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323 lines
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323 lines
12 KiB
Plaintext
<DRAFT!>
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HOWTO proxy certificates
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0. WARNING
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NONE OF THE CODE PRESENTED HERE HAVE BEEN CHECKED! They are just an
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example to show you how things can be done. There may be typos or
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type conflicts, and you will have to resolve them.
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1. Introduction
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Proxy certificates are defined in RFC 3820. They are really usual
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certificates with the mandatory extension proxyCertInfo.
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Proxy certificates are issued by an End Entity (typically a user),
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either directly with the EE certificate as issuing certificate, or by
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extension through an already issued proxy certificate.. They are used
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to extend rights to some other entity (a computer process, typically,
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or sometimes to the user itself), so it can perform operations in the
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name of the owner of the EE certificate.
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See http://www.ietf.org/rfc/rfc3820.txt for more information.
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2. A warning about proxy certificates
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Noone seems to have tested proxy certificates with security in mind.
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Basically, to this date, it seems that proxy certificates have only
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been used in a world that's highly aware of them. What would happen
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if an unsuspecting application is to validate a chain of certificates
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that contains proxy certificates? It would usually consider the leaf
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to be the certificate to check for authorisation data, and since proxy
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certificates are controlled by the EE certificate owner alone, it's
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would be normal to consider what the EE certificate owner could do
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with them.
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subjectAltName and issuerAltName are forbidden in proxy certificates,
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and this is enforced in OpenSSL. The subject must be the same as the
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issuer, with one commonName added on.
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Possible threats are, as far as has been imagined so far:
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- impersonation through commonName (think server certificates).
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- use of additional extensions, possibly non-standard ones used in
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certain environments, that would grant extra or different
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authorisation rights.
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For this reason, OpenSSL requires that the use of proxy certificates
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be explicitely allowed. Currently, this can be done using the
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following methods:
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- if the application calls X509_verify_cert() itself, it can do the
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following prior to that call (ctx is the pointer passed in the call
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to X509_verify_cert()):
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X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
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- in all other cases, proxy certificate validation can be enabled
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before starting the application by setting the envirnoment variable
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OPENSSL_ALLOW_PROXY with some non-empty value.
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There are thoughts to allow proxy certificates with a line in the
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default openssl.cnf, but that's still in the future.
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3. How to create proxy cerificates
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It's quite easy to create proxy certificates, by taking advantage of
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the lack of checks of the 'openssl x509' application (*ahem*). But
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first, you need to create a configuration section that contains a
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definition of the proxyCertInfo extension, a little like this:
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[ v3_proxy ]
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# A proxy certificate MUST NEVER be a CA certificate.
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basicConstraints=CA:FALSE
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# Usual authority key ID
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authorityKeyIdentifier=keyid,issuer:always
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# Now, for the extension that marks this certificate as a proxy one
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proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
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It's also possible to give the proxy extension in a separate section:
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proxyCertInfo=critical,@proxy_ext
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[ proxy_ext ]
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language=id-ppl-anyLanguage
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pathlen=0
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policy=text:BC
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The policy value has a specific syntax, {syntag}:{string}, where the
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syntag determines what will be done with the string. The recognised
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syntags are as follows:
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text indicates that the string is simply the bytes, not
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encoded in any kind of way:
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policy=text:r<>ksm<73>rg<72>s
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Previous versions of this design had a specific tag
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for UTF-8 text. However, since the bytes are copied
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as-is anyway, there's no need for it. Instead, use
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the text: tag, like this:
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policy=text:räksmörgås
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hex indicates the string is encoded in hex, with colons
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between each byte (every second hex digit):
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policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
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Previous versions of this design had a tag to insert a
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complete DER blob. However, the only legal use for
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this would be to surround the bytes that would go with
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the hex: tag with what's needed to construct a correct
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OCTET STRING. Since hex: does that, the DER tag felt
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superfluous, and was therefore removed.
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file indicates that the text of the policy should really be
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taken from a file. The string is then really a file
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name. This is useful for policies that are large
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(more than a few of lines) XML documents, for example.
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The 'policy' setting can be split up in multiple lines like this:
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0.policy=This is
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1.polisy= a multi-
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2.policy=line policy.
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NOTE: the proxy policy value is the part that determines the rights
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granted to the process using the proxy certificate. The value is
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completely dependent on the application reading and interpretting it!
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Now that you have created an extension section for your proxy
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certificate, you can now easily create a proxy certificate like this:
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openssl req -new -config openssl.cnf \
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-out proxy.req -keyout proxy.key
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openssl x509 -req -CAcreateserial -in proxy.req -days 7 \
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-out proxy.crt -CA user.crt -CAkey user.key \
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-extfile openssl.cnf -extensions v3_proxy
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It's just as easy to create a proxy certificate using another proxy
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certificate as issuer (note that I'm using a different configuration
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section for it):
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openssl req -new -config openssl.cnf \
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-out proxy2.req -keyout proxy2.key
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openssl x509 -req -CAcreateserial -in proxy2.req -days 7 \
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-out proxy2.crt -CA proxy.crt -CAkey proxy.key \
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-extfile openssl.cnf -extensions v3_proxy2
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4. How to have your application interpret the policy?
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The basic way to interpret proxy policies is to prepare some default
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rights, then do a check of the proxy certificate against the a chain
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of proxy certificates, user certificate and CA certificates, and see
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what rights came out by the end. Sounds easy, huh? It almost is.
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The slightly complicated part is how to pass data between your
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application and the certificate validation procedure.
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You need the following ingredients:
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- a callback routing that will be called for every certificate that's
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validated. It will be called several times for each certificates,
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so you must be attentive to when it's a good time to do the proxy
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policy interpretation and check, as well as to fill in the defaults
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when the EE certificate is checked.
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- a structure of data that's shared between your application code and
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the callback.
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- a wrapper function that sets it all up.
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- an ex_data index function that creates an index into the generic
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ex_data store that's attached to an X509 validation context.
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This is some cookbook code for you to fill in:
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/* In this example, I will use a view of granted rights as a bit
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array, one bit for each possible right. */
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typedef struct your_rights {
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unsigned char rights[total_rights / 8];
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} YOUR_RIGHTS;
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/* The following procedure will create an index for the ex_data
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store in the X509 validation context the first time it's called.
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Subsequent calls will return the same index. */
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static int get_proxy_auth_ex_data_idx(void)
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{
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static volatile int idx = -1;
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if (idx < 0)
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{
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CRYPTO_w_lock(CRYPTO_LOCK_X509_STORE);
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if (idx < 0)
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{
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idx = X509_STORE_CTX_get_ex_new_index(0,
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"for verify callback",
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NULL,NULL,NULL);
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}
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CRYPTO_w_unlock(CRYPTO_LOCK_X509_STORE);
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}
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return idx;
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}
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/* Callback to be given to the X509 validation procedure. */
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static int verify_callback(int ok, X509_STORE_CTX *ctx)
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{
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if (ok == 1) /* It's REALLY important you keep the proxy policy
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check within this secion. It's important to know
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that when ok is 1, the certificates are checked
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from top to bottom. You get the CA root first,
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followed by the possible chain of intermediate
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CAs, followed by the EE certificate, followed by
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the possible proxy certificates. */
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{
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X509 *xs = ctx->current_cert;
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if (xs->ex_flags & EXFLAG_PROXY)
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{
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YOUR_RIGHTS *rights =
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(YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
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get_proxy_auth_ex_data_idx());
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PROXY_CERT_INFO_EXTENSION *pci =
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X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL);
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switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage))
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{
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case NID_Independent:
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/* Do whatever you need to grant explicit rights to
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this particular proxy certificate, usually by
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pulling them from some database. If there are none
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to be found, clear all rights (making this and any
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subsequent proxy certificate void of any rights).
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*/
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memset(rights->rights, 0, sizeof(rights->rights));
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break;
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case NID_id_ppl_inheritAll:
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/* This is basically a NOP, we simply let the current
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rights stand as they are. */
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break;
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default:
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/* This is usually the most complex section of code.
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You really do whatever you want as long as you
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follow RFC 3820. In the example we use here, the
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simplest thing to do is to build another, temporary
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bit array and fill it with the rights granted by
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the current proxy certificate, then use it as a
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mask on the accumulated rights bit array, and
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voil<69>, you now have a new accumulated rights bit
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array. */
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{
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int i;
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YOUR_RIGHTS tmp_rights;
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memset(tmp_rights.rights, 0, sizeof(tmp_rights.rights));
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/* process_rights() is supposed to be a procedure
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that takes a string and it's length, interprets
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it and sets the bits in the YOUR_RIGHTS pointed
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at by the third argument. */
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process_rights((char *) pci->proxyPolicy->policy->data,
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pci->proxyPolicy->policy->length,
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&tmp_rights);
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for(i = 0; i < total_rights / 8; i++)
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rights->rights[i] &= tmp_rights.rights[i];
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}
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break;
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}
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PROXY_CERT_INFO_EXTENSION_free(pci);
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}
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else if (!(xs->ex_flags & EXFLAG_CA))
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{
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/* We have a EE certificate, let's use it to set default!
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*/
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YOUR_RIGHTS *rights =
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(YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
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get_proxy_auth_ex_data_idx());
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/* The following procedure finds out what rights the owner
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of the current certificate has, and sets them in the
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YOUR_RIGHTS structure pointed at by the second
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argument. */
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set_default_rights(xs, rights);
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}
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}
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return ok;
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}
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static int my_X509_verify_cert(X509_STORE_CTX *ctx,
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YOUR_RIGHTS *needed_rights)
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{
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int i;
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int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) = ctx->verify_cb;
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YOUR_RIGHTS rights;
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X509_STORE_CTX_set_verify_cb(ctx, verify_callback);
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X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(), &rights);
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X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
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ok = X509_verify_cert(ctx);
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if (ok == 1)
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{
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ok = check_needed_rights(rights, needed_rights);
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}
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X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb);
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return ok;
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}
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If you use SSL or TLS, you can easily set up a callback to have the
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certificates checked properly, using the code above:
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SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert, &needed_rights);
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--
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Richard Levitte
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