HOWTO proxy certificates 0. WARNING NONE OF THE CODE PRESENTED HERE HAVE BEEN CHECKED! They are just an example to show you how things can be done. There may be typos or type conflicts, and you will have to resolve them. 1. Introduction Proxy certificates are defined in RFC 3820. They are really usual certificates with the mandatory extension proxyCertInfo. Proxy certificates are issued by an End Entity (typically a user), either directly with the EE certificate as issuing certificate, or by extension through an already issued proxy certificate.. They are used to extend rights to some other entity (a computer process, typically, or sometimes to the user itself), so it can perform operations in the name of the owner of the EE certificate. See http://www.ietf.org/rfc/rfc3820.txt for more information. 2. A warning about proxy certificates Noone seems to have tested proxy certificates with security in mind. Basically, to this date, it seems that proxy certificates have only been used in a world that's highly aware of them. What would happen if an unsuspecting application is to validate a chain of certificates that contains proxy certificates? It would usually consider the leaf to be the certificate to check for authorisation data, and since proxy certificates are controlled by the EE certificate owner alone, it's would be normal to consider what the EE certificate owner could do with them. subjectAltName and issuerAltName are forbidden in proxy certificates, and this is enforced in OpenSSL. The subject must be the same as the issuer, with one commonName added on. Possible threats are, as far as has been imagined so far: - impersonation through commonName (think server certificates). - use of additional extensions, possibly non-standard ones used in certain environments, that would grant extra or different authorisation rights. For this reason, OpenSSL requires that the use of proxy certificates be explicitely allowed. Currently, this can be done using the following methods: - if the application calls X509_verify_cert() itself, it can do the following prior to that call (ctx is the pointer passed in the call to X509_verify_cert()): X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS); - in all other cases, proxy certificate validation can be enabled before starting the application by setting the envirnoment variable OPENSSL_ALLOW_PROXY with some non-empty value. There are thoughts to allow proxy certificates with a line in the default openssl.cnf, but that's still in the future. 3. How to create proxy cerificates It's quite easy to create proxy certificates, by taking advantage of the lack of checks of the 'openssl x509' application (*ahem*). But first, you need to create a configuration section that contains a definition of the proxyCertInfo extension, a little like this: [ v3_proxy ] # A proxy certificate MUST NEVER be a CA certificate. basicConstraints=CA:FALSE # Usual authority key ID authorityKeyIdentifier=keyid,issuer:always # Now, for the extension that marks this certificate as a proxy one proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB It's also possible to give the proxy extension in a separate section: proxyCertInfo=critical,@proxy_ext [ proxy_ext ] language=id-ppl-anyLanguage pathlen=0 policy=text:BC The policy value has a specific syntax, {syntag}:{string}, where the syntag determines what will be done with the string. The recognised syntags are as follows: text indicates that the string is simply the bytes, not encoded in any kind of way: policy=text:räksmörgås Previous versions of this design had a specific tag for UTF-8 text. However, since the bytes are copied as-is anyway, there's no need for it. Instead, use the text: tag, like this: policy=text:räksmörgÃ¥s hex indicates the string is encoded in hex, with colons between each byte (every second hex digit): policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73 Previous versions of this design had a tag to insert a complete DER blob. However, the only legal use for this would be to surround the bytes that would go with the hex: tag with what's needed to construct a correct OCTET STRING. Since hex: does that, the DER tag felt superfluous, and was therefore removed. file indicates that the text of the policy should really be taken from a file. The string is then really a file name. This is useful for policies that are large (more than a few of lines) XML documents, for example. The 'policy' setting can be split up in multiple lines like this: 0.policy=This is 1.polisy= a multi- 2.policy=line policy. NOTE: the proxy policy value is the part that determines the rights granted to the process using the proxy certificate. The value is completely dependent on the application reading and interpretting it! Now that you have created an extension section for your proxy certificate, you can now easily create a proxy certificate like this: openssl req -new -config openssl.cnf \ -out proxy.req -keyout proxy.key openssl x509 -req -CAcreateserial -in proxy.req -days 7 \ -out proxy.crt -CA user.crt -CAkey user.key \ -extfile openssl.cnf -extensions v3_proxy It's just as easy to create a proxy certificate using another proxy certificate as issuer (note that I'm using a different configuration section for it): openssl req -new -config openssl.cnf \ -out proxy2.req -keyout proxy2.key openssl x509 -req -CAcreateserial -in proxy2.req -days 7 \ -out proxy2.crt -CA proxy.crt -CAkey proxy.key \ -extfile openssl.cnf -extensions v3_proxy2 4. How to have your application interpret the policy? The basic way to interpret proxy policies is to prepare some default rights, then do a check of the proxy certificate against the a chain of proxy certificates, user certificate and CA certificates, and see what rights came out by the end. Sounds easy, huh? It almost is. The slightly complicated part is how to pass data between your application and the certificate validation procedure. You need the following ingredients: - a callback routing that will be called for every certificate that's validated. It will be called several times for each certificates, so you must be attentive to when it's a good time to do the proxy policy interpretation and check, as well as to fill in the defaults when the EE certificate is checked. - a structure of data that's shared between your application code and the callback. - a wrapper function that sets it all up. - an ex_data index function that creates an index into the generic ex_data store that's attached to an X509 validation context. This is some cookbook code for you to fill in: /* In this example, I will use a view of granted rights as a bit array, one bit for each possible right. */ typedef struct your_rights { unsigned char rights[total_rights / 8]; } YOUR_RIGHTS; /* The following procedure will create an index for the ex_data store in the X509 validation context the first time it's called. Subsequent calls will return the same index. */ static int get_proxy_auth_ex_data_idx(void) { static volatile int idx = -1; if (idx < 0) { CRYPTO_w_lock(CRYPTO_LOCK_X509_STORE); if (idx < 0) { idx = X509_STORE_CTX_get_ex_new_index(0, "for verify callback", NULL,NULL,NULL); } CRYPTO_w_unlock(CRYPTO_LOCK_X509_STORE); } return idx; } /* Callback to be given to the X509 validation procedure. */ static int verify_callback(int ok, X509_STORE_CTX *ctx) { if (ok == 1) /* It's REALLY important you keep the proxy policy check within this secion. It's important to know that when ok is 1, the certificates are checked from top to bottom. You get the CA root first, followed by the possible chain of intermediate CAs, followed by the EE certificate, followed by the possible proxy certificates. */ { X509 *xs = ctx->current_cert; if (xs->ex_flags & EXFLAG_PROXY) { YOUR_RIGHTS *rights = (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx, get_proxy_auth_ex_data_idx()); PROXY_CERT_INFO_EXTENSION *pci = X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL); switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage)) { case NID_Independent: /* Do whatever you need to grant explicit rights to this particular proxy certificate, usually by pulling them from some database. If there are none to be found, clear all rights (making this and any subsequent proxy certificate void of any rights). */ memset(rights->rights, 0, sizeof(rights->rights)); break; case NID_id_ppl_inheritAll: /* This is basically a NOP, we simply let the current rights stand as they are. */ break; default: /* This is usually the most complex section of code. You really do whatever you want as long as you follow RFC 3820. In the example we use here, the simplest thing to do is to build another, temporary bit array and fill it with the rights granted by the current proxy certificate, then use it as a mask on the accumulated rights bit array, and voilà, you now have a new accumulated rights bit array. */ { int i; YOUR_RIGHTS tmp_rights; memset(tmp_rights.rights, 0, sizeof(tmp_rights.rights)); /* process_rights() is supposed to be a procedure that takes a string and it's length, interprets it and sets the bits in the YOUR_RIGHTS pointed at by the third argument. */ process_rights((char *) pci->proxyPolicy->policy->data, pci->proxyPolicy->policy->length, &tmp_rights); for(i = 0; i < total_rights / 8; i++) rights->rights[i] &= tmp_rights.rights[i]; } break; } PROXY_CERT_INFO_EXTENSION_free(pci); } else if (!(xs->ex_flags & EXFLAG_CA)) { /* We have a EE certificate, let's use it to set default! */ YOUR_RIGHTS *rights = (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx, get_proxy_auth_ex_data_idx()); /* The following procedure finds out what rights the owner of the current certificate has, and sets them in the YOUR_RIGHTS structure pointed at by the second argument. */ set_default_rights(xs, rights); } } return ok; } static int my_X509_verify_cert(X509_STORE_CTX *ctx, YOUR_RIGHTS *needed_rights) { int i; int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) = ctx->verify_cb; YOUR_RIGHTS rights; X509_STORE_CTX_set_verify_cb(ctx, verify_callback); X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(), &rights); X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS); ok = X509_verify_cert(ctx); if (ok == 1) { ok = check_needed_rights(rights, needed_rights); } X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb); return ok; } If you use SSL or TLS, you can easily set up a callback to have the certificates checked properly, using the code above: SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert, &needed_rights); -- Richard Levitte href='#n286'>286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483