Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.70
1.70 ! stevesk 1: /* $OpenBSD: auth-rsa.c,v 1.69 2006/07/22 20:48:22 stevesk Exp $ */
1.1 provos 2: /*
1.13 deraadt 3: * Author: Tatu Ylonen <ylo@cs.hut.fi>
4: * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: * All rights reserved
6: * RSA-based authentication. This code determines whether to admit a login
7: * based on RSA authentication. This file also contains functions to check
8: * validity of the host key.
1.21 markus 9: *
1.28 deraadt 10: * As far as I am concerned, the code I have written for this software
11: * can be used freely for any purpose. Any derived versions of this
12: * software must be clearly marked as such, and if the derived work is
13: * incompatible with the protocol description in the RFC file, it must be
14: * called by a name other than "ssh" or "Secure Shell".
1.13 deraadt 15: */
1.1 provos 16:
17: #include "includes.h"
1.64 stevesk 18:
19: #include <sys/types.h>
20: #include <sys/stat.h>
1.38 markus 21:
22: #include <openssl/rsa.h>
23: #include <openssl/md5.h>
1.68 stevesk 24:
25: #include <pwd.h>
1.70 ! stevesk 26: #include <stdio.h>
1.69 stevesk 27: #include <string.h>
1.1 provos 28:
29: #include "rsa.h"
30: #include "packet.h"
31: #include "xmalloc.h"
1.35 markus 32: #include "ssh1.h"
1.1 provos 33: #include "uidswap.h"
1.19 markus 34: #include "match.h"
1.25 markus 35: #include "auth-options.h"
1.35 markus 36: #include "pathnames.h"
1.38 markus 37: #include "log.h"
38: #include "servconf.h"
39: #include "auth.h"
1.46 jakob 40: #include "hostfile.h"
1.52 provos 41: #include "monitor_wrap.h"
1.55 stevesk 42: #include "ssh.h"
1.62 dtucker 43: #include "misc.h"
1.30 markus 44:
45: /* import */
46: extern ServerOptions options;
47:
1.14 markus 48: /*
49: * Session identifier that is used to bind key exchange and authentication
50: * responses to a particular session.
51: */
1.34 markus 52: extern u_char session_id[16];
1.1 provos 53:
1.14 markus 54: /*
55: * The .ssh/authorized_keys file contains public keys, one per line, in the
56: * following format:
57: * options bits e n comment
58: * where bits, e and n are decimal numbers,
59: * and comment is any string of characters up to newline. The maximum
1.61 dtucker 60: * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
1.14 markus 61: * description of the options.
62: */
63:
1.52 provos 64: BIGNUM *
1.51 markus 65: auth_rsa_generate_challenge(Key *key)
66: {
67: BIGNUM *challenge;
68: BN_CTX *ctx;
69:
70: if ((challenge = BN_new()) == NULL)
71: fatal("auth_rsa_generate_challenge: BN_new() failed");
72: /* Generate a random challenge. */
73: BN_rand(challenge, 256, 0, 0);
74: if ((ctx = BN_CTX_new()) == NULL)
75: fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
76: BN_mod(challenge, challenge, key->rsa->n, ctx);
77: BN_CTX_free(ctx);
78:
79: return challenge;
80: }
81:
1.52 provos 82: int
1.51 markus 83: auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
84: {
85: u_char buf[32], mdbuf[16];
86: MD5_CTX md;
87: int len;
1.54 markus 88:
89: /* don't allow short keys */
1.55 stevesk 90: if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
1.56 stevesk 91: error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
92: BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
1.54 markus 93: return (0);
94: }
1.51 markus 95:
96: /* The response is MD5 of decrypted challenge plus session id. */
97: len = BN_num_bytes(challenge);
98: if (len <= 0 || len > 32)
99: fatal("auth_rsa_verify_response: bad challenge length %d", len);
100: memset(buf, 0, 32);
101: BN_bn2bin(challenge, buf + 32 - len);
102: MD5_Init(&md);
103: MD5_Update(&md, buf, 32);
104: MD5_Update(&md, session_id, 16);
105: MD5_Final(mdbuf, &md);
106:
107: /* Verify that the response is the original challenge. */
108: if (memcmp(response, mdbuf, 16) != 0) {
109: /* Wrong answer. */
110: return (0);
111: }
112: /* Correct answer. */
113: return (1);
114: }
115:
1.14 markus 116: /*
117: * Performs the RSA authentication challenge-response dialog with the client,
118: * and returns true (non-zero) if the client gave the correct answer to
119: * our challenge; returns zero if the client gives a wrong answer.
120: */
1.1 provos 121:
122: int
1.51 markus 123: auth_rsa_challenge_dialog(Key *key)
1.1 provos 124: {
1.18 markus 125: BIGNUM *challenge, *encrypted_challenge;
1.51 markus 126: u_char response[16];
127: int i, success;
1.12 markus 128:
1.47 markus 129: if ((encrypted_challenge = BN_new()) == NULL)
130: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 131:
1.52 provos 132: challenge = PRIVSEP(auth_rsa_generate_challenge(key));
1.12 markus 133:
134: /* Encrypt the challenge with the public key. */
1.51 markus 135: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 136:
137: /* Send the encrypted challenge to the client. */
138: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
139: packet_put_bignum(encrypted_challenge);
140: packet_send();
1.18 markus 141: BN_clear_free(encrypted_challenge);
1.12 markus 142: packet_write_wait();
143:
1.18 markus 144: /* Wait for a response. */
1.50 markus 145: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 146: for (i = 0; i < 16; i++)
1.67 deraadt 147: response[i] = (u_char)packet_get_char();
1.49 markus 148: packet_check_eom();
1.18 markus 149:
1.52 provos 150: success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
1.12 markus 151: BN_clear_free(challenge);
1.51 markus 152: return (success);
1.1 provos 153: }
154:
1.14 markus 155: /*
1.51 markus 156: * check if there's user key matching client_n,
157: * return key if login is allowed, NULL otherwise
1.14 markus 158: */
1.1 provos 159:
1.52 provos 160: int
1.51 markus 161: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
1.1 provos 162: {
1.61 dtucker 163: char line[SSH_MAX_PUBKEY_BYTES], *file;
1.52 provos 164: int allowed = 0;
1.34 markus 165: u_int bits;
1.12 markus 166: FILE *f;
1.34 markus 167: u_long linenum = 0;
1.12 markus 168: struct stat st;
1.46 jakob 169: Key *key;
1.12 markus 170:
171: /* Temporarily use the user's uid. */
1.40 markus 172: temporarily_use_uid(pw);
1.12 markus 173:
174: /* The authorized keys. */
1.41 markus 175: file = authorized_keys_file(pw);
176: debug("trying public RSA key file %s", file);
1.12 markus 177:
178: /* Fail quietly if file does not exist */
179: if (stat(file, &st) < 0) {
180: /* Restore the privileged uid. */
181: restore_uid();
1.41 markus 182: xfree(file);
1.53 markus 183: return (0);
1.12 markus 184: }
185: /* Open the file containing the authorized keys. */
186: f = fopen(file, "r");
187: if (!f) {
188: /* Restore the privileged uid. */
189: restore_uid();
1.41 markus 190: xfree(file);
1.53 markus 191: return (0);
1.1 provos 192: }
1.41 markus 193: if (options.strict_modes &&
1.43 provos 194: secure_filename(f, file, pw, line, sizeof(line)) != 0) {
1.41 markus 195: xfree(file);
196: fclose(f);
1.57 itojun 197: logit("Authentication refused: %s", line);
1.41 markus 198: restore_uid();
1.53 markus 199: return (0);
1.1 provos 200: }
1.51 markus 201:
202: /* Flag indicating whether the key is allowed. */
203: allowed = 0;
1.1 provos 204:
1.46 jakob 205: key = key_new(KEY_RSA1);
1.12 markus 206:
1.14 markus 207: /*
208: * Go though the accepted keys, looking for the current key. If
209: * found, perform a challenge-response dialog to verify that the
210: * user really has the corresponding private key.
211: */
1.61 dtucker 212: while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
1.12 markus 213: char *cp;
1.60 avsm 214: char *key_options;
1.63 djm 215: int keybits;
1.12 markus 216:
1.14 markus 217: /* Skip leading whitespace, empty and comment lines. */
218: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
219: ;
1.12 markus 220: if (!*cp || *cp == '\n' || *cp == '#')
221: continue;
222:
1.14 markus 223: /*
224: * Check if there are options for this key, and if so,
225: * save their starting address and skip the option part
226: * for now. If there are no options, set the starting
227: * address to NULL.
228: */
1.12 markus 229: if (*cp < '0' || *cp > '9') {
230: int quoted = 0;
1.60 avsm 231: key_options = cp;
1.12 markus 232: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
233: if (*cp == '\\' && cp[1] == '"')
234: cp++; /* Skip both */
235: else if (*cp == '"')
236: quoted = !quoted;
237: }
238: } else
1.60 avsm 239: key_options = NULL;
1.1 provos 240:
1.12 markus 241: /* Parse the key from the line. */
1.46 jakob 242: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 243: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 244: file, linenum);
1.12 markus 245: continue;
1.1 provos 246: }
1.12 markus 247: /* cp now points to the comment part. */
248:
1.16 markus 249: /* Check if the we have found the desired key (identified by its modulus). */
1.46 jakob 250: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 251: continue;
252:
1.12 markus 253: /* check the real bits */
1.63 djm 254: keybits = BN_num_bits(key->rsa->n);
255: if (keybits < 0 || bits != (u_int)keybits)
1.57 itojun 256: logit("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 257: "actual %d vs. announced %d.",
1.46 jakob 258: file, linenum, BN_num_bits(key->rsa->n), bits);
1.12 markus 259:
260: /* We have found the desired key. */
1.33 markus 261: /*
262: * If our options do not allow this key to be used,
263: * do not send challenge.
264: */
1.60 avsm 265: if (!auth_parse_options(pw, key_options, file, linenum))
1.33 markus 266: continue;
1.19 markus 267:
1.51 markus 268: /* break out, this key is allowed */
269: allowed = 1;
1.32 markus 270: break;
1.1 provos 271: }
272:
1.12 markus 273: /* Restore the privileged uid. */
274: restore_uid();
275:
276: /* Close the file. */
1.41 markus 277: xfree(file);
1.12 markus 278: fclose(f);
279:
1.51 markus 280: /* return key if allowed */
281: if (allowed && rkey != NULL)
282: *rkey = key;
283: else
284: key_free(key);
285: return (allowed);
286: }
287:
288: /*
289: * Performs the RSA authentication dialog with the client. This returns
290: * 0 if the client could not be authenticated, and 1 if authentication was
291: * successful. This may exit if there is a serious protocol violation.
292: */
293: int
1.58 djm 294: auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
1.51 markus 295: {
296: Key *key;
297: char *fp;
1.58 djm 298: struct passwd *pw = authctxt->pw;
1.51 markus 299:
300: /* no user given */
1.58 djm 301: if (!authctxt->valid)
1.51 markus 302: return 0;
1.1 provos 303:
1.52 provos 304: if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
1.31 markus 305: auth_clear_options();
1.51 markus 306: return (0);
307: }
308:
309: /* Perform the challenge-response dialog for this key. */
310: if (!auth_rsa_challenge_dialog(key)) {
311: /* Wrong response. */
312: verbose("Wrong response to RSA authentication challenge.");
313: packet_send_debug("Wrong response to RSA authentication challenge.");
314: /*
315: * Break out of the loop. Otherwise we might send
316: * another challenge and break the protocol.
317: */
318: key_free(key);
319: return (0);
320: }
321: /*
322: * Correct response. The client has been successfully
323: * authenticated. Note that we have not yet processed the
324: * options; this will be reset if the options cause the
325: * authentication to be rejected.
326: */
327: fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
328: verbose("Found matching %s key: %s",
329: key_type(key), fp);
330: xfree(fp);
331: key_free(key);
1.1 provos 332:
1.51 markus 333: packet_send_debug("RSA authentication accepted.");
334: return (1);
1.1 provos 335: }