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 1.29 2000/09/07 21:13:36 markus Exp $");
28 #include "auth-options.h"
30 #include <openssl/rsa.h>
31 #include <openssl/md5.h>
34 * Session identifier that is used to bind key exchange and authentication
35 * responses to a particular session.
37 extern unsigned char session_id[16];
40 * The .ssh/authorized_keys file contains public keys, one per line, in the
42 * options bits e n comment
43 * where bits, e and n are decimal numbers,
44 * and comment is any string of characters up to newline. The maximum
45 * length of a line is 8000 characters. See the documentation for a
46 * description of the options.
50 * Performs the RSA authentication challenge-response dialog with the client,
51 * and returns true (non-zero) if the client gave the correct answer to
52 * our challenge; returns zero if the client gives a wrong answer.
56 auth_rsa_challenge_dialog(RSA *pk)
58 BIGNUM *challenge, *encrypted_challenge;
60 unsigned char buf[32], mdbuf[16], response[16];
65 encrypted_challenge = BN_new();
68 /* Generate a random challenge. */
69 BN_rand(challenge, 256, 0, 0);
71 BN_mod(challenge, challenge, pk->n, ctx);
74 /* Encrypt the challenge with the public key. */
75 rsa_public_encrypt(encrypted_challenge, challenge, pk);
77 /* Send the encrypted challenge to the client. */
78 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
79 packet_put_bignum(encrypted_challenge);
81 BN_clear_free(encrypted_challenge);
84 /* Wait for a response. */
85 packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE);
86 packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE);
87 for (i = 0; i < 16; i++)
88 response[i] = packet_get_char();
90 /* The response is MD5 of decrypted challenge plus session id. */
91 len = BN_num_bytes(challenge);
92 if (len <= 0 || len > 32)
93 fatal("auth_rsa_challenge_dialog: bad challenge length %d", len);
95 BN_bn2bin(challenge, buf + 32 - len);
97 MD5_Update(&md, buf, 32);
98 MD5_Update(&md, session_id, 16);
99 MD5_Final(mdbuf, &md);
100 BN_clear_free(challenge);
102 /* Verify that the response is the original challenge. */
103 if (memcmp(response, mdbuf, 16) != 0) {
107 /* Correct answer. */
112 * Performs the RSA authentication dialog with the client. This returns
113 * 0 if the client could not be authenticated, and 1 if authentication was
114 * successful. This may exit if there is a serious protocol violation.
118 auth_rsa(struct passwd *pw, BIGNUM *client_n)
120 extern ServerOptions options;
121 char line[8192], file[1024];
125 unsigned long linenum = 0;
129 /* Temporarily use the user's uid. */
130 temporarily_use_uid(pw->pw_uid);
132 /* The authorized keys. */
133 snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir,
134 SSH_USER_PERMITTED_KEYS);
136 /* Fail quietly if file does not exist */
137 if (stat(file, &st) < 0) {
138 /* Restore the privileged uid. */
142 /* Open the file containing the authorized keys. */
143 f = fopen(file, "r");
145 /* Restore the privileged uid. */
147 packet_send_debug("Could not open %.900s for reading.", file);
148 packet_send_debug("If your home is on an NFS volume, it may need to be world-readable.");
151 if (options.strict_modes) {
154 /* Check open file in order to avoid open/stat races */
155 if (fstat(fileno(f), &st) < 0 ||
156 (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
157 (st.st_mode & 022) != 0) {
158 snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
159 "bad ownership or modes for '%s'.", pw->pw_name, file);
162 /* Check path to SSH_USER_PERMITTED_KEYS */
164 static const char *check[] = {
165 "", SSH_USER_DIR, NULL
167 for (i = 0; check[i]; i++) {
168 snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]);
169 if (stat(line, &st) < 0 ||
170 (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
171 (st.st_mode & 022) != 0) {
172 snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
173 "bad ownership or modes for '%s'.", pw->pw_name, line);
182 packet_send_debug("%s",buf);
187 /* Flag indicating whether authentication has succeeded. */
195 * Go though the accepted keys, looking for the current key. If
196 * found, perform a challenge-response dialog to verify that the
197 * user really has the corresponding private key.
199 while (fgets(line, sizeof(line), f)) {
205 /* Skip leading whitespace, empty and comment lines. */
206 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
208 if (!*cp || *cp == '\n' || *cp == '#')
212 * Check if there are options for this key, and if so,
213 * save their starting address and skip the option part
214 * for now. If there are no options, set the starting
217 if (*cp < '0' || *cp > '9') {
220 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
221 if (*cp == '\\' && cp[1] == '"')
222 cp++; /* Skip both */
229 /* Parse the key from the line. */
230 if (!auth_rsa_read_key(&cp, &bits, pk->e, pk->n)) {
231 debug("%.100s, line %lu: bad key syntax",
232 SSH_USER_PERMITTED_KEYS, linenum);
233 packet_send_debug("%.100s, line %lu: bad key syntax",
234 SSH_USER_PERMITTED_KEYS, linenum);
237 /* cp now points to the comment part. */
239 /* Check if the we have found the desired key (identified by its modulus). */
240 if (BN_cmp(pk->n, client_n) != 0)
243 /* check the real bits */
244 if (bits != BN_num_bits(pk->n))
245 log("Warning: %s, line %ld: keysize mismatch: "
246 "actual %d vs. announced %d.",
247 file, linenum, BN_num_bits(pk->n), bits);
249 /* We have found the desired key. */
251 /* Perform the challenge-response dialog for this key. */
252 if (!auth_rsa_challenge_dialog(pk)) {
253 /* Wrong response. */
254 verbose("Wrong response to RSA authentication challenge.");
255 packet_send_debug("Wrong response to RSA authentication challenge.");
259 * Correct response. The client has been successfully
260 * authenticated. Note that we have not yet processed the
261 * options; this will be reset if the options cause the
262 * authentication to be rejected.
263 * Break out of the loop if authentication was successful;
264 * otherwise continue searching.
266 authenticated = auth_parse_options(pw, options, linenum);
271 /* Restore the privileged uid. */
274 /* Close the file. */
280 packet_send_debug("RSA authentication accepted.");
282 /* Return authentication result. */
283 return authenticated;