doc/man3/SSL_CTX_set_verify.pod

   1 =pod
   2
   3 =head1 NAME
   4
   5 SSL_get_ex_data_X509_STORE_CTX_idx,
   6 SSL_CTX_set_verify, SSL_set_verify,
   7 SSL_CTX_set_verify_depth, SSL_set_verify_depth,
   8 SSL_verify_cb,
   9 SSL_verify_client_post_handshake,
  10 SSL_set_post_handshake_auth,
  11 SSL_CTX_set_post_handshake_auth
  12 - set various SSL/TLS parameters for peer certificate verification
  13
  14 =head1 SYNOPSIS
  15
  16  #include <openssl/ssl.h>
  17
  18  typedef int (*SSL_verify_cb)(int preverify_ok, X509_STORE_CTX *x509_ctx);
  19
  20  void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, SSL_verify_cb verify_callback);
  21  void SSL_set_verify(SSL *ssl, int mode, SSL_verify_cb verify_callback);
  22  SSL_get_ex_data_X509_STORE_CTX_idx(void);
  23
  24  void SSL_CTX_set_verify_depth(SSL_CTX *ctx, int depth);
  25  void SSL_set_verify_depth(SSL *ssl, int depth);
  26
  27  int SSL_verify_client_post_handshake(SSL *ssl);
  28  void SSL_CTX_set_post_handshake_auth(SSL_CTX *ctx, int val);
  29  void SSL_set_post_handshake_auth(SSL *ssl, int val);
  30
  31 =head1 DESCRIPTION
  32
  33 SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and
  34 specifies the B<verify_callback> function to be used. If no callback function
  35 shall be specified, the NULL pointer can be used for B<verify_callback>.
  36
  37 SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and
  38 specifies the B<verify_callback> function to be used. If no callback function
  39 shall be specified, the NULL pointer can be used for B<verify_callback>. In
  40 this case last B<verify_callback> set specifically for this B<ssl> remains. If
  41 no special B<callback> was set before, the default callback for the underlying
  42 B<ctx> is used, that was valid at the time B<ssl> was created with
  43 L<SSL_new(3)>. Within the callback function,
  44 B<SSL_get_ex_data_X509_STORE_CTX_idx> can be called to get the data index
  45 of the current SSL object that is doing the verification.
  46
  47 In client mode B<verify_callback> may also call the L<SSL_set_retry_verify(3)>
  48 function on the B<SSL> object set in the I<x509_store_ctx> ex data (see
  49 L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>) and return 1.
  50 This would be typically done in case the certificate verification was not yet
  51 able to succeed.
  52 This makes the handshake suspend and return control to the calling application
  53 with B<SSL_ERROR_WANT_RETRY_VERIFY>.
  54 The application can for instance fetch further certificates or cert status
  55 information needed for the verification.
  56 Calling L<SSL_connect(3)> again resumes the connection attempt by retrying the
  57 server certificate verification step.
  58 This process may even be repeated if need be.
  59 Note that the handshake may still be aborted if a subsequent invocation of the
  60 callback (e.g., at a lower depth, or for a separate error condition) returns 0.
  61
  62 SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain
  63 verification that shall be allowed for B<ctx>.
  64
  65 SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain
  66 verification that shall be allowed for B<ssl>.
  67
  68 SSL_CTX_set_post_handshake_auth() and SSL_set_post_handshake_auth() enable the
  69 Post-Handshake Authentication extension to be added to the ClientHello such that
  70 post-handshake authentication can be requested by the server. If B<val> is 0
  71 then the extension is not sent, otherwise it is. By default the extension is not
  72 sent. A certificate callback will need to be set via
  73 SSL_CTX_set_client_cert_cb() if no certificate is provided at initialization.
  74
  75 SSL_verify_client_post_handshake() causes a CertificateRequest message to be
  76 sent by a server on the given B<ssl> connection. The SSL_VERIFY_PEER flag must
  77 be set; the SSL_VERIFY_POST_HANDSHAKE flag is optional.
  78
  79 =head1 NOTES
  80
  81 The verification of certificates can be controlled by a set of logically
  82 or'ed B<mode> flags:
  83
  84 =over 4
  85
  86 =item SSL_VERIFY_NONE
  87
  88 B<Server mode:> the server will not send a client certificate request to the
  89 client, so the client will not send a certificate.
  90
  91 B<Client mode:> if not using an anonymous cipher (by default disabled), the
  92 server will send a certificate which will be checked. The result of the
  93 certificate verification process can be checked after the TLS/SSL handshake
  94 using the L<SSL_get_verify_result(3)> function.
  95 The handshake will be continued regardless of the verification result.
  96
  97 =item SSL_VERIFY_PEER
  98
  99 B<Server mode:> the server sends a client certificate request to the client.
 100 The certificate returned (if any) is checked. If the verification process
 101 fails, the TLS/SSL handshake is
 102 immediately terminated with an alert message containing the reason for
 103 the verification failure.
 104 The behaviour can be controlled by the additional
 105 SSL_VERIFY_FAIL_IF_NO_PEER_CERT, SSL_VERIFY_CLIENT_ONCE and
 106 SSL_VERIFY_POST_HANDSHAKE flags.
 107
 108 B<Client mode:> the server certificate is verified. If the verification process
 109 fails, the TLS/SSL handshake is
 110 immediately terminated with an alert message containing the reason for
 111 the verification failure. If no server certificate is sent, because an
 112 anonymous cipher is used, SSL_VERIFY_PEER is ignored.
 113
 114 =item SSL_VERIFY_FAIL_IF_NO_PEER_CERT
 115
 116 B<Server mode:> if the client did not return a certificate, the TLS/SSL
 117 handshake is immediately terminated with a "handshake failure" alert.
 118 This flag must be used together with SSL_VERIFY_PEER.
 119
 120 B<Client mode:> ignored (see BUGS)
 121
 122 =item SSL_VERIFY_CLIENT_ONCE
 123
 124 B<Server mode:> only request a client certificate once during the
 125 connection. Do not ask for a client certificate again during
 126 renegotiation or post-authentication if a certificate was requested
 127 during the initial handshake. This flag must be used together with
 128 SSL_VERIFY_PEER.
 129
 130 B<Client mode:> ignored (see BUGS)
 131
 132 =item SSL_VERIFY_POST_HANDSHAKE
 133
 134 B<Server mode:> the server will not send a client certificate request
 135 during the initial handshake, but will send the request via
 136 SSL_verify_client_post_handshake(). This allows the SSL_CTX or SSL
 137 to be configured for post-handshake peer verification before the
 138 handshake occurs. This flag must be used together with
 139 SSL_VERIFY_PEER. TLSv1.3 only; no effect on pre-TLSv1.3 connections.
 140
 141 B<Client mode:> ignored (see BUGS)
 142
 143 =back
 144
 145 If the B<mode> is SSL_VERIFY_NONE none of the other flags may be set.
 146
 147 If verification flags are not modified explicitly by C<SSL_CTX_set_verify()>
 148 or C<SSL_set_verify()>, the default value will be SSL_VERIFY_NONE.
 149
 150 The actual verification procedure is performed either using the built-in
 151 verification procedure or using another application provided verification
 152 function set with
 153 L<SSL_CTX_set_cert_verify_callback(3)>.
 154 The following descriptions apply in the case of the built-in procedure. An
 155 application provided procedure also has access to the verify depth information
 156 and the verify_callback() function, but the way this information is used
 157 may be different.
 158
 159 SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set a limit on the
 160 number of certificates between the end-entity and trust-anchor certificates.
 161 Neither the
 162 end-entity nor the trust-anchor certificates count against B<depth>. If the
 163 certificate chain needed to reach a trusted issuer is longer than B<depth+2>,
 164 X509_V_ERR_CERT_CHAIN_TOO_LONG will be issued.
 165 The depth count is "level 0:peer certificate", "level 1: CA certificate",
 166 "level 2: higher level CA certificate", and so on. Setting the maximum
 167 depth to 2 allows the levels 0, 1, 2 and 3 (0 being the end-entity and 3 the
 168 trust-anchor).
 169 The default depth limit is 100,
 170 allowing for the peer certificate, at most 100 intermediate CA certificates and
 171 a final trust anchor certificate.
 172
 173 The B<verify_callback> function is used to control the behaviour when the
 174 SSL_VERIFY_PEER flag is set. It must be supplied by the application and
 175 receives two arguments: B<preverify_ok> indicates, whether the verification of
 176 the certificate in question was passed (preverify_ok=1) or not
 177 (preverify_ok=0). B<x509_ctx> is a pointer to the complete context used
 178 for the certificate chain verification.
 179
 180 The certificate chain is checked starting with the deepest nesting level
 181 (the root CA certificate) and worked upward to the peer's certificate.
 182 At each level signatures and issuer attributes are checked. Whenever
 183 a verification error is found, the error number is stored in B<x509_ctx>
 184 and B<verify_callback> is called with B<preverify_ok>=0. By applying
 185 X509_CTX_store_* functions B<verify_callback> can locate the certificate
 186 in question and perform additional steps (see EXAMPLES). If no error is
 187 found for a certificate, B<verify_callback> is called with B<preverify_ok>=1
 188 before advancing to the next level.
 189
 190 The return value of B<verify_callback> controls the strategy of the further
 191 verification process. If B<verify_callback> returns 0, the verification
 192 process is immediately stopped with "verification failed" state. If
 193 SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and
 194 the TLS/SSL handshake is terminated. If B<verify_callback> returns 1,
 195 the verification process is continued. If B<verify_callback> always returns
 196 1, the TLS/SSL handshake will not be terminated with respect to verification
 197 failures and the connection will be established. The calling process can
 198 however retrieve the error code of the last verification error using
 199 L<SSL_get_verify_result(3)> or by maintaining its
 200 own error storage managed by B<verify_callback>.
 201
 202 If no B<verify_callback> is specified, the default callback will be used.
 203 Its return value is identical to B<preverify_ok>, so that any verification
 204 failure will lead to a termination of the TLS/SSL handshake with an
 205 alert message, if SSL_VERIFY_PEER is set.
 206
 207 After calling SSL_set_post_handshake_auth(), the client will need to add a
 208 certificate or certificate callback to its configuration before it can
 209 successfully authenticate. This must be called before SSL_connect().
 210
 211 SSL_verify_client_post_handshake() requires that verify flags have been
 212 previously set, and that a client sent the post-handshake authentication
 213 extension. When the client returns a certificate the verify callback will be
 214 invoked. A write operation must take place for the Certificate Request to be
 215 sent to the client, this can be done with SSL_do_handshake() or SSL_write_ex().
 216 Only one certificate request may be outstanding at any time.
 217
 218 When post-handshake authentication occurs, a refreshed NewSessionTicket
 219 message is sent to the client.
 220
 221 Post-handshake authentication cannot be used with QUIC.
 222 SSL_set_post_handshake_auth() has no effect if called on a QUIC SSL object.
 223
 224 =head1 BUGS
 225
 226 In client mode, it is not checked whether the SSL_VERIFY_PEER flag
 227 is set, but whether any flags other than SSL_VERIFY_NONE are set. This can
 228 lead to unexpected behaviour if SSL_VERIFY_PEER and other flags are not used as
 229 required.
 230
 231 =head1 RETURN VALUES
 232
 233 The SSL*_set_verify*() functions do not provide diagnostic information.
 234
 235 The SSL_verify_client_post_handshake() function returns 1 if the request
 236 succeeded, and 0 if the request failed. The error stack can be examined
 237 to determine the failure reason.
 238
 239 =head1 EXAMPLES
 240
 241 The following code sequence realizes an example B<verify_callback> function
 242 that will always continue the TLS/SSL handshake regardless of verification
 243 failure, if wished. The callback realizes a verification depth limit with
 244 more informational output.
 245
 246 All verification errors are printed; information about the certificate chain
 247 is printed on request.
 248 The example is realized for a server that does allow but not require client
 249 certificates.
 250
 251 The example makes use of the ex_data technique to store application data
 252 into/retrieve application data from the SSL structure
 253 (see L<CRYPTO_get_ex_new_index(3)>,
 254 L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>).
 255
 256  ...
 257  typedef struct {
 258    int verbose_mode;
 259    int verify_depth;
 260    int always_continue;
 261  } mydata_t;
 262  int mydata_index;
 263
 264  ...
 265  static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
 266  {
 267      char    buf[256];
 268      X509   *err_cert;
 269      int     err, depth;
 270      SSL    *ssl;
 271      mydata_t *mydata;
 272
 273      err_cert = X509_STORE_CTX_get_current_cert(ctx);
 274      err = X509_STORE_CTX_get_error(ctx);
 275      depth = X509_STORE_CTX_get_error_depth(ctx);
 276
 277      /*
 278       * Retrieve the pointer to the SSL of the connection currently treated
 279       * and the application specific data stored into the SSL object.
 280       */
 281      ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
 282      mydata = SSL_get_ex_data(ssl, mydata_index);
 283
 284      X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);
 285
 286      /*
 287       * Catch a too long certificate chain. The depth limit set using
 288       * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so
 289       * that whenever the "depth>verify_depth" condition is met, we
 290       * have violated the limit and want to log this error condition.
 291       * We must do it here, because the CHAIN_TOO_LONG error would not
 292       * be found explicitly; only errors introduced by cutting off the
 293       * additional certificates would be logged.
 294       */
 295      if (depth > mydata->verify_depth) {
 296          preverify_ok = 0;
 297          err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
 298          X509_STORE_CTX_set_error(ctx, err);
 299      }
 300      if (!preverify_ok) {
 301          printf("verify error:num=%d:%s:depth=%d:%s\n", err,
 302                 X509_verify_cert_error_string(err), depth, buf);
 303      } else if (mydata->verbose_mode) {
 304          printf("depth=%d:%s\n", depth, buf);
 305      }
 306
 307      /*
 308       * At this point, err contains the last verification error. We can use
 309       * it for something special
 310       */
 311      if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)) {
 312          X509_NAME_oneline(X509_get_issuer_name(err_cert), buf, 256);
 313          printf("issuer= %s\n", buf);
 314      }
 315
 316      if (mydata->always_continue)
 317          return 1;
 318      else
 319          return preverify_ok;
 320  }
 321  ...
 322
 323  mydata_t mydata;
 324
 325  ...
 326  mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL);
 327
 328  ...
 329  SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER | SSL_VERIFY_CLIENT_ONCE,
 330                     verify_callback);
 331
 332  /*
 333   * Let the verify_callback catch the verify_depth error so that we get
 334   * an appropriate error in the logfile.
 335   */
 336  SSL_CTX_set_verify_depth(verify_depth + 1);
 337
 338  /*
 339   * Set up the SSL specific data into "mydata" and store it into th SSL
 340   * structure.
 341   */
 342  mydata.verify_depth = verify_depth; ...
 343  SSL_set_ex_data(ssl, mydata_index, &mydata);
 344
 345  ...
 346  SSL_accept(ssl);       /* check of success left out for clarity */
 347  if (peer = SSL_get_peer_certificate(ssl)) {
 348      if (SSL_get_verify_result(ssl) == X509_V_OK) {
 349          /* The client sent a certificate which verified OK */
 350      }
 351  }
 352
 353 =head1 SEE ALSO
 354
 355 L<ssl(7)>, L<SSL_new(3)>,
 356 L<SSL_CTX_get_verify_mode(3)>,
 357 L<SSL_get_verify_result(3)>,
 358 L<SSL_CTX_load_verify_locations(3)>,
 359 L<SSL_get_peer_certificate(3)>,
 360 L<SSL_CTX_set_cert_verify_callback(3)>,
 361 L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>,
 362 L<SSL_CTX_set_client_cert_cb(3)>,
 363 L<CRYPTO_get_ex_new_index(3)>
 364
 365 =head1 HISTORY
 366
 367 The SSL_VERIFY_POST_HANDSHAKE option, and the SSL_verify_client_post_handshake()
 368 and SSL_set_post_handshake_auth() functions were added in OpenSSL 1.1.1.
 369
 370 =head1 COPYRIGHT
 371
 372 Copyright 2000-2023 The OpenSSL Project Authors. All Rights Reserved.
 373
 374 Licensed under the Apache License 2.0 (the "License").  You may not use
 375 this file except in compliance with the License.  You can obtain a copy
 376 in the file LICENSE in the source distribution or at
 377 L<https://www.openssl.org/source/license.html>.
 378
 379 =cut