Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.86
1.86 ! markus 1: /* $OpenBSD: auth-rsa.c,v 1.85 2013/07/12 00:19:58 djm 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>
1.68 stevesk 21:
22: #include <pwd.h>
1.70 stevesk 23: #include <stdio.h>
1.69 stevesk 24: #include <string.h>
1.1 provos 25:
1.71 deraadt 26: #include "xmalloc.h"
1.1 provos 27: #include "rsa.h"
28: #include "packet.h"
1.35 markus 29: #include "ssh1.h"
1.1 provos 30: #include "uidswap.h"
1.19 markus 31: #include "match.h"
1.71 deraadt 32: #include "buffer.h"
1.35 markus 33: #include "pathnames.h"
1.38 markus 34: #include "log.h"
35: #include "servconf.h"
1.71 deraadt 36: #include "key.h"
1.75 djm 37: #include "auth-options.h"
1.71 deraadt 38: #include "hostfile.h"
1.38 markus 39: #include "auth.h"
1.71 deraadt 40: #ifdef GSSAPI
41: #include "ssh-gss.h"
42: #endif
1.52 provos 43: #include "monitor_wrap.h"
1.55 stevesk 44: #include "ssh.h"
1.62 dtucker 45: #include "misc.h"
1.30 markus 46:
1.86 ! markus 47: #include "digest.h"
! 48:
1.30 markus 49: /* import */
50: extern ServerOptions options;
51:
1.14 markus 52: /*
53: * Session identifier that is used to bind key exchange and authentication
54: * responses to a particular session.
55: */
1.34 markus 56: extern u_char session_id[16];
1.1 provos 57:
1.14 markus 58: /*
59: * The .ssh/authorized_keys file contains public keys, one per line, in the
60: * following format:
61: * options bits e n comment
62: * where bits, e and n are decimal numbers,
63: * and comment is any string of characters up to newline. The maximum
1.61 dtucker 64: * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
1.14 markus 65: * description of the options.
66: */
67:
1.52 provos 68: BIGNUM *
1.51 markus 69: auth_rsa_generate_challenge(Key *key)
70: {
71: BIGNUM *challenge;
72: BN_CTX *ctx;
73:
74: if ((challenge = BN_new()) == NULL)
75: fatal("auth_rsa_generate_challenge: BN_new() failed");
76: /* Generate a random challenge. */
1.72 markus 77: if (BN_rand(challenge, 256, 0, 0) == 0)
78: fatal("auth_rsa_generate_challenge: BN_rand failed");
1.51 markus 79: if ((ctx = BN_CTX_new()) == NULL)
1.72 markus 80: fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
81: if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
82: fatal("auth_rsa_generate_challenge: BN_mod failed");
1.51 markus 83: BN_CTX_free(ctx);
84:
85: return challenge;
86: }
87:
1.52 provos 88: int
1.51 markus 89: auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
90: {
91: u_char buf[32], mdbuf[16];
1.86 ! markus 92: struct ssh_digest_ctx *md;
1.51 markus 93: int len;
1.74 djm 94:
1.54 markus 95: /* don't allow short keys */
1.55 stevesk 96: if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
1.86 ! markus 97: error("%s: RSA modulus too small: %d < minimum %d bits",
! 98: __func__,
1.56 stevesk 99: BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
1.54 markus 100: return (0);
101: }
1.51 markus 102:
103: /* The response is MD5 of decrypted challenge plus session id. */
104: len = BN_num_bytes(challenge);
105: if (len <= 0 || len > 32)
1.86 ! markus 106: fatal("%s: bad challenge length %d", __func__, len);
1.51 markus 107: memset(buf, 0, 32);
108: BN_bn2bin(challenge, buf + 32 - len);
1.86 ! markus 109: if ((md = ssh_digest_start(SSH_DIGEST_MD5)) == NULL ||
! 110: ssh_digest_update(md, buf, 32) < 0 ||
! 111: ssh_digest_update(md, session_id, 16) < 0 ||
! 112: ssh_digest_final(md, mdbuf, sizeof(mdbuf)) < 0)
! 113: fatal("%s: md5 failed", __func__);
! 114: ssh_digest_free(md);
1.51 markus 115:
116: /* Verify that the response is the original challenge. */
1.78 djm 117: if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
1.51 markus 118: /* Wrong answer. */
119: return (0);
120: }
121: /* Correct answer. */
122: return (1);
123: }
124:
1.14 markus 125: /*
126: * Performs the RSA authentication challenge-response dialog with the client,
127: * and returns true (non-zero) if the client gave the correct answer to
128: * our challenge; returns zero if the client gives a wrong answer.
129: */
1.1 provos 130:
131: int
1.51 markus 132: auth_rsa_challenge_dialog(Key *key)
1.1 provos 133: {
1.18 markus 134: BIGNUM *challenge, *encrypted_challenge;
1.51 markus 135: u_char response[16];
136: int i, success;
1.12 markus 137:
1.47 markus 138: if ((encrypted_challenge = BN_new()) == NULL)
139: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 140:
1.52 provos 141: challenge = PRIVSEP(auth_rsa_generate_challenge(key));
1.12 markus 142:
143: /* Encrypt the challenge with the public key. */
1.51 markus 144: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 145:
146: /* Send the encrypted challenge to the client. */
147: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
148: packet_put_bignum(encrypted_challenge);
149: packet_send();
1.18 markus 150: BN_clear_free(encrypted_challenge);
1.12 markus 151: packet_write_wait();
152:
1.18 markus 153: /* Wait for a response. */
1.50 markus 154: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 155: for (i = 0; i < 16; i++)
1.67 deraadt 156: response[i] = (u_char)packet_get_char();
1.49 markus 157: packet_check_eom();
1.18 markus 158:
1.52 provos 159: success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
1.12 markus 160: BN_clear_free(challenge);
1.51 markus 161: return (success);
1.1 provos 162: }
163:
1.80 djm 164: static int
165: rsa_key_allowed_in_file(struct passwd *pw, char *file,
166: const BIGNUM *client_n, Key **rkey)
1.1 provos 167: {
1.83 djm 168: char *fp, line[SSH_MAX_PUBKEY_BYTES];
1.85 djm 169: int allowed = 0, bits;
1.12 markus 170: FILE *f;
1.34 markus 171: u_long linenum = 0;
1.46 jakob 172: Key *key;
1.12 markus 173:
1.41 markus 174: debug("trying public RSA key file %s", file);
1.80 djm 175: if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL)
176: return 0;
1.12 markus 177:
1.14 markus 178: /*
179: * Go though the accepted keys, looking for the current key. If
180: * found, perform a challenge-response dialog to verify that the
181: * user really has the corresponding private key.
182: */
1.80 djm 183: key = key_new(KEY_RSA1);
1.61 dtucker 184: while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
1.12 markus 185: char *cp;
1.60 avsm 186: char *key_options;
1.63 djm 187: int keybits;
1.12 markus 188:
1.14 markus 189: /* Skip leading whitespace, empty and comment lines. */
190: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
191: ;
1.12 markus 192: if (!*cp || *cp == '\n' || *cp == '#')
193: continue;
194:
1.14 markus 195: /*
196: * Check if there are options for this key, and if so,
197: * save their starting address and skip the option part
198: * for now. If there are no options, set the starting
199: * address to NULL.
200: */
1.12 markus 201: if (*cp < '0' || *cp > '9') {
202: int quoted = 0;
1.60 avsm 203: key_options = cp;
1.12 markus 204: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
205: if (*cp == '\\' && cp[1] == '"')
206: cp++; /* Skip both */
207: else if (*cp == '"')
208: quoted = !quoted;
209: }
210: } else
1.60 avsm 211: key_options = NULL;
1.1 provos 212:
1.12 markus 213: /* Parse the key from the line. */
1.46 jakob 214: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 215: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 216: file, linenum);
1.12 markus 217: continue;
1.1 provos 218: }
1.12 markus 219: /* cp now points to the comment part. */
220:
1.80 djm 221: /*
222: * Check if the we have found the desired key (identified
223: * by its modulus).
224: */
1.46 jakob 225: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 226: continue;
227:
1.12 markus 228: /* check the real bits */
1.63 djm 229: keybits = BN_num_bits(key->rsa->n);
1.85 djm 230: if (keybits < 0 || bits != keybits)
1.57 itojun 231: logit("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 232: "actual %d vs. announced %d.",
1.46 jakob 233: file, linenum, BN_num_bits(key->rsa->n), bits);
1.79 djm 234:
1.83 djm 235: fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
236: debug("matching key found: file %s, line %lu %s %s",
237: file, linenum, key_type(key), fp);
238: free(fp);
239:
1.79 djm 240: /* Never accept a revoked key */
241: if (auth_key_is_revoked(key))
242: break;
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: */
1.60 avsm 249: if (!auth_parse_options(pw, key_options, file, linenum))
1.33 markus 250: continue;
1.76 djm 251: if (key_is_cert_authority)
252: continue;
1.51 markus 253: /* break out, this key is allowed */
254: allowed = 1;
1.32 markus 255: break;
1.1 provos 256: }
257:
1.12 markus 258: /* Close the file. */
259: fclose(f);
260:
1.51 markus 261: /* return key if allowed */
262: if (allowed && rkey != NULL)
263: *rkey = key;
264: else
265: key_free(key);
1.80 djm 266:
267: return allowed;
268: }
269:
270: /*
271: * check if there's user key matching client_n,
272: * return key if login is allowed, NULL otherwise
273: */
274:
275: int
276: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
277: {
278: char *file;
279: u_int i, allowed = 0;
280:
281: temporarily_use_uid(pw);
282:
283: for (i = 0; !allowed && i < options.num_authkeys_files; i++) {
1.81 djm 284: if (strcasecmp(options.authorized_keys_files[i], "none") == 0)
285: continue;
1.80 djm 286: file = expand_authorized_keys(
287: options.authorized_keys_files[i], pw);
288: allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
1.82 djm 289: free(file);
1.80 djm 290: }
291:
292: restore_uid();
293:
294: return allowed;
1.51 markus 295: }
296:
297: /*
298: * Performs the RSA authentication dialog with the client. This returns
299: * 0 if the client could not be authenticated, and 1 if authentication was
300: * successful. This may exit if there is a serious protocol violation.
301: */
302: int
1.58 djm 303: auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
1.51 markus 304: {
305: Key *key;
1.58 djm 306: struct passwd *pw = authctxt->pw;
1.51 markus 307:
308: /* no user given */
1.58 djm 309: if (!authctxt->valid)
1.51 markus 310: return 0;
1.1 provos 311:
1.52 provos 312: if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
1.31 markus 313: auth_clear_options();
1.51 markus 314: return (0);
315: }
316:
317: /* Perform the challenge-response dialog for this key. */
318: if (!auth_rsa_challenge_dialog(key)) {
319: /* Wrong response. */
320: verbose("Wrong response to RSA authentication challenge.");
321: packet_send_debug("Wrong response to RSA authentication challenge.");
322: /*
323: * Break out of the loop. Otherwise we might send
324: * another challenge and break the protocol.
325: */
326: key_free(key);
327: return (0);
328: }
329: /*
330: * Correct response. The client has been successfully
331: * authenticated. Note that we have not yet processed the
332: * options; this will be reset if the options cause the
333: * authentication to be rejected.
334: */
1.84 djm 335: pubkey_auth_info(authctxt, key, NULL);
1.1 provos 336:
1.51 markus 337: packet_send_debug("RSA authentication accepted.");
338: return (1);
1.1 provos 339: }
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