Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.56
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"
1.56 ! stevesk 17: RCSID("$OpenBSD: auth-rsa.c,v 1.55 2002/03/29 19:18:33 stevesk 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"
1.55 stevesk 36: #include "ssh.h"
1.30 markus 37:
38: /* import */
39: extern ServerOptions options;
40:
1.14 markus 41: /*
42: * Session identifier that is used to bind key exchange and authentication
43: * responses to a particular session.
44: */
1.34 markus 45: extern u_char session_id[16];
1.1 provos 46:
1.14 markus 47: /*
48: * The .ssh/authorized_keys file contains public keys, one per line, in the
49: * following format:
50: * options bits e n comment
51: * where bits, e and n are decimal numbers,
52: * and comment is any string of characters up to newline. The maximum
53: * length of a line is 8000 characters. See the documentation for a
54: * description of the options.
55: */
56:
1.52 provos 57: BIGNUM *
1.51 markus 58: auth_rsa_generate_challenge(Key *key)
59: {
60: BIGNUM *challenge;
61: BN_CTX *ctx;
62:
63: if ((challenge = BN_new()) == NULL)
64: fatal("auth_rsa_generate_challenge: BN_new() failed");
65: /* Generate a random challenge. */
66: BN_rand(challenge, 256, 0, 0);
67: if ((ctx = BN_CTX_new()) == NULL)
68: fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
69: BN_mod(challenge, challenge, key->rsa->n, ctx);
70: BN_CTX_free(ctx);
71:
72: return challenge;
73: }
74:
1.52 provos 75: int
1.51 markus 76: auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
77: {
78: u_char buf[32], mdbuf[16];
79: MD5_CTX md;
80: int len;
1.54 markus 81:
82: /* don't allow short keys */
1.55 stevesk 83: if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
1.56 ! stevesk 84: error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
! 85: BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
1.54 markus 86: return (0);
87: }
1.51 markus 88:
89: /* The response is MD5 of decrypted challenge plus session id. */
90: len = BN_num_bytes(challenge);
91: if (len <= 0 || len > 32)
92: fatal("auth_rsa_verify_response: bad challenge length %d", len);
93: memset(buf, 0, 32);
94: BN_bn2bin(challenge, buf + 32 - len);
95: MD5_Init(&md);
96: MD5_Update(&md, buf, 32);
97: MD5_Update(&md, session_id, 16);
98: MD5_Final(mdbuf, &md);
99:
100: /* Verify that the response is the original challenge. */
101: if (memcmp(response, mdbuf, 16) != 0) {
102: /* Wrong answer. */
103: return (0);
104: }
105: /* Correct answer. */
106: return (1);
107: }
108:
1.14 markus 109: /*
110: * Performs the RSA authentication challenge-response dialog with the client,
111: * and returns true (non-zero) if the client gave the correct answer to
112: * our challenge; returns zero if the client gives a wrong answer.
113: */
1.1 provos 114:
115: int
1.51 markus 116: auth_rsa_challenge_dialog(Key *key)
1.1 provos 117: {
1.18 markus 118: BIGNUM *challenge, *encrypted_challenge;
1.51 markus 119: u_char response[16];
120: int i, success;
1.12 markus 121:
1.47 markus 122: if ((encrypted_challenge = BN_new()) == NULL)
123: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 124:
1.52 provos 125: challenge = PRIVSEP(auth_rsa_generate_challenge(key));
1.12 markus 126:
127: /* Encrypt the challenge with the public key. */
1.51 markus 128: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 129:
130: /* Send the encrypted challenge to the client. */
131: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
132: packet_put_bignum(encrypted_challenge);
133: packet_send();
1.18 markus 134: BN_clear_free(encrypted_challenge);
1.12 markus 135: packet_write_wait();
136:
1.18 markus 137: /* Wait for a response. */
1.50 markus 138: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 139: for (i = 0; i < 16; i++)
140: response[i] = packet_get_char();
1.49 markus 141: packet_check_eom();
1.18 markus 142:
1.52 provos 143: success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
1.12 markus 144: BN_clear_free(challenge);
1.51 markus 145: return (success);
1.1 provos 146: }
147:
1.14 markus 148: /*
1.51 markus 149: * check if there's user key matching client_n,
150: * return key if login is allowed, NULL otherwise
1.14 markus 151: */
1.1 provos 152:
1.52 provos 153: int
1.51 markus 154: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
1.1 provos 155: {
1.41 markus 156: char line[8192], *file;
1.52 provos 157: int allowed = 0;
1.34 markus 158: u_int bits;
1.12 markus 159: FILE *f;
1.34 markus 160: u_long linenum = 0;
1.12 markus 161: struct stat st;
1.46 jakob 162: Key *key;
1.12 markus 163:
164: /* Temporarily use the user's uid. */
1.40 markus 165: temporarily_use_uid(pw);
1.12 markus 166:
167: /* The authorized keys. */
1.41 markus 168: file = authorized_keys_file(pw);
169: debug("trying public RSA key file %s", file);
1.12 markus 170:
171: /* Fail quietly if file does not exist */
172: if (stat(file, &st) < 0) {
173: /* Restore the privileged uid. */
174: restore_uid();
1.41 markus 175: xfree(file);
1.53 markus 176: return (0);
1.12 markus 177: }
178: /* Open the file containing the authorized keys. */
179: f = fopen(file, "r");
180: if (!f) {
181: /* Restore the privileged uid. */
182: restore_uid();
1.41 markus 183: xfree(file);
1.53 markus 184: return (0);
1.1 provos 185: }
1.41 markus 186: if (options.strict_modes &&
1.43 provos 187: secure_filename(f, file, pw, line, sizeof(line)) != 0) {
1.41 markus 188: xfree(file);
189: fclose(f);
190: log("Authentication refused: %s", line);
191: restore_uid();
1.53 markus 192: return (0);
1.1 provos 193: }
1.51 markus 194:
195: /* Flag indicating whether the key is allowed. */
196: allowed = 0;
1.1 provos 197:
1.46 jakob 198: key = key_new(KEY_RSA1);
1.12 markus 199:
1.14 markus 200: /*
201: * Go though the accepted keys, looking for the current key. If
202: * found, perform a challenge-response dialog to verify that the
203: * user really has the corresponding private key.
204: */
1.12 markus 205: while (fgets(line, sizeof(line), f)) {
206: char *cp;
207: char *options;
208:
209: linenum++;
210:
1.14 markus 211: /* Skip leading whitespace, empty and comment lines. */
212: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
213: ;
1.12 markus 214: if (!*cp || *cp == '\n' || *cp == '#')
215: continue;
216:
1.14 markus 217: /*
218: * Check if there are options for this key, and if so,
219: * save their starting address and skip the option part
220: * for now. If there are no options, set the starting
221: * address to NULL.
222: */
1.12 markus 223: if (*cp < '0' || *cp > '9') {
224: int quoted = 0;
225: options = cp;
226: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
227: if (*cp == '\\' && cp[1] == '"')
228: cp++; /* Skip both */
229: else if (*cp == '"')
230: quoted = !quoted;
231: }
232: } else
233: options = NULL;
1.1 provos 234:
1.12 markus 235: /* Parse the key from the line. */
1.46 jakob 236: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 237: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 238: file, linenum);
1.12 markus 239: continue;
1.1 provos 240: }
1.12 markus 241: /* cp now points to the comment part. */
242:
1.16 markus 243: /* Check if the we have found the desired key (identified by its modulus). */
1.46 jakob 244: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 245: continue;
246:
1.12 markus 247: /* check the real bits */
1.46 jakob 248: if (bits != BN_num_bits(key->rsa->n))
1.44 stevesk 249: log("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 250: "actual %d vs. announced %d.",
1.46 jakob 251: file, linenum, BN_num_bits(key->rsa->n), bits);
1.12 markus 252:
253: /* We have found the desired key. */
1.33 markus 254: /*
255: * If our options do not allow this key to be used,
256: * do not send challenge.
257: */
1.36 markus 258: if (!auth_parse_options(pw, options, file, linenum))
1.33 markus 259: continue;
1.19 markus 260:
1.51 markus 261: /* break out, this key is allowed */
262: allowed = 1;
1.32 markus 263: break;
1.1 provos 264: }
265:
1.12 markus 266: /* Restore the privileged uid. */
267: restore_uid();
268:
269: /* Close the file. */
1.41 markus 270: xfree(file);
1.12 markus 271: fclose(f);
272:
1.51 markus 273: /* return key if allowed */
274: if (allowed && rkey != NULL)
275: *rkey = key;
276: else
277: key_free(key);
278: return (allowed);
279: }
280:
281: /*
282: * Performs the RSA authentication dialog with the client. This returns
283: * 0 if the client could not be authenticated, and 1 if authentication was
284: * successful. This may exit if there is a serious protocol violation.
285: */
286: int
287: auth_rsa(struct passwd *pw, BIGNUM *client_n)
288: {
289: Key *key;
290: char *fp;
291:
292: /* no user given */
293: if (pw == NULL)
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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