Even though it is not really practical people should know about it.

doc/ssl/SSL_CTX_add_extra_chain_cert.pod

@@ -33,6 +33,7 @@ error stack to find out the reason for failure otherwise.
 
 L<ssl(3)|ssl(3)>,
 L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)>,
+L<SSL_CTX_set_client_cert_cb(3)|SSL_CTX_set_client_cert_cb(3)>,
 L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>
 
 =cut

doc/ssl/SSL_CTX_set_client_cert_cb.pod

@@ -0,0 +1,90 @@
+=pod
+
+=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);
+
+=head1 DESCRIPTION
+
+SSL_CTX_set_client_cert_cb() sets the B<client_cert_cb()> callback, that is
+called when a client certificate is requested by a server.
+When B<client_cert_cb()> is NULL, not 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 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 the default
+certificate will be sent. A fatal error can be indicated by returning
+a negative value, in which case the handshake will be canceled.
+
+=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 no callback function is set, an OpenSSL client will send the certificate
+that was set using the
+L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)> family of functions.
+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.
+The callback function can obtain the list of acceptable CAs using the
+L<SSL_get_client_CA_list(3)|SSL_get_client_CA_list(3)> function.
+
+If a callback function is defined, 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 SSL_use_certificate() and SSL_use_private_key() functions.
+Thus it will permanently override the certificate and key previously
+installed and will not be reset by calling L<SSL_clear(3)|SSL_clear(3)>.
+If the callback returns no certificate, the OpenSSL library will send
+the certificate previously installed for the SSL_CTX object or the specific
+certificate of the SSL object, if available.
+
+=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
+L<SSL_CTX_add_extra_chain_cert(3)|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 L<SSL_clear(3)|SSL_clear(3)> is being called. It is therefore
+mandatory to destroy the SSL object using L<SSL_free(3)|SSL_free(3)>
+and create a new one to return to the previous state.
+
+=head1 SEE ALSO
+
+L<ssl(3)|ssl(3)>, L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)>,
+L<SSL_CTX_add_extra_chain_cert(3)|SSL_CTX_add_extra_chain_cert(3)>,
+L<SSL_get_client_CA_list(3)|SSL_get_client_CA_list(3)>,
+L<SSL_clear(3)|SSL_clear(3)>, L<SSL_free(3)|SSL_free(3)>
+
+=cut

doc/ssl/SSL_CTX_use_certificate.pod

@@ -149,6 +149,7 @@ L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>,
 L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>,
 L<SSL_CTX_set_default_passwd_cb(3)|SSL_CTX_set_default_passwd_cb(3)>,
 L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
+L<SSL_CTX_set_client_cert_cb(3)|SSL_CTX_set_client_cert_cb(3)>,
 L<SSL_CTX_add_extra_chain_cert(3)|SSL_CTX_add_extra_chain_cert(3)>
 
 =cut

doc/ssl/SSL_clear.pod

@@ -44,6 +44,7 @@ The SSL_clear() operation was successful.
 
 L<SSL_new(3)|SSL_new(3)>, L<SSL_free(3)|SSL_free(3)>,
 L<SSL_shutdown(3)|SSL_shutdown(3)>, L<SSL_set_shutdown(3)|SSL_set_shutdown(3)>,
-L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, L<ssl(3)|ssl(3)>
+L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, L<ssl(3)|ssl(3)>,
+L<SSL_CTX_set_client_cert_cb(3)|SSL_CTX_set_client_cert_cb(3)>
 
 =cut

doc/ssl/SSL_get_client_CA_list.pod

@@ -47,6 +47,7 @@ the server did not send a list of CAs (client mode).
 =head1 SEE ALSO
 
 L<ssl(3)|ssl(3)>,
-L<SSL_CTX_set_client_CA_list(3)|SSL_CTX_set_client_CA_list(3)>
+L<SSL_CTX_set_client_CA_list(3)|SSL_CTX_set_client_CA_list(3)>,
+L<SSL_CTX_set_client_cert_cb(3)|SSL_CTX_set_client_cert_cb(3)>
 
 =cut

doc/ssl/ssl.pod

@@ -675,6 +675,7 @@ L<SSL_CTX_set_cert_store(3)|SSL_CTX_set_cert_store(3)>,
 L<SSL_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(3)>,
 L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
 L<SSL_CTX_set_client_CA_list(3)|SSL_CTX_set_client_CA_list(3)>,
+L<SSL_CTX_set_client_cert_cb(3)|SSL_CTX_set_client_cert_cb(3)>,
 L<SSL_CTX_set_default_passwd_cb(3)|SSL_CTX_set_default_passwd_cb(3)>,
 L<SSL_CTX_set_generate_session_id(3)|SSL_CTX_set_generate_session_id(3)>,
 L<SSL_CTX_set_info_callback(3)|SSL_CTX_set_info_callback(3)>,