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".
18 #include <sys/types.h>
21 #include <openssl/rsa.h>
22 #include <openssl/md5.h>
30 #include "auth-options.h"
31 #include "pathnames.h"
36 #include "monitor_wrap.h"
41 extern ServerOptions options;
44 * Session identifier that is used to bind key exchange and authentication
45 * responses to a particular session.
47 extern u_char session_id[16];
50 * The .ssh/authorized_keys file contains public keys, one per line, in the
52 * options bits e n comment
53 * where bits, e and n are decimal numbers,
54 * and comment is any string of characters up to newline. The maximum
55 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
56 * description of the options.
60 auth_rsa_generate_challenge(Key *key)
65 if ((challenge = BN_new()) == NULL)
66 fatal("auth_rsa_generate_challenge: BN_new() failed");
67 /* Generate a random challenge. */
68 BN_rand(challenge, 256, 0, 0);
69 if ((ctx = BN_CTX_new()) == NULL)
70 fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
71 BN_mod(challenge, challenge, key->rsa->n, ctx);
78 auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
80 u_char buf[32], mdbuf[16];
84 /* don't allow short keys */
85 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
86 error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
87 BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
91 /* The response is MD5 of decrypted challenge plus session id. */
92 len = BN_num_bytes(challenge);
93 if (len <= 0 || len > 32)
94 fatal("auth_rsa_verify_response: bad challenge length %d", len);
96 BN_bn2bin(challenge, buf + 32 - len);
98 MD5_Update(&md, buf, 32);
99 MD5_Update(&md, session_id, 16);
100 MD5_Final(mdbuf, &md);
102 /* Verify that the response is the original challenge. */
103 if (memcmp(response, mdbuf, 16) != 0) {
107 /* Correct answer. */
112 * Performs the RSA authentication challenge-response dialog with the client,
113 * and returns true (non-zero) if the client gave the correct answer to
114 * our challenge; returns zero if the client gives a wrong answer.
118 auth_rsa_challenge_dialog(Key *key)
120 BIGNUM *challenge, *encrypted_challenge;
124 if ((encrypted_challenge = BN_new()) == NULL)
125 fatal("auth_rsa_challenge_dialog: BN_new() failed");
127 challenge = PRIVSEP(auth_rsa_generate_challenge(key));
129 /* Encrypt the challenge with the public key. */
130 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
132 /* Send the encrypted challenge to the client. */
133 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
134 packet_put_bignum(encrypted_challenge);
136 BN_clear_free(encrypted_challenge);
139 /* Wait for a response. */
140 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
141 for (i = 0; i < 16; i++)
142 response[i] = packet_get_char();
145 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
146 BN_clear_free(challenge);
151 * check if there's user key matching client_n,
152 * return key if login is allowed, NULL otherwise
156 auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
158 char line[SSH_MAX_PUBKEY_BYTES], *file;
166 /* Temporarily use the user's uid. */
167 temporarily_use_uid(pw);
169 /* The authorized keys. */
170 file = authorized_keys_file(pw);
171 debug("trying public RSA key file %s", file);
173 /* Fail quietly if file does not exist */
174 if (stat(file, &st) < 0) {
175 /* Restore the privileged uid. */
180 /* Open the file containing the authorized keys. */
181 f = fopen(file, "r");
183 /* Restore the privileged uid. */
188 if (options.strict_modes &&
189 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
192 logit("Authentication refused: %s", line);
197 /* Flag indicating whether the key is allowed. */
200 key = key_new(KEY_RSA1);
203 * Go though the accepted keys, looking for the current key. If
204 * found, perform a challenge-response dialog to verify that the
205 * user really has the corresponding private key.
207 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
212 /* Skip leading whitespace, empty and comment lines. */
213 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
215 if (!*cp || *cp == '\n' || *cp == '#')
219 * Check if there are options for this key, and if so,
220 * save their starting address and skip the option part
221 * for now. If there are no options, set the starting
224 if (*cp < '0' || *cp > '9') {
227 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
228 if (*cp == '\\' && cp[1] == '"')
229 cp++; /* Skip both */
236 /* Parse the key from the line. */
237 if (hostfile_read_key(&cp, &bits, key) == 0) {
238 debug("%.100s, line %lu: non ssh1 key syntax",
242 /* cp now points to the comment part. */
244 /* Check if the we have found the desired key (identified by its modulus). */
245 if (BN_cmp(key->rsa->n, client_n) != 0)
248 /* check the real bits */
249 keybits = BN_num_bits(key->rsa->n);
250 if (keybits < 0 || bits != (u_int)keybits)
251 logit("Warning: %s, line %lu: keysize mismatch: "
252 "actual %d vs. announced %d.",
253 file, linenum, BN_num_bits(key->rsa->n), bits);
255 /* We have found the desired key. */
257 * If our options do not allow this key to be used,
258 * do not send challenge.
260 if (!auth_parse_options(pw, key_options, file, linenum))
263 /* break out, this key is allowed */
268 /* Restore the privileged uid. */
271 /* Close the file. */
275 /* return key if allowed */
276 if (allowed && rkey != NULL)
284 * Performs the RSA authentication dialog with the client. This returns
285 * 0 if the client could not be authenticated, and 1 if authentication was
286 * successful. This may exit if there is a serious protocol violation.
289 auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
293 struct passwd *pw = authctxt->pw;
296 if (!authctxt->valid)
299 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
300 auth_clear_options();
304 /* Perform the challenge-response dialog for this key. */
305 if (!auth_rsa_challenge_dialog(key)) {
306 /* Wrong response. */
307 verbose("Wrong response to RSA authentication challenge.");
308 packet_send_debug("Wrong response to RSA authentication challenge.");
310 * Break out of the loop. Otherwise we might send
311 * another challenge and break the protocol.
317 * Correct response. The client has been successfully
318 * authenticated. Note that we have not yet processed the
319 * options; this will be reset if the options cause the
320 * authentication to be rejected.
322 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
323 verbose("Found matching %s key: %s",
328 packet_send_debug("RSA authentication accepted.");