auth-rsa.c revision 255767
1/* $OpenBSD: auth-rsa.c,v 1.85 2013/07/12 00:19:58 djm Exp $ */ 2/* 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. 9 * 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". 15 */ 16 17#include "includes.h" 18 19#include <sys/types.h> 20#include <sys/stat.h> 21 22#include <openssl/rsa.h> 23#include <openssl/md5.h> 24 25#include <pwd.h> 26#include <stdio.h> 27#include <stdarg.h> 28#include <string.h> 29 30#include "xmalloc.h" 31#include "rsa.h" 32#include "packet.h" 33#include "ssh1.h" 34#include "uidswap.h" 35#include "match.h" 36#include "buffer.h" 37#include "pathnames.h" 38#include "log.h" 39#include "servconf.h" 40#include "key.h" 41#include "auth-options.h" 42#include "hostfile.h" 43#include "auth.h" 44#ifdef GSSAPI 45#include "ssh-gss.h" 46#endif 47#include "monitor_wrap.h" 48#include "ssh.h" 49#include "misc.h" 50 51/* import */ 52extern ServerOptions options; 53 54/* 55 * Session identifier that is used to bind key exchange and authentication 56 * responses to a particular session. 57 */ 58extern u_char session_id[16]; 59 60/* 61 * The .ssh/authorized_keys file contains public keys, one per line, in the 62 * following format: 63 * options bits e n comment 64 * where bits, e and n are decimal numbers, 65 * and comment is any string of characters up to newline. The maximum 66 * length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a 67 * description of the options. 68 */ 69 70BIGNUM * 71auth_rsa_generate_challenge(Key *key) 72{ 73 BIGNUM *challenge; 74 BN_CTX *ctx; 75 76 if ((challenge = BN_new()) == NULL) 77 fatal("auth_rsa_generate_challenge: BN_new() failed"); 78 /* Generate a random challenge. */ 79 if (BN_rand(challenge, 256, 0, 0) == 0) 80 fatal("auth_rsa_generate_challenge: BN_rand failed"); 81 if ((ctx = BN_CTX_new()) == NULL) 82 fatal("auth_rsa_generate_challenge: BN_CTX_new failed"); 83 if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0) 84 fatal("auth_rsa_generate_challenge: BN_mod failed"); 85 BN_CTX_free(ctx); 86 87 return challenge; 88} 89 90int 91auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16]) 92{ 93 u_char buf[32], mdbuf[16]; 94 MD5_CTX md; 95 int len; 96 97 /* don't allow short keys */ 98 if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) { 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); 101 return (0); 102 } 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. */ 116 if (timingsafe_bcmp(response, mdbuf, 16) != 0) { 117 /* Wrong answer. */ 118 return (0); 119 } 120 /* Correct answer. */ 121 return (1); 122} 123 124/* 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 */ 129 130int 131auth_rsa_challenge_dialog(Key *key) 132{ 133 BIGNUM *challenge, *encrypted_challenge; 134 u_char response[16]; 135 int i, success; 136 137 if ((encrypted_challenge = BN_new()) == NULL) 138 fatal("auth_rsa_challenge_dialog: BN_new() failed"); 139 140 challenge = PRIVSEP(auth_rsa_generate_challenge(key)); 141 142 /* Encrypt the challenge with the public key. */ 143 rsa_public_encrypt(encrypted_challenge, challenge, key->rsa); 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(); 149 BN_clear_free(encrypted_challenge); 150 packet_write_wait(); 151 152 /* Wait for a response. */ 153 packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE); 154 for (i = 0; i < 16; i++) 155 response[i] = (u_char)packet_get_char(); 156 packet_check_eom(); 157 158 success = PRIVSEP(auth_rsa_verify_response(key, challenge, response)); 159 BN_clear_free(challenge); 160 return (success); 161} 162 163static int 164rsa_key_allowed_in_file(struct passwd *pw, char *file, 165 const BIGNUM *client_n, Key **rkey) 166{ 167 char *fp, line[SSH_MAX_PUBKEY_BYTES]; 168 int allowed = 0, bits; 169 FILE *f; 170 u_long linenum = 0; 171 Key *key; 172 173 debug("trying public RSA key file %s", file); 174 if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL) 175 return 0; 176 177 /* 178 * Go though the accepted keys, looking for the current key. If 179 * found, perform a challenge-response dialog to verify that the 180 * user really has the corresponding private key. 181 */ 182 key = key_new(KEY_RSA1); 183 while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) { 184 char *cp; 185 char *key_options; 186 int keybits; 187 188 /* Skip leading whitespace, empty and comment lines. */ 189 for (cp = line; *cp == ' ' || *cp == '\t'; cp++) 190 ; 191 if (!*cp || *cp == '\n' || *cp == '#') 192 continue; 193 194 /* 195 * Check if there are options for this key, and if so, 196 * save their starting address and skip the option part 197 * for now. If there are no options, set the starting 198 * address to NULL. 199 */ 200 if (*cp < '0' || *cp > '9') { 201 int quoted = 0; 202 key_options = cp; 203 for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) { 204 if (*cp == '\\' && cp[1] == '"') 205 cp++; /* Skip both */ 206 else if (*cp == '"') 207 quoted = !quoted; 208 } 209 } else 210 key_options = NULL; 211 212 /* Parse the key from the line. */ 213 if (hostfile_read_key(&cp, &bits, key) == 0) { 214 debug("%.100s, line %lu: non ssh1 key syntax", 215 file, linenum); 216 continue; 217 } 218 /* cp now points to the comment part. */ 219 220 /* 221 * Check if the we have found the desired key (identified 222 * by its modulus). 223 */ 224 if (BN_cmp(key->rsa->n, client_n) != 0) 225 continue; 226 227 /* check the real bits */ 228 keybits = BN_num_bits(key->rsa->n); 229 if (keybits < 0 || bits != keybits) 230 logit("Warning: %s, line %lu: keysize mismatch: " 231 "actual %d vs. announced %d.", 232 file, linenum, BN_num_bits(key->rsa->n), bits); 233 234 fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX); 235 debug("matching key found: file %s, line %lu %s %s", 236 file, linenum, key_type(key), fp); 237 free(fp); 238 239 /* Never accept a revoked key */ 240 if (auth_key_is_revoked(key)) 241 break; 242 243 /* We have found the desired key. */ 244 /* 245 * If our options do not allow this key to be used, 246 * do not send challenge. 247 */ 248 if (!auth_parse_options(pw, key_options, file, linenum)) 249 continue; 250 if (key_is_cert_authority) 251 continue; 252 /* break out, this key is allowed */ 253 allowed = 1; 254 break; 255 } 256 257 /* Close the file. */ 258 fclose(f); 259 260 /* return key if allowed */ 261 if (allowed && rkey != NULL) 262 *rkey = key; 263 else 264 key_free(key); 265 266 return allowed; 267} 268 269/* 270 * check if there's user key matching client_n, 271 * return key if login is allowed, NULL otherwise 272 */ 273 274int 275auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey) 276{ 277 char *file; 278 u_int i, allowed = 0; 279 280 temporarily_use_uid(pw); 281 282 for (i = 0; !allowed && i < options.num_authkeys_files; i++) { 283 if (strcasecmp(options.authorized_keys_files[i], "none") == 0) 284 continue; 285 file = expand_authorized_keys( 286 options.authorized_keys_files[i], pw); 287 allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey); 288 free(file); 289 } 290 291 restore_uid(); 292 293 return allowed; 294} 295 296/* 297 * Performs the RSA authentication dialog with the client. This returns 298 * 0 if the client could not be authenticated, and 1 if authentication was 299 * successful. This may exit if there is a serious protocol violation. 300 */ 301int 302auth_rsa(Authctxt *authctxt, BIGNUM *client_n) 303{ 304 Key *key; 305 struct passwd *pw = authctxt->pw; 306 307 /* no user given */ 308 if (!authctxt->valid) 309 return 0; 310 311 if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) { 312 auth_clear_options(); 313 return (0); 314 } 315 316 /* Perform the challenge-response dialog for this key. */ 317 if (!auth_rsa_challenge_dialog(key)) { 318 /* Wrong response. */ 319 verbose("Wrong response to RSA authentication challenge."); 320 packet_send_debug("Wrong response to RSA authentication challenge."); 321 /* 322 * Break out of the loop. Otherwise we might send 323 * another challenge and break the protocol. 324 */ 325 key_free(key); 326 return (0); 327 } 328 /* 329 * Correct response. The client has been successfully 330 * authenticated. Note that we have not yet processed the 331 * options; this will be reset if the options cause the 332 * authentication to be rejected. 333 */ 334 pubkey_auth_info(authctxt, key, NULL); 335 336 packet_send_debug("RSA authentication accepted."); 337 return (1); 338} 339