1=pod 2 3=encoding UTF-8 4 5=head1 NAME 6 7proxy-certificates - Proxy certificates in OpenSSL 8 9=head1 DESCRIPTION 10 11Proxy certificates are defined in RFC 3820. They are used to 12extend rights to some other entity (a computer process, typically, or 13sometimes to the user itself). This allows the entity to perform 14operations on behalf of the owner of the EE (End Entity) certificate. 15 16The requirements for a valid proxy certificate are: 17 18=over 4 19 20=item * 21 22They are issued by an End Entity, either a normal EE certificate, or 23another proxy certificate. 24 25=item * 26 27They must not have the B<subjectAltName> or B<issuerAltName> 28extensions. 29 30=item * 31 32They must have the B<proxyCertInfo> extension. 33 34=item * 35 36They must have the subject of their issuer, with one B<commonName> 37added. 38 39=back 40 41=head2 Enabling proxy certificate verification 42 43OpenSSL expects applications that want to use proxy certificates to be 44specially aware of them, and make that explicit. This is done by 45setting an X509 verification flag: 46 47 X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS); 48 49or 50 51 X509_VERIFY_PARAM_set_flags(param, X509_V_FLAG_ALLOW_PROXY_CERTS); 52 53See L</NOTES> for a discussion on this requirement. 54 55=head2 Creating proxy certificates 56 57Creating proxy certificates can be done using the L<openssl-x509(1)> 58command, with some extra extensions: 59 60 [ proxy ] 61 # A proxy certificate MUST NEVER be a CA certificate. 62 basicConstraints = CA:FALSE 63 # Usual authority key ID 64 authorityKeyIdentifier = keyid,issuer:always 65 # The extension which marks this certificate as a proxy 66 proxyCertInfo = critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB 67 68It's also possible to specify the proxy extension in a separate section: 69 70 proxyCertInfo = critical,@proxy_ext 71 72 [ proxy_ext ] 73 language = id-ppl-anyLanguage 74 pathlen = 0 75 policy = text:BC 76 77The policy value has a specific syntax, I<syntag>:I<string>, where the 78I<syntag> determines what will be done with the string. The following 79I<syntag>s are recognised: 80 81=over 4 82 83=item B<text> 84 85indicates that the string is a byte sequence, without any encoding: 86 87 policy=text:räksmörgås 88 89=item B<hex> 90 91indicates the string is encoded hexadecimal encoded binary data, with 92colons 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 98indicates that the text of the policy should be taken from a file. 99The string is then a filename. This is useful for policies that are 100more than a few lines, such as XML or other markup. 101 102=back 103 104Note that the proxy policy value is what determines the rights granted 105to the process during the proxy certificate, and it is up to the 106application to interpret and combine these policies.> 107 108With a proxy extension, creating a proxy certificate is a matter of 109two commands: 110 111 openssl req -new -config proxy.cnf \ 112 -out proxy.req -keyout proxy.key \ 113 -subj "/DC=org/DC=openssl/DC=users/CN=proxy" 114 115 openssl x509 -req -CAcreateserial -in proxy.req -out proxy.crt \ 116 -CA user.crt -CAkey user.key -days 7 \ 117 -extfile proxy.cnf -extensions proxy 118 119You can also create a proxy certificate using another proxy 120certificate as issuer. Note that this example uses a different 121configuration section for the proxy extensions: 122 123 openssl req -new -config proxy.cnf \ 124 -out proxy2.req -keyout proxy2.key \ 125 -subj "/DC=org/DC=openssl/DC=users/CN=proxy/CN=proxy 2" 126 127 openssl x509 -req -CAcreateserial -in proxy2.req -out proxy2.crt \ 128 -CA proxy.crt -CAkey proxy.key -days 7 \ 129 -extfile proxy.cnf -extensions proxy_2 130 131=head2 Using proxy certs in applications 132 133To interpret proxy policies, the application would normally start with 134some default rights (perhaps none at all), then compute the resulting 135rights by checking the rights against the chain of proxy certificates, 136user certificate and CA certificates. 137 138The complicated part is figuring out how to pass data between your 139application and the certificate validation procedure. 140 141The following ingredients are needed for such processing: 142 143=over 4 144 145=item * 146 147a callback function that will be called for every certificate being 148validated. The callback is called several times for each certificate, 149so you must be careful to do the proxy policy interpretation at the 150right time. You also need to fill in the defaults when the EE 151certificate is checked. 152 153=item * 154 155a data structure that is shared between your application code and the 156callback. 157 158=item * 159 160a wrapper function that sets it all up. 161 162=item * 163 164an ex_data index function that creates an index into the generic 165ex_data store that is attached to an X509 validation context. 166 167=back 168 169The 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 218 * certificates. 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 237 * of any rights). 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 323If you use SSL or TLS, you can easily set up a callback to have the 324certificates 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 331To this date, it seems that proxy certificates have only been used in 332environments that are aware of them, and no one seems to have 333investigated how they can be used or misused outside of such an 334environment. 335 336For that reason, OpenSSL requires that applications aware of proxy 337certificates must also make that explicit. 338 339B<subjectAltName> and B<issuerAltName> are forbidden in proxy 340certificates, and this is enforced in OpenSSL. The subject must be 341the same as the issuer, with one commonName added on. 342 343=head1 SEE ALSO 344 345L<X509_STORE_CTX_set_flags(3)>, 346L<X509_STORE_CTX_set_verify_cb(3)>, 347L<X509_VERIFY_PARAM_set_flags(3)>, 348L<SSL_CTX_set_cert_verify_callback(3)>, 349L<openssl-req(1)>, L<openssl-x509(1)>, 350L<RFC 3820|https://tools.ietf.org/html/rfc3820> 351 352=head1 COPYRIGHT 353 354Copyright 2019-2020 The OpenSSL Project Authors. All Rights Reserved. 355 356Licensed under the Apache License 2.0 (the "License"). You may not use 357this file except in compliance with the License. You can obtain a copy 358in the file LICENSE in the source distribution or at 359L<https://www.openssl.org/source/license.html>. 360 361=cut 362
