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+=head1 NAME
+SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client certificate callback function
+=head1 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);
+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
+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 B<x509> and B<pkey> arguments and "1" must be returned. The
+certificate will be installed into B<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. 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 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.
+=head1 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
+L<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 L<SSL_clear(3)>.
+If the callback returns no certificate, the OpenSSL library will not send
+a certificate.
+SSL_CTX_get_client_cert_cb() returns function pointer of client_cert_cb() or
+NULL if the callback is not set.
+=head1 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
+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 L<SSL_clear(3)> is being called. It is therefore
+mandatory to destroy the SSL object using L<SSL_free(3)>
+and create a new one to return to the previous state.
+=head1 SEE ALSO
+L<ssl(7)>, L<SSL_CTX_use_certificate(3)>,
+L<SSL_clear(3)>, L<SSL_free(3)>
+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