Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.71
1.71 ! deraadt 1: /* $OpenBSD: auth-rsa.c,v 1.70 2006/08/01 23:22:47 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:
1.64 stevesk 17: #include <sys/types.h>
18: #include <sys/stat.h>
1.38 markus 19:
20: #include <openssl/rsa.h>
21: #include <openssl/md5.h>
1.68 stevesk 22:
23: #include <pwd.h>
1.70 stevesk 24: #include <stdio.h>
1.69 stevesk 25: #include <string.h>
1.1 provos 26:
1.71 ! deraadt 27: #include "xmalloc.h"
1.1 provos 28: #include "rsa.h"
29: #include "packet.h"
1.35 markus 30: #include "ssh1.h"
1.1 provos 31: #include "uidswap.h"
1.19 markus 32: #include "match.h"
1.71 ! deraadt 33: #include "buffer.h"
1.25 markus 34: #include "auth-options.h"
1.35 markus 35: #include "pathnames.h"
1.38 markus 36: #include "log.h"
37: #include "servconf.h"
1.71 ! deraadt 38: #include "key.h"
! 39: #include "hostfile.h"
1.38 markus 40: #include "auth.h"
1.71 ! deraadt 41: #ifdef GSSAPI
! 42: #include "ssh-gss.h"
! 43: #endif
1.52 provos 44: #include "monitor_wrap.h"
1.55 stevesk 45: #include "ssh.h"
1.62 dtucker 46: #include "misc.h"
1.30 markus 47:
48: /* import */
49: extern ServerOptions options;
50:
1.14 markus 51: /*
52: * Session identifier that is used to bind key exchange and authentication
53: * responses to a particular session.
54: */
1.34 markus 55: extern u_char session_id[16];
1.1 provos 56:
1.14 markus 57: /*
58: * The .ssh/authorized_keys file contains public keys, one per line, in the
59: * following format:
60: * options bits e n comment
61: * where bits, e and n are decimal numbers,
62: * and comment is any string of characters up to newline. The maximum
1.61 dtucker 63: * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
1.14 markus 64: * description of the options.
65: */
66:
1.52 provos 67: BIGNUM *
1.51 markus 68: auth_rsa_generate_challenge(Key *key)
69: {
70: BIGNUM *challenge;
71: BN_CTX *ctx;
72:
73: if ((challenge = BN_new()) == NULL)
74: fatal("auth_rsa_generate_challenge: BN_new() failed");
75: /* Generate a random challenge. */
76: BN_rand(challenge, 256, 0, 0);
77: if ((ctx = BN_CTX_new()) == NULL)
78: fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
79: BN_mod(challenge, challenge, key->rsa->n, ctx);
80: BN_CTX_free(ctx);
81:
82: return challenge;
83: }
84:
1.52 provos 85: int
1.51 markus 86: auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
87: {
88: u_char buf[32], mdbuf[16];
89: MD5_CTX md;
90: int len;
1.54 markus 91:
92: /* don't allow short keys */
1.55 stevesk 93: if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
1.56 stevesk 94: error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
95: BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
1.54 markus 96: return (0);
97: }
1.51 markus 98:
99: /* The response is MD5 of decrypted challenge plus session id. */
100: len = BN_num_bytes(challenge);
101: if (len <= 0 || len > 32)
102: fatal("auth_rsa_verify_response: bad challenge length %d", len);
103: memset(buf, 0, 32);
104: BN_bn2bin(challenge, buf + 32 - len);
105: MD5_Init(&md);
106: MD5_Update(&md, buf, 32);
107: MD5_Update(&md, session_id, 16);
108: MD5_Final(mdbuf, &md);
109:
110: /* Verify that the response is the original challenge. */
111: if (memcmp(response, mdbuf, 16) != 0) {
112: /* Wrong answer. */
113: return (0);
114: }
115: /* Correct answer. */
116: return (1);
117: }
118:
1.14 markus 119: /*
120: * Performs the RSA authentication challenge-response dialog with the client,
121: * and returns true (non-zero) if the client gave the correct answer to
122: * our challenge; returns zero if the client gives a wrong answer.
123: */
1.1 provos 124:
125: int
1.51 markus 126: auth_rsa_challenge_dialog(Key *key)
1.1 provos 127: {
1.18 markus 128: BIGNUM *challenge, *encrypted_challenge;
1.51 markus 129: u_char response[16];
130: int i, success;
1.12 markus 131:
1.47 markus 132: if ((encrypted_challenge = BN_new()) == NULL)
133: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 134:
1.52 provos 135: challenge = PRIVSEP(auth_rsa_generate_challenge(key));
1.12 markus 136:
137: /* Encrypt the challenge with the public key. */
1.51 markus 138: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 139:
140: /* Send the encrypted challenge to the client. */
141: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
142: packet_put_bignum(encrypted_challenge);
143: packet_send();
1.18 markus 144: BN_clear_free(encrypted_challenge);
1.12 markus 145: packet_write_wait();
146:
1.18 markus 147: /* Wait for a response. */
1.50 markus 148: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 149: for (i = 0; i < 16; i++)
1.67 deraadt 150: response[i] = (u_char)packet_get_char();
1.49 markus 151: packet_check_eom();
1.18 markus 152:
1.52 provos 153: success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
1.12 markus 154: BN_clear_free(challenge);
1.51 markus 155: return (success);
1.1 provos 156: }
157:
1.14 markus 158: /*
1.51 markus 159: * check if there's user key matching client_n,
160: * return key if login is allowed, NULL otherwise
1.14 markus 161: */
1.1 provos 162:
1.52 provos 163: int
1.51 markus 164: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
1.1 provos 165: {
1.61 dtucker 166: char line[SSH_MAX_PUBKEY_BYTES], *file;
1.52 provos 167: int allowed = 0;
1.34 markus 168: u_int bits;
1.12 markus 169: FILE *f;
1.34 markus 170: u_long linenum = 0;
1.12 markus 171: struct stat st;
1.46 jakob 172: Key *key;
1.12 markus 173:
174: /* Temporarily use the user's uid. */
1.40 markus 175: temporarily_use_uid(pw);
1.12 markus 176:
177: /* The authorized keys. */
1.41 markus 178: file = authorized_keys_file(pw);
179: debug("trying public RSA key file %s", file);
1.12 markus 180:
181: /* Fail quietly if file does not exist */
182: if (stat(file, &st) < 0) {
183: /* Restore the privileged uid. */
184: restore_uid();
1.41 markus 185: xfree(file);
1.53 markus 186: return (0);
1.12 markus 187: }
188: /* Open the file containing the authorized keys. */
189: f = fopen(file, "r");
190: if (!f) {
191: /* Restore the privileged uid. */
192: restore_uid();
1.41 markus 193: xfree(file);
1.53 markus 194: return (0);
1.1 provos 195: }
1.41 markus 196: if (options.strict_modes &&
1.43 provos 197: secure_filename(f, file, pw, line, sizeof(line)) != 0) {
1.41 markus 198: xfree(file);
199: fclose(f);
1.57 itojun 200: logit("Authentication refused: %s", line);
1.41 markus 201: restore_uid();
1.53 markus 202: return (0);
1.1 provos 203: }
1.51 markus 204:
205: /* Flag indicating whether the key is allowed. */
206: allowed = 0;
1.1 provos 207:
1.46 jakob 208: key = key_new(KEY_RSA1);
1.12 markus 209:
1.14 markus 210: /*
211: * Go though the accepted keys, looking for the current key. If
212: * found, perform a challenge-response dialog to verify that the
213: * user really has the corresponding private key.
214: */
1.61 dtucker 215: while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
1.12 markus 216: char *cp;
1.60 avsm 217: char *key_options;
1.63 djm 218: int keybits;
1.12 markus 219:
1.14 markus 220: /* Skip leading whitespace, empty and comment lines. */
221: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
222: ;
1.12 markus 223: if (!*cp || *cp == '\n' || *cp == '#')
224: continue;
225:
1.14 markus 226: /*
227: * Check if there are options for this key, and if so,
228: * save their starting address and skip the option part
229: * for now. If there are no options, set the starting
230: * address to NULL.
231: */
1.12 markus 232: if (*cp < '0' || *cp > '9') {
233: int quoted = 0;
1.60 avsm 234: key_options = cp;
1.12 markus 235: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
236: if (*cp == '\\' && cp[1] == '"')
237: cp++; /* Skip both */
238: else if (*cp == '"')
239: quoted = !quoted;
240: }
241: } else
1.60 avsm 242: key_options = NULL;
1.1 provos 243:
1.12 markus 244: /* Parse the key from the line. */
1.46 jakob 245: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 246: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 247: file, linenum);
1.12 markus 248: continue;
1.1 provos 249: }
1.12 markus 250: /* cp now points to the comment part. */
251:
1.16 markus 252: /* Check if the we have found the desired key (identified by its modulus). */
1.46 jakob 253: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 254: continue;
255:
1.12 markus 256: /* check the real bits */
1.63 djm 257: keybits = BN_num_bits(key->rsa->n);
258: if (keybits < 0 || bits != (u_int)keybits)
1.57 itojun 259: logit("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 260: "actual %d vs. announced %d.",
1.46 jakob 261: file, linenum, BN_num_bits(key->rsa->n), bits);
1.12 markus 262:
263: /* We have found the desired key. */
1.33 markus 264: /*
265: * If our options do not allow this key to be used,
266: * do not send challenge.
267: */
1.60 avsm 268: if (!auth_parse_options(pw, key_options, file, linenum))
1.33 markus 269: continue;
1.19 markus 270:
1.51 markus 271: /* break out, this key is allowed */
272: allowed = 1;
1.32 markus 273: break;
1.1 provos 274: }
275:
1.12 markus 276: /* Restore the privileged uid. */
277: restore_uid();
278:
279: /* Close the file. */
1.41 markus 280: xfree(file);
1.12 markus 281: fclose(f);
282:
1.51 markus 283: /* return key if allowed */
284: if (allowed && rkey != NULL)
285: *rkey = key;
286: else
287: key_free(key);
288: return (allowed);
289: }
290:
291: /*
292: * Performs the RSA authentication dialog with the client. This returns
293: * 0 if the client could not be authenticated, and 1 if authentication was
294: * successful. This may exit if there is a serious protocol violation.
295: */
296: int
1.58 djm 297: auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
1.51 markus 298: {
299: Key *key;
300: char *fp;
1.58 djm 301: struct passwd *pw = authctxt->pw;
1.51 markus 302:
303: /* no user given */
1.58 djm 304: if (!authctxt->valid)
1.51 markus 305: return 0;
1.1 provos 306:
1.52 provos 307: if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
1.31 markus 308: auth_clear_options();
1.51 markus 309: return (0);
310: }
311:
312: /* Perform the challenge-response dialog for this key. */
313: if (!auth_rsa_challenge_dialog(key)) {
314: /* Wrong response. */
315: verbose("Wrong response to RSA authentication challenge.");
316: packet_send_debug("Wrong response to RSA authentication challenge.");
317: /*
318: * Break out of the loop. Otherwise we might send
319: * another challenge and break the protocol.
320: */
321: key_free(key);
322: return (0);
323: }
324: /*
325: * Correct response. The client has been successfully
326: * authenticated. Note that we have not yet processed the
327: * options; this will be reset if the options cause the
328: * authentication to be rejected.
329: */
330: fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
331: verbose("Found matching %s key: %s",
332: key_type(key), fp);
333: xfree(fp);
334: key_free(key);
1.1 provos 335:
1.51 markus 336: packet_send_debug("RSA authentication accepted.");
337: return (1);
1.1 provos 338: }
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