1PROXY-CERTIFICATES(7ossl) OpenSSL PROXY-CERTIFICATES(7ossl)
2
6 proxy-certificates - Proxy certificates in OpenSSL
7
9 Proxy certificates are defined in RFC 3820. They are used to extend
10 rights to some other entity (a computer process, typically, or
11 sometimes to the user itself). This allows the entity to perform
12 operations on behalf of the owner of the EE (End Entity) certificate.
13 The requirements for a valid proxy certificate are:
15 • They are issued by an End Entity, either a normal EE certificate,
17 or another proxy certificate.
18 • They must not have the subjectAltName or issuerAltName extensions.
20 • They must have the proxyCertInfo extension.
22 • They must have the subject of their issuer, with one commonName
24 added.
25 Enabling proxy certificate verification
27 OpenSSL expects applications that want to use proxy certificates to be
28 specially aware of them, and make that explicit. This is done by
29 setting an X509 verification flag:
30 X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
32 or
34 X509_VERIFY_PARAM_set_flags(param, X509_V_FLAG_ALLOW_PROXY_CERTS);
36 See "NOTES" for a discussion on this requirement.
38 Creating proxy certificates
40 Creating proxy certificates can be done using the openssl-x509(1)
41 command, with some extra extensions:
42 [ proxy ]
44 # A proxy certificate MUST NEVER be a CA certificate.
45 basicConstraints = CA:FALSE
46 # Usual authority key ID
47 authorityKeyIdentifier = keyid,issuer:always
48 # The extension which marks this certificate as a proxy
49 proxyCertInfo = critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
50 It's also possible to specify the proxy extension in a separate
52 section:
53 proxyCertInfo = critical,@proxy_ext
55 [ proxy_ext ]
57 language = id-ppl-anyLanguage
58 pathlen = 0
59 policy = text:BC
60 The policy value has a specific syntax, syntag:string, where the syntag
62 determines what will be done with the string. The following syntags
63 are recognised:
64 text
66 indicates that the string is a byte sequence, without any encoding:
67 policy=text:räksmörgås
69 hex indicates the string is encoded hexadecimal encoded binary data,
71 with colons between each byte (every second hex digit):
72 policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
74 file
76 indicates that the text of the policy should be taken from a file.
77 The string is then a filename. This is useful for policies that
78 are more than a few lines, such as XML or other markup.
79 Note that the proxy policy value is what determines the rights granted
81 to the process during the proxy certificate, and it is up to the
82 application to interpret and combine these policies.>
83 With a proxy extension, creating a proxy certificate is a matter of two
85 commands:
86 openssl req -new -config proxy.cnf \
88 -out proxy.req -keyout proxy.key \
89 -subj "/DC=org/DC=openssl/DC=users/CN=proxy"
90 openssl x509 -req -CAcreateserial -in proxy.req -out proxy.crt \
92 -CA user.crt -CAkey user.key -days 7 \
93 -extfile proxy.cnf -extensions proxy
94 You can also create a proxy certificate using another proxy certificate
96 as issuer. Note that this example uses a different configuration
97 section for the proxy extensions:
98 openssl req -new -config proxy.cnf \
100 -out proxy2.req -keyout proxy2.key \
101 -subj "/DC=org/DC=openssl/DC=users/CN=proxy/CN=proxy 2"
102 openssl x509 -req -CAcreateserial -in proxy2.req -out proxy2.crt \
104 -CA proxy.crt -CAkey proxy.key -days 7 \
105 -extfile proxy.cnf -extensions proxy_2
106 Using proxy certs in applications
108 To interpret proxy policies, the application would normally start with
109 some default rights (perhaps none at all), then compute the resulting
110 rights by checking the rights against the chain of proxy certificates,
111 user certificate and CA certificates.
112 The complicated part is figuring out how to pass data between your
114 application and the certificate validation procedure.
115 The following ingredients are needed for such processing:
117 • a callback function that will be called for every certificate being
119 validated. The callback is called several times for each
120 certificate, so you must be careful to do the proxy policy
121 interpretation at the right time. You also need to fill in the
122 defaults when the EE certificate is checked.
123 • a data structure that is shared between your application code and
125 the callback.
126 • a wrapper function that sets it all up.
128 • an ex_data index function that creates an index into the generic
130 ex_data store that is attached to an X509 validation context.
131 The following skeleton code can be used as a starting point:
133 #include <string.h>
135 #include <netdb.h>
136 #include <openssl/x509.h>
137 #include <openssl/x509v3.h>
138 #define total_rights 25
140 /*
142 * In this example, I will use a view of granted rights as a bit
143 * array, one bit for each possible right.
144 */
145 typedef struct your_rights {
146 unsigned char rights[(total_rights + 7) / 8];
147 } YOUR_RIGHTS;
148 /*
150 * The following procedure will create an index for the ex_data
151 * store in the X509 validation context the first time it's
152 * called. Subsequent calls will return the same index.
153 */
154 static int get_proxy_auth_ex_data_idx(X509_STORE_CTX *ctx)
155 {
156 static volatile int idx = -1;
157 if (idx < 0) {
159 X509_STORE_lock(X509_STORE_CTX_get0_store(ctx));
160 if (idx < 0) {
161 idx = X509_STORE_CTX_get_ex_new_index(0,
162 "for verify callback",
163 NULL,NULL,NULL);
164 }
165 X509_STORE_unlock(X509_STORE_CTX_get0_store(ctx));
166 }
167 return idx;
168 }
169 /* Callback to be given to the X509 validation procedure. */
171 static int verify_callback(int ok, X509_STORE_CTX *ctx)
172 {
173 if (ok == 1) {
174 /*
175 * It's REALLY important you keep the proxy policy check
176 * within this section. It's important to know that when
177 * ok is 1, the certificates are checked from top to
178 * bottom. You get the CA root first, followed by the
179 * possible chain of intermediate CAs, followed by the EE
180 * certificate, followed by the possible proxy
181 * certificates.
182 */
183 X509 *xs = X509_STORE_CTX_get_current_cert(ctx);
184 if (X509_get_extension_flags(xs) & EXFLAG_PROXY) {
186 YOUR_RIGHTS *rights =
187 (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
188 get_proxy_auth_ex_data_idx(ctx));
189 PROXY_CERT_INFO_EXTENSION *pci =
190 X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL);
191 switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage)) {
193 case NID_Independent:
194 /*
195 * Do whatever you need to grant explicit rights
196 * to this particular proxy certificate, usually
197 * by pulling them from some database. If there
198 * are none to be found, clear all rights (making
199 * this and any subsequent proxy certificate void
200 * of any rights).
201 */
202 memset(rights->rights, 0, sizeof(rights->rights));
203 break;
204 case NID_id_ppl_inheritAll:
205 /*
206 * This is basically a NOP, we simply let the
207 * current rights stand as they are.
208 */
209 break;
210 default:
211 /*
212 * This is usually the most complex section of
213 * code. You really do whatever you want as long
214 * as you follow RFC 3820. In the example we use
215 * here, the simplest thing to do is to build
216 * another, temporary bit array and fill it with
217 * the rights granted by the current proxy
218 * certificate, then use it as a mask on the
219 * accumulated rights bit array, and voilà, you
220 * now have a new accumulated rights bit array.
221 */
222 {
223 int i;
224 YOUR_RIGHTS tmp_rights;
225 memset(tmp_rights.rights, 0,
226 sizeof(tmp_rights.rights));
227 /*
229 * process_rights() is supposed to be a
230 * procedure that takes a string and its
231 * length, interprets it and sets the bits
232 * in the YOUR_RIGHTS pointed at by the
233 * third argument.
234 */
235 process_rights((char *) pci->proxyPolicy->policy->data,
236 pci->proxyPolicy->policy->length,
237 &tmp_rights);
238 for(i = 0; i < total_rights / 8; i++)
240 rights->rights[i] &= tmp_rights.rights[i];
241 }
242 break;
243 }
244 PROXY_CERT_INFO_EXTENSION_free(pci);
245 } else if (!(X509_get_extension_flags(xs) & EXFLAG_CA)) {
246 /* We have an EE certificate, let's use it to set default! */
247 YOUR_RIGHTS *rights =
248 (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
249 get_proxy_auth_ex_data_idx(ctx));
250 /*
252 * The following procedure finds out what rights the
253 * owner of the current certificate has, and sets them
254 * in the YOUR_RIGHTS structure pointed at by the
255 * second argument.
256 */
257 set_default_rights(xs, rights);
258 }
259 }
260 return ok;
261 }
262 static int my_X509_verify_cert(X509_STORE_CTX *ctx,
264 YOUR_RIGHTS *needed_rights)
265 {
266 int ok;
267 int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) =
268 X509_STORE_CTX_get_verify_cb(ctx);
269 YOUR_RIGHTS rights;
270 X509_STORE_CTX_set_verify_cb(ctx, verify_callback);
272 X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(ctx),
273 &rights);
274 X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
275 ok = X509_verify_cert(ctx);
276 if (ok == 1) {
278 ok = check_needed_rights(rights, needed_rights);
279 }
280 X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb);
282 return ok;
284 }
285 If you use SSL or TLS, you can easily set up a callback to have the
287 certificates checked properly, using the code above:
288 SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert,
290 &needed_rights);
291
293 To this date, it seems that proxy certificates have only been used in
294 environments that are aware of them, and no one seems to have
295 investigated how they can be used or misused outside of such an
296 environment.
297 For that reason, OpenSSL requires that applications aware of proxy
299 certificates must also make that explicit.
300 subjectAltName and issuerAltName are forbidden in proxy certificates,
302 and this is enforced in OpenSSL. The subject must be the same as the
303 issuer, with one commonName added on.
304
306 X509_STORE_CTX_set_flags(3), X509_STORE_CTX_set_verify_cb(3),
307 X509_VERIFY_PARAM_set_flags(3), SSL_CTX_set_cert_verify_callback(3),
308 openssl-req(1), openssl-x509(1), RFC 3820
309 <https://tools.ietf.org/html/rfc3820>
310
312 Copyright 2019-2021 The OpenSSL Project Authors. All Rights Reserved.
313 Licensed under the Apache License 2.0 (the "License"). You may not use
315 this file except in compliance with the License. You can obtain a copy
316 in the file LICENSE in the source distribution or at
317 <https://www.openssl.org/source/license.html>.
3183.1.1 2023-08-31 PROXY-CERTIFICATES(7ossl)