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    1 			HOWTO proxy certificates
    2 
    3 0. WARNING
    4 
    5 NONE OF THE CODE PRESENTED HERE HAS BEEN CHECKED!  The code is just examples to
    6 show you how things could be done.  There might be typos or type conflicts, and
    7 you will have to resolve them.
    8 
    9 1. Introduction
   10 
   11 Proxy certificates are defined in RFC 3820.  They are really usual certificates
   12 with the mandatory extension proxyCertInfo.
   13 
   14 Proxy certificates are issued by an End Entity (typically a user), either
   15 directly with the EE certificate as issuing certificate, or by extension through
   16 an already issued proxy certificate.  Proxy certificates are used to extend
   17 rights to some other entity (a computer process, typically, or sometimes to the
   18 user itself).  This allows the entity to perform operations on behalf of the
   19 owner of the EE certificate.
   20 
   21 See https://www.ietf.org/rfc/rfc3820.txt for more information.
   22 
   23 
   24 2. A warning about proxy certificates
   25 
   26 No one seems to have tested proxy certificates with security in mind.  To this
   27 date, it seems that proxy certificates have only been used in a context highly
   28 aware of them.
   29 
   30 Existing applications might misbehave when trying to validate a chain of
   31 certificates which use a proxy certificate.  They might incorrectly consider the
   32 leaf to be the certificate to check for authorisation data, which is controlled
   33 by the EE certificate owner.
   34 
   35 subjectAltName and issuerAltName are forbidden in proxy certificates, and this
   36 is enforced in OpenSSL.  The subject must be the same as the issuer, with one
   37 commonName added on.
   38 
   39 Possible threats we can think of at this time include:
   40 
   41  - impersonation through commonName (think server certificates).
   42  - use of additional extensions, possibly non-standard ones used in certain
   43    environments, that would grant extra or different authorisation rights.
   44 
   45 For these reasons, OpenSSL requires that the use of proxy certificates be
   46 explicitly allowed.  Currently, this can be done using the following methods:
   47 
   48  - if the application directly calls X509_verify_cert(), it can first call:
   49 
   50    X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
   51 
   52    Where ctx is the pointer which then gets passed to X509_verify_cert().
   53 
   54  - proxy certificate validation can be enabled before starting the application
   55    by setting the environment variable OPENSSL_ALLOW_PROXY_CERTS.
   56 
   57 In the future, it might be possible to enable proxy certificates by editing
   58 openssl.cnf.
   59 
   60 
   61 3. How to create proxy certificates
   62 
   63 Creating proxy certificates is quite easy, by taking advantage of a lack of
   64 checks in the 'openssl x509' application (*ahem*).  You must first create a
   65 configuration section that contains a definition of the proxyCertInfo extension,
   66 for example:
   67 
   68   [ v3_proxy ]
   69   # A proxy certificate MUST NEVER be a CA certificate.
   70   basicConstraints=CA:FALSE
   71 
   72   # Usual authority key ID
   73   authorityKeyIdentifier=keyid,issuer:always
   74 
   75   # The extension which marks this certificate as a proxy
   76   proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
   77 
   78 It's also possible to specify the proxy extension in a separate section:
   79 
   80   proxyCertInfo=critical,@proxy_ext
   81 
   82   [ proxy_ext ]
   83   language=id-ppl-anyLanguage
   84   pathlen=0
   85   policy=text:BC
   86 
   87 The policy value has a specific syntax, {syntag}:{string}, where the syntag
   88 determines what will be done with the string.  The following syntags are
   89 recognised:
   90 
   91   text  indicates that the string is simply bytes, without any encoding:
   92 
   93           policy=text:räksmörgås
   94 
   95         Previous versions of this design had a specific tag for UTF-8 text.
   96         However, since the bytes are copied as-is anyway, there is no need for
   97         such a specific tag.
   98 
   99   hex   indicates the string is encoded in hex, with colons between each byte
  100         (every second hex digit):
  101 
  102           policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
  103 
  104         Previous versions of this design had a tag to insert a complete DER
  105         blob.  However, the only legal use for this would be to surround the
  106         bytes that would go with the hex: tag with whatever is needed to
  107         construct a correct OCTET STRING.  The DER tag therefore felt
  108         superfluous, and was removed.
  109 
  110   file  indicates that the text of the policy should really be taken from a
  111         file.  The string is then really a file name.  This is useful for
  112         policies that are large (more than a few lines, e.g. XML documents).
  113 
  114 The 'policy' setting can be split up in multiple lines like this:
  115 
  116   0.policy=This is
  117   1.policy= a multi-
  118   2.policy=line policy.
  119 
  120 NOTE: the proxy policy value is the part which determines the rights granted to
  121 the process using the proxy certificate.  The value is completely dependent on
  122 the application reading and interpreting it!
  123 
  124 Now that you have created an extension section for your proxy certificate, you
  125 can easily create a proxy certificate by doing:
  126 
  127   openssl req -new -config openssl.cnf -out proxy.req -keyout proxy.key
  128   openssl x509 -req -CAcreateserial -in proxy.req -days 7 -out proxy.crt \
  129     -CA user.crt -CAkey user.key -extfile openssl.cnf -extensions v3_proxy
  130 
  131 You can also create a proxy certificate using another proxy certificate as
  132 issuer (note: I'm using a different configuration section for it):
  133 
  134   openssl req -new -config openssl.cnf -out proxy2.req -keyout proxy2.key
  135   openssl x509 -req -CAcreateserial -in proxy2.req -days 7 -out proxy2.crt \
  136     -CA proxy.crt -CAkey proxy.key -extfile openssl.cnf -extensions v3_proxy2
  137 
  138 
  139 4. How to have your application interpret the policy?
  140 
  141 The basic way to interpret proxy policies is to start with some default rights,
  142 then compute the resulting rights by checking the proxy certificate against
  143 the chain of proxy certificates, user certificate and CA certificates. You then
  144 use the final computed rights.  Sounds easy, huh?  It almost is.
  145 
  146 The slightly complicated part is figuring out how to pass data between your
  147 application and the certificate validation procedure.
  148 
  149 You need the following ingredients:
  150 
  151  - a callback function that will be called for every certificate being
  152    validated.  The callback be called several times for each certificate,
  153    so you must be careful to do the proxy policy interpretation at the right
  154    time.  You also need to fill in the defaults when the EE certificate is
  155    checked.
  156 
  157  - a data structure that is shared between your application code and the
  158    callback.
  159 
  160  - a wrapper function that sets it all up.
  161 
  162  - an ex_data index function that creates an index into the generic ex_data
  163    store that is attached to an X509 validation context.
  164 
  165 Here is some skeleton code you can fill in:
  166 
  167   #include <string.h>
  168   #include <netdb.h>
  169   #include <openssl/x509.h>
  170   #include <openssl/x509v3.h>
  171 
  172   #define total_rights 25
  173 
  174   /*
  175    * In this example, I will use a view of granted rights as a bit
  176    * array, one bit for each possible right.
  177    */
  178   typedef struct your_rights {
  179       unsigned char rights[(total_rights + 7) / 8];
  180   } YOUR_RIGHTS;
  181 
  182   /*
  183    * The following procedure will create an index for the ex_data
  184    * store in the X509 validation context the first time it's called.
  185    * Subsequent calls will return the same index.  */
  186   static int get_proxy_auth_ex_data_idx(X509_STORE_CTX *ctx)
  187   {
  188       static volatile int idx = -1;
  189       if (idx < 0) {
  190           X509_STORE_lock(X509_STORE_CTX_get0_store(ctx));
  191           if (idx < 0) {
  192               idx = X509_STORE_CTX_get_ex_new_index(0,
  193                                                     "for verify callback",
  194                                                     NULL,NULL,NULL);
  195           }
  196           X509_STORE_unlock(X509_STORE_CTX_get0_store(ctx));
  197       }
  198       return idx;
  199   }
  200 
  201   /* Callback to be given to the X509 validation procedure.  */
  202   static int verify_callback(int ok, X509_STORE_CTX *ctx)
  203   {
  204       if (ok == 1) {
  205           /*
  206            * It's REALLY important you keep the proxy policy
  207            * check within this section.  It's important to know
  208            * that when ok is 1, the certificates are checked
  209            * from top to bottom.  You get the CA root first,
  210            * followed by the possible chain of intermediate
  211            * CAs, followed by the EE certificate, followed by
  212            * the possible proxy certificates.
  213            */
  214           X509 *xs = X509_STORE_CTX_get_current_cert(ctx);
  215 
  216           if (X509_get_extension_flags(xs) & EXFLAG_PROXY) {
  217               YOUR_RIGHTS *rights =
  218                   (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
  219                       get_proxy_auth_ex_data_idx(ctx));
  220               PROXY_CERT_INFO_EXTENSION *pci =
  221                   X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL);
  222 
  223               switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage)) {
  224               case NID_Independent:
  225                   /*
  226                    * Do whatever you need to grant explicit rights to
  227                    * this particular proxy certificate, usually by
  228                    * pulling them from some database.  If there are none
  229                    * to be found, clear all rights (making this and any
  230                    * subsequent proxy certificate void of any rights).
  231                    */
  232                   memset(rights->rights, 0, sizeof(rights->rights));
  233                   break;
  234               case NID_id_ppl_inheritAll:
  235                   /*
  236                    * This is basically a NOP, we simply let the current
  237                    * rights stand as they are.
  238                    */
  239                   break;
  240               default:
  241                   /* This is usually the most complex section of code.
  242                    * You really do whatever you want as long as you
  243                    * follow RFC 3820.  In the example we use here, the
  244                    * simplest thing to do is to build another, temporary
  245                    * bit array and fill it with the rights granted by
  246                    * the current proxy certificate, then use it as a
  247                    * mask on the accumulated rights bit array, and
  248                    * voilà, you now have a new accumulated rights bit
  249                    * array.
  250                    */
  251                   {
  252                       int i;
  253                       YOUR_RIGHTS tmp_rights;
  254                       memset(tmp_rights.rights, 0, sizeof(tmp_rights.rights));
  255 
  256                       /*
  257                        * process_rights() is supposed to be a procedure
  258                        * that takes a string and it's length, interprets
  259                        * it and sets the bits in the YOUR_RIGHTS pointed
  260                        * at by the third argument.
  261                        */
  262                       process_rights((char *) pci->proxyPolicy->policy->data,
  263                                      pci->proxyPolicy->policy->length,
  264                                      &tmp_rights);
  265 
  266                       for(i = 0; i < total_rights / 8; i++)
  267                           rights->rights[i] &= tmp_rights.rights[i];
  268                   }
  269                   break;
  270               }
  271               PROXY_CERT_INFO_EXTENSION_free(pci);
  272           } else if (!(X509_get_extension_flags(xs) & EXFLAG_CA)) {
  273               /* We have an EE certificate, let's use it to set default! */
  274               YOUR_RIGHTS *rights =
  275                   (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
  276                       get_proxy_auth_ex_data_idx(ctx));
  277 
  278               /* The following procedure finds out what rights the owner
  279                * of the current certificate has, and sets them in the
  280                * YOUR_RIGHTS structure pointed at by the second
  281                * argument.
  282                */
  283               set_default_rights(xs, rights);
  284           }
  285       }
  286       return ok;
  287   }
  288 
  289   static int my_X509_verify_cert(X509_STORE_CTX *ctx,
  290                                  YOUR_RIGHTS *needed_rights)
  291   {
  292       int ok;
  293       int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) =
  294           X509_STORE_CTX_get_verify_cb(ctx);
  295       YOUR_RIGHTS rights;
  296 
  297       X509_STORE_CTX_set_verify_cb(ctx, verify_callback);
  298       X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(ctx), &rights);
  299       X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
  300       ok = X509_verify_cert(ctx);
  301 
  302       if (ok == 1) {
  303           ok = check_needed_rights(rights, needed_rights);
  304       }
  305 
  306       X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb);
  307 
  308       return ok;
  309   }
  310 
  311 
  312 If you use SSL or TLS, you can easily set up a callback to have the
  313 certificates checked properly, using the code above:
  314 
  315   SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert, &needed_rights);
  316 
  317 
  318 --
  319 Richard Levitte