210 | continue; | |
211 | ||
aa3378df | 212 | /* |
213 | * Check if there are options for this key, and if so, | |
214 | * save their starting address and skip the option part | |
215 | * for now. If there are no options, set the starting | |
216 | * address to NULL. | |
217 | */ | |
5260325f | 218 | if (*cp < '0' || *cp > '9') { |
219 | int quoted = 0; | |
220 | options = cp; | |
221 | for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) { | |
222 | if (*cp == '\\' && cp[1] == '"') | |
223 | cp++; /* Skip both */ | |
224 | else if (*cp == '"') | |
225 | quoted = !quoted; | |
8efc0c15 | 226 | } |
5260325f | 227 | } else |
228 | options = NULL; | |
229 | ||
230 | /* Parse the key from the line. */ | |
4fe2af09 | 231 | if (!auth_rsa_read_key(&cp, &bits, pk->e, pk->n)) { |
5260325f | 232 | debug("%.100s, line %lu: bad key syntax", |
233 | SSH_USER_PERMITTED_KEYS, linenum); | |
234 | packet_send_debug("%.100s, line %lu: bad key syntax", | |
6ae2364d | 235 | SSH_USER_PERMITTED_KEYS, linenum); |
5260325f | 236 | continue; |
8efc0c15 | 237 | } |
5260325f | 238 | /* cp now points to the comment part. */ |
239 | ||
5260325f | 240 | /* Check if the we have found the desired key (identified by its modulus). */ |
4fe2af09 | 241 | if (BN_cmp(pk->n, client_n) != 0) |
aa3378df | 242 | continue; |
5260325f | 243 | |
57112b5a | 244 | /* check the real bits */ |
4fe2af09 | 245 | if (bits != BN_num_bits(pk->n)) |
57112b5a | 246 | log("Warning: %s, line %ld: keysize mismatch: " |
247 | "actual %d vs. announced %d.", | |
4fe2af09 | 248 | file, linenum, BN_num_bits(pk->n), bits); |
57112b5a | 249 | |
5260325f | 250 | /* We have found the desired key. */ |
251 | ||
252 | /* Perform the challenge-response dialog for this key. */ | |
4fe2af09 | 253 | if (!auth_rsa_challenge_dialog(pk)) { |
5260325f | 254 | /* Wrong response. */ |
255 | verbose("Wrong response to RSA authentication challenge."); | |
256 | packet_send_debug("Wrong response to RSA authentication challenge."); | |
257 | continue; | |
8efc0c15 | 258 | } |
aa3378df | 259 | /* |
260 | * Correct response. The client has been successfully | |
261 | * authenticated. Note that we have not yet processed the | |
262 | * options; this will be reset if the options cause the | |
263 | * authentication to be rejected. | |
aa3378df | 264 | * Break out of the loop if authentication was successful; |
265 | * otherwise continue searching. | |
266 | */ | |
38c295d6 | 267 | authenticated = auth_parse_options(pw, options, linenum); |
5260325f | 268 | if (authenticated) |
269 | break; | |
8efc0c15 | 270 | } |
271 | ||
5260325f | 272 | /* Restore the privileged uid. */ |
273 | restore_uid(); | |
8efc0c15 | 274 | |
5260325f | 275 | /* Close the file. */ |
276 | fclose(f); | |
8efc0c15 | 277 | |
4fe2af09 | 278 | RSA_free(pk); |
8efc0c15 | 279 | |
5260325f | 280 | if (authenticated) |
281 | packet_send_debug("RSA authentication accepted."); | |
8efc0c15 | 282 | |
5260325f | 283 | /* Return authentication result. */ |
284 | return authenticated; | |
8efc0c15 | 285 | } |