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 int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey);
17
18=head1 DESCRIPTION
19
20SSL_CTX_set_client_cert_cb() sets the client_cert_cb() callback, that is
21called when a client certificate is requested by a server and no certificate
22was yet set for the SSL object.
23
24When client_cert_cb() is NULL, no callback function is used.
25
26SSL_CTX_get_client_cert_cb() returns a pointer to the currently set callback
27function.
28
29client_cert_cb() is the application defined callback. If it wants to
30set a certificate, a certificate/private key combination must be set
31using the B<x509> and B<pkey> arguments and "1" must be returned. The
32certificate will be installed into B<ssl>, see the NOTES and BUGS sections.
33If no certificate should be set, "0" has to be returned and no certificate
34will be sent. A negative return value will suspend the handshake and the
35handshake function will return immediately. L<SSL_get_error(3)>
36will return SSL_ERROR_WANT_X509_LOOKUP to indicate, that the handshake was
37suspended. The next call to the handshake function will again lead to the call
38of client_cert_cb(). It is the job of the client_cert_cb() to store information
39about the state of the last call, if required to continue.
40
41=head1 NOTES
42
43During a handshake (or renegotiation) a server may request a certificate
44from the client. A client certificate must only be sent, when the server
45did send the request.
46
47When a certificate was set using the
48L<SSL_CTX_use_certificate(3)> family of functions,
49it will be sent to the server. The TLS standard requires that only a
50certificate is sent, if it matches the list of acceptable CAs sent by the
51server. This constraint is violated by the default behavior of the OpenSSL
52library. Using the callback function it is possible to implement a proper
53selection routine or to allow a user interaction to choose the certificate to
54be sent.
55
56If a callback function is defined and no certificate was yet defined for the
57SSL object, the callback function will be called.
58If the callback function returns a certificate, the OpenSSL library
59will try to load the private key and certificate data into the SSL
60object using the SSL_use_certificate() and SSL_use_private_key() functions.
61Thus it will permanently install the certificate and key for this SSL
62object. It will not be reset by calling L<SSL_clear(3)>.
63If the callback returns no certificate, the OpenSSL library will not send
64a certificate.
65
66=head1 RETURN VALUES
67
68SSL_CTX_get_client_cert_cb() returns function pointer of client_cert_cb() or
69NULL if the callback is not set.
70
71=head1 BUGS
72
73The client_cert_cb() cannot return a complete certificate chain, it can
74only return one client certificate. If the chain only has a length of 2,
75the root CA certificate may be omitted according to the TLS standard and
76thus a standard conforming answer can be sent to the server. For a
77longer chain, the client must send the complete chain (with the option
78to leave out the root CA certificate). This can only be accomplished by
79either adding the intermediate CA certificates into the trusted
80certificate store for the SSL_CTX object (resulting in having to add
81CA certificates that otherwise maybe would not be trusted), or by adding
82the chain certificates using the
83L<SSL_CTX_add_extra_chain_cert(3)>
84function, which is only available for the SSL_CTX object as a whole and that
85therefore probably can only apply for one client certificate, making
86the concept of the callback function (to allow the choice from several
87certificates) questionable.
88
89Once the SSL object has been used in conjunction with the callback function,
90the certificate will be set for the SSL object and will not be cleared
91even when L<SSL_clear(3)> is being called. It is therefore
92mandatory to destroy the SSL object using L<SSL_free(3)>
93and create a new one to return to the previous state.
94
95=head1 SEE ALSO
96
97L<ssl(7)>, L<SSL_CTX_use_certificate(3)>,
98L<SSL_CTX_add_extra_chain_cert(3)>,
99L<SSL_get_client_CA_list(3)>,
100L<SSL_clear(3)>, L<SSL_free(3)>
101
102=head1 COPYRIGHT
103
104Copyright 2002-2018 The OpenSSL Project Authors. All Rights Reserved.
105
106Licensed under the OpenSSL license (the "License").  You may not use
107this file except in compliance with the License.  You can obtain a copy
108in the file LICENSE in the source distribution or at
109L<https://www.openssl.org/source/license.html>.
110
111=cut
112