Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame^] | 1 | /* $OpenBSD: auth-rsa.c,v 1.80 2011/05/23 03:30:07 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 */ |
| 52 | extern ServerOptions options; |
| 53 | |
| 54 | /* |
| 55 | * Session identifier that is used to bind key exchange and authentication |
| 56 | * responses to a particular session. |
| 57 | */ |
| 58 | extern 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 | |
| 70 | BIGNUM * |
| 71 | auth_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 | |
| 90 | int |
| 91 | auth_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 | |
| 130 | int |
| 131 | auth_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 | |
| 163 | static int |
| 164 | rsa_key_allowed_in_file(struct passwd *pw, char *file, |
| 165 | const BIGNUM *client_n, Key **rkey) |
| 166 | { |
| 167 | char line[SSH_MAX_PUBKEY_BYTES]; |
| 168 | int allowed = 0; |
| 169 | u_int bits; |
| 170 | FILE *f; |
| 171 | u_long linenum = 0; |
| 172 | Key *key; |
| 173 | |
| 174 | debug("trying public RSA key file %s", file); |
| 175 | if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL) |
| 176 | return 0; |
| 177 | |
| 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 | */ |
| 183 | key = key_new(KEY_RSA1); |
| 184 | while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) { |
| 185 | char *cp; |
| 186 | char *key_options; |
| 187 | int keybits; |
| 188 | |
| 189 | /* Skip leading whitespace, empty and comment lines. */ |
| 190 | for (cp = line; *cp == ' ' || *cp == '\t'; cp++) |
| 191 | ; |
| 192 | if (!*cp || *cp == '\n' || *cp == '#') |
| 193 | continue; |
| 194 | |
| 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 | */ |
| 201 | if (*cp < '0' || *cp > '9') { |
| 202 | int quoted = 0; |
| 203 | key_options = cp; |
| 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 |
| 211 | key_options = NULL; |
| 212 | |
| 213 | /* Parse the key from the line. */ |
| 214 | if (hostfile_read_key(&cp, &bits, key) == 0) { |
| 215 | debug("%.100s, line %lu: non ssh1 key syntax", |
| 216 | file, linenum); |
| 217 | continue; |
| 218 | } |
| 219 | /* cp now points to the comment part. */ |
| 220 | |
| 221 | /* |
| 222 | * Check if the we have found the desired key (identified |
| 223 | * by its modulus). |
| 224 | */ |
| 225 | if (BN_cmp(key->rsa->n, client_n) != 0) |
| 226 | continue; |
| 227 | |
| 228 | /* check the real bits */ |
| 229 | keybits = BN_num_bits(key->rsa->n); |
| 230 | if (keybits < 0 || bits != (u_int)keybits) |
| 231 | logit("Warning: %s, line %lu: keysize mismatch: " |
| 232 | "actual %d vs. announced %d.", |
| 233 | file, linenum, BN_num_bits(key->rsa->n), bits); |
| 234 | |
| 235 | /* Never accept a revoked key */ |
| 236 | if (auth_key_is_revoked(key)) |
| 237 | break; |
| 238 | |
| 239 | /* We have found the desired key. */ |
| 240 | /* |
| 241 | * If our options do not allow this key to be used, |
| 242 | * do not send challenge. |
| 243 | */ |
| 244 | if (!auth_parse_options(pw, key_options, file, linenum)) |
| 245 | continue; |
| 246 | if (key_is_cert_authority) |
| 247 | continue; |
| 248 | /* break out, this key is allowed */ |
| 249 | allowed = 1; |
| 250 | break; |
| 251 | } |
| 252 | |
| 253 | /* Close the file. */ |
| 254 | fclose(f); |
| 255 | |
| 256 | /* return key if allowed */ |
| 257 | if (allowed && rkey != NULL) |
| 258 | *rkey = key; |
| 259 | else |
| 260 | key_free(key); |
| 261 | |
| 262 | return allowed; |
| 263 | } |
| 264 | |
| 265 | /* |
| 266 | * check if there's user key matching client_n, |
| 267 | * return key if login is allowed, NULL otherwise |
| 268 | */ |
| 269 | |
| 270 | int |
| 271 | auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey) |
| 272 | { |
| 273 | char *file; |
| 274 | u_int i, allowed = 0; |
| 275 | |
| 276 | temporarily_use_uid(pw); |
| 277 | |
| 278 | for (i = 0; !allowed && i < options.num_authkeys_files; i++) { |
| 279 | file = expand_authorized_keys( |
| 280 | options.authorized_keys_files[i], pw); |
| 281 | allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey); |
| 282 | xfree(file); |
| 283 | } |
| 284 | |
| 285 | restore_uid(); |
| 286 | |
| 287 | return allowed; |
| 288 | } |
| 289 | |
| 290 | /* |
| 291 | * Performs the RSA authentication dialog with the client. This returns |
| 292 | * 0 if the client could not be authenticated, and 1 if authentication was |
| 293 | * successful. This may exit if there is a serious protocol violation. |
| 294 | */ |
| 295 | int |
| 296 | auth_rsa(Authctxt *authctxt, BIGNUM *client_n) |
| 297 | { |
| 298 | Key *key; |
| 299 | char *fp; |
| 300 | struct passwd *pw = authctxt->pw; |
| 301 | |
| 302 | /* no user given */ |
| 303 | if (!authctxt->valid) |
| 304 | return 0; |
| 305 | |
| 306 | if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) { |
| 307 | auth_clear_options(); |
| 308 | return (0); |
| 309 | } |
| 310 | |
| 311 | /* Perform the challenge-response dialog for this key. */ |
| 312 | if (!auth_rsa_challenge_dialog(key)) { |
| 313 | /* Wrong response. */ |
| 314 | verbose("Wrong response to RSA authentication challenge."); |
| 315 | packet_send_debug("Wrong response to RSA authentication challenge."); |
| 316 | /* |
| 317 | * Break out of the loop. Otherwise we might send |
| 318 | * another challenge and break the protocol. |
| 319 | */ |
| 320 | key_free(key); |
| 321 | return (0); |
| 322 | } |
| 323 | /* |
| 324 | * Correct response. The client has been successfully |
| 325 | * authenticated. Note that we have not yet processed the |
| 326 | * options; this will be reset if the options cause the |
| 327 | * authentication to be rejected. |
| 328 | */ |
| 329 | fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX); |
| 330 | verbose("Found matching %s key: %s", |
| 331 | key_type(key), fp); |
| 332 | xfree(fp); |
| 333 | key_free(key); |
| 334 | |
| 335 | packet_send_debug("RSA authentication accepted."); |
| 336 | return (1); |
| 337 | } |