2 HOWTO proxy certificates
6 NONE OF THE CODE PRESENTED HERE HAS BEEN CHECKED! The code is just examples to
7 show you how things could be done. There might be typos or type conflicts, and
8 you will have to resolve them.
12 Proxy certificates are defined in RFC 3820. They are really usual certificates
13 with the mandatory extension proxyCertInfo.
15 Proxy certificates are issued by an End Entity (typically a user), either
16 directly with the EE certificate as issuing certificate, or by extension through
17 an already issued proxy certificate. Proxy certificates are used to extend
18 rights to some other entity (a computer process, typically, or sometimes to the
19 user itself). This allows the entity to perform operations on behalf of the
20 owner of the EE certificate.
22 See http://www.ietf.org/rfc/rfc3820.txt for more information.
25 2. A warning about proxy certificates
27 No one seems to have tested proxy certificates with security in mind. To this
28 date, it seems that proxy certificates have only been used in a context highly
31 Existing applications might misbehave when trying to validate a chain of
32 certificates which use a proxy certificate. They might incorrectly consider the
33 leaf to be the certificate to check for authorisation data, which is controlled
34 by the EE certificate owner.
36 subjectAltName and issuerAltName are forbidden in proxy certificates, and this
37 is enforced in OpenSSL. The subject must be the same as the issuer, with one
40 Possible threats we can think of at this time include:
42 - impersonation through commonName (think server certificates).
43 - use of additional extensions, possibly non-standard ones used in certain
44 environments, that would grant extra or different authorisation rights.
46 For these reasons, OpenSSL requires that the use of proxy certificates be
47 explicitly allowed. Currently, this can be done using the following methods:
49 - if the application directly calls X509_verify_cert(), it can first call:
51 X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
53 Where ctx is the pointer which then gets passed to X509_verify_cert().
55 - proxy certificate validation can be enabled before starting the application
56 by setting the environment variable OPENSSL_ALLOW_PROXY_CERTS.
58 In the future, it might be possible to enable proxy certificates by editing
62 3. How to create proxy certificates
64 Creating proxy certificates is quite easy, by taking advantage of a lack of
65 checks in the 'openssl x509' application (*ahem*). You must first create a
66 configuration section that contains a definition of the proxyCertInfo extension,
70 # A proxy certificate MUST NEVER be a CA certificate.
71 basicConstraints=CA:FALSE
73 # Usual authority key ID
74 authorityKeyIdentifier=keyid,issuer:always
76 # The extension which marks this certificate as a proxy
77 proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
79 It's also possible to specify the proxy extension in a separate section:
81 proxyCertInfo=critical,@proxy_ext
84 language=id-ppl-anyLanguage
88 The policy value has a specific syntax, {syntag}:{string}, where the syntag
89 determines what will be done with the string. The following syntags are
92 text indicates that the string is simply bytes, without any encoding:
94 policy=text:räksmörgås
96 Previous versions of this design had a specific tag for UTF-8 text.
97 However, since the bytes are copied as-is anyway, there is no need for
100 hex indicates the string is encoded in hex, with colons between each byte
101 (every second hex digit):
103 policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
105 Previous versions of this design had a tag to insert a complete DER
106 blob. However, the only legal use for this would be to surround the
107 bytes that would go with the hex: tag with whatever is needed to
108 construct a correct OCTET STRING. The DER tag therefore felt
109 superfluous, and was removed.
111 file indicates that the text of the policy should really be taken from a
112 file. The string is then really a file name. This is useful for
113 policies that are large (more than a few lines, e.g. XML documents).
115 The 'policy' setting can be split up in multiple lines like this:
119 2.policy=line policy.
121 NOTE: the proxy policy value is the part which determines the rights granted to
122 the process using the proxy certificate. The value is completely dependent on
123 the application reading and interpreting it!
125 Now that you have created an extension section for your proxy certificate, you
126 can easily create a proxy certificate by doing:
128 openssl req -new -config openssl.cnf -out proxy.req -keyout proxy.key
129 openssl x509 -req -CAcreateserial -in proxy.req -days 7 -out proxy.crt \
130 -CA user.crt -CAkey user.key -extfile openssl.cnf -extensions v3_proxy
132 You can also create a proxy certificate using another proxy certificate as
133 issuer (note: I'm using a different configuration section for it):
135 openssl req -new -config openssl.cnf -out proxy2.req -keyout proxy2.key
136 openssl x509 -req -CAcreateserial -in proxy2.req -days 7 -out proxy2.crt \
137 -CA proxy.crt -CAkey proxy.key -extfile openssl.cnf -extensions v3_proxy2
140 4. How to have your application interpret the policy?
142 The basic way to interpret proxy policies is to start with some default rights,
143 then compute the resulting rights by checking the proxy certificate against
144 the chain of proxy certificates, user certificate and CA certificates. You then
145 use the final computed rights. Sounds easy, huh? It almost is.
147 The slightly complicated part is figuring out how to pass data between your
148 application and the certificate validation procedure.
150 You need the following ingredients:
152 - a callback function that will be called for every certificate being
153 validated. The callback be called several times for each certificate,
154 so you must be careful to do the proxy policy interpretation at the right
155 time. You also need to fill in the defaults when the EE certificate is
158 - a data structure that is shared between your application code and the
161 - a wrapper function that sets it all up.
163 - an ex_data index function that creates an index into the generic ex_data
164 store that is attached to an X509 validation context.
166 Here is some skeleton code you can fill in:
168 /* In this example, I will use a view of granted rights as a bit
169 array, one bit for each possible right. */
170 typedef struct your_rights {
171 unsigned char rights[total_rights / 8];
174 /* The following procedure will create an index for the ex_data
175 store in the X509 validation context the first time it's called.
176 Subsequent calls will return the same index. */
177 static int get_proxy_auth_ex_data_idx(void)
179 static volatile int idx = -1;
182 CRYPTO_w_lock(CRYPTO_LOCK_X509_STORE);
185 idx = X509_STORE_CTX_get_ex_new_index(0,
186 "for verify callback",
189 CRYPTO_w_unlock(CRYPTO_LOCK_X509_STORE);
194 /* Callback to be given to the X509 validation procedure. */
195 static int verify_callback(int ok, X509_STORE_CTX *ctx)
197 if (ok == 1) /* It's REALLY important you keep the proxy policy
198 check within this section. It's important to know
199 that when ok is 1, the certificates are checked
200 from top to bottom. You get the CA root first,
201 followed by the possible chain of intermediate
202 CAs, followed by the EE certificate, followed by
203 the possible proxy certificates. */
205 X509 *xs = ctx->current_cert;
207 if (xs->ex_flags & EXFLAG_PROXY)
209 YOUR_RIGHTS *rights =
210 (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
211 get_proxy_auth_ex_data_idx());
212 PROXY_CERT_INFO_EXTENSION *pci =
213 X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL);
215 switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage))
217 case NID_Independent:
218 /* Do whatever you need to grant explicit rights to
219 this particular proxy certificate, usually by
220 pulling them from some database. If there are none
221 to be found, clear all rights (making this and any
222 subsequent proxy certificate void of any rights).
224 memset(rights->rights, 0, sizeof(rights->rights));
226 case NID_id_ppl_inheritAll:
227 /* This is basically a NOP, we simply let the current
228 rights stand as they are. */
231 /* This is usually the most complex section of code.
232 You really do whatever you want as long as you
233 follow RFC 3820. In the example we use here, the
234 simplest thing to do is to build another, temporary
235 bit array and fill it with the rights granted by
236 the current proxy certificate, then use it as a
237 mask on the accumulated rights bit array, and
238 voilà, you now have a new accumulated rights bit
242 YOUR_RIGHTS tmp_rights;
243 memset(tmp_rights.rights, 0, sizeof(tmp_rights.rights));
245 /* process_rights() is supposed to be a procedure
246 that takes a string and it's length, interprets
247 it and sets the bits in the YOUR_RIGHTS pointed
248 at by the third argument. */
249 process_rights((char *) pci->proxyPolicy->policy->data,
250 pci->proxyPolicy->policy->length,
253 for(i = 0; i < total_rights / 8; i++)
254 rights->rights[i] &= tmp_rights.rights[i];
258 PROXY_CERT_INFO_EXTENSION_free(pci);
260 else if (!(xs->ex_flags & EXFLAG_CA))
262 /* We have a EE certificate, let's use it to set default!
264 YOUR_RIGHTS *rights =
265 (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
266 get_proxy_auth_ex_data_idx());
268 /* The following procedure finds out what rights the owner
269 of the current certificate has, and sets them in the
270 YOUR_RIGHTS structure pointed at by the second
272 set_default_rights(xs, rights);
278 static int my_X509_verify_cert(X509_STORE_CTX *ctx,
279 YOUR_RIGHTS *needed_rights)
282 int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) = ctx->verify_cb;
285 X509_STORE_CTX_set_verify_cb(ctx, verify_callback);
286 X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(), &rights);
287 X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
288 ok = X509_verify_cert(ctx);
292 ok = check_needed_rights(rights, needed_rights);
295 X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb);
300 If you use SSL or TLS, you can easily set up a callback to have the
301 certificates checked properly, using the code above:
303 SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert, &needed_rights);