Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.51
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"
1.51 ! markus 17: RCSID("$OpenBSD: auth-rsa.c,v 1.50 2001/12/28 14:50:54 markus 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"
1.30 markus 35:
36: /* import */
37: extern ServerOptions options;
38:
1.14 markus 39: /*
40: * Session identifier that is used to bind key exchange and authentication
41: * responses to a particular session.
42: */
1.34 markus 43: extern u_char session_id[16];
1.1 provos 44:
1.14 markus 45: /*
46: * The .ssh/authorized_keys file contains public keys, one per line, in the
47: * following format:
48: * options bits e n comment
49: * where bits, e and n are decimal numbers,
50: * and comment is any string of characters up to newline. The maximum
51: * length of a line is 8000 characters. See the documentation for a
52: * description of the options.
53: */
54:
1.51 ! markus 55: static BIGNUM *
! 56: auth_rsa_generate_challenge(Key *key)
! 57: {
! 58: BIGNUM *challenge;
! 59: BN_CTX *ctx;
! 60:
! 61: if ((challenge = BN_new()) == NULL)
! 62: fatal("auth_rsa_generate_challenge: BN_new() failed");
! 63: /* Generate a random challenge. */
! 64: BN_rand(challenge, 256, 0, 0);
! 65: if ((ctx = BN_CTX_new()) == NULL)
! 66: fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
! 67: BN_mod(challenge, challenge, key->rsa->n, ctx);
! 68: BN_CTX_free(ctx);
! 69:
! 70: return challenge;
! 71: }
! 72:
! 73: static int
! 74: auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
! 75: {
! 76: u_char buf[32], mdbuf[16];
! 77: MD5_CTX md;
! 78: int len;
! 79:
! 80: /* The response is MD5 of decrypted challenge plus session id. */
! 81: len = BN_num_bytes(challenge);
! 82: if (len <= 0 || len > 32)
! 83: fatal("auth_rsa_verify_response: bad challenge length %d", len);
! 84: memset(buf, 0, 32);
! 85: BN_bn2bin(challenge, buf + 32 - len);
! 86: MD5_Init(&md);
! 87: MD5_Update(&md, buf, 32);
! 88: MD5_Update(&md, session_id, 16);
! 89: MD5_Final(mdbuf, &md);
! 90:
! 91: /* Verify that the response is the original challenge. */
! 92: if (memcmp(response, mdbuf, 16) != 0) {
! 93: /* Wrong answer. */
! 94: return (0);
! 95: }
! 96: /* Correct answer. */
! 97: return (1);
! 98: }
! 99:
1.14 markus 100: /*
101: * Performs the RSA authentication challenge-response dialog with the client,
102: * and returns true (non-zero) if the client gave the correct answer to
103: * our challenge; returns zero if the client gives a wrong answer.
104: */
1.1 provos 105:
106: int
1.51 ! markus 107: auth_rsa_challenge_dialog(Key *key)
1.1 provos 108: {
1.18 markus 109: BIGNUM *challenge, *encrypted_challenge;
1.51 ! markus 110: u_char response[16];
! 111: int i, success;
1.12 markus 112:
1.47 markus 113: if ((encrypted_challenge = BN_new()) == NULL)
114: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 115:
1.51 ! markus 116: challenge = auth_rsa_generate_challenge(key);
1.12 markus 117:
118: /* Encrypt the challenge with the public key. */
1.51 ! markus 119: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 120:
121: /* Send the encrypted challenge to the client. */
122: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
123: packet_put_bignum(encrypted_challenge);
124: packet_send();
1.18 markus 125: BN_clear_free(encrypted_challenge);
1.12 markus 126: packet_write_wait();
127:
1.18 markus 128: /* Wait for a response. */
1.50 markus 129: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 130: for (i = 0; i < 16; i++)
131: response[i] = packet_get_char();
1.49 markus 132: packet_check_eom();
1.18 markus 133:
1.51 ! markus 134: success = auth_rsa_verify_response(key, challenge, response);
1.12 markus 135: BN_clear_free(challenge);
1.51 ! markus 136: return (success);
1.1 provos 137: }
138:
1.14 markus 139: /*
1.51 ! markus 140: * check if there's user key matching client_n,
! 141: * return key if login is allowed, NULL otherwise
1.14 markus 142: */
1.1 provos 143:
1.51 ! markus 144: static int
! 145: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
1.1 provos 146: {
1.41 markus 147: char line[8192], *file;
1.51 ! markus 148: int allowed;
1.34 markus 149: u_int bits;
1.12 markus 150: FILE *f;
1.34 markus 151: u_long linenum = 0;
1.12 markus 152: struct stat st;
1.46 jakob 153: Key *key;
1.12 markus 154:
155: /* Temporarily use the user's uid. */
1.40 markus 156: temporarily_use_uid(pw);
1.12 markus 157:
158: /* The authorized keys. */
1.41 markus 159: file = authorized_keys_file(pw);
160: debug("trying public RSA key file %s", file);
1.12 markus 161:
162: /* Fail quietly if file does not exist */
163: if (stat(file, &st) < 0) {
164: /* Restore the privileged uid. */
165: restore_uid();
1.41 markus 166: xfree(file);
1.51 ! markus 167: return (NULL);
1.12 markus 168: }
169: /* Open the file containing the authorized keys. */
170: f = fopen(file, "r");
171: if (!f) {
172: /* Restore the privileged uid. */
173: restore_uid();
1.41 markus 174: xfree(file);
1.51 ! markus 175: return (NULL);
1.1 provos 176: }
1.41 markus 177: if (options.strict_modes &&
1.43 provos 178: secure_filename(f, file, pw, line, sizeof(line)) != 0) {
1.41 markus 179: xfree(file);
180: fclose(f);
181: log("Authentication refused: %s", line);
182: restore_uid();
1.51 ! markus 183: return (NULL);
1.1 provos 184: }
1.51 ! markus 185:
! 186: /* Flag indicating whether the key is allowed. */
! 187: allowed = 0;
1.1 provos 188:
1.46 jakob 189: key = key_new(KEY_RSA1);
1.12 markus 190:
1.14 markus 191: /*
192: * Go though the accepted keys, looking for the current key. If
193: * found, perform a challenge-response dialog to verify that the
194: * user really has the corresponding private key.
195: */
1.12 markus 196: while (fgets(line, sizeof(line), f)) {
197: char *cp;
198: char *options;
199:
200: linenum++;
201:
1.14 markus 202: /* Skip leading whitespace, empty and comment lines. */
203: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
204: ;
1.12 markus 205: if (!*cp || *cp == '\n' || *cp == '#')
206: continue;
207:
1.14 markus 208: /*
209: * Check if there are options for this key, and if so,
210: * save their starting address and skip the option part
211: * for now. If there are no options, set the starting
212: * address to NULL.
213: */
1.12 markus 214: if (*cp < '0' || *cp > '9') {
215: int quoted = 0;
216: options = cp;
217: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
218: if (*cp == '\\' && cp[1] == '"')
219: cp++; /* Skip both */
220: else if (*cp == '"')
221: quoted = !quoted;
222: }
223: } else
224: options = NULL;
1.1 provos 225:
1.12 markus 226: /* Parse the key from the line. */
1.46 jakob 227: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 228: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 229: file, linenum);
1.12 markus 230: continue;
1.1 provos 231: }
1.12 markus 232: /* cp now points to the comment part. */
233:
1.16 markus 234: /* Check if the we have found the desired key (identified by its modulus). */
1.46 jakob 235: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 236: continue;
237:
1.12 markus 238: /* check the real bits */
1.46 jakob 239: if (bits != BN_num_bits(key->rsa->n))
1.44 stevesk 240: log("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 241: "actual %d vs. announced %d.",
1.46 jakob 242: file, linenum, BN_num_bits(key->rsa->n), bits);
1.12 markus 243:
244: /* We have found the desired key. */
1.33 markus 245: /*
246: * If our options do not allow this key to be used,
247: * do not send challenge.
248: */
1.36 markus 249: if (!auth_parse_options(pw, options, file, linenum))
1.33 markus 250: continue;
1.19 markus 251:
1.51 ! markus 252: /* break out, this key is allowed */
! 253: allowed = 1;
1.32 markus 254: break;
1.1 provos 255: }
256:
1.12 markus 257: /* Restore the privileged uid. */
258: restore_uid();
259:
260: /* Close the file. */
1.41 markus 261: xfree(file);
1.12 markus 262: fclose(f);
263:
1.51 ! markus 264: /* return key if allowed */
! 265: if (allowed && rkey != NULL)
! 266: *rkey = key;
! 267: else
! 268: key_free(key);
! 269: return (allowed);
! 270: }
! 271:
! 272: /*
! 273: * Performs the RSA authentication dialog with the client. This returns
! 274: * 0 if the client could not be authenticated, and 1 if authentication was
! 275: * successful. This may exit if there is a serious protocol violation.
! 276: */
! 277: int
! 278: auth_rsa(struct passwd *pw, BIGNUM *client_n)
! 279: {
! 280: Key *key;
! 281: char *fp;
! 282:
! 283: /* no user given */
! 284: if (pw == NULL)
! 285: return 0;
1.1 provos 286:
1.51 ! markus 287: if (auth_rsa_key_allowed(pw, client_n, &key) == 0) {
1.31 markus 288: auth_clear_options();
1.51 ! markus 289: return (0);
! 290: }
! 291:
! 292: /* Perform the challenge-response dialog for this key. */
! 293: if (!auth_rsa_challenge_dialog(key)) {
! 294: /* Wrong response. */
! 295: verbose("Wrong response to RSA authentication challenge.");
! 296: packet_send_debug("Wrong response to RSA authentication challenge.");
! 297: /*
! 298: * Break out of the loop. Otherwise we might send
! 299: * another challenge and break the protocol.
! 300: */
! 301: key_free(key);
! 302: return (0);
! 303: }
! 304: /*
! 305: * Correct response. The client has been successfully
! 306: * authenticated. Note that we have not yet processed the
! 307: * options; this will be reset if the options cause the
! 308: * authentication to be rejected.
! 309: */
! 310: fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
! 311: verbose("Found matching %s key: %s",
! 312: key_type(key), fp);
! 313: xfree(fp);
! 314: key_free(key);
1.1 provos 315:
1.51 ! markus 316: packet_send_debug("RSA authentication accepted.");
! 317: return (1);
1.1 provos 318: }
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