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SSL_CTX_SET_CLIENT_CERT_CB(3)	    OpenSSL	 SSL_CTX_SET_CLIENT_CERT_CB(3)

NAME
       SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb -	handle client
       certificate callback function

SYNOPSIS
	#include <openssl/ssl.h>

	void SSL_CTX_set_client_cert_cb(SSL_CTX	*ctx,
					int (*client_cert_cb)(SSL *ssl,	X509 **x509,
							      EVP_PKEY **pkey));
	int (*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509,
							EVP_PKEY **pkey);
	int (*client_cert_cb)(SSL *ssl,	X509 **x509, EVP_PKEY **pkey);

DESCRIPTION
       SSL_CTX_set_client_cert_cb() sets the client_cert_cb() callback,	that
       is called when a	client certificate is requested	by a server and	no
       certificate was yet set for the SSL object.

       When client_cert_cb() is	NULL, no callback function is used.

       SSL_CTX_get_client_cert_cb() returns a pointer to the currently set
       callback	function.

       client_cert_cb()	is the application defined callback. If	it wants to
       set a certificate, a certificate/private	key combination	must be	set
       using the x509 and pkey arguments and "1" must be returned. The
       certificate will	be installed into ssl, see the NOTES and BUGS
       sections.  If no	certificate should be set, "0" has to be returned and
       no certificate will be sent. A negative return value will suspend the
       handshake and the handshake function will return	immediately.
       SSL_get_error(3)	will return SSL_ERROR_WANT_X509_LOOKUP to indicate,
       that the	handshake was suspended. The next call to the handshake
       function	will again lead	to the call of client_cert_cb(). It is the job
       of the client_cert_cb() to store	information about the state of the
       last call, if required to continue.

NOTES
       During a	handshake (or renegotiation) a server may request a
       certificate from	the client. A client certificate must only be sent,
       when the	server did send	the request.

       When a certificate was set using	the SSL_CTX_use_certificate(3) family
       of functions, it	will be	sent to	the server. The	TLS standard requires
       that only a certificate is sent,	if it matches the list of acceptable
       CAs sent	by the server. This constraint is violated by the default
       behavior	of the OpenSSL library.	Using the callback function it is
       possible	to implement a proper selection	routine	or to allow a user
       interaction to choose the certificate to	be sent.

       If a callback function is defined and no	certificate was	yet defined
       for the SSL object, the callback	function will be called.  If the
       callback	function returns a certificate,	the OpenSSL library will try
       to load the private key and certificate data into the SSL object	using
       the SSL_use_certificate() and SSL_use_private_key() functions.  Thus it
       will permanently	install	the certificate	and key	for this SSL object.
       It will not be reset by calling SSL_clear(3).  If the callback returns
       no certificate, the OpenSSL library will	not send a certificate.

RETURN VALUES
       SSL_CTX_get_client_cert_cb() returns function pointer of
       client_cert_cb()	or NULL	if the callback	is not set.

BUGS
       The client_cert_cb() cannot return a complete certificate chain,	it can
       only return one client certificate. If the chain	only has a length of
       2, the root CA certificate may be omitted according to the TLS standard
       and thus	a standard conforming answer can be sent to the	server.	For a
       longer chain, the client	must send the complete chain (with the option
       to leave	out the	root CA	certificate). This can only be accomplished by
       either adding the intermediate CA certificates into the trusted
       certificate store for the SSL_CTX object	(resulting in having to	add CA
       certificates that otherwise maybe would not be trusted),	or by adding
       the chain certificates using the	SSL_CTX_add_extra_chain_cert(3)
       function, which is only available for the SSL_CTX object	as a whole and
       that therefore probably can only	apply for one client certificate,
       making the concept of the callback function (to allow the choice	from
       several certificates) questionable.

       Once the	SSL object has been used in conjunction	with the callback
       function, the certificate will be set for the SSL object	and will not
       be cleared even when SSL_clear(3) is being called. It is	therefore
       mandatory to destroy the	SSL object using SSL_free(3) and create	a new
       one to return to	the previous state.

SEE ALSO
       ssl(7), SSL_CTX_use_certificate(3), SSL_CTX_add_extra_chain_cert(3),
       SSL_get_client_CA_list(3), SSL_clear(3),	SSL_free(3)

COPYRIGHT
       Copyright 2002-2018 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
       <https://www.openssl.org/source/license.html>.

1.1.1d				  2019-09-10	 SSL_CTX_SET_CLIENT_CERT_CB(3)

NAME | SYNOPSIS | DESCRIPTION | NOTES | RETURN VALUES | BUGS | SEE ALSO | COPYRIGHT

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