1=pod
2
3=head1 NAME
4
5SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client certificate callback function
6
7=head1 SYNOPSIS
8
9 #include <openssl/ssl.h>
10
11 void SSL_CTX_set_client_cert_cb(SSL_CTX *ctx,
12                                 int (*client_cert_cb)(SSL *ssl, X509 **x509,
13                                                       EVP_PKEY **pkey));
14 int (*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509,
15                                                 EVP_PKEY **pkey);
16
17=head1 DESCRIPTION
18
19SSL_CTX_set_client_cert_cb() sets the I<client_cert_cb> callback, that is
20called when a client certificate is requested by a server and no certificate
21was yet set for the SSL object.
22
23When I<client_cert_cb> is NULL, no callback function is used.
24
25SSL_CTX_get_client_cert_cb() returns a pointer to the currently set callback
26function.
27
28I<client_cert_cb> is the application defined callback. If it wants to
29set a certificate, a certificate/private key combination must be set
30using the I<x509> and I<pkey> arguments and "1" must be returned. The
31certificate will be installed into I<ssl>, see the NOTES and BUGS sections.
32If no certificate should be set, "0" has to be returned and no certificate
33will be sent. A negative return value will suspend the handshake and the
34handshake function will return immediately. L<SSL_get_error(3)>
35will return SSL_ERROR_WANT_X509_LOOKUP to indicate, that the handshake was
36suspended. The next call to the handshake function will again lead to the call
37of I<client_cert_cb>. It is the job of the I<client_cert_cb> to store information
38about the state of the last call, if required to continue.
39
40=head1 NOTES
41
42During a handshake (or renegotiation) a server may request a certificate
43from the client. A client certificate must only be sent, when the server
44did send the request.
45
46When a certificate was set using the
47L<SSL_CTX_use_certificate(3)> family of functions,
48it will be sent to the server. The TLS standard requires that only a
49certificate is sent, if it matches the list of acceptable CAs sent by the
50server. This constraint is violated by the default behavior of the OpenSSL
51library. Using the callback function it is possible to implement a proper
52selection routine or to allow a user interaction to choose the certificate to
53be sent.
54
55If a callback function is defined and no certificate was yet defined for the
56SSL object, the callback function will be called.
57If the callback function returns a certificate, the OpenSSL library
58will try to load the private key and certificate data into the SSL
59object using the SSL_use_certificate() and SSL_use_private_key() functions.
60Thus it will permanently install the certificate and key for this SSL
61object. It will not be reset by calling L<SSL_clear(3)>.
62If the callback returns no certificate, the OpenSSL library will not send
63a certificate.
64
65=head1 RETURN VALUES
66
67SSL_CTX_get_client_cert_cb() returns function pointer of I<client_cert_cb> or
68NULL if the callback is not set.
69
70=head1 BUGS
71
72The I<client_cert_cb> cannot return a complete certificate chain, it can
73only return one client certificate. If the chain only has a length of 2,
74the root CA certificate may be omitted according to the TLS standard and
75thus a standard conforming answer can be sent to the server. For a
76longer chain, the client must send the complete chain (with the option
77to leave out the root CA certificate). This can only be accomplished by
78either adding the intermediate CA certificates into the trusted
79certificate store for the SSL_CTX object (resulting in having to add
80CA certificates that otherwise maybe would not be trusted), or by adding
81the chain certificates using the
82L<SSL_CTX_add_extra_chain_cert(3)>
83function, which is only available for the SSL_CTX object as a whole and that
84therefore probably can only apply for one client certificate, making
85the concept of the callback function (to allow the choice from several
86certificates) questionable.
87
88Once the SSL object has been used in conjunction with the callback function,
89the certificate will be set for the SSL object and will not be cleared
90even when L<SSL_clear(3)> is being called. It is therefore
91mandatory to destroy the SSL object using L<SSL_free(3)>
92and create a new one to return to the previous state.
93
94=head1 SEE ALSO
95
96L<ssl(7)>, L<SSL_CTX_use_certificate(3)>,
97L<SSL_CTX_add_extra_chain_cert(3)>,
98L<SSL_get_client_CA_list(3)>,
99L<SSL_clear(3)>, L<SSL_free(3)>
100
101=head1 COPYRIGHT
102
103Copyright 2002-2020 The OpenSSL Project Authors. All Rights Reserved.
104
105Licensed under the Apache License 2.0 (the "License").  You may not use
106this file except in compliance with the License.  You can obtain a copy
107in the file LICENSE in the source distribution or at
108L<https://www.openssl.org/source/license.html>.
109
110=cut
111