Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.64
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"
1.64 ! stevesk 17: RCSID("$OpenBSD: auth-rsa.c,v 1.63 2005/06/17 02:44:32 djm Exp $");
! 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>
1.1 provos 24:
25: #include "rsa.h"
26: #include "packet.h"
27: #include "xmalloc.h"
1.35 markus 28: #include "ssh1.h"
1.1 provos 29: #include "uidswap.h"
1.19 markus 30: #include "match.h"
1.25 markus 31: #include "auth-options.h"
1.35 markus 32: #include "pathnames.h"
1.38 markus 33: #include "log.h"
34: #include "servconf.h"
35: #include "auth.h"
1.46 jakob 36: #include "hostfile.h"
1.52 provos 37: #include "monitor_wrap.h"
1.55 stevesk 38: #include "ssh.h"
1.62 dtucker 39: #include "misc.h"
1.30 markus 40:
41: /* import */
42: extern ServerOptions options;
43:
1.14 markus 44: /*
45: * Session identifier that is used to bind key exchange and authentication
46: * responses to a particular session.
47: */
1.34 markus 48: extern u_char session_id[16];
1.1 provos 49:
1.14 markus 50: /*
51: * The .ssh/authorized_keys file contains public keys, one per line, in the
52: * following format:
53: * options bits e n comment
54: * where bits, e and n are decimal numbers,
55: * and comment is any string of characters up to newline. The maximum
1.61 dtucker 56: * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
1.14 markus 57: * description of the options.
58: */
59:
1.52 provos 60: BIGNUM *
1.51 markus 61: auth_rsa_generate_challenge(Key *key)
62: {
63: BIGNUM *challenge;
64: BN_CTX *ctx;
65:
66: if ((challenge = BN_new()) == NULL)
67: fatal("auth_rsa_generate_challenge: BN_new() failed");
68: /* Generate a random challenge. */
69: BN_rand(challenge, 256, 0, 0);
70: if ((ctx = BN_CTX_new()) == NULL)
71: fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
72: BN_mod(challenge, challenge, key->rsa->n, ctx);
73: BN_CTX_free(ctx);
74:
75: return challenge;
76: }
77:
1.52 provos 78: int
1.51 markus 79: auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
80: {
81: u_char buf[32], mdbuf[16];
82: MD5_CTX md;
83: int len;
1.54 markus 84:
85: /* don't allow short keys */
1.55 stevesk 86: if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
1.56 stevesk 87: error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
88: BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
1.54 markus 89: return (0);
90: }
1.51 markus 91:
92: /* The response is MD5 of decrypted challenge plus session id. */
93: len = BN_num_bytes(challenge);
94: if (len <= 0 || len > 32)
95: fatal("auth_rsa_verify_response: bad challenge length %d", len);
96: memset(buf, 0, 32);
97: BN_bn2bin(challenge, buf + 32 - len);
98: MD5_Init(&md);
99: MD5_Update(&md, buf, 32);
100: MD5_Update(&md, session_id, 16);
101: MD5_Final(mdbuf, &md);
102:
103: /* Verify that the response is the original challenge. */
104: if (memcmp(response, mdbuf, 16) != 0) {
105: /* Wrong answer. */
106: return (0);
107: }
108: /* Correct answer. */
109: return (1);
110: }
111:
1.14 markus 112: /*
113: * Performs the RSA authentication challenge-response dialog with the client,
114: * and returns true (non-zero) if the client gave the correct answer to
115: * our challenge; returns zero if the client gives a wrong answer.
116: */
1.1 provos 117:
118: int
1.51 markus 119: auth_rsa_challenge_dialog(Key *key)
1.1 provos 120: {
1.18 markus 121: BIGNUM *challenge, *encrypted_challenge;
1.51 markus 122: u_char response[16];
123: int i, success;
1.12 markus 124:
1.47 markus 125: if ((encrypted_challenge = BN_new()) == NULL)
126: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 127:
1.52 provos 128: challenge = PRIVSEP(auth_rsa_generate_challenge(key));
1.12 markus 129:
130: /* Encrypt the challenge with the public key. */
1.51 markus 131: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 132:
133: /* Send the encrypted challenge to the client. */
134: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
135: packet_put_bignum(encrypted_challenge);
136: packet_send();
1.18 markus 137: BN_clear_free(encrypted_challenge);
1.12 markus 138: packet_write_wait();
139:
1.18 markus 140: /* Wait for a response. */
1.50 markus 141: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 142: for (i = 0; i < 16; i++)
143: response[i] = packet_get_char();
1.49 markus 144: packet_check_eom();
1.18 markus 145:
1.52 provos 146: success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
1.12 markus 147: BN_clear_free(challenge);
1.51 markus 148: return (success);
1.1 provos 149: }
150:
1.14 markus 151: /*
1.51 markus 152: * check if there's user key matching client_n,
153: * return key if login is allowed, NULL otherwise
1.14 markus 154: */
1.1 provos 155:
1.52 provos 156: int
1.51 markus 157: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
1.1 provos 158: {
1.61 dtucker 159: char line[SSH_MAX_PUBKEY_BYTES], *file;
1.52 provos 160: int allowed = 0;
1.34 markus 161: u_int bits;
1.12 markus 162: FILE *f;
1.34 markus 163: u_long linenum = 0;
1.12 markus 164: struct stat st;
1.46 jakob 165: Key *key;
1.12 markus 166:
167: /* Temporarily use the user's uid. */
1.40 markus 168: temporarily_use_uid(pw);
1.12 markus 169:
170: /* The authorized keys. */
1.41 markus 171: file = authorized_keys_file(pw);
172: debug("trying public RSA key file %s", file);
1.12 markus 173:
174: /* Fail quietly if file does not exist */
175: if (stat(file, &st) < 0) {
176: /* Restore the privileged uid. */
177: restore_uid();
1.41 markus 178: xfree(file);
1.53 markus 179: return (0);
1.12 markus 180: }
181: /* Open the file containing the authorized keys. */
182: f = fopen(file, "r");
183: if (!f) {
184: /* Restore the privileged uid. */
185: restore_uid();
1.41 markus 186: xfree(file);
1.53 markus 187: return (0);
1.1 provos 188: }
1.41 markus 189: if (options.strict_modes &&
1.43 provos 190: secure_filename(f, file, pw, line, sizeof(line)) != 0) {
1.41 markus 191: xfree(file);
192: fclose(f);
1.57 itojun 193: logit("Authentication refused: %s", line);
1.41 markus 194: restore_uid();
1.53 markus 195: return (0);
1.1 provos 196: }
1.51 markus 197:
198: /* Flag indicating whether the key is allowed. */
199: allowed = 0;
1.1 provos 200:
1.46 jakob 201: key = key_new(KEY_RSA1);
1.12 markus 202:
1.14 markus 203: /*
204: * Go though the accepted keys, looking for the current key. If
205: * found, perform a challenge-response dialog to verify that the
206: * user really has the corresponding private key.
207: */
1.61 dtucker 208: while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
1.12 markus 209: char *cp;
1.60 avsm 210: char *key_options;
1.63 djm 211: int keybits;
1.12 markus 212:
1.14 markus 213: /* Skip leading whitespace, empty and comment lines. */
214: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
215: ;
1.12 markus 216: if (!*cp || *cp == '\n' || *cp == '#')
217: continue;
218:
1.14 markus 219: /*
220: * Check if there are options for this key, and if so,
221: * save their starting address and skip the option part
222: * for now. If there are no options, set the starting
223: * address to NULL.
224: */
1.12 markus 225: if (*cp < '0' || *cp > '9') {
226: int quoted = 0;
1.60 avsm 227: key_options = cp;
1.12 markus 228: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
229: if (*cp == '\\' && cp[1] == '"')
230: cp++; /* Skip both */
231: else if (*cp == '"')
232: quoted = !quoted;
233: }
234: } else
1.60 avsm 235: key_options = NULL;
1.1 provos 236:
1.12 markus 237: /* Parse the key from the line. */
1.46 jakob 238: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 239: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 240: file, linenum);
1.12 markus 241: continue;
1.1 provos 242: }
1.12 markus 243: /* cp now points to the comment part. */
244:
1.16 markus 245: /* Check if the we have found the desired key (identified by its modulus). */
1.46 jakob 246: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 247: continue;
248:
1.12 markus 249: /* check the real bits */
1.63 djm 250: keybits = BN_num_bits(key->rsa->n);
251: if (keybits < 0 || bits != (u_int)keybits)
1.57 itojun 252: logit("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 253: "actual %d vs. announced %d.",
1.46 jakob 254: file, linenum, BN_num_bits(key->rsa->n), bits);
1.12 markus 255:
256: /* We have found the desired key. */
1.33 markus 257: /*
258: * If our options do not allow this key to be used,
259: * do not send challenge.
260: */
1.60 avsm 261: if (!auth_parse_options(pw, key_options, file, linenum))
1.33 markus 262: continue;
1.19 markus 263:
1.51 markus 264: /* break out, this key is allowed */
265: allowed = 1;
1.32 markus 266: break;
1.1 provos 267: }
268:
1.12 markus 269: /* Restore the privileged uid. */
270: restore_uid();
271:
272: /* Close the file. */
1.41 markus 273: xfree(file);
1.12 markus 274: fclose(f);
275:
1.51 markus 276: /* return key if allowed */
277: if (allowed && rkey != NULL)
278: *rkey = key;
279: else
280: key_free(key);
281: return (allowed);
282: }
283:
284: /*
285: * Performs the RSA authentication dialog with the client. This returns
286: * 0 if the client could not be authenticated, and 1 if authentication was
287: * successful. This may exit if there is a serious protocol violation.
288: */
289: int
1.58 djm 290: auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
1.51 markus 291: {
292: Key *key;
293: char *fp;
1.58 djm 294: struct passwd *pw = authctxt->pw;
1.51 markus 295:
296: /* no user given */
1.58 djm 297: if (!authctxt->valid)
1.51 markus 298: return 0;
1.1 provos 299:
1.52 provos 300: if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
1.31 markus 301: auth_clear_options();
1.51 markus 302: return (0);
303: }
304:
305: /* Perform the challenge-response dialog for this key. */
306: if (!auth_rsa_challenge_dialog(key)) {
307: /* Wrong response. */
308: verbose("Wrong response to RSA authentication challenge.");
309: packet_send_debug("Wrong response to RSA authentication challenge.");
310: /*
311: * Break out of the loop. Otherwise we might send
312: * another challenge and break the protocol.
313: */
314: key_free(key);
315: return (0);
316: }
317: /*
318: * Correct response. The client has been successfully
319: * authenticated. Note that we have not yet processed the
320: * options; this will be reset if the options cause the
321: * authentication to be rejected.
322: */
323: fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
324: verbose("Found matching %s key: %s",
325: key_type(key), fp);
326: xfree(fp);
327: key_free(key);
1.1 provos 328:
1.51 markus 329: packet_send_debug("RSA authentication accepted.");
330: return (1);
1.1 provos 331: }
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