2 * Author: Tatu Ylonen <ylo@cs.hut.fi>
3 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
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.
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".
17 RCSID("$OpenBSD: auth-rsa.c,v 1.39 2001/03/01 02:45:10 deraadt Exp $");
19 #include <openssl/rsa.h>
20 #include <openssl/md5.h>
29 #include "auth-options.h"
30 #include "pathnames.h"
36 extern ServerOptions options;
39 * Session identifier that is used to bind key exchange and authentication
40 * responses to a particular session.
42 extern u_char session_id[16];
45 * The .ssh/authorized_keys file contains public keys, one per line, in the
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.
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.
61 auth_rsa_challenge_dialog(RSA *pk)
63 BIGNUM *challenge, *encrypted_challenge;
65 u_char buf[32], mdbuf[16], response[16];
70 encrypted_challenge = BN_new();
73 /* Generate a random challenge. */
74 BN_rand(challenge, 256, 0, 0);
76 BN_mod(challenge, challenge, pk->n, ctx);
79 /* Encrypt the challenge with the public key. */
80 rsa_public_encrypt(encrypted_challenge, challenge, pk);
82 /* Send the encrypted challenge to the client. */
83 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
84 packet_put_bignum(encrypted_challenge);
86 BN_clear_free(encrypted_challenge);
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();
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);
100 BN_bn2bin(challenge, buf + 32 - len);
102 MD5_Update(&md, buf, 32);
103 MD5_Update(&md, session_id, 16);
104 MD5_Final(mdbuf, &md);
105 BN_clear_free(challenge);
107 /* Verify that the response is the original challenge. */
108 if (memcmp(response, mdbuf, 16) != 0) {
112 /* Correct answer. */
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.
123 auth_rsa(struct passwd *pw, BIGNUM *client_n)
125 char line[8192], file[MAXPATHLEN];
137 /* Temporarily use the user's uid. */
138 temporarily_use_uid(pw->pw_uid);
140 /* The authorized keys. */
141 snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir,
142 _PATH_SSH_USER_PERMITTED_KEYS);
144 /* Fail quietly if file does not exist */
145 if (stat(file, &st) < 0) {
146 /* Restore the privileged uid. */
150 /* Open the file containing the authorized keys. */
151 f = fopen(file, "r");
153 /* Restore the privileged uid. */
155 packet_send_debug("Could not open %.900s for reading.", file);
156 packet_send_debug("If your home is on an NFS volume, it may need to be world-readable.");
159 if (options.strict_modes) {
162 /* Check open file in order to avoid open/stat races */
163 if (fstat(fileno(f), &st) < 0 ||
164 (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
165 (st.st_mode & 022) != 0) {
166 snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
167 "bad ownership or modes for '%s'.", pw->pw_name, file);
170 /* Check path to _PATH_SSH_USER_PERMITTED_KEYS */
172 static const char *check[] = {
173 "", _PATH_SSH_USER_DIR, NULL
175 for (i = 0; check[i]; i++) {
176 snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]);
177 if (stat(line, &st) < 0 ||
178 (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
179 (st.st_mode & 022) != 0) {
180 snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
181 "bad ownership or modes for '%s'.", pw->pw_name, line);
190 packet_send_debug("%s", buf);
195 /* Flag indicating whether authentication has succeeded. */
203 * Go though the accepted keys, looking for the current key. If
204 * found, perform a challenge-response dialog to verify that the
205 * user really has the corresponding private key.
207 while (fgets(line, sizeof(line), f)) {
213 /* Skip leading whitespace, empty and comment lines. */
214 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
216 if (!*cp || *cp == '\n' || *cp == '#')
220 * Check if there are options for this key, and if so,
221 * save their starting address and skip the option part
222 * for now. If there are no options, set the starting
225 if (*cp < '0' || *cp > '9') {
228 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
229 if (*cp == '\\' && cp[1] == '"')
230 cp++; /* Skip both */
237 /* Parse the key from the line. */
238 if (!auth_rsa_read_key(&cp, &bits, pk->e, pk->n)) {
239 debug("%.100s, line %lu: bad key syntax",
241 packet_send_debug("%.100s, line %lu: bad key syntax",
245 /* cp now points to the comment part. */
247 /* Check if the we have found the desired key (identified by its modulus). */
248 if (BN_cmp(pk->n, client_n) != 0)
251 /* check the real bits */
252 if (bits != BN_num_bits(pk->n))
253 log("Warning: %s, line %ld: keysize mismatch: "
254 "actual %d vs. announced %d.",
255 file, linenum, BN_num_bits(pk->n), bits);
257 /* We have found the desired key. */
259 * If our options do not allow this key to be used,
260 * do not send challenge.
262 if (!auth_parse_options(pw, options, file, linenum))
265 /* Perform the challenge-response dialog for this key. */
266 if (!auth_rsa_challenge_dialog(pk)) {
267 /* Wrong response. */
268 verbose("Wrong response to RSA authentication challenge.");
269 packet_send_debug("Wrong response to RSA authentication challenge.");
273 * Correct response. The client has been successfully
274 * authenticated. Note that we have not yet processed the
275 * options; this will be reset if the options cause the
276 * authentication to be rejected.
277 * Break out of the loop if authentication was successful;
278 * otherwise continue searching.
284 /* Restore the privileged uid. */
287 /* Close the file. */
293 packet_send_debug("RSA authentication accepted.");
295 auth_clear_options();
297 /* Return authentication result. */
298 return authenticated;