proxy-certificates - Proxy certificates in OpenSSL

       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:

       o   They are issued by an End Entity, either a normal EE certificate,
           or another proxy certificate.

       o   They must not have the subjectAltName or issuerAltName extensions.

       o   They must have the proxyCertInfo extension.

       o   They must have the subject of their issuer, with one commonName

   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);


           X509_VERIFY_PARAM_set_flags(param, X509_V_FLAG_ALLOW_PROXY_CERTS);

       See "NOTES" for a discussion on this requirement.

   Creating proxy certificates
       Creating proxy certificates can be done using the openssl-x509(1)
       command, with some extra extensions:

           [ v3_proxy ]
           # A proxy certificate MUST NEVER be a CA certificate.

           # Usual authority key ID

           # The extension which marks this certificate as a proxy

       It's also possible to specify the proxy extension in a separate


           [ proxy_ext ]

       The policy value has a specific syntax, syntag:string, where the syntag
       determines what will be done with the string.  The following syntags
       are recognised:

           indicates that the string is a byte sequence, without any encoding:


       hex indicates the string is encoded hexadecimal encoded binary data,
           with colons between each byte (every second hex digit):


           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 large (more than a few lines, e.g. XML documents).

       NOTE: The proxy policy value is what determines the rights granted to
       the process during the proxy certificate.  It's up to the application
       to interpret and combine these policies.

       With a proxy extension, creating a proxy certificate is a matter of two

           openssl req -new -config proxy.cnf \
               -out proxy.req -keyout proxy.key \
               -subj "/DC=org/DC=openssl/DC=users/CN=proxy 1"

           openssl x509 -req -CAcreateserial -in proxy.req -out proxy.crt \
               -CA user.crt -CAkey user.key -days 7 \
               -extfile proxy.cnf -extensions v3_proxy1

       You can also create a proxy certificate using another proxy certificate
       as issuer (note: using a different configuration section for the proxy

           openssl req -new -config proxy.cnf \
               -out proxy2.req -keyout proxy2.key \
               -subj "/DC=org/DC=openssl/DC=users/CN=proxy 1/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 v3_proxy2

   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:

       o   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.

       o   a data structure that is shared between your application code and
           the callback.

       o   a wrapper function that sets it all up.

       o   an ex_data index function that creates an index into the generic
           ex_data store that is attached to an X509 validation context.

       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) {
                   if (idx < 0) {
                       idx = X509_STORE_CTX_get_ex_new_index(0,
                                                             "for verify callback",
               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,
                       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));
                       case NID_id_ppl_inheritAll:
                            * This is basically a NOP, we simply let the
                            * current rights stand as they are.
                            * 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 voila, you
                            * now have a new accumulated rights bit array.
                               int i;
                               YOUR_RIGHTS tmp_rights;
                               memset(tmp_rights.rights, 0,

                                * 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,

                               for(i = 0; i < total_rights / 8; i++)
                                   rights->rights[i] &= tmp_rights.rights[i];
                   } 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,

                        * 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) =
               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),
               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,

       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

       For that reason, OpenSSL requires that applications aware of proxy
       certificates must also make that explicit.

       subjectAltName and 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.

       X509_STORE_CTX_set_flags(3), X509_STORE_CTX_set_verify_cb(3),
       X509_VERIFY_PARAM_set_flags(3), SSL_CTX_set_cert_verify_callback(3),
       openssl-req(1), openssl-x509(1), RFC 3820

       Copyright 2019 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

1.1.1f                            2023-10-10          PROXY-CERTIFICATES(7SSL)
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