The requirements for a valid proxy certificate are:
X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
or
X509_VERIFY_PARAM_set_flags(param, X509_V_FLAG_ALLOW_PROXY_CERTS);
See ``NOTES'' for a discussion on this requirement.
[ v3_proxy ]
# A proxy certificate MUST NEVER be a CA certificate.
basicConstraints=CA:FALSE
# Usual authority key ID
authorityKeyIdentifier=keyid,issuer:always
# The extension which marks this certificate as a proxy
proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
It's also possible to specify 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 following syntags are recognised:
policy=text:ra.ksmo.rga°s
policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
NOTE: The proxy policy value is what determines the rights granted to the process during the proxy certificate. It's up to the application to interpret and combine these policies.
With a proxy extension, creating a proxy certificate is a matter of two commands:
openssl req -new -config proxy.cnf \
-out proxy.req -keyout proxy.key \
-subj "/DC=org/DC=openssl/DC=users/CN=proxy 1"
openssl x509 -req -CAcreateserial -in proxy.req -out proxy.crt \
-CA user.crt -CAkey user.key -days 7 \
-extfile proxy.cnf -extensions v3_proxy1
You can also create a proxy certificate using another proxy certificate as issuer (note: using a different configuration section for the proxy extensions):
openssl req -new -config proxy.cnf \
-out proxy2.req -keyout proxy2.key \
-subj "/DC=org/DC=openssl/DC=users/CN=proxy 1/CN=proxy 2"
openssl x509 -req -CAcreateserial -in proxy2.req -out proxy2.crt \
-CA proxy.crt -CAkey proxy.key -days 7 \
-extfile proxy.cnf -extensions v3_proxy2
The complicated part is figuring out how to pass data between your application and the certificate validation procedure.
The following ingredients are needed for such processing:
The following skeleton code can be used as a starting point:
#include <string.h>
#include <netdb.h>
#include <openssl/x509.h>
#include <openssl/x509v3.h>
#define total_rights 25
/*
* 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 + 7) / 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(X509_STORE_CTX *ctx)
{
static volatile int idx = -1;
if (idx < 0) {
X509_STORE_lock(X509_STORE_CTX_get0_store(ctx));
if (idx < 0) {
idx = X509_STORE_CTX_get_ex_new_index(0,
"for verify callback",
NULL,NULL,NULL);
}
X509_STORE_unlock(X509_STORE_CTX_get0_store(ctx));
}
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 section. 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 = X509_STORE_CTX_get_current_cert(ctx);
if (X509_get_extension_flags(xs) & EXFLAG_PROXY) {
YOUR_RIGHTS *rights =
(YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
get_proxy_auth_ex_data_idx(ctx));
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 voila, 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 its
* 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 (!(X509_get_extension_flags(xs) & EXFLAG_CA)) {
/* We have an 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(ctx));
/*
* 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 ok;
int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) =
X509_STORE_CTX_get_verify_cb(ctx);
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(ctx),
&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);
For that reason, OpenSSL requires that applications aware of proxy certificates must also make that explicit.
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.
Licensed under the Apache License 2.0 (the ``License''). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.