doc/HOWTO/proxy_certificates.txt

   1 <DRAFT!>
   2                         HOWTO proxy certificates
   3
   4 0. WARNING
   5
   6 NONE OF THE CODE PRESENTED HERE HAVE BEEN CHECKED!  They are just an
   7 example to show you how things can be done.  There may be typos or
   8 type conflicts, and you will have to resolve them.
   9
  10 1. Introduction
  11
  12 Proxy certificates are defined in RFC 3820.  They are really usual
  13 certificates with the mandatory extension proxyCertInfo.
  14
  15 Proxy certificates are issued by an End Entity (typically a user),
  16 either directly with the EE certificate as issuing certificate, or by
  17 extension through an already issued proxy certificate..  They are used
  18 to extend rights to some other entity (a computer process, typically,
  19 or sometimes to the user itself), so it can perform operations in the
  20 name of the owner of the EE certificate.
  21
  22 See http://www.ietf.org/rfc/rfc3820.txt for more information.
  23
  24
  25 2. A warning about proxy certificates
  26
  27 Noone seems to have tested proxy certificates with security in mind.
  28 Basically, to this date, it seems that proxy certificates have only
  29 been used in a world that's highly aware of them.  What would happen
  30 if an unsuspecting application is to validate a chain of certificates
  31 that contains proxy certificates?  It would usually consider the leaf
  32 to be the certificate to check for authorisation data, and since proxy
  33 certificates are controlled by the EE certificate owner alone, it's
  34 would be normal to consider what the EE certificate owner could do
  35 with them.
  36
  37 subjectAltName and issuerAltName are forbidden in proxy certificates,
  38 and this is enforced in OpenSSL.  The subject must be the same as the
  39 issuer, with one commonName added on.
  40
  41 Possible threats are, as far as has been imagined so far:
  42
  43  - impersonation through commonName (think server certificates).
  44  - use of additional extensions, possibly non-standard ones used in
  45    certain environments, that would grant extra or different
  46    authorisation rights.
  47
  48 For this reason, OpenSSL requires that the use of proxy certificates
  49 be explicitely allowed.  Currently, this can be done using the
  50 following methods:
  51
  52  - if the application calls X509_verify_cert() itself, it can do the
  53    following prior to that call (ctx is the pointer passed in the call
  54    to X509_verify_cert()):
  55
  56         X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
  57
  58  - in all other cases, proxy certificate validation can be enabled
  59    before starting the application by setting the envirnoment variable
  60    OPENSSL_ALLOW_PROXY_CERTS with some non-empty value.
  61
  62 There are thoughts to allow proxy certificates with a line in the
  63 default openssl.cnf, but that's still in the future.
  64
  65
  66 3. How to create proxy cerificates
  67
  68 It's quite easy to create proxy certificates, by taking advantage of
  69 the lack of checks of the 'openssl x509' application (*ahem*).  But
  70 first, you need to create a configuration section that contains a
  71 definition of the proxyCertInfo extension, a little like this:
  72
  73   [ v3_proxy ]
  74   # A proxy certificate MUST NEVER be a CA certificate.
  75   basicConstraints=CA:FALSE
  76
  77   # Usual authority key ID
  78   authorityKeyIdentifier=keyid,issuer:always
  79
  80   # Now, for the extension that marks this certificate as a proxy one
  81   proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
  82
  83 It's also possible to give the proxy extension in a separate section:
  84
  85   proxyCertInfo=critical,@proxy_ext
  86
  87   [ proxy_ext ]
  88   language=id-ppl-anyLanguage
  89   pathlen=0
  90   policy=text:BC
  91
  92 The policy value has a specific syntax, {syntag}:{string}, where the
  93 syntag determines what will be done with the string.  The recognised
  94 syntags are as follows:
  95
  96   text  indicates that the string is simply the bytes, not
  97         encoded in any kind of way:
  98
  99                 policy=text:räksmörgås
 100
 101         Previous versions of this design had a specific tag
 102         for UTF-8 text.  However, since the bytes are copied
 103         as-is anyway, there's no need for it.  Instead, use
 104         the text: tag, like this:
 105
 106                 policy=text:räksmörgås
 107
 108   hex   indicates the string is encoded in hex, with colons
 109         between each byte (every second hex digit):
 110
 111                 policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
 112
 113         Previous versions of this design had a tag to insert a
 114         complete DER blob.  However, the only legal use for
 115         this would be to surround the bytes that would go with
 116         the hex: tag with what's needed to construct a correct
 117         OCTET STRING.  Since hex: does that, the DER tag felt
 118         superfluous, and was therefore removed.
 119
 120   file  indicates that the text of the policy should really be
 121         taken from a file.  The string is then really a file
 122         name.  This is useful for policies that are large
 123         (more than a few of lines) XML documents, for example.
 124
 125 The 'policy' setting can be split up in multiple lines like this:
 126
 127   0.policy=This is
 128   1.polisy= a multi-
 129   2.policy=line policy.
 130
 131 NOTE: the proxy policy value is the part that determines the rights
 132 granted to the process using the proxy certificate.  The value is
 133 completely dependent on the application reading and interpretting it!
 134
 135 Now that you have created an extension section for your proxy
 136 certificate, you can now easily create a proxy certificate like this:
 137
 138   openssl req -new -config openssl.cnf \
 139           -out proxy.req -keyout proxy.key
 140   openssl x509 -req -CAcreateserial -in proxy.req -days 7 \
 141           -out proxy.crt -CA user.crt -CAkey user.key \
 142           -extfile openssl.cnf -extensions v3_proxy
 143
 144 It's just as easy to create a proxy certificate using another proxy
 145 certificate as issuer (note that I'm using a different configuration
 146 section for it):
 147
 148   openssl req -new -config openssl.cnf \
 149           -out proxy2.req -keyout proxy2.key
 150   openssl x509 -req -CAcreateserial -in proxy2.req -days 7 \
 151           -out proxy2.crt -CA proxy.crt -CAkey proxy.key \
 152           -extfile openssl.cnf -extensions v3_proxy2
 153
 154
 155 4. How to have your application interpret the policy?
 156
 157 The basic way to interpret proxy policies is to prepare some default
 158 rights, then do a check of the proxy certificate against the a chain
 159 of proxy certificates, user certificate and CA certificates, and see
 160 what rights came out by the end.  Sounds easy, huh?  It almost is.
 161
 162 The slightly complicated part is how to pass data between your
 163 application and the certificate validation procedure.
 164
 165 You need the following ingredients:
 166
 167  - a callback routing that will be called for every certificate that's
 168    validated.  It will be called several times for each certificates,
 169    so you must be attentive to when it's a good time to do the proxy
 170    policy interpretation and check, as well as to fill in the defaults
 171    when the EE certificate is checked.
 172
 173  - a structure of data that's shared between your application code and
 174    the callback.
 175
 176  - a wrapper function that sets it all up.
 177
 178  - an ex_data index function that creates an index into the generic
 179    ex_data store that's attached to an X509 validation context.
 180
 181 This is some cookbook code for you to fill in:
 182
 183   /* In this example, I will use a view of granted rights as a bit
 184      array, one bit for each possible right.  */
 185   typedef struct your_rights {
 186     unsigned char rights[total_rights / 8];
 187   } YOUR_RIGHTS;
 188
 189   /* The following procedure will create an index for the ex_data
 190      store in the X509 validation context the first time it's called.
 191      Subsequent calls will return the same index.  */
 192   static int get_proxy_auth_ex_data_idx(void)
 193   {
 194     static volatile int idx = -1;
 195     if (idx < 0)
 196       {
 197         CRYPTO_w_lock(CRYPTO_LOCK_X509_STORE);
 198         if (idx < 0)
 199           {
 200             idx = X509_STORE_CTX_get_ex_new_index(0,
 201                                                   "for verify callback",
 202                                                   NULL,NULL,NULL);
 203           }
 204         CRYPTO_w_unlock(CRYPTO_LOCK_X509_STORE);
 205       }
 206     return idx;
 207   }
 208
 209   /* Callback to be given to the X509 validation procedure.  */
 210   static int verify_callback(int ok, X509_STORE_CTX *ctx)
 211   {
 212     if (ok == 1) /* It's REALLY important you keep the proxy policy
 213                     check within this secion.  It's important to know
 214                     that when ok is 1, the certificates are checked
 215                     from top to bottom.  You get the CA root first,
 216                     followed by the possible chain of intermediate
 217                     CAs, followed by the EE certificate, followed by
 218                     the possible proxy certificates.  */
 219       {
 220         X509 *xs = ctx->current_cert;
 221
 222         if (xs->ex_flags & EXFLAG_PROXY)
 223           {
 224             YOUR_RIGHTS *rights =
 225               (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
 226                 get_proxy_auth_ex_data_idx());
 227             PROXY_CERT_INFO_EXTENSION *pci =
 228               X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL);
 229
 230             switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage))
 231               {
 232               case NID_Independent:
 233                 /* Do whatever you need to grant explicit rights to
 234                    this particular proxy certificate, usually by
 235                    pulling them from some database.  If there are none
 236                    to be found, clear all rights (making this and any
 237                    subsequent proxy certificate void of any rights).
 238                 */
 239                 memset(rights->rights, 0, sizeof(rights->rights));
 240                 break;
 241               case NID_id_ppl_inheritAll:
 242                 /* This is basically a NOP, we simply let the current
 243                    rights stand as they are. */
 244                 break;
 245               default:
 246                 /* This is usually the most complex section of code.
 247                    You really do whatever you want as long as you
 248                    follow RFC 3820.  In the example we use here, the
 249                    simplest thing to do is to build another, temporary
 250                    bit array and fill it with the rights granted by
 251                    the current proxy certificate, then use it as a
 252                    mask on the accumulated rights bit array, and
 253                    voilà, you now have a new accumulated rights bit
 254                    array.  */
 255                 {
 256                   int i;
 257                   YOUR_RIGHTS tmp_rights;
 258                   memset(tmp_rights.rights, 0, sizeof(tmp_rights.rights));
 259
 260                   /* process_rights() is supposed to be a procedure
 261                      that takes a string and it's length, interprets
 262                      it and sets the bits in the YOUR_RIGHTS pointed
 263                      at by the third argument.  */
 264                   process_rights((char *) pci->proxyPolicy->policy->data,
 265                                  pci->proxyPolicy->policy->length,
 266                                  &tmp_rights);
 267
 268                   for(i = 0; i < total_rights / 8; i++)
 269                     rights->rights[i] &= tmp_rights.rights[i];
 270                 }
 271                 break;
 272               }
 273             PROXY_CERT_INFO_EXTENSION_free(pci);
 274           }
 275         else if (!(xs->ex_flags & EXFLAG_CA))
 276           {
 277             /* We have a EE certificate, let's use it to set default!
 278             */
 279             YOUR_RIGHTS *rights =
 280               (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
 281                 get_proxy_auth_ex_data_idx());
 282
 283             /* The following procedure finds out what rights the owner
 284                of the current certificate has, and sets them in the
 285                YOUR_RIGHTS structure pointed at by the second
 286                argument.  */
 287             set_default_rights(xs, rights);
 288           }
 289       }
 290     return ok;
 291   }
 292
 293   static int my_X509_verify_cert(X509_STORE_CTX *ctx,
 294                                  YOUR_RIGHTS *needed_rights)
 295   {
 296     int i;
 297     int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) = ctx->verify_cb;
 298     YOUR_RIGHTS rights;
 299
 300     X509_STORE_CTX_set_verify_cb(ctx, verify_callback);
 301     X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(), &rights);
 302     X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
 303     ok = X509_verify_cert(ctx);
 304
 305     if (ok == 1)
 306       {
 307         ok = check_needed_rights(rights, needed_rights);
 308       }
 309
 310     X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb);
 311
 312     return ok;
 313   }
 314
 315 If you use SSL or TLS, you can easily set up a callback to have the
 316 certificates checked properly, using the code above:
 317
 318   SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert, &needed_rights);
 319
 320
 321 --
 322 Richard Levitte