Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.69
1.69 ! stevesk 1: /* $OpenBSD: auth-rsa.c,v 1.68 2006/07/06 16:03:53 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>
1.69 ! stevesk 26: #include <string.h>
1.1 provos 27:
28: #include "rsa.h"
29: #include "packet.h"
30: #include "xmalloc.h"
1.35 markus 31: #include "ssh1.h"
1.1 provos 32: #include "uidswap.h"
1.19 markus 33: #include "match.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"
38: #include "auth.h"
1.46 jakob 39: #include "hostfile.h"
1.52 provos 40: #include "monitor_wrap.h"
1.55 stevesk 41: #include "ssh.h"
1.62 dtucker 42: #include "misc.h"
1.30 markus 43:
44: /* import */
45: extern ServerOptions options;
46:
1.14 markus 47: /*
48: * Session identifier that is used to bind key exchange and authentication
49: * responses to a particular session.
50: */
1.34 markus 51: extern u_char session_id[16];
1.1 provos 52:
1.14 markus 53: /*
54: * The .ssh/authorized_keys file contains public keys, one per line, in the
55: * following format:
56: * options bits e n comment
57: * where bits, e and n are decimal numbers,
58: * and comment is any string of characters up to newline. The maximum
1.61 dtucker 59: * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
1.14 markus 60: * description of the options.
61: */
62:
1.52 provos 63: BIGNUM *
1.51 markus 64: auth_rsa_generate_challenge(Key *key)
65: {
66: BIGNUM *challenge;
67: BN_CTX *ctx;
68:
69: if ((challenge = BN_new()) == NULL)
70: fatal("auth_rsa_generate_challenge: BN_new() failed");
71: /* Generate a random challenge. */
72: BN_rand(challenge, 256, 0, 0);
73: if ((ctx = BN_CTX_new()) == NULL)
74: fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
75: BN_mod(challenge, challenge, key->rsa->n, ctx);
76: BN_CTX_free(ctx);
77:
78: return challenge;
79: }
80:
1.52 provos 81: int
1.51 markus 82: auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
83: {
84: u_char buf[32], mdbuf[16];
85: MD5_CTX md;
86: int len;
1.54 markus 87:
88: /* don't allow short keys */
1.55 stevesk 89: if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
1.56 stevesk 90: error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
91: BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
1.54 markus 92: return (0);
93: }
1.51 markus 94:
95: /* The response is MD5 of decrypted challenge plus session id. */
96: len = BN_num_bytes(challenge);
97: if (len <= 0 || len > 32)
98: fatal("auth_rsa_verify_response: bad challenge length %d", len);
99: memset(buf, 0, 32);
100: BN_bn2bin(challenge, buf + 32 - len);
101: MD5_Init(&md);
102: MD5_Update(&md, buf, 32);
103: MD5_Update(&md, session_id, 16);
104: MD5_Final(mdbuf, &md);
105:
106: /* Verify that the response is the original challenge. */
107: if (memcmp(response, mdbuf, 16) != 0) {
108: /* Wrong answer. */
109: return (0);
110: }
111: /* Correct answer. */
112: return (1);
113: }
114:
1.14 markus 115: /*
116: * Performs the RSA authentication challenge-response dialog with the client,
117: * and returns true (non-zero) if the client gave the correct answer to
118: * our challenge; returns zero if the client gives a wrong answer.
119: */
1.1 provos 120:
121: int
1.51 markus 122: auth_rsa_challenge_dialog(Key *key)
1.1 provos 123: {
1.18 markus 124: BIGNUM *challenge, *encrypted_challenge;
1.51 markus 125: u_char response[16];
126: int i, success;
1.12 markus 127:
1.47 markus 128: if ((encrypted_challenge = BN_new()) == NULL)
129: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 130:
1.52 provos 131: challenge = PRIVSEP(auth_rsa_generate_challenge(key));
1.12 markus 132:
133: /* Encrypt the challenge with the public key. */
1.51 markus 134: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 135:
136: /* Send the encrypted challenge to the client. */
137: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
138: packet_put_bignum(encrypted_challenge);
139: packet_send();
1.18 markus 140: BN_clear_free(encrypted_challenge);
1.12 markus 141: packet_write_wait();
142:
1.18 markus 143: /* Wait for a response. */
1.50 markus 144: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 145: for (i = 0; i < 16; i++)
1.67 deraadt 146: response[i] = (u_char)packet_get_char();
1.49 markus 147: packet_check_eom();
1.18 markus 148:
1.52 provos 149: success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
1.12 markus 150: BN_clear_free(challenge);
1.51 markus 151: return (success);
1.1 provos 152: }
153:
1.14 markus 154: /*
1.51 markus 155: * check if there's user key matching client_n,
156: * return key if login is allowed, NULL otherwise
1.14 markus 157: */
1.1 provos 158:
1.52 provos 159: int
1.51 markus 160: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
1.1 provos 161: {
1.61 dtucker 162: char line[SSH_MAX_PUBKEY_BYTES], *file;
1.52 provos 163: int allowed = 0;
1.34 markus 164: u_int bits;
1.12 markus 165: FILE *f;
1.34 markus 166: u_long linenum = 0;
1.12 markus 167: struct stat st;
1.46 jakob 168: Key *key;
1.12 markus 169:
170: /* Temporarily use the user's uid. */
1.40 markus 171: temporarily_use_uid(pw);
1.12 markus 172:
173: /* The authorized keys. */
1.41 markus 174: file = authorized_keys_file(pw);
175: debug("trying public RSA key file %s", file);
1.12 markus 176:
177: /* Fail quietly if file does not exist */
178: if (stat(file, &st) < 0) {
179: /* Restore the privileged uid. */
180: restore_uid();
1.41 markus 181: xfree(file);
1.53 markus 182: return (0);
1.12 markus 183: }
184: /* Open the file containing the authorized keys. */
185: f = fopen(file, "r");
186: if (!f) {
187: /* Restore the privileged uid. */
188: restore_uid();
1.41 markus 189: xfree(file);
1.53 markus 190: return (0);
1.1 provos 191: }
1.41 markus 192: if (options.strict_modes &&
1.43 provos 193: secure_filename(f, file, pw, line, sizeof(line)) != 0) {
1.41 markus 194: xfree(file);
195: fclose(f);
1.57 itojun 196: logit("Authentication refused: %s", line);
1.41 markus 197: restore_uid();
1.53 markus 198: return (0);
1.1 provos 199: }
1.51 markus 200:
201: /* Flag indicating whether the key is allowed. */
202: allowed = 0;
1.1 provos 203:
1.46 jakob 204: key = key_new(KEY_RSA1);
1.12 markus 205:
1.14 markus 206: /*
207: * Go though the accepted keys, looking for the current key. If
208: * found, perform a challenge-response dialog to verify that the
209: * user really has the corresponding private key.
210: */
1.61 dtucker 211: while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
1.12 markus 212: char *cp;
1.60 avsm 213: char *key_options;
1.63 djm 214: int keybits;
1.12 markus 215:
1.14 markus 216: /* Skip leading whitespace, empty and comment lines. */
217: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
218: ;
1.12 markus 219: if (!*cp || *cp == '\n' || *cp == '#')
220: continue;
221:
1.14 markus 222: /*
223: * Check if there are options for this key, and if so,
224: * save their starting address and skip the option part
225: * for now. If there are no options, set the starting
226: * address to NULL.
227: */
1.12 markus 228: if (*cp < '0' || *cp > '9') {
229: int quoted = 0;
1.60 avsm 230: key_options = cp;
1.12 markus 231: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
232: if (*cp == '\\' && cp[1] == '"')
233: cp++; /* Skip both */
234: else if (*cp == '"')
235: quoted = !quoted;
236: }
237: } else
1.60 avsm 238: key_options = NULL;
1.1 provos 239:
1.12 markus 240: /* Parse the key from the line. */
1.46 jakob 241: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 242: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 243: file, linenum);
1.12 markus 244: continue;
1.1 provos 245: }
1.12 markus 246: /* cp now points to the comment part. */
247:
1.16 markus 248: /* Check if the we have found the desired key (identified by its modulus). */
1.46 jakob 249: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 250: continue;
251:
1.12 markus 252: /* check the real bits */
1.63 djm 253: keybits = BN_num_bits(key->rsa->n);
254: if (keybits < 0 || bits != (u_int)keybits)
1.57 itojun 255: logit("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 256: "actual %d vs. announced %d.",
1.46 jakob 257: file, linenum, BN_num_bits(key->rsa->n), bits);
1.12 markus 258:
259: /* We have found the desired key. */
1.33 markus 260: /*
261: * If our options do not allow this key to be used,
262: * do not send challenge.
263: */
1.60 avsm 264: if (!auth_parse_options(pw, key_options, file, linenum))
1.33 markus 265: continue;
1.19 markus 266:
1.51 markus 267: /* break out, this key is allowed */
268: allowed = 1;
1.32 markus 269: break;
1.1 provos 270: }
271:
1.12 markus 272: /* Restore the privileged uid. */
273: restore_uid();
274:
275: /* Close the file. */
1.41 markus 276: xfree(file);
1.12 markus 277: fclose(f);
278:
1.51 markus 279: /* return key if allowed */
280: if (allowed && rkey != NULL)
281: *rkey = key;
282: else
283: key_free(key);
284: return (allowed);
285: }
286:
287: /*
288: * Performs the RSA authentication dialog with the client. This returns
289: * 0 if the client could not be authenticated, and 1 if authentication was
290: * successful. This may exit if there is a serious protocol violation.
291: */
292: int
1.58 djm 293: auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
1.51 markus 294: {
295: Key *key;
296: char *fp;
1.58 djm 297: struct passwd *pw = authctxt->pw;
1.51 markus 298:
299: /* no user given */
1.58 djm 300: if (!authctxt->valid)
1.51 markus 301: return 0;
1.1 provos 302:
1.52 provos 303: if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
1.31 markus 304: auth_clear_options();
1.51 markus 305: return (0);
306: }
307:
308: /* Perform the challenge-response dialog for this key. */
309: if (!auth_rsa_challenge_dialog(key)) {
310: /* Wrong response. */
311: verbose("Wrong response to RSA authentication challenge.");
312: packet_send_debug("Wrong response to RSA authentication challenge.");
313: /*
314: * Break out of the loop. Otherwise we might send
315: * another challenge and break the protocol.
316: */
317: key_free(key);
318: return (0);
319: }
320: /*
321: * Correct response. The client has been successfully
322: * authenticated. Note that we have not yet processed the
323: * options; this will be reset if the options cause the
324: * authentication to be rejected.
325: */
326: fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
327: verbose("Found matching %s key: %s",
328: key_type(key), fp);
329: xfree(fp);
330: key_free(key);
1.1 provos 331:
1.51 markus 332: packet_send_debug("RSA authentication accepted.");
333: return (1);
1.1 provos 334: }