Annotation of src/usr.bin/ssh/auth-rsa.c, Revision 1.78
1.78 ! djm 1: /* $OpenBSD: auth-rsa.c,v 1.77 2010/07/13 11:52:06 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>
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"
1.35 markus 34: #include "pathnames.h"
1.38 markus 35: #include "log.h"
36: #include "servconf.h"
1.71 deraadt 37: #include "key.h"
1.75 djm 38: #include "auth-options.h"
1.71 deraadt 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. */
1.72 markus 76: if (BN_rand(challenge, 256, 0, 0) == 0)
77: fatal("auth_rsa_generate_challenge: BN_rand failed");
1.51 markus 78: if ((ctx = BN_CTX_new()) == NULL)
1.72 markus 79: fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
80: if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
81: fatal("auth_rsa_generate_challenge: BN_mod failed");
1.51 markus 82: BN_CTX_free(ctx);
83:
84: return challenge;
85: }
86:
1.52 provos 87: int
1.51 markus 88: auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
89: {
90: u_char buf[32], mdbuf[16];
91: MD5_CTX md;
92: int len;
1.74 djm 93:
94: if (auth_key_is_revoked(key))
95: return 0;
1.54 markus 96:
97: /* don't allow short keys */
1.55 stevesk 98: if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
1.56 stevesk 99: error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
100: BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
1.54 markus 101: return (0);
102: }
1.51 markus 103:
104: /* The response is MD5 of decrypted challenge plus session id. */
105: len = BN_num_bytes(challenge);
106: if (len <= 0 || len > 32)
107: fatal("auth_rsa_verify_response: bad challenge length %d", len);
108: memset(buf, 0, 32);
109: BN_bn2bin(challenge, buf + 32 - len);
110: MD5_Init(&md);
111: MD5_Update(&md, buf, 32);
112: MD5_Update(&md, session_id, 16);
113: MD5_Final(mdbuf, &md);
114:
115: /* Verify that the response is the original challenge. */
1.78 ! djm 116: if (timingsafe_bcmp(response, mdbuf, 16) != 0) {
1.51 markus 117: /* Wrong answer. */
118: return (0);
119: }
120: /* Correct answer. */
121: return (1);
122: }
123:
1.14 markus 124: /*
125: * Performs the RSA authentication challenge-response dialog with the client,
126: * and returns true (non-zero) if the client gave the correct answer to
127: * our challenge; returns zero if the client gives a wrong answer.
128: */
1.1 provos 129:
130: int
1.51 markus 131: auth_rsa_challenge_dialog(Key *key)
1.1 provos 132: {
1.18 markus 133: BIGNUM *challenge, *encrypted_challenge;
1.51 markus 134: u_char response[16];
135: int i, success;
1.12 markus 136:
1.47 markus 137: if ((encrypted_challenge = BN_new()) == NULL)
138: fatal("auth_rsa_challenge_dialog: BN_new() failed");
1.12 markus 139:
1.52 provos 140: challenge = PRIVSEP(auth_rsa_generate_challenge(key));
1.12 markus 141:
142: /* Encrypt the challenge with the public key. */
1.51 markus 143: rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
1.12 markus 144:
145: /* Send the encrypted challenge to the client. */
146: packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
147: packet_put_bignum(encrypted_challenge);
148: packet_send();
1.18 markus 149: BN_clear_free(encrypted_challenge);
1.12 markus 150: packet_write_wait();
151:
1.18 markus 152: /* Wait for a response. */
1.50 markus 153: packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
1.18 markus 154: for (i = 0; i < 16; i++)
1.67 deraadt 155: response[i] = (u_char)packet_get_char();
1.49 markus 156: packet_check_eom();
1.18 markus 157:
1.52 provos 158: success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
1.12 markus 159: BN_clear_free(challenge);
1.51 markus 160: return (success);
1.1 provos 161: }
162:
1.14 markus 163: /*
1.51 markus 164: * check if there's user key matching client_n,
165: * return key if login is allowed, NULL otherwise
1.14 markus 166: */
1.1 provos 167:
1.52 provos 168: int
1.51 markus 169: auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
1.1 provos 170: {
1.61 dtucker 171: char line[SSH_MAX_PUBKEY_BYTES], *file;
1.52 provos 172: int allowed = 0;
1.34 markus 173: u_int bits;
1.12 markus 174: FILE *f;
1.34 markus 175: u_long linenum = 0;
1.46 jakob 176: Key *key;
1.12 markus 177:
178: /* Temporarily use the user's uid. */
1.40 markus 179: temporarily_use_uid(pw);
1.12 markus 180:
181: /* The authorized keys. */
1.41 markus 182: file = authorized_keys_file(pw);
183: debug("trying public RSA key file %s", file);
1.73 dtucker 184: f = auth_openkeyfile(file, pw, options.strict_modes);
1.12 markus 185: if (!f) {
1.41 markus 186: xfree(file);
187: restore_uid();
1.53 markus 188: return (0);
1.1 provos 189: }
1.51 markus 190:
191: /* Flag indicating whether the key is allowed. */
192: allowed = 0;
1.1 provos 193:
1.46 jakob 194: key = key_new(KEY_RSA1);
1.12 markus 195:
1.14 markus 196: /*
197: * Go though the accepted keys, looking for the current key. If
198: * found, perform a challenge-response dialog to verify that the
199: * user really has the corresponding private key.
200: */
1.61 dtucker 201: while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
1.12 markus 202: char *cp;
1.60 avsm 203: char *key_options;
1.63 djm 204: int keybits;
1.12 markus 205:
1.14 markus 206: /* Skip leading whitespace, empty and comment lines. */
207: for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
208: ;
1.12 markus 209: if (!*cp || *cp == '\n' || *cp == '#')
210: continue;
211:
1.14 markus 212: /*
213: * Check if there are options for this key, and if so,
214: * save their starting address and skip the option part
215: * for now. If there are no options, set the starting
216: * address to NULL.
217: */
1.12 markus 218: if (*cp < '0' || *cp > '9') {
219: int quoted = 0;
1.60 avsm 220: key_options = cp;
1.12 markus 221: for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
222: if (*cp == '\\' && cp[1] == '"')
223: cp++; /* Skip both */
224: else if (*cp == '"')
225: quoted = !quoted;
226: }
227: } else
1.60 avsm 228: key_options = NULL;
1.1 provos 229:
1.12 markus 230: /* Parse the key from the line. */
1.46 jakob 231: if (hostfile_read_key(&cp, &bits, key) == 0) {
1.42 markus 232: debug("%.100s, line %lu: non ssh1 key syntax",
1.36 markus 233: file, linenum);
1.12 markus 234: continue;
1.1 provos 235: }
1.12 markus 236: /* cp now points to the comment part. */
237:
1.16 markus 238: /* Check if the we have found the desired key (identified by its modulus). */
1.46 jakob 239: if (BN_cmp(key->rsa->n, client_n) != 0)
1.16 markus 240: continue;
241:
1.12 markus 242: /* check the real bits */
1.63 djm 243: keybits = BN_num_bits(key->rsa->n);
244: if (keybits < 0 || bits != (u_int)keybits)
1.57 itojun 245: logit("Warning: %s, line %lu: keysize mismatch: "
1.15 markus 246: "actual %d vs. announced %d.",
1.46 jakob 247: file, linenum, BN_num_bits(key->rsa->n), bits);
1.12 markus 248:
249: /* We have found the desired key. */
1.33 markus 250: /*
251: * If our options do not allow this key to be used,
252: * do not send challenge.
253: */
1.60 avsm 254: if (!auth_parse_options(pw, key_options, file, linenum))
1.33 markus 255: continue;
1.76 djm 256: if (key_is_cert_authority)
257: continue;
1.51 markus 258: /* break out, this key is allowed */
259: allowed = 1;
1.32 markus 260: break;
1.1 provos 261: }
262:
1.12 markus 263: /* Restore the privileged uid. */
264: restore_uid();
265:
266: /* Close the file. */
1.41 markus 267: xfree(file);
1.12 markus 268: fclose(f);
269:
1.51 markus 270: /* return key if allowed */
271: if (allowed && rkey != NULL)
272: *rkey = key;
273: else
274: key_free(key);
275: return (allowed);
276: }
277:
278: /*
279: * Performs the RSA authentication dialog with the client. This returns
280: * 0 if the client could not be authenticated, and 1 if authentication was
281: * successful. This may exit if there is a serious protocol violation.
282: */
283: int
1.58 djm 284: auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
1.51 markus 285: {
286: Key *key;
287: char *fp;
1.58 djm 288: struct passwd *pw = authctxt->pw;
1.51 markus 289:
290: /* no user given */
1.58 djm 291: if (!authctxt->valid)
1.51 markus 292: return 0;
1.1 provos 293:
1.52 provos 294: if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
1.31 markus 295: auth_clear_options();
1.51 markus 296: return (0);
297: }
298:
299: /* Perform the challenge-response dialog for this key. */
300: if (!auth_rsa_challenge_dialog(key)) {
301: /* Wrong response. */
302: verbose("Wrong response to RSA authentication challenge.");
303: packet_send_debug("Wrong response to RSA authentication challenge.");
304: /*
305: * Break out of the loop. Otherwise we might send
306: * another challenge and break the protocol.
307: */
308: key_free(key);
309: return (0);
310: }
311: /*
312: * Correct response. The client has been successfully
313: * authenticated. Note that we have not yet processed the
314: * options; this will be reset if the options cause the
315: * authentication to be rejected.
316: */
317: fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
318: verbose("Found matching %s key: %s",
319: key_type(key), fp);
320: xfree(fp);
321: key_free(key);
1.1 provos 322:
1.51 markus 323: packet_send_debug("RSA authentication accepted.");
324: return (1);
1.1 provos 325: }
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