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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);

=head1 DESCRIPTION

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

When I<cert_cb> is NULL, no callback function is used.

I<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 I<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 I<cert_cb>. It is the job of the
I<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 2014-2020 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>.

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