1PROXY-CERTIFICATES(7) OpenSSL PROXY-CERTIFICATES(7)
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 [ v3_proxy ]
44 # A proxy certificate MUST NEVER be a CA certificate.
45 basicConstraints=CA:FALSE
46 # Usual authority key ID
48 authorityKeyIdentifier=keyid,issuer:always
49 # The extension which marks this certificate as a proxy
51 proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
52 It's also possible to specify the proxy extension in a separate
54 section:
55 proxyCertInfo=critical,@proxy_ext
57 [ proxy_ext ]
59 language=id-ppl-anyLanguage
60 pathlen=0
61 policy=text:BC
62 The policy value has a specific syntax, syntag:string, where the syntag
64 determines what will be done with the string. The following syntags
65 are recognised:
66 text
68 indicates that the string is a byte sequence, without any encoding:
69 policy=text:raeksmoergaas
71 hex indicates the string is encoded hexadecimal encoded binary data,
73 with colons between each byte (every second hex digit):
74 policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
76 file
78 indicates that the text of the policy should be taken from a file.
79 The string is then a filename. This is useful for policies that
80 are large (more than a few lines, e.g. XML documents).
81 NOTE: The proxy policy value is what determines the rights granted to
83 the process during the proxy certificate. It's up to the application
84 to interpret and combine these policies.
85 With a proxy extension, creating a proxy certificate is a matter of two
87 commands:
88 openssl req -new -config proxy.cnf \
90 -out proxy.req -keyout proxy.key \
91 -subj "/DC=org/DC=openssl/DC=users/CN=proxy 1"
92 openssl x509 -req -CAcreateserial -in proxy.req -out proxy.crt \
94 -CA user.crt -CAkey user.key -days 7 \
95 -extfile proxy.cnf -extensions v3_proxy1
96 You can also create a proxy certificate using another proxy certificate
98 as issuer (note: using a different configuration section for the proxy
99 extensions):
100 openssl req -new -config proxy.cnf \
102 -out proxy2.req -keyout proxy2.key \
103 -subj "/DC=org/DC=openssl/DC=users/CN=proxy 1/CN=proxy 2"
104 openssl x509 -req -CAcreateserial -in proxy2.req -out proxy2.crt \
106 -CA proxy.crt -CAkey proxy.key -days 7 \
107 -extfile proxy.cnf -extensions v3_proxy2
108 Using proxy certs in applications
110 To interpret proxy policies, the application would normally start with
111 some default rights (perhaps none at all), then compute the resulting
112 rights by checking the rights against the chain of proxy certificates,
113 user certificate and CA certificates.
114 The complicated part is figuring out how to pass data between your
116 application and the certificate validation procedure.
117 The following ingredients are needed for such processing:
119 • a callback function that will be called for every certificate being
121 validated. The callback is called several times for each
122 certificate, so you must be careful to do the proxy policy
123 interpretation at the right time. You also need to fill in the
124 defaults when the EE certificate is checked.
125 • a data structure that is shared between your application code and
127 the callback.
128 • a wrapper function that sets it all up.
130 • an ex_data index function that creates an index into the generic
132 ex_data store that is attached to an X509 validation context.
133 The following skeleton code can be used as a starting point:
135 #include <string.h>
137 #include <netdb.h>
138 #include <openssl/x509.h>
139 #include <openssl/x509v3.h>
140 #define total_rights 25
142 /*
144 * In this example, I will use a view of granted rights as a bit
145 * array, one bit for each possible right.
146 */
147 typedef struct your_rights {
148 unsigned char rights[(total_rights + 7) / 8];
149 } YOUR_RIGHTS;
150 /*
152 * The following procedure will create an index for the ex_data
153 * store in the X509 validation context the first time it's
154 * called. Subsequent calls will return the same index.
155 */
156 static int get_proxy_auth_ex_data_idx(X509_STORE_CTX *ctx)
157 {
158 static volatile int idx = -1;
159 if (idx < 0) {
161 X509_STORE_lock(X509_STORE_CTX_get0_store(ctx));
162 if (idx < 0) {
163 idx = X509_STORE_CTX_get_ex_new_index(0,
164 "for verify callback",
165 NULL,NULL,NULL);
166 }
167 X509_STORE_unlock(X509_STORE_CTX_get0_store(ctx));
168 }
169 return idx;
170 }
171 /* Callback to be given to the X509 validation procedure. */
173 static int verify_callback(int ok, X509_STORE_CTX *ctx)
174 {
175 if (ok == 1) {
176 /*
177 * It's REALLY important you keep the proxy policy check
178 * within this section. It's important to know that when
179 * ok is 1, the certificates are checked from top to
180 * bottom. You get the CA root first, followed by the
181 * possible chain of intermediate CAs, followed by the EE
182 * certificate, followed by the possible proxy
183 * certificates.
184 */
185 X509 *xs = X509_STORE_CTX_get_current_cert(ctx);
186 if (X509_get_extension_flags(xs) & EXFLAG_PROXY) {
188 YOUR_RIGHTS *rights =
189 (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
190 get_proxy_auth_ex_data_idx(ctx));
191 PROXY_CERT_INFO_EXTENSION *pci =
192 X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL);
193 switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage)) {
195 case NID_Independent:
196 /*
197 * Do whatever you need to grant explicit rights
198 * to this particular proxy certificate, usually
199 * by pulling them from some database. If there
200 * are none to be found, clear all rights (making
201 * this and any subsequent proxy certificate void
202 * of any rights).
203 */
204 memset(rights->rights, 0, sizeof(rights->rights));
205 break;
206 case NID_id_ppl_inheritAll:
207 /*
208 * This is basically a NOP, we simply let the
209 * current rights stand as they are.
210 */
211 break;
212 default:
213 /*
214 * This is usually the most complex section of
215 * code. You really do whatever you want as long
216 * as you follow RFC 3820. In the example we use
217 * here, the simplest thing to do is to build
218 * another, temporary bit array and fill it with
219 * the rights granted by the current proxy
220 * certificate, then use it as a mask on the
221 * accumulated rights bit array, and voila, you
222 * now have a new accumulated rights bit array.
223 */
224 {
225 int i;
226 YOUR_RIGHTS tmp_rights;
227 memset(tmp_rights.rights, 0,
228 sizeof(tmp_rights.rights));
229 /*
231 * process_rights() is supposed to be a
232 * procedure that takes a string and its
233 * length, interprets it and sets the bits
234 * in the YOUR_RIGHTS pointed at by the
235 * third argument.
236 */
237 process_rights((char *) pci->proxyPolicy->policy->data,
238 pci->proxyPolicy->policy->length,
239 &tmp_rights);
240 for(i = 0; i < total_rights / 8; i++)
242 rights->rights[i] &= tmp_rights.rights[i];
243 }
244 break;
245 }
246 PROXY_CERT_INFO_EXTENSION_free(pci);
247 } else if (!(X509_get_extension_flags(xs) & EXFLAG_CA)) {
248 /* We have an EE certificate, let's use it to set default! */
249 YOUR_RIGHTS *rights =
250 (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
251 get_proxy_auth_ex_data_idx(ctx));
252 /*
254 * The following procedure finds out what rights the
255 * owner of the current certificate has, and sets them
256 * in the YOUR_RIGHTS structure pointed at by the
257 * second argument.
258 */
259 set_default_rights(xs, rights);
260 }
261 }
262 return ok;
263 }
264 static int my_X509_verify_cert(X509_STORE_CTX *ctx,
266 YOUR_RIGHTS *needed_rights)
267 {
268 int ok;
269 int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) =
270 X509_STORE_CTX_get_verify_cb(ctx);
271 YOUR_RIGHTS rights;
272 X509_STORE_CTX_set_verify_cb(ctx, verify_callback);
274 X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(ctx),
275 &rights);
276 X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
277 ok = X509_verify_cert(ctx);
278 if (ok == 1) {
280 ok = check_needed_rights(rights, needed_rights);
281 }
282 X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb);
284 return ok;
286 }
287 If you use SSL or TLS, you can easily set up a callback to have the
289 certificates checked properly, using the code above:
290 SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert,
292 &needed_rights);
293
295 To this date, it seems that proxy certificates have only been used in
296 environments that are aware of them, and no one seems to have
297 investigated how they can be used or misused outside of such an
298 environment.
299 For that reason, OpenSSL requires that applications aware of proxy
301 certificates must also make that explicit.
302 subjectAltName and issuerAltName are forbidden in proxy certificates,
304 and this is enforced in OpenSSL. The subject must be the same as the
305 issuer, with one commonName added on.
306
308 X509_STORE_CTX_set_flags(3), X509_STORE_CTX_set_verify_cb(3),
309 X509_VERIFY_PARAM_set_flags(3), SSL_CTX_set_cert_verify_callback(3),
310 openssl-req(1), openssl-x509(1), RFC 3820
311 <https://tools.ietf.org/html/rfc3820>
312
314 Copyright 2019 The OpenSSL Project Authors. All Rights Reserved.
315 Licensed under the Apache License 2.0 (the "License"). You may not use
317 this file except in compliance with the License. You can obtain a copy
318 in the file LICENSE in the source distribution or at
319 <https://www.openssl.org/source/license.html>.
3201.1.1l 2021-09-15 PROXY-CERTIFICATES(7)