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 1.50 2001/12/28 14:50:54 markus Exp $");
19 #include <openssl/rsa.h>
20 #include <openssl/md5.h>
29 #include "auth-options.h"
30 #include "pathnames.h"
37 extern ServerOptions options;
40 * Session identifier that is used to bind key exchange and authentication
41 * responses to a particular session.
43 extern u_char session_id[16];
46 * The .ssh/authorized_keys file contains public keys, one per line, in the
48 * options bits e n comment
49 * where bits, e and n are decimal numbers,
50 * and comment is any string of characters up to newline. The maximum
51 * length of a line is 8000 characters. See the documentation for a
52 * description of the options.
56 * Performs the RSA authentication challenge-response dialog with the client,
57 * and returns true (non-zero) if the client gave the correct answer to
58 * our challenge; returns zero if the client gives a wrong answer.
62 auth_rsa_challenge_dialog(RSA *pk)
64 BIGNUM *challenge, *encrypted_challenge;
66 u_char buf[32], mdbuf[16], response[16];
71 if ((encrypted_challenge = BN_new()) == NULL)
72 fatal("auth_rsa_challenge_dialog: BN_new() failed");
73 if ((challenge = BN_new()) == NULL)
74 fatal("auth_rsa_challenge_dialog: BN_new() failed");
76 /* Generate a random challenge. */
77 BN_rand(challenge, 256, 0, 0);
78 if ((ctx = BN_CTX_new()) == NULL)
79 fatal("auth_rsa_challenge_dialog: BN_CTX_new() failed");
80 BN_mod(challenge, challenge, pk->n, ctx);
83 /* Encrypt the challenge with the public key. */
84 rsa_public_encrypt(encrypted_challenge, challenge, pk);
86 /* Send the encrypted challenge to the client. */
87 packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
88 packet_put_bignum(encrypted_challenge);
90 BN_clear_free(encrypted_challenge);
93 /* Wait for a response. */
94 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
95 for (i = 0; i < 16; i++)
96 response[i] = packet_get_char();
99 /* The response is MD5 of decrypted challenge plus session id. */
100 len = BN_num_bytes(challenge);
101 if (len <= 0 || len > 32)
102 fatal("auth_rsa_challenge_dialog: bad challenge length %d", len);
104 BN_bn2bin(challenge, buf + 32 - len);
106 MD5_Update(&md, buf, 32);
107 MD5_Update(&md, session_id, 16);
108 MD5_Final(mdbuf, &md);
109 BN_clear_free(challenge);
111 /* Verify that the response is the original challenge. */
112 if (memcmp(response, mdbuf, 16) != 0) {
116 /* Correct answer. */
121 * Performs the RSA authentication dialog with the client. This returns
122 * 0 if the client could not be authenticated, and 1 if authentication was
123 * successful. This may exit if there is a serious protocol violation.
127 auth_rsa(struct passwd *pw, BIGNUM *client_n)
129 char line[8192], *file;
142 /* Temporarily use the user's uid. */
143 temporarily_use_uid(pw);
145 /* The authorized keys. */
146 file = authorized_keys_file(pw);
147 debug("trying public RSA key file %s", file);
149 /* Fail quietly if file does not exist */
150 if (stat(file, &st) < 0) {
151 /* Restore the privileged uid. */
156 /* Open the file containing the authorized keys. */
157 f = fopen(file, "r");
159 /* Restore the privileged uid. */
161 packet_send_debug("Could not open %.900s for reading.", file);
162 packet_send_debug("If your home is on an NFS volume, it may need to be world-readable.");
166 if (options.strict_modes &&
167 secure_filename(f, file, pw, line, sizeof(line)) != 0) {
170 log("Authentication refused: %s", line);
171 packet_send_debug("Authentication refused: %s", line);
175 /* Flag indicating whether authentication has succeeded. */
178 key = key_new(KEY_RSA1);
181 * Go though the accepted keys, looking for the current key. If
182 * found, perform a challenge-response dialog to verify that the
183 * user really has the corresponding private key.
185 while (fgets(line, sizeof(line), f)) {
191 /* Skip leading whitespace, empty and comment lines. */
192 for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
194 if (!*cp || *cp == '\n' || *cp == '#')
198 * Check if there are options for this key, and if so,
199 * save their starting address and skip the option part
200 * for now. If there are no options, set the starting
203 if (*cp < '0' || *cp > '9') {
206 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
207 if (*cp == '\\' && cp[1] == '"')
208 cp++; /* Skip both */
215 /* Parse the key from the line. */
216 if (hostfile_read_key(&cp, &bits, key) == 0) {
217 debug("%.100s, line %lu: non ssh1 key syntax",
221 /* cp now points to the comment part. */
223 /* Check if the we have found the desired key (identified by its modulus). */
224 if (BN_cmp(key->rsa->n, client_n) != 0)
227 /* check the real bits */
228 if (bits != BN_num_bits(key->rsa->n))
229 log("Warning: %s, line %lu: keysize mismatch: "
230 "actual %d vs. announced %d.",
231 file, linenum, BN_num_bits(key->rsa->n), bits);
233 /* We have found the desired key. */
235 * If our options do not allow this key to be used,
236 * do not send challenge.
238 if (!auth_parse_options(pw, options, file, linenum))
241 /* Perform the challenge-response dialog for this key. */
242 if (!auth_rsa_challenge_dialog(key->rsa)) {
243 /* Wrong response. */
244 verbose("Wrong response to RSA authentication challenge.");
245 packet_send_debug("Wrong response to RSA authentication challenge.");
247 * Break out of the loop. Otherwise we might send
248 * another challenge and break the protocol.
253 * Correct response. The client has been successfully
254 * authenticated. Note that we have not yet processed the
255 * options; this will be reset if the options cause the
256 * authentication to be rejected.
257 * Break out of the loop if authentication was successful;
258 * otherwise continue searching.
262 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
263 verbose("Found matching %s key: %s",
270 /* Restore the privileged uid. */
273 /* Close the file. */
280 packet_send_debug("RSA authentication accepted.");
282 auth_clear_options();
284 /* Return authentication result. */
285 return authenticated;