1 /* $OpenBSD: auth-rsa.c,v 1.68 2006/07/06 16:03:53 stevesk 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>
33 #include "auth-options.h"
34 #include "pathnames.h"
39 #include "monitor_wrap.h"
44 extern ServerOptions options;
47 * Session identifier that is used to bind key exchange and authentication
48 * responses to a particular session.
50 extern u_char session_id[16];
53 * The .ssh/authorized_keys file contains public keys, one per line, in the
55 * options bits e n comment
56 * where bits, e and n are decimal numbers,
57 * and comment is any string of characters up to newline. The maximum
58 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
59 * description of the options.
63 auth_rsa_generate_challenge(Key *key)
68 if ((challenge = BN_new()) == NULL)
69 fatal("auth_rsa_generate_challenge: BN_new() failed");
70 /* Generate a random challenge. */
71 BN_rand(challenge, 256, 0, 0);
72 if ((ctx = BN_CTX_new()) == NULL)
73 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
74 BN_mod(challenge, challenge, key->rsa->n, ctx);
81 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
83 u_char buf[32], mdbuf[16];
87 /* don't allow short keys */
88 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
89 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
90 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
94 /* The response is MD5 of decrypted challenge plus session id. */
95 len = BN_num_bytes(challenge);
96 if (len <= 0 || len > 32)
97 fatal("auth_rsa_verify_response: bad challenge length %d", len);
99 BN_bn2bin(challenge, buf + 32 - len);
101 MD5_Update(&md, buf, 32);
102 MD5_Update(&md, session_id, 16);
103 MD5_Final(mdbuf, &md);
105 /* Verify that the response is the original challenge. */
106 if (memcmp(response, mdbuf, 16) != 0) {
110 /* Correct answer. */
115 * Performs the RSA authentication challenge-response dialog with the client,
116 * and returns true (non-zero) if the client gave the correct answer to
117 * our challenge; returns zero if the client gives a wrong answer.
121 auth_rsa_challenge_dialog(Key *key)
123 BIGNUM *challenge, *encrypted_challenge;
127 if ((encrypted_challenge = BN_new()) == NULL)
128 fatal("auth_rsa_challenge_dialog: BN_new() failed");
130 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
132 /* Encrypt the challenge with the public key. */
133 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
135 /* Send the encrypted challenge to the client. */
136 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
137 packet_put_bignum(encrypted_challenge);
139 BN_clear_free(encrypted_challenge);
142 /* Wait for a response. */
143 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
144 for (i = 0; i < 16; i++)
145 response[i] = (u_char)packet_get_char();
148 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
149 BN_clear_free(challenge);
154 * check if there's user key matching client_n,
155 * return key if login is allowed, NULL otherwise
159 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
161 char line[SSH_MAX_PUBKEY_BYTES], *file;
169 /* Temporarily use the user's uid. */
170 temporarily_use_uid(pw);
172 /* The authorized keys. */
173 file = authorized_keys_file(pw);
174 debug("trying public RSA key file %s", file);
176 /* Fail quietly if file does not exist */
177 if (stat(file, &st) < 0) {
178 /* Restore the privileged uid. */
183 /* Open the file containing the authorized keys. */
184 f = fopen(file, "r");
186 /* Restore the privileged uid. */
191 if (options.strict_modes &&
192 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
195 logit("Authentication refused: %s", line);
200 /* Flag indicating whether the key is allowed. */
203 key = key_new(KEY_RSA1);
206 * Go though the accepted keys, looking for the current key. If
207 * found, perform a challenge-response dialog to verify that the
208 * user really has the corresponding private key.
210 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
215 /* Skip leading whitespace, empty and comment lines. */
216 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
218 if (!*cp || *cp == '\n' || *cp == '#')
222 * Check if there are options for this key, and if so,
223 * save their starting address and skip the option part
224 * for now. If there are no options, set the starting
227 if (*cp < '0' || *cp > '9') {
230 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
231 if (*cp == '\\' && cp[1] == '"')
232 cp++; /* Skip both */
239 /* Parse the key from the line. */
240 if (hostfile_read_key(&cp, &bits, key) == 0) {
241 debug("%.100s, line %lu: non ssh1 key syntax",
245 /* cp now points to the comment part. */
247 /* Check if the we have found the desired key (identified by its modulus). */
248 if (BN_cmp(key->rsa->n, client_n) != 0)
251 /* check the real bits */
252 keybits = BN_num_bits(key->rsa->n);
253 if (keybits < 0 || bits != (u_int)keybits)
254 logit("Warning: %s, line %lu: keysize mismatch: "
255 "actual %d vs. announced %d.",
256 file, linenum, BN_num_bits(key->rsa->n), bits);
258 /* We have found the desired key. */
260 * If our options do not allow this key to be used,
261 * do not send challenge.
263 if (!auth_parse_options(pw, key_options, file, linenum))
266 /* break out, this key is allowed */
271 /* Restore the privileged uid. */
274 /* Close the file. */
278 /* return key if allowed */
279 if (allowed && rkey != NULL)
287 * Performs the RSA authentication dialog with the client. This returns
288 * 0 if the client could not be authenticated, and 1 if authentication was
289 * successful. This may exit if there is a serious protocol violation.
292 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
296 struct passwd *pw = authctxt->pw;
299 if (!authctxt->valid)
302 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
303 auth_clear_options();
307 /* Perform the challenge-response dialog for this key. */
308 if (!auth_rsa_challenge_dialog(key)) {
309 /* Wrong response. */
310 verbose("Wrong response to RSA authentication challenge.");
311 packet_send_debug("Wrong response to RSA authentication challenge.");
313 * Break out of the loop. Otherwise we might send
314 * another challenge and break the protocol.
320 * Correct response. The client has been successfully
321 * authenticated. Note that we have not yet processed the
322 * options; this will be reset if the options cause the
323 * authentication to be rejected.
325 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
326 verbose("Found matching %s key: %s",
331 packet_send_debug("RSA authentication accepted.");