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 1.64 2006/02/20 17:19:53 stevesk Exp $");
19 #include <sys/types.h>
22 #include <openssl/rsa.h>
23 #include <openssl/md5.h>
31 #include "auth-options.h"
32 #include "pathnames.h"
37 #include "monitor_wrap.h"
42 extern ServerOptions options;
45 * Session identifier that is used to bind key exchange and authentication
46 * responses to a particular session.
48 extern u_char session_id[16];
51 * The .ssh/authorized_keys file contains public keys, one per line, in the
53 * options bits e n comment
54 * where bits, e and n are decimal numbers,
55 * and comment is any string of characters up to newline. The maximum
56 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
57 * description of the options.
61 auth_rsa_generate_challenge(Key *key)
66 if ((challenge = BN_new()) == NULL)
67 fatal("auth_rsa_generate_challenge: BN_new() failed");
68 /* Generate a random challenge. */
69 BN_rand(challenge, 256, 0, 0);
70 if ((ctx = BN_CTX_new()) == NULL)
71 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
72 BN_mod(challenge, challenge, key->rsa->n, ctx);
79 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
81 u_char buf[32], mdbuf[16];
85 /* don't allow short keys */
86 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
87 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
88 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
92 /* The response is MD5 of decrypted challenge plus session id. */
93 len = BN_num_bytes(challenge);
94 if (len <= 0 || len > 32)
95 fatal("auth_rsa_verify_response: bad challenge length %d", len);
97 BN_bn2bin(challenge, buf + 32 - len);
99 MD5_Update(&md, buf, 32);
100 MD5_Update(&md, session_id, 16);
101 MD5_Final(mdbuf, &md);
103 /* Verify that the response is the original challenge. */
104 if (memcmp(response, mdbuf, 16) != 0) {
108 /* Correct answer. */
113 * Performs the RSA authentication challenge-response dialog with the client,
114 * and returns true (non-zero) if the client gave the correct answer to
115 * our challenge; returns zero if the client gives a wrong answer.
119 auth_rsa_challenge_dialog(Key *key)
121 BIGNUM *challenge, *encrypted_challenge;
125 if ((encrypted_challenge = BN_new()) == NULL)
126 fatal("auth_rsa_challenge_dialog: BN_new() failed");
128 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
130 /* Encrypt the challenge with the public key. */
131 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
133 /* Send the encrypted challenge to the client. */
134 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
135 packet_put_bignum(encrypted_challenge);
137 BN_clear_free(encrypted_challenge);
140 /* Wait for a response. */
141 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
142 for (i = 0; i < 16; i++)
143 response[i] = packet_get_char();
146 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
147 BN_clear_free(challenge);
152 * check if there's user key matching client_n,
153 * return key if login is allowed, NULL otherwise
157 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
159 char line[SSH_MAX_PUBKEY_BYTES], *file;
167 /* Temporarily use the user's uid. */
168 temporarily_use_uid(pw);
170 /* The authorized keys. */
171 file = authorized_keys_file(pw);
172 debug("trying public RSA key file %s", file);
174 /* Fail quietly if file does not exist */
175 if (stat(file, &st) < 0) {
176 /* Restore the privileged uid. */
181 /* Open the file containing the authorized keys. */
182 f = fopen(file, "r");
184 /* Restore the privileged uid. */
189 if (options.strict_modes &&
190 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
193 logit("Authentication refused: %s", line);
198 /* Flag indicating whether the key is allowed. */
201 key = key_new(KEY_RSA1);
204 * Go though the accepted keys, looking for the current key. If
205 * found, perform a challenge-response dialog to verify that the
206 * user really has the corresponding private key.
208 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
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 (hostfile_read_key(&cp, &bits, key) == 0) {
239 debug("%.100s, line %lu: non ssh1 key syntax",
243 /* cp now points to the comment part. */
245 /* Check if the we have found the desired key (identified by its modulus). */
246 if (BN_cmp(key->rsa->n, client_n) != 0)
249 /* check the real bits */
250 keybits = BN_num_bits(key->rsa->n);
251 if (keybits < 0 || bits != (u_int)keybits)
252 logit("Warning: %s, line %lu: keysize mismatch: "
253 "actual %d vs. announced %d.",
254 file, linenum, BN_num_bits(key->rsa->n), bits);
256 /* We have found the desired key. */
258 * If our options do not allow this key to be used,
259 * do not send challenge.
261 if (!auth_parse_options(pw, key_options, file, linenum))
264 /* break out, this key is allowed */
269 /* Restore the privileged uid. */
272 /* Close the file. */
276 /* return key if allowed */
277 if (allowed && rkey != NULL)
285 * Performs the RSA authentication dialog with the client. This returns
286 * 0 if the client could not be authenticated, and 1 if authentication was
287 * successful. This may exit if there is a serious protocol violation.
290 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
294 struct passwd *pw = authctxt->pw;
297 if (!authctxt->valid)
300 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
301 auth_clear_options();
305 /* Perform the challenge-response dialog for this key. */
306 if (!auth_rsa_challenge_dialog(key)) {
307 /* Wrong response. */
308 verbose("Wrong response to RSA authentication challenge.");
309 packet_send_debug("Wrong response to RSA authentication challenge.");
311 * Break out of the loop. Otherwise we might send
312 * another challenge and break the protocol.
318 * Correct response. The client has been successfully
319 * authenticated. Note that we have not yet processed the
320 * options; this will be reset if the options cause the
321 * authentication to be rejected.
323 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
324 verbose("Found matching %s key: %s",
329 packet_send_debug("RSA authentication accepted.");