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Diffstat (limited to 'openssl/trunk/doc/HOWTO/proxy_certificates.txt')
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diff --git a/openssl/trunk/doc/HOWTO/proxy_certificates.txt b/openssl/trunk/doc/HOWTO/proxy_certificates.txt deleted file mode 100644 index 3d36b02f..00000000 --- a/openssl/trunk/doc/HOWTO/proxy_certificates.txt +++ /dev/null @@ -1,322 +0,0 @@ -<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. 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 |