Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.60
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"
1.60 ! avsm 17: RCSID("$OpenBSD: auth-rsa.c,v 1.59 2004/05/09 01:19:27 djm 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 "uidswap.h"
1.19 markus 27: #include "match.h"
1.25 markus 28: #include "auth-options.h"
1.35 markus 29: #include "pathnames.h"
1.38 markus 30: #include "log.h"
31: #include "servconf.h"
32: #include "auth.h"
1.46 jakob 33: #include "hostfile.h"
1.52 provos 34: #include "monitor_wrap.h"
1.55 stevesk 35: #include "ssh.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;
1.54 markus 80:
81: /* don't allow short keys */
1.55 stevesk 82: if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
1.56 stevesk 83: error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
84: BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
1.54 markus 85: return (0);
86: }
1.51 markus 87:
88: /* The response is MD5 of decrypted challenge plus session id. */
89: len = BN_num_bytes(challenge);
90: if (len <= 0 || len > 32)
91: fatal("auth_rsa_verify_response: bad challenge length %d", len);
92: memset(buf, 0, 32);
93: BN_bn2bin(challenge, buf + 32 - len);
94: MD5_Init(&md);
95: MD5_Update(&md, buf, 32);
96: MD5_Update(&md, session_id, 16);
97: MD5_Final(mdbuf, &md);
98:
99: /* Verify that the response is the original challenge. */
100: if (memcmp(response, mdbuf, 16) != 0) {
101: /* Wrong answer. */
102: return (0);
103: }
104: /* Correct answer. */
105: return (1);
106: }
107:
1.14 markus 108: /*
109: * Performs the RSA authentication challenge-response dialog with the client,
110: * and returns true (non-zero) if the client gave the correct answer to
111: * our challenge; returns zero if the client gives a wrong answer.
112: */
1.1 provos 113:
114: int
1.51 markus 115: auth_rsa_challenge_dialog(Key *key)
1.1 provos 116: {
1.18 markus 117: BIGNUM *challenge, *encrypted_challenge;
1.51 markus 118: u_char response[16];
119: int i, success;
1.12 markus 120:
1.47 markus 121: if ((encrypted_challenge = BN_new()) == NULL)
122: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 123:
1.52 provos 124: challenge = PRIVSEP(auth_rsa_generate_challenge(key));
1.12 markus 125:
126: /* Encrypt the challenge with the public key. */
1.51 markus 127: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 128:
129: /* Send the encrypted challenge to the client. */
130: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
131: packet_put_bignum(encrypted_challenge);
132: packet_send();
1.18 markus 133: BN_clear_free(encrypted_challenge);
1.12 markus 134: packet_write_wait();
135:
1.18 markus 136: /* Wait for a response. */
1.50 markus 137: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 138: for (i = 0; i < 16; i++)
139: response[i] = packet_get_char();
1.49 markus 140: packet_check_eom();
1.18 markus 141:
1.52 provos 142: success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
1.12 markus 143: BN_clear_free(challenge);
1.51 markus 144: return (success);
1.1 provos 145: }
146:
1.14 markus 147: /*
1.51 markus 148: * check if there's user key matching client_n,
149: * return key if login is allowed, NULL otherwise
1.14 markus 150: */
1.1 provos 151:
1.52 provos 152: int
1.51 markus 153: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
1.1 provos 154: {
1.41 markus 155: char line[8192], *file;
1.52 provos 156: int allowed = 0;
1.34 markus 157: u_int bits;
1.12 markus 158: FILE *f;
1.34 markus 159: u_long linenum = 0;
1.12 markus 160: struct stat st;
1.46 jakob 161: Key *key;
1.12 markus 162:
163: /* Temporarily use the user's uid. */
1.40 markus 164: temporarily_use_uid(pw);
1.12 markus 165:
166: /* The authorized keys. */
1.41 markus 167: file = authorized_keys_file(pw);
168: debug("trying public RSA key file %s", file);
1.12 markus 169:
170: /* Fail quietly if file does not exist */
171: if (stat(file, &st) < 0) {
172: /* Restore the privileged uid. */
173: restore_uid();
1.41 markus 174: xfree(file);
1.53 markus 175: return (0);
1.12 markus 176: }
177: /* Open the file containing the authorized keys. */
178: f = fopen(file, "r");
179: if (!f) {
180: /* Restore the privileged uid. */
181: restore_uid();
1.41 markus 182: xfree(file);
1.53 markus 183: return (0);
1.1 provos 184: }
1.41 markus 185: if (options.strict_modes &&
1.43 provos 186: secure_filename(f, file, pw, line, sizeof(line)) != 0) {
1.41 markus 187: xfree(file);
188: fclose(f);
1.57 itojun 189: logit("Authentication refused: %s", line);
1.41 markus 190: restore_uid();
1.53 markus 191: return (0);
1.1 provos 192: }
1.51 markus 193:
194: /* Flag indicating whether the key is allowed. */
195: allowed = 0;
1.1 provos 196:
1.46 jakob 197: key = key_new(KEY_RSA1);
1.12 markus 198:
1.14 markus 199: /*
200: * Go though the accepted keys, looking for the current key. If
201: * found, perform a challenge-response dialog to verify that the
202: * user really has the corresponding private key.
203: */
1.12 markus 204: while (fgets(line, sizeof(line), f)) {
205: char *cp;
1.60 ! avsm 206: char *key_options;
1.12 markus 207:
208: linenum++;
209:
1.14 markus 210: /* Skip leading whitespace, empty and comment lines. */
211: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
212: ;
1.12 markus 213: if (!*cp || *cp == '\n' || *cp == '#')
214: continue;
215:
1.14 markus 216: /*
217: * Check if there are options for this key, and if so,
218: * save their starting address and skip the option part
219: * for now. If there are no options, set the starting
220: * address to NULL.
221: */
1.12 markus 222: if (*cp < '0' || *cp > '9') {
223: int quoted = 0;
1.60 ! avsm 224: key_options = cp;
1.12 markus 225: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
226: if (*cp == '\\' && cp[1] == '"')
227: cp++; /* Skip both */
228: else if (*cp == '"')
229: quoted = !quoted;
230: }
231: } else
1.60 ! avsm 232: key_options = NULL;
1.1 provos 233:
1.12 markus 234: /* Parse the key from the line. */
1.46 jakob 235: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 236: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 237: file, linenum);
1.12 markus 238: continue;
1.1 provos 239: }
1.12 markus 240: /* cp now points to the comment part. */
241:
1.16 markus 242: /* Check if the we have found the desired key (identified by its modulus). */
1.46 jakob 243: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 244: continue;
245:
1.12 markus 246: /* check the real bits */
1.46 jakob 247: if (bits != BN_num_bits(key->rsa->n))
1.57 itojun 248: logit("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 249: "actual %d vs. announced %d.",
1.46 jakob 250: file, linenum, BN_num_bits(key->rsa->n), bits);
1.12 markus 251:
252: /* We have found the desired key. */
1.33 markus 253: /*
254: * If our options do not allow this key to be used,
255: * do not send challenge.
256: */
1.60 ! avsm 257: if (!auth_parse_options(pw, key_options, file, linenum))
1.33 markus 258: continue;
1.19 markus 259:
1.51 markus 260: /* break out, this key is allowed */
261: allowed = 1;
1.32 markus 262: break;
1.1 provos 263: }
264:
1.12 markus 265: /* Restore the privileged uid. */
266: restore_uid();
267:
268: /* Close the file. */
1.41 markus 269: xfree(file);
1.12 markus 270: fclose(f);
271:
1.51 markus 272: /* return key if allowed */
273: if (allowed && rkey != NULL)
274: *rkey = key;
275: else
276: key_free(key);
277: return (allowed);
278: }
279:
280: /*
281: * Performs the RSA authentication dialog with the client. This returns
282: * 0 if the client could not be authenticated, and 1 if authentication was
283: * successful. This may exit if there is a serious protocol violation.
284: */
285: int
1.58 djm 286: auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
1.51 markus 287: {
288: Key *key;
289: char *fp;
1.58 djm 290: struct passwd *pw = authctxt->pw;
1.51 markus 291:
292: /* no user given */
1.58 djm 293: if (!authctxt->valid)
1.51 markus 294: return 0;
1.1 provos 295:
1.52 provos 296: if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
1.31 markus 297: auth_clear_options();
1.51 markus 298: return (0);
299: }
300:
301: /* Perform the challenge-response dialog for this key. */
302: if (!auth_rsa_challenge_dialog(key)) {
303: /* Wrong response. */
304: verbose("Wrong response to RSA authentication challenge.");
305: packet_send_debug("Wrong response to RSA authentication challenge.");
306: /*
307: * Break out of the loop. Otherwise we might send
308: * another challenge and break the protocol.
309: */
310: key_free(key);
311: return (0);
312: }
313: /*
314: * Correct response. The client has been successfully
315: * authenticated. Note that we have not yet processed the
316: * options; this will be reset if the options cause the
317: * authentication to be rejected.
318: */
319: fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
320: verbose("Found matching %s key: %s",
321: key_type(key), fp);
322: xfree(fp);
323: key_free(key);
1.1 provos 324:
1.51 markus 325: packet_send_debug("RSA authentication accepted.");
326: return (1);
1.1 provos 327: }
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