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       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 peer certificate verification

	#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);

	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);

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

       SSL_set_verify()	sets the verification flags for	ssl to be mode and
       specifies the verify_callback function to be used. If no	callback
       function	shall be specified, the	NULL pointer can be used for
       verify_callback.	In this	case last verify_callback set specifically for
       this ssl	remains. If no special callback	was set	before,	the default
       callback	for the	underlying ctx is used,	that was valid at the time ssl
       was created with	SSL_new(3). Within the callback	function,
       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 depth for the certificate
       chain verification that shall be	allowed	for ctx.

       SSL_set_verify_depth() sets the maximum depth for the certificate chain
       verification that shall be allowed for 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 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 ssl connection. The SSL_VERIFY_PEER
       flag must be set; the SSL_VERIFY_POST_HANDSHAKE flag is optional.

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

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

	   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
	   SSL_get_verify_result(3) function.  The handshake will be continued
	   regardless of the verification result.

	   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

	   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.

	   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

	   Client mode:	ignored	(see BUGS)

	   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

	   Client mode:	ignored	(see BUGS)

	   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

	   Client mode:	ignored	(see BUGS)

       If the 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 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 depth. If the certificate chain needed to reach a trusted
       issuer is longer	than 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 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: preverify_ok indicates, whether the
       verification of the certificate in question was passed (preverify_ok=1)
       or not (preverify_ok=0).	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 x509_ctx and verify_callback is called	with preverify_ok=0.
       By applying X509_CTX_store_* functions verify_callback can locate the
       certificate in question and perform additional steps (see EXAMPLES). If
       no error	is found for a certificate, verify_callback is called with
       preverify_ok=1 before advancing to the next level.

       The return value	of verify_callback controls the	strategy of the
       further verification process. If	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 verify_callback
       returns 1, the verification process is continued. If 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 SSL_get_verify_result(3) or by
       maintaining its own error storage managed by verify_callback.

       If no verify_callback is	specified, the default callback	will be	used.
       Its return value	is identical to	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_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.

       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.

       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.

       The following code sequence realizes an example 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
       CRYPTO_get_ex_new_index(3), 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;
		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;
		return preverify_ok;

	mydata_t mydata;

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


	 * 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 */

       ssl(7), SSL_new(3), SSL_CTX_get_verify_mode(3),
       SSL_get_verify_result(3), SSL_CTX_load_verify_locations(3),
       SSL_get_peer_certificate(3), SSL_CTX_set_cert_verify_callback(3),
       SSL_get_ex_data_X509_STORE_CTX_idx(3), SSL_CTX_set_client_cert_cb(3),

       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.

       Copyright 2000-2019 The OpenSSL Project Authors.	All Rights Reserved.

       Licensed	under the OpenSSL license (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

1.1.1k				  2021-03-25		 SSL_CTX_SET_VERIFY(3)


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