364 lines
12 KiB
Text
364 lines
12 KiB
Text
|
=pod
|
||
|
|
|
||
|
|
=encoding UTF-8
|
||
|
|
|
||
|
|
=head1 NAME
|
||
|
|
|
||
|
|
proxy-certificates - Proxy certificates in OpenSSL
|
||
|
|
|
||
|
|
=head1 DESCRIPTION
|
||
|
|
|
||
|
|
Proxy certificates are defined in RFC 3820. They are used to
|
||
|
|
extend rights to some other entity (a computer process, typically, or
|
||
|
|
sometimes to the user itself). This allows the entity to perform
|
||
|
|
operations on behalf of the owner of the EE (End Entity) certificate.
|
||
|
|
|
||
|
|
The requirements for a valid proxy certificate are:
|
||
|
|
|
||
|
|
=over 4
|
||
|
|
|
||
|
|
=item *
|
||
|
|
|
||
|
|
They are issued by an End Entity, either a normal EE certificate, or
|
||
|
|
another proxy certificate.
|
||
|
|
|
||
|
|
=item *
|
||
|
|
|
||
|
|
They must not have the B<subjectAltName> or B<issuerAltName>
|
||
|
|
extensions.
|
||
|
|
|
||
|
|
=item *
|
||
|
|
|
||
|
|
They must have the B<proxyCertInfo> extension.
|
||
|
|
|
||
|
|
=item *
|
||
|
|
|
||
|
|
They must have the subject of their issuer, with one B<commonName>
|
||
|
|
added.
|
||
|
|
|
||
|
|
=back
|
||
|
|
|
||
|
|
=head2 Enabling proxy certificate verification
|
||
|
|
|
||
|
|
OpenSSL expects applications that want to use proxy certificates to be
|
||
|
|
specially aware of them, and make that explicit. This is done by
|
||
|
|
setting an X509 verification flag:
|
||
|
|
|
||
|
|
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 L</NOTES> for a discussion on this requirement.
|
||
|
|
|
||
|
|
=head2 Creating proxy certificates
|
||
|
|
|
||
|
|
Creating proxy certificates can be done using the L<openssl-x509(1)>
|
||
|
|
command, with some extra extensions:
|
||
|
|
|
||
|
|
[ 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, I<syntag>:I<string>, where the
|
||
|
|
I<syntag> determines what will be done with the string. The following
|
||
|
|
I<syntag>s are recognised:
|
||
|
|
|
||
|
|
=over 4
|
||
|
|
|
||
|
|
=item B<text>
|
||
|
|
|
||
|
|
indicates that the string is a byte sequence, without any encoding:
|
||
|
|
|
||
|
|
policy=text:räksmörgås
|
||
|
|
|
||
|
|
=item B<hex>
|
||
|
|
|
||
|
|
indicates the string is encoded hexadecimal encoded binary data, with
|
||
|
|
colons between each byte (every second hex digit):
|
||
|
|
|
||
|
|
policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
|
||
|
|
|
||
|
|
=item B<file>
|
||
|
|
|
||
|
|
indicates that the text of the policy should be taken from a file.
|
||
|
|
The string is then a filename. This is useful for policies that are
|
||
|
|
large (more than a few lines, e.g. XML documents).
|
||
|
|
|
||
|
|
=back
|
||
|
|
|
||
|
|
I<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
|
||
|
|
|
||
|
|
=head2 Using proxy certs in applications
|
||
|
|
|
||
|
|
To interpret proxy policies, the application would normally start with
|
||
|
|
some default rights (perhaps none at all), then compute the resulting
|
||
|
|
rights by checking the rights against the chain of proxy certificates,
|
||
|
|
user certificate and CA certificates.
|
||
|
|
|
||
|
|
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:
|
||
|
|
|
||
|
|
=over 4
|
||
|
|
|
||
|
|
=item *
|
||
|
|
|
||
|
|
a callback function that will be called for every certificate being
|
||
|
|
validated. The callback is called several times for each certificate,
|
||
|
|
so you must be careful to do the proxy policy interpretation at the
|
||
|
|
right time. You also need to fill in the defaults when the EE
|
||
|
|
certificate is checked.
|
||
|
|
|
||
|
|
=item *
|
||
|
|
|
||
|
|
a data structure that is shared between your application code and the
|
||
|
|
callback.
|
||
|
|
|
||
|
|
=item *
|
||
|
|
|
||
|
|
a wrapper function that sets it all up.
|
||
|
|
|
||
|
|
=item *
|
||
|
|
|
||
|
|
an ex_data index function that creates an index into the generic
|
||
|
|
ex_data store that is attached to an X509 validation context.
|
||
|
|
|
||
|
|
=back
|
||
|
|
|
||
|
|
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 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 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);
|
||
|
|
|
||
|
|
=head1 NOTES
|
||
|
|
|
||
|
|
To this date, it seems that proxy certificates have only been used in
|
||
|
|
environments that are aware of them, and no one seems to have
|
||
|
|
investigated how they can be used or misused outside of such an
|
||
|
|
environment.
|
||
|
|
|
||
|
|
For that reason, OpenSSL requires that applications aware of proxy
|
||
|
|
certificates must also make that explicit.
|
||
|
|
|
||
|
|
B<subjectAltName> and B<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.
|
||
|
|
|
||
|
|
=head1 SEE ALSO
|
||
|
|
|
||
|
|
L<X509_STORE_CTX_set_flags(3)>,
|
||
|
|
L<X509_STORE_CTX_set_verify_cb(3)>,
|
||
|
|
L<X509_VERIFY_PARAM_set_flags(3)>,
|
||
|
|
L<SSL_CTX_set_cert_verify_callback(3)>,
|
||
|
|
L<openssl-req(1)>, L<openssl-x509(1)>,
|
||
|
|
L<RFC 3820|https://tools.ietf.org/html/rfc3820>
|
||
|
|
|
||
|
|
=head1 COPYRIGHT
|
||
|
|
|
||
|
|
Copyright 2019 The OpenSSL Project Authors. All Rights Reserved.
|
||
|
|
|
||
|
|
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
|
||
|
|
L<https://www.openssl.org/source/license.html>.
|
||
|
|
|
||
|
|
=cut
|