1 /* $OpenBSD: auth-rsa.c,v 1.73 2008/07/02 12:03:51 dtucker 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>
37 #include "auth-options.h"
38 #include "pathnames.h"
47 #include "monitor_wrap.h"
52 extern ServerOptions options;
55 * Session identifier that is used to bind key exchange and authentication
56 * responses to a particular session.
58 extern u_char session_id[16];
61 * The .ssh/authorized_keys file contains public keys, one per line, in the
63 * options bits e n comment
64 * where bits, e and n are decimal numbers,
65 * and comment is any string of characters up to newline. The maximum
66 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
67 * description of the options.
71 auth_rsa_generate_challenge(Key *key)
76 if ((challenge = BN_new()) == NULL)
77 fatal("auth_rsa_generate_challenge: BN_new() failed");
78 /* Generate a random challenge. */
79 if (BN_rand(challenge, 256, 0, 0) == 0)
80 fatal("auth_rsa_generate_challenge: BN_rand failed");
81 if ((ctx = BN_CTX_new()) == NULL)
82 fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
83 if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
84 fatal("auth_rsa_generate_challenge: BN_mod failed");
91 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
93 u_char buf[32], mdbuf[16];
97 /* don't allow short keys */
98 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
99 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
100 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
104 /* The response is MD5 of decrypted challenge plus session id. */
105 len = BN_num_bytes(challenge);
106 if (len <= 0 || len > 32)
107 fatal("auth_rsa_verify_response: bad challenge length %d", len);
109 BN_bn2bin(challenge, buf + 32 - len);
111 MD5_Update(&md, buf, 32);
112 MD5_Update(&md, session_id, 16);
113 MD5_Final(mdbuf, &md);
115 /* Verify that the response is the original challenge. */
116 if (memcmp(response, mdbuf, 16) != 0) {
120 /* Correct answer. */
125 * Performs the RSA authentication challenge-response dialog with the client,
126 * and returns true (non-zero) if the client gave the correct answer to
127 * our challenge; returns zero if the client gives a wrong answer.
131 auth_rsa_challenge_dialog(Key *key)
133 BIGNUM *challenge, *encrypted_challenge;
137 if ((encrypted_challenge = BN_new()) == NULL)
138 fatal("auth_rsa_challenge_dialog: BN_new() failed");
140 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
142 /* Encrypt the challenge with the public key. */
143 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
145 /* Send the encrypted challenge to the client. */
146 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
147 packet_put_bignum(encrypted_challenge);
149 BN_clear_free(encrypted_challenge);
152 /* Wait for a response. */
153 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
154 for (i = 0; i < 16; i++)
155 response[i] = (u_char)packet_get_char();
158 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
159 BN_clear_free(challenge);
164 * check if there's user key matching client_n,
165 * return key if login is allowed, NULL otherwise
169 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
171 char line[SSH_MAX_PUBKEY_BYTES], *file;
178 /* Temporarily use the user's uid. */
179 temporarily_use_uid(pw);
181 /* The authorized keys. */
182 file = authorized_keys_file(pw);
183 debug("trying public RSA key file %s", file);
184 f = auth_openkeyfile(file, pw, options.strict_modes);
191 /* Flag indicating whether the key is allowed. */
194 key = key_new(KEY_RSA1);
197 * Go though the accepted keys, looking for the current key. If
198 * found, perform a challenge-response dialog to verify that the
199 * user really has the corresponding private key.
201 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
206 /* Skip leading whitespace, empty and comment lines. */
207 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
209 if (!*cp || *cp == '\n' || *cp == '#')
213 * Check if there are options for this key, and if so,
214 * save their starting address and skip the option part
215 * for now. If there are no options, set the starting
218 if (*cp < '0' || *cp > '9') {
221 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
222 if (*cp == '\\' && cp[1] == '"')
223 cp++; /* Skip both */
230 /* Parse the key from the line. */
231 if (hostfile_read_key(&cp, &bits, key) == 0) {
232 debug("%.100s, line %lu: non ssh1 key syntax",
236 /* cp now points to the comment part. */
238 /* Check if the we have found the desired key (identified by its modulus). */
239 if (BN_cmp(key->rsa->n, client_n) != 0)
242 /* check the real bits */
243 keybits = BN_num_bits(key->rsa->n);
244 if (keybits < 0 || bits != (u_int)keybits)
245 logit("Warning: %s, line %lu: keysize mismatch: "
246 "actual %d vs. announced %d.",
247 file, linenum, BN_num_bits(key->rsa->n), bits);
249 /* We have found the desired key. */
251 * If our options do not allow this key to be used,
252 * do not send challenge.
254 if (!auth_parse_options(pw, key_options, file, linenum))
257 /* break out, this key is allowed */
262 /* Restore the privileged uid. */
265 /* Close the file. */
269 /* return key if allowed */
270 if (allowed && rkey != NULL)
278 * Performs the RSA authentication dialog with the client. This returns
279 * 0 if the client could not be authenticated, and 1 if authentication was
280 * successful. This may exit if there is a serious protocol violation.
283 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
287 struct passwd *pw = authctxt->pw;
290 if (!authctxt->valid)
293 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
294 auth_clear_options();
298 /* Perform the challenge-response dialog for this key. */
299 if (!auth_rsa_challenge_dialog(key)) {
300 /* Wrong response. */
301 verbose("Wrong response to RSA authentication challenge.");
302 packet_send_debug("Wrong response to RSA authentication challenge.");
304 * Break out of the loop. Otherwise we might send
305 * another challenge and break the protocol.
311 * Correct response. The client has been successfully
312 * authenticated. Note that we have not yet processed the
313 * options; this will be reset if the options cause the
314 * authentication to be rejected.
316 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
317 verbose("Found matching %s key: %s",
322 packet_send_debug("RSA authentication accepted.");