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=head1 NAME

SSL_CTX_set_cert_cb, SSL_set_cert_cb - handle certificate callback function

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 void SSL_CTX_set_cert_cb(SSL_CTX *c, int (*cert_cb)(SSL *ssl, void *arg),
                          void *arg);
 void SSL_set_cert_cb(SSL *s, int (*cert_cb)(SSL *ssl, void *arg), void *arg);

 int (*cert_cb)(SSL *ssl, void *arg);

=head1 DESCRIPTION

SSL_CTX_set_cert_cb() and SSL_set_cert_cb() sets the cert_cb() callback,
B<arg> value is pointer which is passed to the application callback.

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

cert_cb() is the application defined callback. It is called before a
certificate will be used by a client or server. The callback can then inspect
the passed B<ssl> structure and set or clear any appropriate certificates. If
the callback is successful it B<MUST> return 1 even if no certificates have
been set. A zero is returned on error which will abort the handshake with a
fatal internal error alert. A negative return value will suspend the handshake
and the handshake function will return immediately.
L<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 cert_cb(). It is the job of the
cert_cb() to store information about the state of the last call,
if required to continue.

=head1 NOTES

An application will typically call SSL_use_certificate() and
SSL_use_PrivateKey() to set the end entity certificate and private key.
It can add intermediate and optionally the root CA certificates using
SSL_add1_chain_cert().

It might also call SSL_certs_clear() to delete any certificates associated
with the B<SSL> object.

The certificate callback functionality supersedes the (largely broken)
functionality provided by the old client certificate callback interface.
It is B<always> called even is a certificate is already set so the callback
can modify or delete the existing certificate.

A more advanced callback might examine the handshake parameters and set
whatever chain is appropriate. For example a legacy client supporting only
TLSv1.0 might receive a certificate chain signed using SHA1 whereas a
TLSv1.2 or later client which advertises support for SHA256 could receive a
chain using SHA256.

Normal server sanity checks are performed on any certificates set
by the callback. So if an EC chain is set for a curve the client does not
support it will B<not> be used.

=head1 RETURN VALUES

SSL_CTX_set_cert_cb() and SSL_set_cert_cb() do not return values.

=head1 SEE ALSO

L<ssl(7)>, L<SSL_use_certificate(3)>,
L<SSL_add1_chain_cert(3)>,
L<SSL_get_client_CA_list(3)>,
L<SSL_clear(3)>, L<SSL_free(3)>

=head1 COPYRIGHT

Copyright 2014-2018 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
L<https://www.openssl.org/source/license.html>.

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