306 lines
12 KiB
Text
306 lines
12 KiB
Text
HOWTO proxy certificates
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0. WARNING
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NONE OF THE CODE PRESENTED HERE HAS BEEN CHECKED! The code is just examples to
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show you how things could be done. There might be typos or type conflicts, and
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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 certificates
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with the mandatory extension proxyCertInfo.
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Proxy certificates are issued by an End Entity (typically a user), either
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directly with the EE certificate as issuing certificate, or by extension through
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an already issued proxy certificate. Proxy certificates are used to extend
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rights to some other entity (a computer process, typically, or sometimes to the
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user itself). This allows the entity to perform operations on behalf of the
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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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No one seems to have tested proxy certificates with security in mind. To this
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date, it seems that proxy certificates have only been used in a context highly
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aware of them.
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Existing applications might misbehave when trying to validate a chain of
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certificates which use a proxy certificate. They might incorrectly consider the
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leaf to be the certificate to check for authorisation data, which is controlled
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by the EE certificate owner.
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subjectAltName and issuerAltName are forbidden in proxy certificates, and this
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is enforced in OpenSSL. The subject must be the same as the issuer, with one
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commonName added on.
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Possible threats we can think of at this time include:
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- impersonation through commonName (think server certificates).
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- use of additional extensions, possibly non-standard ones used in certain
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environments, that would grant extra or different authorisation rights.
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For these reasons, OpenSSL requires that the use of proxy certificates be
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explicitly allowed. Currently, this can be done using the following methods:
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- if the application directly calls X509_verify_cert(), it can first call:
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X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
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Where ctx is the pointer which then gets passed to X509_verify_cert().
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- proxy certificate validation can be enabled before starting the application
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by setting the environment variable OPENSSL_ALLOW_PROXY_CERTS.
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In the future, it might be possible to enable proxy certificates by editing
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openssl.cnf.
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3. How to create proxy certificates
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Creating proxy certificates is quite easy, by taking advantage of a lack of
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checks in the 'openssl x509' application (*ahem*). You must first create a
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configuration section that contains a definition of the proxyCertInfo extension,
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for example:
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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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# The extension which marks this certificate as a proxy
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proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
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It's also possible to specify 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 syntag
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determines what will be done with the string. The following syntags are
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recognised:
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text indicates that the string is simply bytes, without any encoding:
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policy=text:räksmörgås
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Previous versions of this design had a specific tag for UTF-8 text.
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However, since the bytes are copied as-is anyway, there is no need for
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such a specific tag.
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hex indicates the string is encoded in hex, with colons between each byte
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(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 complete DER
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blob. However, the only legal use for this would be to surround the
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bytes that would go with the hex: tag with whatever is needed to
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construct a correct OCTET STRING. The DER tag therefore felt
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superfluous, and was removed.
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file indicates that the text of the policy should really be taken from a
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file. The string is then really a file name. This is useful for
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policies that are large (more than a few lines, e.g. XML documents).
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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.policy= a multi-
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2.policy=line policy.
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NOTE: the proxy policy value is the part which determines the rights granted to
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the process using the proxy certificate. The value is completely dependent on
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the application reading and interpreting it!
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Now that you have created an extension section for your proxy certificate, you
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can easily create a proxy certificate by doing:
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openssl req -new -config openssl.cnf -out proxy.req -keyout proxy.key
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openssl x509 -req -CAcreateserial -in proxy.req -days 7 -out proxy.crt \
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-CA user.crt -CAkey user.key -extfile openssl.cnf -extensions v3_proxy
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You can also create a proxy certificate using another proxy certificate as
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issuer (note: I'm using a different configuration section for it):
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openssl req -new -config openssl.cnf -out proxy2.req -keyout proxy2.key
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openssl x509 -req -CAcreateserial -in proxy2.req -days 7 -out proxy2.crt \
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-CA proxy.crt -CAkey proxy.key -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 start with some default rights,
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then compute the resulting rights by checking the proxy certificate against
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the chain of proxy certificates, user certificate and CA certificates. You then
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use the final computed rights. Sounds easy, huh? It almost is.
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The slightly complicated part is figuring out 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 function that will be called for every certificate being
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validated. The callback be called several times for each certificate,
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so you must be careful to do the proxy policy interpretation at the right
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time. You also need to fill in the defaults when the EE certificate is
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checked.
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- a data structure that is shared between your application code and the
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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 ex_data
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store that is attached to an X509 validation context.
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Here is some skeleton code you can 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 section. 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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voilà, 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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