[thirdparty/openssl.git] / doc / man7 / proxy-certificates.pod

=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:

    [ 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
more than a few lines, such as XML or other markup.

=back

Note that the proxy policy value is what determines the rights granted
to the process during the proxy certificate, and it is 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"

    openssl x509 -req -CAcreateserial -in proxy.req -out proxy.crt \
        -CA user.crt -CAkey user.key -days 7 \
        -extfile proxy.cnf -extensions proxy

You can also create a proxy certificate using another proxy
certificate as issuer. Note that this example uses 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/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 proxy_2

=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 2019-2020 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
Commit	Line	Data
2a038236 RL	1	=pod
	2
	3	=encoding UTF-8
	4
	5	=head1 NAME
	6
	7	proxy-certificates - Proxy certificates in OpenSSL
	8
	9	=head1 DESCRIPTION
	10
	11	Proxy certificates are defined in RFC 3820. They are used to
	12	extend rights to some other entity (a computer process, typically, or
	13	sometimes to the user itself). This allows the entity to perform
	14	operations on behalf of the owner of the EE (End Entity) certificate.
	15
	16	The requirements for a valid proxy certificate are:
	17
	18	=over 4
	19
	20	=item *
	21
	22	They are issued by an End Entity, either a normal EE certificate, or
	23	another proxy certificate.
	24
	25	=item *
	26
	27	They must not have the B<subjectAltName> or B<issuerAltName>
	28	extensions.
	29
	30	=item *
	31
	32	They must have the B<proxyCertInfo> extension.
	33
	34	=item *
	35
	36	They must have the subject of their issuer, with one B<commonName>
	37	added.
	38
	39	=back
	40
	41	=head2 Enabling proxy certificate verification
	42
	43	OpenSSL expects applications that want to use proxy certificates to be
	44	specially aware of them, and make that explicit. This is done by
	45	setting an X509 verification flag:
	46
	47	X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
	48
	49	or
	50
	51	X509_VERIFY_PARAM_set_flags(param, X509_V_FLAG_ALLOW_PROXY_CERTS);
	52
	53	See L</NOTES> for a discussion on this requirement.
	54
	55	=head2 Creating proxy certificates
	56
	57	Creating proxy certificates can be done using the L<openssl-x509(1)>
	58	command, with some extra extensions:
	59
2b584ff3	60	[ proxy ]
2a038236	61	# A proxy certificate MUST NEVER be a CA certificate.
2b584ff3	62	basicConstraints = CA:FALSE
2a038236	63	# Usual authority key ID
2b584ff3	64	authorityKeyIdentifier = keyid,issuer:always
2a038236	65	# The extension which marks this certificate as a proxy
2b584ff3	66	proxyCertInfo = critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
2a038236 RL	67
	68	It's also possible to specify the proxy extension in a separate section:
	69
2b584ff3	70	proxyCertInfo = critical,@proxy_ext
2a038236 RL	71
2a038236 RL	72	[ proxy_ext ]
2b584ff3 RS	73	language = id-ppl-anyLanguage
	74	pathlen = 0
	75	policy = text:BC
2a038236 RL	76
	77	The policy value has a specific syntax, I<syntag>:I<string>, where the
	78	I<syntag> determines what will be done with the string. The following
	79	I<syntag>s are recognised:
	80
	81	=over 4
	82
	83	=item B<text>
	84
	85	indicates that the string is a byte sequence, without any encoding:
	86
	87	policy=text:räksmörgås
	88
	89	=item B<hex>
	90
	91	indicates the string is encoded hexadecimal encoded binary data, with
	92	colons between each byte (every second hex digit):
	93
	94	policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
	95
	96	=item B<file>
	97
	98	indicates that the text of the policy should be taken from a file.
	99	The string is then a filename. This is useful for policies that are
2b584ff3	100	more than a few lines, such as XML or other markup.
2a038236 RL	101
	102	=back
	103
2b584ff3 RS	104	Note that the proxy policy value is what determines the rights granted
2b584ff3 RS	105	to the process during the proxy certificate, and it is up to the
2a038236 RL	106	application to interpret and combine these policies.>
	107
	108	With a proxy extension, creating a proxy certificate is a matter of
	109	two commands:
	110
	111	openssl req -new -config proxy.cnf \
	112	-out proxy.req -keyout proxy.key \
2b584ff3	113	-subj "/DC=org/DC=openssl/DC=users/CN=proxy"
2a038236 RL	114
	115	openssl x509 -req -CAcreateserial -in proxy.req -out proxy.crt \
	116	-CA user.crt -CAkey user.key -days 7 \
4e6e57cf	117	-extfile proxy.cnf -extensions proxy
2a038236 RL	118
2a038236 RL	119	You can also create a proxy certificate using another proxy
2b584ff3 RS	120	certificate as issuer. Note that this example uses a different
2b584ff3 RS	121	configuration section for the proxy extensions:
2a038236 RL	122
	123	openssl req -new -config proxy.cnf \
	124	-out proxy2.req -keyout proxy2.key \
2b584ff3	125	-subj "/DC=org/DC=openssl/DC=users/CN=proxy/CN=proxy 2"
2a038236 RL	126
	127	openssl x509 -req -CAcreateserial -in proxy2.req -out proxy2.crt \
	128	-CA proxy.crt -CAkey proxy.key -days 7 \
4e6e57cf	129	-extfile proxy.cnf -extensions proxy_2
2a038236 RL	130
	131	=head2 Using proxy certs in applications
	132
	133	To interpret proxy policies, the application would normally start with
	134	some default rights (perhaps none at all), then compute the resulting
	135	rights by checking the rights against the chain of proxy certificates,
	136	user certificate and CA certificates.
	137
	138	The complicated part is figuring out how to pass data between your
	139	application and the certificate validation procedure.
	140
	141	The following ingredients are needed for such processing:
	142
	143	=over 4
	144
	145	=item *
	146
	147	a callback function that will be called for every certificate being
	148	validated. The callback is called several times for each certificate,
	149	so you must be careful to do the proxy policy interpretation at the
	150	right time. You also need to fill in the defaults when the EE
	151	certificate is checked.
	152
	153	=item *
	154
	155	a data structure that is shared between your application code and the
	156	callback.
	157
	158	=item *
	159
	160	a wrapper function that sets it all up.
	161
	162	=item *
	163
	164	an ex_data index function that creates an index into the generic
	165	ex_data store that is attached to an X509 validation context.
	166
	167	=back
	168
	169	The following skeleton code can be used as a starting point:
	170
	171	#include <string.h>
	172	#include <netdb.h>
	173	#include <openssl/x509.h>
	174	#include <openssl/x509v3.h>
	175
	176	#define total_rights 25
	177
	178	/*
	179	* In this example, I will use a view of granted rights as a bit
	180	* array, one bit for each possible right.
	181	*/
	182	typedef struct your_rights {
	183	unsigned char rights[(total_rights + 7) / 8];
	184	} YOUR_RIGHTS;
	185
	186	/*
	187	* The following procedure will create an index for the ex_data
	188	* store in the X509 validation context the first time it's
	189	* called. Subsequent calls will return the same index.
	190	*/
	191	static int get_proxy_auth_ex_data_idx(X509_STORE_CTX *ctx)
	192	{
	193	static volatile int idx = -1;
194
195	if (idx < 0) {
196	X509_STORE_lock(X509_STORE_CTX_get0_store(ctx));
197	if (idx < 0) {
198	idx = X509_STORE_CTX_get_ex_new_index(0,
199	"for verify callback",
200	NULL,NULL,NULL);
201	}
202	X509_STORE_unlock(X509_STORE_CTX_get0_store(ctx));
203	}
204	return idx;
205	}
206
207	/* Callback to be given to the X509 validation procedure. */
208	static int verify_callback(int ok, X509_STORE_CTX *ctx)
209	{
210	if (ok == 1) {
211	/*
212	* It's REALLY important you keep the proxy policy check
213	* within this section. It's important to know that when
214	* ok is 1, the certificates are checked from top to
215	* bottom. You get the CA root first, followed by the
216	* possible chain of intermediate CAs, followed by the EE
217	* certificate, followed by the possible proxy
57cd10dd	218	* certificates.
2a038236 RL	219	*/
	220	X509 *xs = X509_STORE_CTX_get_current_cert(ctx);
	221
	222	if (X509_get_extension_flags(xs) & EXFLAG_PROXY) {
	223	YOUR_RIGHTS *rights =
	224	(YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
	225	get_proxy_auth_ex_data_idx(ctx));
	226	PROXY_CERT_INFO_EXTENSION *pci =
	227	X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL);
	228
	229	switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage)) {
	230	case NID_Independent:
	231	/*
	232	* Do whatever you need to grant explicit rights
	233	* to this particular proxy certificate, usually
	234	* by pulling them from some database. If there
	235	* are none to be found, clear all rights (making
	236	* this and any subsequent proxy certificate void
57cd10dd	237	* of any rights).
2a038236 RL	238	*/
	239	memset(rights->rights, 0, sizeof(rights->rights));
	240	break;
	241	case NID_id_ppl_inheritAll:
	242	/*
	243	* This is basically a NOP, we simply let the
	244	* current rights stand as they are.
	245	*/
	246	break;
	247	default:
	248	/*
	249	* This is usually the most complex section of
	250	* code. You really do whatever you want as long
	251	* as you follow RFC 3820. In the example we use
	252	* here, the simplest thing to do is to build
	253	* another, temporary bit array and fill it with
	254	* the rights granted by the current proxy
	255	* certificate, then use it as a mask on the
	256	* accumulated rights bit array, and voilà, you
	257	* now have a new accumulated rights bit array.
	258	*/
	259	{
	260	int i;
	261	YOUR_RIGHTS tmp_rights;
	262	memset(tmp_rights.rights, 0,
	263	sizeof(tmp_rights.rights));
	264
	265	/*
	266	* process_rights() is supposed to be a
	267	* procedure that takes a string and its
	268	* length, interprets it and sets the bits
	269	* in the YOUR_RIGHTS pointed at by the
	270	* third argument.
	271	*/
	272	process_rights((char *) pci->proxyPolicy->policy->data,
	273	pci->proxyPolicy->policy->length,
	274	&tmp_rights);
	275
	276	for(i = 0; i < total_rights / 8; i++)
	277	rights->rights[i] &= tmp_rights.rights[i];
	278	}
	279	break;
	280	}
	281	PROXY_CERT_INFO_EXTENSION_free(pci);
	282	} else if (!(X509_get_extension_flags(xs) & EXFLAG_CA)) {
	283	/* We have an EE certificate, let's use it to set default! */
	284	YOUR_RIGHTS *rights =
	285	(YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
	286	get_proxy_auth_ex_data_idx(ctx));
	287
	288	/*
	289	* The following procedure finds out what rights the
	290	* owner of the current certificate has, and sets them
	291	* in the YOUR_RIGHTS structure pointed at by the
	292	* second argument.
	293	*/
	294	set_default_rights(xs, rights);
	295	}
	296	}
	297	return ok;
	298	}
	299
	300	static int my_X509_verify_cert(X509_STORE_CTX *ctx,
	301	YOUR_RIGHTS *needed_rights)
302	{
303	int ok;
304	int (save_verify_cb)(int ok,X509_STORE_CTX ctx) =
305	X509_STORE_CTX_get_verify_cb(ctx);
306	YOUR_RIGHTS rights;
307
308	X509_STORE_CTX_set_verify_cb(ctx, verify_callback);
309	X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(ctx),
310	&rights);
311	X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
312	ok = X509_verify_cert(ctx);
313
314	if (ok == 1) {
315	ok = check_needed_rights(rights, needed_rights);
316	}
317
318	X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb);
319
320	return ok;
321	}
322
323	If you use SSL or TLS, you can easily set up a callback to have the
324	certificates checked properly, using the code above:
325
326	SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert,
327	&needed_rights);
328
329	=head1 NOTES
330
331	To this date, it seems that proxy certificates have only been used in
332	environments that are aware of them, and no one seems to have
333	investigated how they can be used or misused outside of such an
334	environment.
335
336	For that reason, OpenSSL requires that applications aware of proxy
337	certificates must also make that explicit.
338
339	B<subjectAltName> and B<issuerAltName> are forbidden in proxy
340	certificates, and this is enforced in OpenSSL. The subject must be
341	the same as the issuer, with one commonName added on.
342
343	=head1 SEE ALSO
344
345	L<X509_STORE_CTX_set_flags(3)>,
346	L<X509_STORE_CTX_set_verify_cb(3)>,
347	L<X509_VERIFY_PARAM_set_flags(3)>,
348	L<SSL_CTX_set_cert_verify_callback(3)>,
349	L<openssl-req(1)>, L<openssl-x509(1)>,
350	L<RFC 3820\|https://tools.ietf.org/html/rfc3820>
351
352	=head1 COPYRIGHT
353
00c405b3	354	Copyright 2019-2020 The OpenSSL Project Authors. All Rights Reserved.
2a038236 RL	355
	356	Licensed under the Apache License 2.0 (the "License"). You may not use
	357	this file except in compliance with the License. You can obtain a copy
	358	in the file LICENSE in the source distribution or at
	359	L<https://www.openssl.org/source/license.html>.
	360
	361	=cut