Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.52
1.1 provos 1: /*
1.13 deraadt 2: * Author: Tatu Ylonen <ylo@cs.hut.fi>
3: * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: * All rights reserved
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.
1.21 markus 8: *
1.28 deraadt 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".
1.13 deraadt 14: */
1.1 provos 15:
16: #include "includes.h"
1.51 markus 17: RCSID("$OpenBSD: auth-rsa.c,v 1.50 2001/12/28 14:50:54 markus Exp $");
1.38 markus 18:
19: #include <openssl/rsa.h>
20: #include <openssl/md5.h>
1.1 provos 21:
22: #include "rsa.h"
23: #include "packet.h"
24: #include "xmalloc.h"
1.35 markus 25: #include "ssh1.h"
1.1 provos 26: #include "mpaux.h"
27: #include "uidswap.h"
1.19 markus 28: #include "match.h"
1.25 markus 29: #include "auth-options.h"
1.35 markus 30: #include "pathnames.h"
1.38 markus 31: #include "log.h"
32: #include "servconf.h"
33: #include "auth.h"
1.46 jakob 34: #include "hostfile.h"
1.52 ! provos 35: #include "monitor_wrap.h"
1.30 markus 36:
37: /* import */
38: extern ServerOptions options;
39:
1.14 markus 40: /*
41: * Session identifier that is used to bind key exchange and authentication
42: * responses to a particular session.
43: */
1.34 markus 44: extern u_char session_id[16];
1.1 provos 45:
1.14 markus 46: /*
47: * The .ssh/authorized_keys file contains public keys, one per line, in the
48: * following format:
49: * options bits e n comment
50: * where bits, e and n are decimal numbers,
51: * and comment is any string of characters up to newline. The maximum
52: * length of a line is 8000 characters. See the documentation for a
53: * description of the options.
54: */
55:
1.52 ! provos 56: BIGNUM *
1.51 markus 57: auth_rsa_generate_challenge(Key *key)
58: {
59: BIGNUM *challenge;
60: BN_CTX *ctx;
61:
62: if ((challenge = BN_new()) == NULL)
63: fatal("auth_rsa_generate_challenge: BN_new() failed");
64: /* Generate a random challenge. */
65: BN_rand(challenge, 256, 0, 0);
66: if ((ctx = BN_CTX_new()) == NULL)
67: fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
68: BN_mod(challenge, challenge, key->rsa->n, ctx);
69: BN_CTX_free(ctx);
70:
71: return challenge;
72: }
73:
1.52 ! provos 74: int
1.51 markus 75: auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
76: {
77: u_char buf[32], mdbuf[16];
78: MD5_CTX md;
79: int len;
80:
81: /* The response is MD5 of decrypted challenge plus session id. */
82: len = BN_num_bytes(challenge);
83: if (len <= 0 || len > 32)
84: fatal("auth_rsa_verify_response: bad challenge length %d", len);
85: memset(buf, 0, 32);
86: BN_bn2bin(challenge, buf + 32 - len);
87: MD5_Init(&md);
88: MD5_Update(&md, buf, 32);
89: MD5_Update(&md, session_id, 16);
90: MD5_Final(mdbuf, &md);
91:
92: /* Verify that the response is the original challenge. */
93: if (memcmp(response, mdbuf, 16) != 0) {
94: /* Wrong answer. */
95: return (0);
96: }
97: /* Correct answer. */
98: return (1);
99: }
100:
1.14 markus 101: /*
102: * Performs the RSA authentication challenge-response dialog with the client,
103: * and returns true (non-zero) if the client gave the correct answer to
104: * our challenge; returns zero if the client gives a wrong answer.
105: */
1.1 provos 106:
107: int
1.51 markus 108: auth_rsa_challenge_dialog(Key *key)
1.1 provos 109: {
1.18 markus 110: BIGNUM *challenge, *encrypted_challenge;
1.51 markus 111: u_char response[16];
112: int i, success;
1.12 markus 113:
1.47 markus 114: if ((encrypted_challenge = BN_new()) == NULL)
115: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 116:
1.52 ! provos 117: challenge = PRIVSEP(auth_rsa_generate_challenge(key));
1.12 markus 118:
119: /* Encrypt the challenge with the public key. */
1.51 markus 120: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 121:
122: /* Send the encrypted challenge to the client. */
123: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
124: packet_put_bignum(encrypted_challenge);
125: packet_send();
1.18 markus 126: BN_clear_free(encrypted_challenge);
1.12 markus 127: packet_write_wait();
128:
1.18 markus 129: /* Wait for a response. */
1.50 markus 130: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 131: for (i = 0; i < 16; i++)
132: response[i] = packet_get_char();
1.49 markus 133: packet_check_eom();
1.18 markus 134:
1.52 ! provos 135: success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
1.12 markus 136: BN_clear_free(challenge);
1.51 markus 137: return (success);
1.1 provos 138: }
139:
1.14 markus 140: /*
1.51 markus 141: * check if there's user key matching client_n,
142: * return key if login is allowed, NULL otherwise
1.14 markus 143: */
1.1 provos 144:
1.52 ! provos 145: int
1.51 markus 146: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
1.1 provos 147: {
1.41 markus 148: char line[8192], *file;
1.52 ! provos 149: int allowed = 0;
1.34 markus 150: u_int bits;
1.12 markus 151: FILE *f;
1.34 markus 152: u_long linenum = 0;
1.12 markus 153: struct stat st;
1.46 jakob 154: Key *key;
1.12 markus 155:
156: /* Temporarily use the user's uid. */
1.40 markus 157: temporarily_use_uid(pw);
1.12 markus 158:
159: /* The authorized keys. */
1.41 markus 160: file = authorized_keys_file(pw);
161: debug("trying public RSA key file %s", file);
1.12 markus 162:
163: /* Fail quietly if file does not exist */
164: if (stat(file, &st) < 0) {
165: /* Restore the privileged uid. */
166: restore_uid();
1.41 markus 167: xfree(file);
1.51 markus 168: return (NULL);
1.12 markus 169: }
170: /* Open the file containing the authorized keys. */
171: f = fopen(file, "r");
172: if (!f) {
173: /* Restore the privileged uid. */
174: restore_uid();
1.41 markus 175: xfree(file);
1.51 markus 176: return (NULL);
1.1 provos 177: }
1.41 markus 178: if (options.strict_modes &&
1.43 provos 179: secure_filename(f, file, pw, line, sizeof(line)) != 0) {
1.41 markus 180: xfree(file);
181: fclose(f);
182: log("Authentication refused: %s", line);
183: restore_uid();
1.51 markus 184: return (NULL);
1.1 provos 185: }
1.51 markus 186:
187: /* Flag indicating whether the key is allowed. */
188: allowed = 0;
1.1 provos 189:
1.46 jakob 190: key = key_new(KEY_RSA1);
1.12 markus 191:
1.14 markus 192: /*
193: * Go though the accepted keys, looking for the current key. If
194: * found, perform a challenge-response dialog to verify that the
195: * user really has the corresponding private key.
196: */
1.12 markus 197: while (fgets(line, sizeof(line), f)) {
198: char *cp;
199: char *options;
200:
201: linenum++;
202:
1.14 markus 203: /* Skip leading whitespace, empty and comment lines. */
204: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
205: ;
1.12 markus 206: if (!*cp || *cp == '\n' || *cp == '#')
207: continue;
208:
1.14 markus 209: /*
210: * Check if there are options for this key, and if so,
211: * save their starting address and skip the option part
212: * for now. If there are no options, set the starting
213: * address to NULL.
214: */
1.12 markus 215: if (*cp < '0' || *cp > '9') {
216: int quoted = 0;
217: options = cp;
218: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
219: if (*cp == '\\' && cp[1] == '"')
220: cp++; /* Skip both */
221: else if (*cp == '"')
222: quoted = !quoted;
223: }
224: } else
225: options = NULL;
1.1 provos 226:
1.12 markus 227: /* Parse the key from the line. */
1.46 jakob 228: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 229: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 230: file, linenum);
1.12 markus 231: continue;
1.1 provos 232: }
1.12 markus 233: /* cp now points to the comment part. */
234:
1.16 markus 235: /* Check if the we have found the desired key (identified by its modulus). */
1.46 jakob 236: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 237: continue;
238:
1.12 markus 239: /* check the real bits */
1.46 jakob 240: if (bits != BN_num_bits(key->rsa->n))
1.44 stevesk 241: log("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 242: "actual %d vs. announced %d.",
1.46 jakob 243: file, linenum, BN_num_bits(key->rsa->n), bits);
1.12 markus 244:
245: /* We have found the desired key. */
1.33 markus 246: /*
247: * If our options do not allow this key to be used,
248: * do not send challenge.
249: */
1.36 markus 250: if (!auth_parse_options(pw, options, file, linenum))
1.33 markus 251: continue;
1.19 markus 252:
1.51 markus 253: /* break out, this key is allowed */
254: allowed = 1;
1.32 markus 255: break;
1.1 provos 256: }
257:
1.12 markus 258: /* Restore the privileged uid. */
259: restore_uid();
260:
261: /* Close the file. */
1.41 markus 262: xfree(file);
1.12 markus 263: fclose(f);
264:
1.51 markus 265: /* return key if allowed */
266: if (allowed && rkey != NULL)
267: *rkey = key;
268: else
269: key_free(key);
270: return (allowed);
271: }
272:
273: /*
274: * Performs the RSA authentication dialog with the client. This returns
275: * 0 if the client could not be authenticated, and 1 if authentication was
276: * successful. This may exit if there is a serious protocol violation.
277: */
278: int
279: auth_rsa(struct passwd *pw, BIGNUM *client_n)
280: {
281: Key *key;
282: char *fp;
283:
284: /* no user given */
285: if (pw == NULL)
286: return 0;
1.1 provos 287:
1.52 ! provos 288: if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
1.31 markus 289: auth_clear_options();
1.51 markus 290: return (0);
291: }
292:
293: /* Perform the challenge-response dialog for this key. */
294: if (!auth_rsa_challenge_dialog(key)) {
295: /* Wrong response. */
296: verbose("Wrong response to RSA authentication challenge.");
297: packet_send_debug("Wrong response to RSA authentication challenge.");
298: /*
299: * Break out of the loop. Otherwise we might send
300: * another challenge and break the protocol.
301: */
302: key_free(key);
303: return (0);
304: }
305: /*
306: * Correct response. The client has been successfully
307: * authenticated. Note that we have not yet processed the
308: * options; this will be reset if the options cause the
309: * authentication to be rejected.
310: */
311: fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
312: verbose("Found matching %s key: %s",
313: key_type(key), fp);
314: xfree(fp);
315: key_free(key);
1.1 provos 316:
1.51 markus 317: packet_send_debug("RSA authentication accepted.");
318: return (1);
1.1 provos 319: }
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