--- /dev/null
+<DRAFT!>
+ 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. 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:räksmörgås
+
+ 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
+
+
+3. 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);
+ 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
projects / openssl / commitdiff