doc/man3/SSL_CTX_set_verify.pod - third_party/openssl - Git at Google

 =pod

 =head1 NAME

 SSL_get_ex_data_X509_STORE_CTX_idx,
 SSL_CTX_set_verify, SSL_set_verify,
 SSL_CTX_set_verify_depth, SSL_set_verify_depth,
 SSL_verify_cb,
 SSL_verify_client_post_handshake,
 SSL_set_post_handshake_auth,
 SSL_CTX_set_post_handshake_auth
 - set various SSL/TLS parameters for peer certificate verification

 =head1 SYNOPSIS

  #include <openssl/ssl.h>

  typedef int (*SSL_verify_cb)(int preverify_ok, X509_STORE_CTX *x509_ctx);

  void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, SSL_verify_cb verify_callback);
  void SSL_set_verify(SSL *ssl, int mode, SSL_verify_cb verify_callback);
  SSL_get_ex_data_X509_STORE_CTX_idx(void);

  void SSL_CTX_set_verify_depth(SSL_CTX *ctx, int depth);
  void SSL_set_verify_depth(SSL *ssl, int depth);

  int SSL_verify_client_post_handshake(SSL *ssl);
  void SSL_CTX_set_post_handshake_auth(SSL_CTX *ctx, int val);
  void SSL_set_post_handshake_auth(SSL *ssl, int val);

 =head1 DESCRIPTION

 SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and
 specifies the B<verify_callback> function to be used. If no callback function
 shall be specified, the NULL pointer can be used for B<verify_callback>.

 SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and
 specifies the B<verify_callback> function to be used. If no callback function
 shall be specified, the NULL pointer can be used for B<verify_callback>. In
 this case last B<verify_callback> set specifically for this B<ssl> remains. If
 no special B<callback> was set before, the default callback for the underlying
 B<ctx> is used, that was valid at the time B<ssl> was created with
 L<SSL_new(3)>. Within the callback function,
 B<SSL_get_ex_data_X509_STORE_CTX_idx> can be called to get the data index
 of the current SSL object that is doing the verification.

 SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain
 verification that shall be allowed for B<ctx>.

 SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain
 verification that shall be allowed for B<ssl>.

 SSL_CTX_set_post_handshake_auth() and SSL_set_post_handshake_auth() enable the
 Post-Handshake Authentication extension to be added to the ClientHello such that
 post-handshake authentication can be requested by the server. If B<val> is 0
 then the extension is not sent, otherwise it is. By default the extension is not
 sent. A certificate callback will need to be set via
 SSL_CTX_set_client_cert_cb() if no certificate is provided at initialization.

 SSL_verify_client_post_handshake() causes a CertificateRequest message to be
 sent by a server on the given B<ssl> connection. The SSL_VERIFY_PEER flag must
 be set; the SSL_VERIFY_POST_HANDSHAKE flag is optional.

 =head1 NOTES

 The verification of certificates can be controlled by a set of logically
 or'ed B<mode> flags:

 =over 4

 =item SSL_VERIFY_NONE

 B<Server mode:> the server will not send a client certificate request to the
 client, so the client will not send a certificate.

 B<Client mode:> if not using an anonymous cipher (by default disabled), the
 server will send a certificate which will be checked. The result of the
 certificate verification process can be checked after the TLS/SSL handshake
 using the L<SSL_get_verify_result(3)> function.
 The handshake will be continued regardless of the verification result.

 =item SSL_VERIFY_PEER

 B<Server mode:> the server sends a client certificate request to the client.
 The certificate returned (if any) is checked. If the verification process
 fails, the TLS/SSL handshake is
 immediately terminated with an alert message containing the reason for
 the verification failure.
 The behaviour can be controlled by the additional
 SSL_VERIFY_FAIL_IF_NO_PEER_CERT, SSL_VERIFY_CLIENT_ONCE and
 SSL_VERIFY_POST_HANDSHAKE flags.

 B<Client mode:> the server certificate is verified. If the verification process
 fails, the TLS/SSL handshake is
 immediately terminated with an alert message containing the reason for
 the verification failure. If no server certificate is sent, because an
 anonymous cipher is used, SSL_VERIFY_PEER is ignored.

 =item SSL_VERIFY_FAIL_IF_NO_PEER_CERT

 B<Server mode:> if the client did not return a certificate, the TLS/SSL
 handshake is immediately terminated with a "handshake failure" alert.
 This flag must be used together with SSL_VERIFY_PEER.

 B<Client mode:> ignored (see BUGS)

 =item SSL_VERIFY_CLIENT_ONCE

 B<Server mode:> only request a client certificate once during the
 connection. Do not ask for a client certificate again during
 renegotiation or post-authentication if a certificate was requested
 during the initial handshake. This flag must be used together with
 SSL_VERIFY_PEER.

 B<Client mode:> ignored (see BUGS)

 =item SSL_VERIFY_POST_HANDSHAKE

 B<Server mode:> the server will not send a client certificate request
 during the initial handshake, but will send the request via
 SSL_verify_client_post_handshake(). This allows the SSL_CTX or SSL
 to be configured for post-handshake peer verification before the
 handshake occurs. This flag must be used together with
 SSL_VERIFY_PEER. TLSv1.3 only; no effect on pre-TLSv1.3 connections.

 B<Client mode:> ignored (see BUGS)

 =back

 If the B<mode> is SSL_VERIFY_NONE none of the other flags may be set.

 The actual verification procedure is performed either using the built-in
 verification procedure or using another application provided verification
 function set with
 L<SSL_CTX_set_cert_verify_callback(3)>.
 The following descriptions apply in the case of the built-in procedure. An
 application provided procedure also has access to the verify depth information
 and the verify_callback() function, but the way this information is used
 may be different.

 SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set a limit on the
 number of certificates between the end-entity and trust-anchor certificates.
 Neither the
 end-entity nor the trust-anchor certificates count against B<depth>. If the
 certificate chain needed to reach a trusted issuer is longer than B<depth+2>,
 X509_V_ERR_CERT_CHAIN_TOO_LONG will be issued.
 The depth count is "level 0:peer certificate", "level 1: CA certificate",
 "level 2: higher level CA certificate", and so on. Setting the maximum
 depth to 2 allows the levels 0, 1, 2 and 3 (0 being the end-entity and 3 the
 trust-anchor).
 The default depth limit is 100,
 allowing for the peer certificate, at most 100 intermediate CA certificates and
 a final trust anchor certificate.

 The B<verify_callback> function is used to control the behaviour when the
 SSL_VERIFY_PEER flag is set. It must be supplied by the application and
 receives two arguments: B<preverify_ok> indicates, whether the verification of
 the certificate in question was passed (preverify_ok=1) or not
 (preverify_ok=0). B<x509_ctx> is a pointer to the complete context used
 for the certificate chain verification.

 The certificate chain is checked starting with the deepest nesting level
 (the root CA certificate) and worked upward to the peer's certificate.
 At each level signatures and issuer attributes are checked. Whenever
 a verification error is found, the error number is stored in B<x509_ctx>
 and B<verify_callback> is called with B<preverify_ok>=0. By applying
 X509_CTX_store_* functions B<verify_callback> can locate the certificate
 in question and perform additional steps (see EXAMPLES). If no error is
 found for a certificate, B<verify_callback> is called with B<preverify_ok>=1
 before advancing to the next level.

 The return value of B<verify_callback> controls the strategy of the further
 verification process. If B<verify_callback> returns 0, the verification
 process is immediately stopped with "verification failed" state. If
 SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and
 the TLS/SSL handshake is terminated. If B<verify_callback> returns 1,
 the verification process is continued. If B<verify_callback> always returns
 1, the TLS/SSL handshake will not be terminated with respect to verification
 failures and the connection will be established. The calling process can
 however retrieve the error code of the last verification error using
 L<SSL_get_verify_result(3)> or by maintaining its
 own error storage managed by B<verify_callback>.

 If no B<verify_callback> is specified, the default callback will be used.
 Its return value is identical to B<preverify_ok>, so that any verification
 failure will lead to a termination of the TLS/SSL handshake with an
 alert message, if SSL_VERIFY_PEER is set.

 After calling SSL_set_post_handshake_auth(), the client will need to add a
 certificate or certificate callback to its configuration before it can
 successfully authenticate. This must be called before SSL_connect().

 SSL_verify_client_post_handshake() requires that verify flags have been
 previously set, and that a client sent the post-handshake authentication
 extension. When the client returns a certificate the verify callback will be
 invoked. A write operation must take place for the Certificate Request to be
 sent to the client, this can be done with SSL_do_handshake() or SSL_write_ex().
 Only one certificate request may be outstanding at any time.

 When post-handshake authentication occurs, a refreshed NewSessionTicket
 message is sent to the client.

 =head1 BUGS

 In client mode, it is not checked whether the SSL_VERIFY_PEER flag
 is set, but whether any flags other than SSL_VERIFY_NONE are set. This can
 lead to unexpected behaviour if SSL_VERIFY_PEER and other flags are not used as
 required.

 =head1 RETURN VALUES

 The SSL*_set_verify*() functions do not provide diagnostic information.

 The SSL_verify_client_post_handshake() function returns 1 if the request
 succeeded, and 0 if the request failed. The error stack can be examined
 to determine the failure reason.

 =head1 EXAMPLES

 The following code sequence realizes an example B<verify_callback> function
 that will always continue the TLS/SSL handshake regardless of verification
 failure, if wished. The callback realizes a verification depth limit with
 more informational output.

 All verification errors are printed; information about the certificate chain
 is printed on request.
 The example is realized for a server that does allow but not require client
 certificates.

 The example makes use of the ex_data technique to store application data
 into/retrieve application data from the SSL structure
 (see L<CRYPTO_get_ex_new_index(3)>,
 L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>).

  ...
  typedef struct {
    int verbose_mode;
    int verify_depth;
    int always_continue;
  } mydata_t;
  int mydata_index;

  ...
  static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
  {
      char    buf[256];
      X509   *err_cert;
      int     err, depth;
      SSL    *ssl;
      mydata_t *mydata;

      err_cert = X509_STORE_CTX_get_current_cert(ctx);
      err = X509_STORE_CTX_get_error(ctx);
      depth = X509_STORE_CTX_get_error_depth(ctx);

      /*
       * Retrieve the pointer to the SSL of the connection currently treated
       * and the application specific data stored into the SSL object.
       */
      ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
      mydata = SSL_get_ex_data(ssl, mydata_index);

      X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);

      /*
       * Catch a too long certificate chain. The depth limit set using
       * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so
       * that whenever the "depth>verify_depth" condition is met, we
       * have violated the limit and want to log this error condition.
       * We must do it here, because the CHAIN_TOO_LONG error would not
       * be found explicitly; only errors introduced by cutting off the
       * additional certificates would be logged.
       */
      if (depth > mydata->verify_depth) {
          preverify_ok = 0;
          err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
          X509_STORE_CTX_set_error(ctx, err);
      }
      if (!preverify_ok) {
          printf("verify error:num=%d:%s:depth=%d:%s\n", err,
                 X509_verify_cert_error_string(err), depth, buf);
      } else if (mydata->verbose_mode) {
          printf("depth=%d:%s\n", depth, buf);
      }

      /*
       * At this point, err contains the last verification error. We can use
       * it for something special
       */
      if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)) {
          X509_NAME_oneline(X509_get_issuer_name(err_cert), buf, 256);
          printf("issuer= %s\n", buf);
      }

      if (mydata->always_continue)
          return 1;
      else
          return preverify_ok;
  }
  ...

  mydata_t mydata;

  ...
  mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL);

  ...
  SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER | SSL_VERIFY_CLIENT_ONCE,
                     verify_callback);

  /*
   * Let the verify_callback catch the verify_depth error so that we get
   * an appropriate error in the logfile.
   */
  SSL_CTX_set_verify_depth(verify_depth + 1);

  /*
   * Set up the SSL specific data into "mydata" and store it into th SSL
   * structure.
   */
  mydata.verify_depth = verify_depth; ...
  SSL_set_ex_data(ssl, mydata_index, &mydata);

  ...
  SSL_accept(ssl);       /* check of success left out for clarity */
  if (peer = SSL_get_peer_certificate(ssl)) {
      if (SSL_get_verify_result(ssl) == X509_V_OK) {
          /* The client sent a certificate which verified OK */
      }
  }

 =head1 SEE ALSO

 L<ssl(7)>, L<SSL_new(3)>,
 L<SSL_CTX_get_verify_mode(3)>,
 L<SSL_get_verify_result(3)>,
 L<SSL_CTX_load_verify_locations(3)>,
 L<SSL_get_peer_certificate(3)>,
 L<SSL_CTX_set_cert_verify_callback(3)>,
 L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>,
 L<SSL_CTX_set_client_cert_cb(3)>,
 L<CRYPTO_get_ex_new_index(3)>

 =head1 HISTORY

 The SSL_VERIFY_POST_HANDSHAKE option, and the SSL_verify_client_post_handshake()
 and SSL_set_post_handshake_auth() functions were added in OpenSSL 1.1.1.

 =head1 COPYRIGHT

 Copyright 2000-2021 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
 L<https://www.openssl.org/source/license.html>.

 =cut
	=pod

	=head1 NAME

	SSL_get_ex_data_X509_STORE_CTX_idx,
	SSL_CTX_set_verify, SSL_set_verify,
	SSL_CTX_set_verify_depth, SSL_set_verify_depth,
	SSL_verify_cb,
	SSL_verify_client_post_handshake,
	SSL_set_post_handshake_auth,
	SSL_CTX_set_post_handshake_auth
	- set various SSL/TLS parameters for peer certificate verification

	=head1 SYNOPSIS

	#include <openssl/ssl.h>

	typedef int (SSL_verify_cb)(int preverify_ok, X509_STORE_CTX x509_ctx);

	void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, SSL_verify_cb verify_callback);
	void SSL_set_verify(SSL *ssl, int mode, SSL_verify_cb verify_callback);
	SSL_get_ex_data_X509_STORE_CTX_idx(void);

	void SSL_CTX_set_verify_depth(SSL_CTX *ctx, int depth);
	void SSL_set_verify_depth(SSL *ssl, int depth);

	int SSL_verify_client_post_handshake(SSL *ssl);
	void SSL_CTX_set_post_handshake_auth(SSL_CTX *ctx, int val);
	void SSL_set_post_handshake_auth(SSL *ssl, int val);

	=head1 DESCRIPTION

	SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and
	specifies the B<verify_callback> function to be used. If no callback function
	shall be specified, the NULL pointer can be used for B<verify_callback>.

	SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and
	specifies the B<verify_callback> function to be used. If no callback function
	shall be specified, the NULL pointer can be used for B<verify_callback>. In
	this case last B<verify_callback> set specifically for this B<ssl> remains. If
	no special B<callback> was set before, the default callback for the underlying
	B<ctx> is used, that was valid at the time B<ssl> was created with
	L<SSL_new(3)>. Within the callback function,
	B<SSL_get_ex_data_X509_STORE_CTX_idx> can be called to get the data index
	of the current SSL object that is doing the verification.

	SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain
	verification that shall be allowed for B<ctx>.

	SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain
	verification that shall be allowed for B<ssl>.

	SSL_CTX_set_post_handshake_auth() and SSL_set_post_handshake_auth() enable the
	Post-Handshake Authentication extension to be added to the ClientHello such that
	post-handshake authentication can be requested by the server. If B<val> is 0
	then the extension is not sent, otherwise it is. By default the extension is not
	sent. A certificate callback will need to be set via
	SSL_CTX_set_client_cert_cb() if no certificate is provided at initialization.

	SSL_verify_client_post_handshake() causes a CertificateRequest message to be
	sent by a server on the given B<ssl> connection. The SSL_VERIFY_PEER flag must
	be set; the SSL_VERIFY_POST_HANDSHAKE flag is optional.

	=head1 NOTES

	The verification of certificates can be controlled by a set of logically
	or'ed B<mode> flags:

	=over 4

	=item SSL_VERIFY_NONE

	B<Server mode:> the server will not send a client certificate request to the
	client, so the client will not send a certificate.

	B<Client mode:> if not using an anonymous cipher (by default disabled), the
	server will send a certificate which will be checked. The result of the
	certificate verification process can be checked after the TLS/SSL handshake
	using the L<SSL_get_verify_result(3)> function.
	The handshake will be continued regardless of the verification result.

	=item SSL_VERIFY_PEER

	B<Server mode:> the server sends a client certificate request to the client.
	The certificate returned (if any) is checked. If the verification process
	fails, the TLS/SSL handshake is
	immediately terminated with an alert message containing the reason for
	the verification failure.
	The behaviour can be controlled by the additional
	SSL_VERIFY_FAIL_IF_NO_PEER_CERT, SSL_VERIFY_CLIENT_ONCE and
	SSL_VERIFY_POST_HANDSHAKE flags.

	B<Client mode:> the server certificate is verified. If the verification process
	fails, the TLS/SSL handshake is
	immediately terminated with an alert message containing the reason for
	the verification failure. If no server certificate is sent, because an
	anonymous cipher is used, SSL_VERIFY_PEER is ignored.

	=item SSL_VERIFY_FAIL_IF_NO_PEER_CERT

	B<Server mode:> if the client did not return a certificate, the TLS/SSL
	handshake is immediately terminated with a "handshake failure" alert.
	This flag must be used together with SSL_VERIFY_PEER.

	B<Client mode:> ignored (see BUGS)

	=item SSL_VERIFY_CLIENT_ONCE

	B<Server mode:> only request a client certificate once during the
	connection. Do not ask for a client certificate again during
	renegotiation or post-authentication if a certificate was requested
	during the initial handshake. This flag must be used together with
	SSL_VERIFY_PEER.

	B<Client mode:> ignored (see BUGS)

	=item SSL_VERIFY_POST_HANDSHAKE

	B<Server mode:> the server will not send a client certificate request
	during the initial handshake, but will send the request via
	SSL_verify_client_post_handshake(). This allows the SSL_CTX or SSL
	to be configured for post-handshake peer verification before the
	handshake occurs. This flag must be used together with
	SSL_VERIFY_PEER. TLSv1.3 only; no effect on pre-TLSv1.3 connections.

	B<Client mode:> ignored (see BUGS)

	=back

	If the B<mode> is SSL_VERIFY_NONE none of the other flags may be set.

	The actual verification procedure is performed either using the built-in
	verification procedure or using another application provided verification
	function set with
	L<SSL_CTX_set_cert_verify_callback(3)>.
	The following descriptions apply in the case of the built-in procedure. An
	application provided procedure also has access to the verify depth information
	and the verify_callback() function, but the way this information is used
	may be different.

	SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set a limit on the
	number of certificates between the end-entity and trust-anchor certificates.
	Neither the
	end-entity nor the trust-anchor certificates count against B<depth>. If the
	certificate chain needed to reach a trusted issuer is longer than B<depth+2>,
	X509_V_ERR_CERT_CHAIN_TOO_LONG will be issued.
	The depth count is "level 0:peer certificate", "level 1: CA certificate",
	"level 2: higher level CA certificate", and so on. Setting the maximum
	depth to 2 allows the levels 0, 1, 2 and 3 (0 being the end-entity and 3 the
	trust-anchor).
	The default depth limit is 100,
	allowing for the peer certificate, at most 100 intermediate CA certificates and
	a final trust anchor certificate.

	The B<verify_callback> function is used to control the behaviour when the
	SSL_VERIFY_PEER flag is set. It must be supplied by the application and
	receives two arguments: B<preverify_ok> indicates, whether the verification of
	the certificate in question was passed (preverify_ok=1) or not
	(preverify_ok=0). B<x509_ctx> is a pointer to the complete context used
	for the certificate chain verification.

	The certificate chain is checked starting with the deepest nesting level
	(the root CA certificate) and worked upward to the peer's certificate.
	At each level signatures and issuer attributes are checked. Whenever
	a verification error is found, the error number is stored in B<x509_ctx>
	and B<verify_callback> is called with B<preverify_ok>=0. By applying
	X509_CTX_store_* functions B<verify_callback> can locate the certificate
	in question and perform additional steps (see EXAMPLES). If no error is
	found for a certificate, B<verify_callback> is called with B<preverify_ok>=1
	before advancing to the next level.

	The return value of B<verify_callback> controls the strategy of the further
	verification process. If B<verify_callback> returns 0, the verification
	process is immediately stopped with "verification failed" state. If
	SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and
	the TLS/SSL handshake is terminated. If B<verify_callback> returns 1,
	the verification process is continued. If B<verify_callback> always returns
	1, the TLS/SSL handshake will not be terminated with respect to verification
	failures and the connection will be established. The calling process can
	however retrieve the error code of the last verification error using
	L<SSL_get_verify_result(3)> or by maintaining its
	own error storage managed by B<verify_callback>.

	If no B<verify_callback> is specified, the default callback will be used.
	Its return value is identical to B<preverify_ok>, so that any verification
	failure will lead to a termination of the TLS/SSL handshake with an
	alert message, if SSL_VERIFY_PEER is set.

	After calling SSL_set_post_handshake_auth(), the client will need to add a
	certificate or certificate callback to its configuration before it can
	successfully authenticate. This must be called before SSL_connect().

	SSL_verify_client_post_handshake() requires that verify flags have been
	previously set, and that a client sent the post-handshake authentication
	extension. When the client returns a certificate the verify callback will be
	invoked. A write operation must take place for the Certificate Request to be
	sent to the client, this can be done with SSL_do_handshake() or SSL_write_ex().
	Only one certificate request may be outstanding at any time.

	When post-handshake authentication occurs, a refreshed NewSessionTicket
	message is sent to the client.

	=head1 BUGS

	In client mode, it is not checked whether the SSL_VERIFY_PEER flag
	is set, but whether any flags other than SSL_VERIFY_NONE are set. This can
	lead to unexpected behaviour if SSL_VERIFY_PEER and other flags are not used as
	required.

	=head1 RETURN VALUES

	The SSL_set_verify() functions do not provide diagnostic information.

	The SSL_verify_client_post_handshake() function returns 1 if the request
	succeeded, and 0 if the request failed. The error stack can be examined
	to determine the failure reason.

	=head1 EXAMPLES

	The following code sequence realizes an example B<verify_callback> function
	that will always continue the TLS/SSL handshake regardless of verification
	failure, if wished. The callback realizes a verification depth limit with
	more informational output.

	All verification errors are printed; information about the certificate chain
	is printed on request.
	The example is realized for a server that does allow but not require client
	certificates.

	The example makes use of the ex_data technique to store application data
	into/retrieve application data from the SSL structure
	(see L<CRYPTO_get_ex_new_index(3)>,
	L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>).

	...
	typedef struct {
	int verbose_mode;
	int verify_depth;
	int always_continue;
	} mydata_t;
	int mydata_index;

	...
	static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
	{
	char buf[256];
	X509 *err_cert;
	int err, depth;
	SSL *ssl;
	mydata_t *mydata;

	err_cert = X509_STORE_CTX_get_current_cert(ctx);
	err = X509_STORE_CTX_get_error(ctx);
	depth = X509_STORE_CTX_get_error_depth(ctx);

	/*
	* Retrieve the pointer to the SSL of the connection currently treated
	* and the application specific data stored into the SSL object.
	*/
	ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
	mydata = SSL_get_ex_data(ssl, mydata_index);

	X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);

	/*
	* Catch a too long certificate chain. The depth limit set using
	* SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so
	* that whenever the "depth>verify_depth" condition is met, we
	* have violated the limit and want to log this error condition.
	* We must do it here, because the CHAIN_TOO_LONG error would not
	* be found explicitly; only errors introduced by cutting off the
	* additional certificates would be logged.
	*/
	if (depth > mydata->verify_depth) {
	preverify_ok = 0;
	err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
	X509_STORE_CTX_set_error(ctx, err);
	}
	if (!preverify_ok) {
	printf("verify error:num=%d:%s:depth=%d:%s\n", err,
	X509_verify_cert_error_string(err), depth, buf);
	} else if (mydata->verbose_mode) {
	printf("depth=%d:%s\n", depth, buf);
	}

	/*
	* At this point, err contains the last verification error. We can use
	* it for something special
	*/
	if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)) {
	X509_NAME_oneline(X509_get_issuer_name(err_cert), buf, 256);
	printf("issuer= %s\n", buf);
	}

	if (mydata->always_continue)
	return 1;
	else
	return preverify_ok;
	}
	...

	mydata_t mydata;

	...
	mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL);

	...
	SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER \| SSL_VERIFY_CLIENT_ONCE,
	verify_callback);

	/*
	* Let the verify_callback catch the verify_depth error so that we get
	* an appropriate error in the logfile.
	*/
	SSL_CTX_set_verify_depth(verify_depth + 1);

	/*
	* Set up the SSL specific data into "mydata" and store it into th SSL
	* structure.
	*/
	mydata.verify_depth = verify_depth; ...
	SSL_set_ex_data(ssl, mydata_index, &mydata);

	...
	SSL_accept(ssl); /* check of success left out for clarity */
	if (peer = SSL_get_peer_certificate(ssl)) {
	if (SSL_get_verify_result(ssl) == X509_V_OK) {
	/* The client sent a certificate which verified OK */
	}
	}

	=head1 SEE ALSO

	L<ssl(7)>, L<SSL_new(3)>,
	L<SSL_CTX_get_verify_mode(3)>,
	L<SSL_get_verify_result(3)>,
	L<SSL_CTX_load_verify_locations(3)>,
	L<SSL_get_peer_certificate(3)>,
	L<SSL_CTX_set_cert_verify_callback(3)>,
	L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>,
	L<SSL_CTX_set_client_cert_cb(3)>,
	L<CRYPTO_get_ex_new_index(3)>

	=head1 HISTORY

	The SSL_VERIFY_POST_HANDSHAKE option, and the SSL_verify_client_post_handshake()
	and SSL_set_post_handshake_auth() functions were added in OpenSSL 1.1.1.

	=head1 COPYRIGHT

	Copyright 2000-2021 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
	L<https://www.openssl.org/source/license.html>.

	=cut