1 /* $OpenBSD: auth-rsa.c,v 1.71 2006/08/03 03:34:41 deraadt Exp $ */
3 * Author: Tatu Ylonen <ylo@cs.hut.fi>
4 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
6 * RSA-based authentication. This code determines whether to admit a login
7 * based on RSA authentication. This file also contains functions to check
8 * validity of the host key.
10 * As far as I am concerned, the code I have written for this software
11 * can be used freely for any purpose. Any derived versions of this
12 * software must be clearly marked as such, and if the derived work is
13 * incompatible with the protocol description in the RFC file, it must be
14 * called by a name other than "ssh" or "Secure Shell".
19 #include <sys/types.h>
22 #include <openssl/rsa.h>
23 #include <openssl/md5.h>
36 #include "auth-options.h"
37 #include "pathnames.h"
46 #include "monitor_wrap.h"
51 extern ServerOptions options;
54 * Session identifier that is used to bind key exchange and authentication
55 * responses to a particular session.
57 extern u_char session_id[16];
60 * The .ssh/authorized_keys file contains public keys, one per line, in the
62 * options bits e n comment
63 * where bits, e and n are decimal numbers,
64 * and comment is any string of characters up to newline. The maximum
65 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
66 * description of the options.
70 auth_rsa_generate_challenge(Key *key)
75 if ((challenge = BN_new()) == NULL)
76 fatal("auth_rsa_generate_challenge: BN_new() failed");
77 /* Generate a random challenge. */
78 BN_rand(challenge, 256, 0, 0);
79 if ((ctx = BN_CTX_new()) == NULL)
80 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
81 BN_mod(challenge, challenge, key->rsa->n, ctx);
88 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
90 u_char buf[32], mdbuf[16];
94 /* don't allow short keys */
95 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
96 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
97 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
101 /* The response is MD5 of decrypted challenge plus session id. */
102 len = BN_num_bytes(challenge);
103 if (len <= 0 || len > 32)
104 fatal("auth_rsa_verify_response: bad challenge length %d", len);
106 BN_bn2bin(challenge, buf + 32 - len);
108 MD5_Update(&md, buf, 32);
109 MD5_Update(&md, session_id, 16);
110 MD5_Final(mdbuf, &md);
112 /* Verify that the response is the original challenge. */
113 if (memcmp(response, mdbuf, 16) != 0) {
117 /* Correct answer. */
122 * Performs the RSA authentication challenge-response dialog with the client,
123 * and returns true (non-zero) if the client gave the correct answer to
124 * our challenge; returns zero if the client gives a wrong answer.
128 auth_rsa_challenge_dialog(Key *key)
130 BIGNUM *challenge, *encrypted_challenge;
134 if ((encrypted_challenge = BN_new()) == NULL)
135 fatal("auth_rsa_challenge_dialog: BN_new() failed");
137 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
139 /* Encrypt the challenge with the public key. */
140 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
142 /* Send the encrypted challenge to the client. */
143 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
144 packet_put_bignum(encrypted_challenge);
146 BN_clear_free(encrypted_challenge);
149 /* Wait for a response. */
150 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
151 for (i = 0; i < 16; i++)
152 response[i] = (u_char)packet_get_char();
155 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
156 BN_clear_free(challenge);
161 * check if there's user key matching client_n,
162 * return key if login is allowed, NULL otherwise
166 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
168 char line[SSH_MAX_PUBKEY_BYTES], *file;
176 /* Temporarily use the user's uid. */
177 temporarily_use_uid(pw);
179 /* The authorized keys. */
180 file = authorized_keys_file(pw);
181 debug("trying public RSA key file %s", file);
183 /* Fail quietly if file does not exist */
184 if (stat(file, &st) < 0) {
185 /* Restore the privileged uid. */
190 /* Open the file containing the authorized keys. */
191 f = fopen(file, "r");
193 /* Restore the privileged uid. */
198 if (options.strict_modes &&
199 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
202 logit("Authentication refused: %s", line);
207 /* Flag indicating whether the key is allowed. */
210 key = key_new(KEY_RSA1);
213 * Go though the accepted keys, looking for the current key. If
214 * found, perform a challenge-response dialog to verify that the
215 * user really has the corresponding private key.
217 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
222 /* Skip leading whitespace, empty and comment lines. */
223 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
225 if (!*cp || *cp == '\n' || *cp == '#')
229 * Check if there are options for this key, and if so,
230 * save their starting address and skip the option part
231 * for now. If there are no options, set the starting
234 if (*cp < '0' || *cp > '9') {
237 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
238 if (*cp == '\\' && cp[1] == '"')
239 cp++; /* Skip both */
246 /* Parse the key from the line. */
247 if (hostfile_read_key(&cp, &bits, key) == 0) {
248 debug("%.100s, line %lu: non ssh1 key syntax",
252 /* cp now points to the comment part. */
254 /* Check if the we have found the desired key (identified by its modulus). */
255 if (BN_cmp(key->rsa->n, client_n) != 0)
258 /* check the real bits */
259 keybits = BN_num_bits(key->rsa->n);
260 if (keybits < 0 || bits != (u_int)keybits)
261 logit("Warning: %s, line %lu: keysize mismatch: "
262 "actual %d vs. announced %d.",
263 file, linenum, BN_num_bits(key->rsa->n), bits);
265 /* We have found the desired key. */
267 * If our options do not allow this key to be used,
268 * do not send challenge.
270 if (!auth_parse_options(pw, key_options, file, linenum))
273 /* break out, this key is allowed */
278 /* Restore the privileged uid. */
281 /* Close the file. */
285 /* return key if allowed */
286 if (allowed && rkey != NULL)
294 * Performs the RSA authentication dialog with the client. This returns
295 * 0 if the client could not be authenticated, and 1 if authentication was
296 * successful. This may exit if there is a serious protocol violation.
299 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
303 struct passwd *pw = authctxt->pw;
306 if (!authctxt->valid)
309 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
310 auth_clear_options();
314 /* Perform the challenge-response dialog for this key. */
315 if (!auth_rsa_challenge_dialog(key)) {
316 /* Wrong response. */
317 verbose("Wrong response to RSA authentication challenge.");
318 packet_send_debug("Wrong response to RSA authentication challenge.");
320 * Break out of the loop. Otherwise we might send
321 * another challenge and break the protocol.
327 * Correct response. The client has been successfully
328 * authenticated. Note that we have not yet processed the
329 * options; this will be reset if the options cause the
330 * authentication to be rejected.
332 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
333 verbose("Found matching %s key: %s",
338 packet_send_debug("RSA authentication accepted.");