Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.65
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:
18: #include <sys/types.h>
19: #include <sys/stat.h>
1.38 markus 20:
21: #include <openssl/rsa.h>
22: #include <openssl/md5.h>
1.1 provos 23:
24: #include "rsa.h"
25: #include "packet.h"
26: #include "xmalloc.h"
1.35 markus 27: #include "ssh1.h"
1.1 provos 28: #include "uidswap.h"
1.19 markus 29: #include "match.h"
1.25 markus 30: #include "auth-options.h"
1.35 markus 31: #include "pathnames.h"
1.38 markus 32: #include "log.h"
33: #include "servconf.h"
34: #include "auth.h"
1.46 jakob 35: #include "hostfile.h"
1.52 provos 36: #include "monitor_wrap.h"
1.55 stevesk 37: #include "ssh.h"
1.62 dtucker 38: #include "misc.h"
1.30 markus 39:
40: /* import */
41: extern ServerOptions options;
42:
1.14 markus 43: /*
44: * Session identifier that is used to bind key exchange and authentication
45: * responses to a particular session.
46: */
1.34 markus 47: extern u_char session_id[16];
1.1 provos 48:
1.14 markus 49: /*
50: * The .ssh/authorized_keys file contains public keys, one per line, in the
51: * following format:
52: * options bits e n comment
53: * where bits, e and n are decimal numbers,
54: * and comment is any string of characters up to newline. The maximum
1.61 dtucker 55: * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
1.14 markus 56: * description of the options.
57: */
58:
1.52 provos 59: BIGNUM *
1.51 markus 60: auth_rsa_generate_challenge(Key *key)
61: {
62: BIGNUM *challenge;
63: BN_CTX *ctx;
64:
65: if ((challenge = BN_new()) == NULL)
66: fatal("auth_rsa_generate_challenge: BN_new() failed");
67: /* Generate a random challenge. */
68: BN_rand(challenge, 256, 0, 0);
69: if ((ctx = BN_CTX_new()) == NULL)
70: fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
71: BN_mod(challenge, challenge, key->rsa->n, ctx);
72: BN_CTX_free(ctx);
73:
74: return challenge;
75: }
76:
1.52 provos 77: int
1.51 markus 78: auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
79: {
80: u_char buf[32], mdbuf[16];
81: MD5_CTX md;
82: int len;
1.54 markus 83:
84: /* don't allow short keys */
1.55 stevesk 85: if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
1.56 stevesk 86: error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
87: BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
1.54 markus 88: return (0);
89: }
1.51 markus 90:
91: /* The response is MD5 of decrypted challenge plus session id. */
92: len = BN_num_bytes(challenge);
93: if (len <= 0 || len > 32)
94: fatal("auth_rsa_verify_response: bad challenge length %d", len);
95: memset(buf, 0, 32);
96: BN_bn2bin(challenge, buf + 32 - len);
97: MD5_Init(&md);
98: MD5_Update(&md, buf, 32);
99: MD5_Update(&md, session_id, 16);
100: MD5_Final(mdbuf, &md);
101:
102: /* Verify that the response is the original challenge. */
103: if (memcmp(response, mdbuf, 16) != 0) {
104: /* Wrong answer. */
105: return (0);
106: }
107: /* Correct answer. */
108: return (1);
109: }
110:
1.14 markus 111: /*
112: * Performs the RSA authentication challenge-response dialog with the client,
113: * and returns true (non-zero) if the client gave the correct answer to
114: * our challenge; returns zero if the client gives a wrong answer.
115: */
1.1 provos 116:
117: int
1.51 markus 118: auth_rsa_challenge_dialog(Key *key)
1.1 provos 119: {
1.18 markus 120: BIGNUM *challenge, *encrypted_challenge;
1.51 markus 121: u_char response[16];
122: int i, success;
1.12 markus 123:
1.47 markus 124: if ((encrypted_challenge = BN_new()) == NULL)
125: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 126:
1.52 provos 127: challenge = PRIVSEP(auth_rsa_generate_challenge(key));
1.12 markus 128:
129: /* Encrypt the challenge with the public key. */
1.51 markus 130: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 131:
132: /* Send the encrypted challenge to the client. */
133: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
134: packet_put_bignum(encrypted_challenge);
135: packet_send();
1.18 markus 136: BN_clear_free(encrypted_challenge);
1.12 markus 137: packet_write_wait();
138:
1.18 markus 139: /* Wait for a response. */
1.50 markus 140: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 141: for (i = 0; i < 16; i++)
142: response[i] = packet_get_char();
1.49 markus 143: packet_check_eom();
1.18 markus 144:
1.52 provos 145: success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
1.12 markus 146: BN_clear_free(challenge);
1.51 markus 147: return (success);
1.1 provos 148: }
149:
1.14 markus 150: /*
1.51 markus 151: * check if there's user key matching client_n,
152: * return key if login is allowed, NULL otherwise
1.14 markus 153: */
1.1 provos 154:
1.52 provos 155: int
1.51 markus 156: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
1.1 provos 157: {
1.61 dtucker 158: char line[SSH_MAX_PUBKEY_BYTES], *file;
1.52 provos 159: int allowed = 0;
1.34 markus 160: u_int bits;
1.12 markus 161: FILE *f;
1.34 markus 162: u_long linenum = 0;
1.12 markus 163: struct stat st;
1.46 jakob 164: Key *key;
1.12 markus 165:
166: /* Temporarily use the user's uid. */
1.40 markus 167: temporarily_use_uid(pw);
1.12 markus 168:
169: /* The authorized keys. */
1.41 markus 170: file = authorized_keys_file(pw);
171: debug("trying public RSA key file %s", file);
1.12 markus 172:
173: /* Fail quietly if file does not exist */
174: if (stat(file, &st) < 0) {
175: /* Restore the privileged uid. */
176: restore_uid();
1.41 markus 177: xfree(file);
1.53 markus 178: return (0);
1.12 markus 179: }
180: /* Open the file containing the authorized keys. */
181: f = fopen(file, "r");
182: if (!f) {
183: /* Restore the privileged uid. */
184: restore_uid();
1.41 markus 185: xfree(file);
1.53 markus 186: return (0);
1.1 provos 187: }
1.41 markus 188: if (options.strict_modes &&
1.43 provos 189: secure_filename(f, file, pw, line, sizeof(line)) != 0) {
1.41 markus 190: xfree(file);
191: fclose(f);
1.57 itojun 192: logit("Authentication refused: %s", line);
1.41 markus 193: restore_uid();
1.53 markus 194: return (0);
1.1 provos 195: }
1.51 markus 196:
197: /* Flag indicating whether the key is allowed. */
198: allowed = 0;
1.1 provos 199:
1.46 jakob 200: key = key_new(KEY_RSA1);
1.12 markus 201:
1.14 markus 202: /*
203: * Go though the accepted keys, looking for the current key. If
204: * found, perform a challenge-response dialog to verify that the
205: * user really has the corresponding private key.
206: */
1.61 dtucker 207: while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
1.12 markus 208: char *cp;
1.60 avsm 209: char *key_options;
1.63 djm 210: int keybits;
1.12 markus 211:
1.14 markus 212: /* Skip leading whitespace, empty and comment lines. */
213: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
214: ;
1.12 markus 215: if (!*cp || *cp == '\n' || *cp == '#')
216: continue;
217:
1.14 markus 218: /*
219: * Check if there are options for this key, and if so,
220: * save their starting address and skip the option part
221: * for now. If there are no options, set the starting
222: * address to NULL.
223: */
1.12 markus 224: if (*cp < '0' || *cp > '9') {
225: int quoted = 0;
1.60 avsm 226: key_options = cp;
1.12 markus 227: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
228: if (*cp == '\\' && cp[1] == '"')
229: cp++; /* Skip both */
230: else if (*cp == '"')
231: quoted = !quoted;
232: }
233: } else
1.60 avsm 234: key_options = NULL;
1.1 provos 235:
1.12 markus 236: /* Parse the key from the line. */
1.46 jakob 237: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 238: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 239: file, linenum);
1.12 markus 240: continue;
1.1 provos 241: }
1.12 markus 242: /* cp now points to the comment part. */
243:
1.16 markus 244: /* Check if the we have found the desired key (identified by its modulus). */
1.46 jakob 245: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 246: continue;
247:
1.12 markus 248: /* check the real bits */
1.63 djm 249: keybits = BN_num_bits(key->rsa->n);
250: if (keybits < 0 || bits != (u_int)keybits)
1.57 itojun 251: logit("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 252: "actual %d vs. announced %d.",
1.46 jakob 253: file, linenum, BN_num_bits(key->rsa->n), bits);
1.12 markus 254:
255: /* We have found the desired key. */
1.33 markus 256: /*
257: * If our options do not allow this key to be used,
258: * do not send challenge.
259: */
1.60 avsm 260: if (!auth_parse_options(pw, key_options, file, linenum))
1.33 markus 261: continue;
1.19 markus 262:
1.51 markus 263: /* break out, this key is allowed */
264: allowed = 1;
1.32 markus 265: break;
1.1 provos 266: }
267:
1.12 markus 268: /* Restore the privileged uid. */
269: restore_uid();
270:
271: /* Close the file. */
1.41 markus 272: xfree(file);
1.12 markus 273: fclose(f);
274:
1.51 markus 275: /* return key if allowed */
276: if (allowed && rkey != NULL)
277: *rkey = key;
278: else
279: key_free(key);
280: return (allowed);
281: }
282:
283: /*
284: * Performs the RSA authentication dialog with the client. This returns
285: * 0 if the client could not be authenticated, and 1 if authentication was
286: * successful. This may exit if there is a serious protocol violation.
287: */
288: int
1.58 djm 289: auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
1.51 markus 290: {
291: Key *key;
292: char *fp;
1.58 djm 293: struct passwd *pw = authctxt->pw;
1.51 markus 294:
295: /* no user given */
1.58 djm 296: if (!authctxt->valid)
1.51 markus 297: return 0;
1.1 provos 298:
1.52 provos 299: if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
1.31 markus 300: auth_clear_options();
1.51 markus 301: return (0);
302: }
303:
304: /* Perform the challenge-response dialog for this key. */
305: if (!auth_rsa_challenge_dialog(key)) {
306: /* Wrong response. */
307: verbose("Wrong response to RSA authentication challenge.");
308: packet_send_debug("Wrong response to RSA authentication challenge.");
309: /*
310: * Break out of the loop. Otherwise we might send
311: * another challenge and break the protocol.
312: */
313: key_free(key);
314: return (0);
315: }
316: /*
317: * Correct response. The client has been successfully
318: * authenticated. Note that we have not yet processed the
319: * options; this will be reset if the options cause the
320: * authentication to be rejected.
321: */
322: fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
323: verbose("Found matching %s key: %s",
324: key_type(key), fp);
325: xfree(fp);
326: key_free(key);
1.1 provos 327:
1.51 markus 328: packet_send_debug("RSA authentication accepted.");
329: return (1);
1.1 provos 330: }