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 1.53 2002/03/25 09:21:13 markus Exp $");
19 #include <openssl/rsa.h>
20 #include <openssl/md5.h>
29 #include "auth-options.h"
30 #include "pathnames.h"
35 #include "monitor_wrap.h"
38 extern ServerOptions options;
41 * Session identifier that is used to bind key exchange and authentication
42 * responses to a particular session.
44 extern u_char session_id[16];
47 * The .ssh/authorized_keys file contains public keys, one per line, in the
49 * options bits e n comment
50 * where bits, e and n are decimal numbers,
51 * and comment is any string of characters up to newline. The maximum
52 * length of a line is 8000 characters. See the documentation for a
53 * description of the options.
57 auth_rsa_generate_challenge(Key *key)
62 if ((challenge = BN_new()) == NULL)
63 fatal("auth_rsa_generate_challenge: BN_new() failed");
64 /* Generate a random challenge. */
65 BN_rand(challenge, 256, 0, 0);
66 if ((ctx = BN_CTX_new()) == NULL)
67 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
68 BN_mod(challenge, challenge, key->rsa->n, ctx);
75 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
77 u_char buf[32], mdbuf[16];
81 /* The response is MD5 of decrypted challenge plus session id. */
82 len = BN_num_bytes(challenge);
83 if (len <= 0 || len > 32)
84 fatal("auth_rsa_verify_response: bad challenge length %d", len);
86 BN_bn2bin(challenge, buf + 32 - len);
88 MD5_Update(&md, buf, 32);
89 MD5_Update(&md, session_id, 16);
90 MD5_Final(mdbuf, &md);
92 /* Verify that the response is the original challenge. */
93 if (memcmp(response, mdbuf, 16) != 0) {
102 * Performs the RSA authentication challenge-response dialog with the client,
103 * and returns true (non-zero) if the client gave the correct answer to
104 * our challenge; returns zero if the client gives a wrong answer.
108 auth_rsa_challenge_dialog(Key *key)
110 BIGNUM *challenge, *encrypted_challenge;
114 if ((encrypted_challenge = BN_new()) == NULL)
115 fatal("auth_rsa_challenge_dialog: BN_new() failed");
117 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
119 /* Encrypt the challenge with the public key. */
120 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
122 /* Send the encrypted challenge to the client. */
123 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
124 packet_put_bignum(encrypted_challenge);
126 BN_clear_free(encrypted_challenge);
129 /* Wait for a response. */
130 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
131 for (i = 0; i < 16; i++)
132 response[i] = packet_get_char();
135 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
136 BN_clear_free(challenge);
141 * check if there's user key matching client_n,
142 * return key if login is allowed, NULL otherwise
146 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
148 char line[8192], *file;
156 /* Temporarily use the user's uid. */
157 temporarily_use_uid(pw);
159 /* The authorized keys. */
160 file = authorized_keys_file(pw);
161 debug("trying public RSA key file %s", file);
163 /* Fail quietly if file does not exist */
164 if (stat(file, &st) < 0) {
165 /* Restore the privileged uid. */
170 /* Open the file containing the authorized keys. */
171 f = fopen(file, "r");
173 /* Restore the privileged uid. */
178 if (options.strict_modes &&
179 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
182 log("Authentication refused: %s", line);
187 /* Flag indicating whether the key is allowed. */
190 key = key_new(KEY_RSA1);
193 * Go though the accepted keys, looking for the current key. If
194 * found, perform a challenge-response dialog to verify that the
195 * user really has the corresponding private key.
197 while (fgets(line, sizeof(line), f)) {
203 /* Skip leading whitespace, empty and comment lines. */
204 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
206 if (!*cp || *cp == '\n' || *cp == '#')
210 * Check if there are options for this key, and if so,
211 * save their starting address and skip the option part
212 * for now. If there are no options, set the starting
215 if (*cp < '0' || *cp > '9') {
218 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
219 if (*cp == '\\' && cp[1] == '"')
220 cp++; /* Skip both */
227 /* Parse the key from the line. */
228 if (hostfile_read_key(&cp, &bits, key) == 0) {
229 debug("%.100s, line %lu: non ssh1 key syntax",
233 /* cp now points to the comment part. */
235 /* Check if the we have found the desired key (identified by its modulus). */
236 if (BN_cmp(key->rsa->n, client_n) != 0)
239 /* check the real bits */
240 if (bits != BN_num_bits(key->rsa->n))
241 log("Warning: %s, line %lu: keysize mismatch: "
242 "actual %d vs. announced %d.",
243 file, linenum, BN_num_bits(key->rsa->n), bits);
245 /* We have found the desired key. */
247 * If our options do not allow this key to be used,
248 * do not send challenge.
250 if (!auth_parse_options(pw, options, file, linenum))
253 /* break out, this key is allowed */
258 /* Restore the privileged uid. */
261 /* Close the file. */
265 /* return key if allowed */
266 if (allowed && rkey != NULL)
274 * Performs the RSA authentication dialog with the client. This returns
275 * 0 if the client could not be authenticated, and 1 if authentication was
276 * successful. This may exit if there is a serious protocol violation.
279 auth_rsa(struct passwd *pw, BIGNUM *client_n)
288 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
289 auth_clear_options();
293 /* Perform the challenge-response dialog for this key. */
294 if (!auth_rsa_challenge_dialog(key)) {
295 /* Wrong response. */
296 verbose("Wrong response to RSA authentication challenge.");
297 packet_send_debug("Wrong response to RSA authentication challenge.");
299 * Break out of the loop. Otherwise we might send
300 * another challenge and break the protocol.
306 * Correct response. The client has been successfully
307 * authenticated. Note that we have not yet processed the
308 * options; this will be reset if the options cause the
309 * authentication to be rejected.
311 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
312 verbose("Found matching %s key: %s",
317 packet_send_debug("RSA authentication accepted.");