doc/HOWTO/proxy_certificates.txt

   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 http://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.polisy= 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   /* In this example, I will use a view of granted rights as a bit
 168      array, one bit for each possible right.  */
 169   typedef struct your_rights {
 170     unsigned char rights[total_rights / 8];
 171   } YOUR_RIGHTS;
 172
 173   /* The following procedure will create an index for the ex_data
 174      store in the X509 validation context the first time it's called.
 175      Subsequent calls will return the same index.  */
 176   static int get_proxy_auth_ex_data_idx(void)
 177   {
 178     static volatile int idx = -1;
 179     if (idx < 0)
 180       {
 181         CRYPTO_w_lock(CRYPTO_LOCK_X509_STORE);
 182         if (idx < 0)
 183           {
 184             idx = X509_STORE_CTX_get_ex_new_index(0,
 185                                                   "for verify callback",
 186                                                   NULL,NULL,NULL);
 187           }
 188         CRYPTO_w_unlock(CRYPTO_LOCK_X509_STORE);
 189       }
 190     return idx;
 191   }
 192
 193   /* Callback to be given to the X509 validation procedure.  */
 194   static int verify_callback(int ok, X509_STORE_CTX *ctx)
 195   {
 196     if (ok == 1) /* It's REALLY important you keep the proxy policy
 197                     check within this section.  It's important to know
 198                     that when ok is 1, the certificates are checked
 199                     from top to bottom.  You get the CA root first,
 200                     followed by the possible chain of intermediate
 201                     CAs, followed by the EE certificate, followed by
 202                     the possible proxy certificates.  */
 203       {
 204         X509 *xs = ctx->current_cert;
 205
 206         if (xs->ex_flags & EXFLAG_PROXY)
 207           {
 208             YOUR_RIGHTS *rights =
 209               (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
 210                 get_proxy_auth_ex_data_idx());
 211             PROXY_CERT_INFO_EXTENSION *pci =
 212               X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL);
 213
 214             switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage))
 215               {
 216               case NID_Independent:
 217                 /* Do whatever you need to grant explicit rights to
 218                    this particular proxy certificate, usually by
 219                    pulling them from some database.  If there are none
 220                    to be found, clear all rights (making this and any
 221                    subsequent proxy certificate void of any rights).
 222                 */
 223                 memset(rights->rights, 0, sizeof(rights->rights));
 224                 break;
 225               case NID_id_ppl_inheritAll:
 226                 /* This is basically a NOP, we simply let the current
 227                    rights stand as they are. */
 228                 break;
 229               default:
 230                 /* This is usually the most complex section of code.
 231                    You really do whatever you want as long as you
 232                    follow RFC 3820.  In the example we use here, the
 233                    simplest thing to do is to build another, temporary
 234                    bit array and fill it with the rights granted by
 235                    the current proxy certificate, then use it as a
 236                    mask on the accumulated rights bit array, and
 237                    voilà, you now have a new accumulated rights bit
 238                    array.  */
 239                 {
 240                   int i;
 241                   YOUR_RIGHTS tmp_rights;
 242                   memset(tmp_rights.rights, 0, sizeof(tmp_rights.rights));
 243
 244                   /* process_rights() is supposed to be a procedure
 245                      that takes a string and it's length, interprets
 246                      it and sets the bits in the YOUR_RIGHTS pointed
 247                      at by the third argument.  */
 248                   process_rights((char *) pci->proxyPolicy->policy->data,
 249                                  pci->proxyPolicy->policy->length,
 250                                  &tmp_rights);
 251
 252                   for(i = 0; i < total_rights / 8; i++)
 253                     rights->rights[i] &= tmp_rights.rights[i];
 254                 }
 255                 break;
 256               }
 257             PROXY_CERT_INFO_EXTENSION_free(pci);
 258           }
 259         else if (!(xs->ex_flags & EXFLAG_CA))
 260           {
 261             /* We have a EE certificate, let's use it to set default!
 262             */
 263             YOUR_RIGHTS *rights =
 264               (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
 265                 get_proxy_auth_ex_data_idx());
 266
 267             /* The following procedure finds out what rights the owner
 268                of the current certificate has, and sets them in the
 269                YOUR_RIGHTS structure pointed at by the second
 270                argument.  */
 271             set_default_rights(xs, rights);
 272           }
 273       }
 274     return ok;
 275   }
 276
 277   static int my_X509_verify_cert(X509_STORE_CTX *ctx,
 278                                  YOUR_RIGHTS *needed_rights)
 279   {
 280     int i;
 281     int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) = ctx->verify_cb;
 282     YOUR_RIGHTS rights;
 283
 284     X509_STORE_CTX_set_verify_cb(ctx, verify_callback);
 285     X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(), &rights);
 286     X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
 287     ok = X509_verify_cert(ctx);
 288
 289     if (ok == 1)
 290       {
 291         ok = check_needed_rights(rights, needed_rights);
 292       }
 293
 294     X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb);
 295
 296     return ok;
 297   }
 298
 299 If you use SSL or TLS, you can easily set up a callback to have the
 300 certificates checked properly, using the code above:
 301
 302   SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert, &needed_rights);
 303
 304
 305 --
 306 Richard Levitte