1SSL_CTX_SET_CLIENT_CERT_CB(3) OpenSSL SSL_CTX_SET_CLIENT_CERT_CB(3)
2
6 SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb - handle client
7 certificate callback function
8
10 #include <openssl/ssl.h>
11 void SSL_CTX_set_client_cert_cb(SSL_CTX *ctx,
13 int (*client_cert_cb)(SSL *ssl, X509 **x509,
14 EVP_PKEY **pkey));
15 int (*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509,
16 EVP_PKEY **pkey);
17 int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey);
18
20 SSL_CTX_set_client_cert_cb() sets the client_cert_cb() callback, that
21 is called when a client certificate is requested by a server and no
22 certificate was yet set for the SSL object.
23 When client_cert_cb() is NULL, no callback function is used.
25 SSL_CTX_get_client_cert_cb() returns a pointer to the currently set
27 callback function.
28 client_cert_cb() is the application defined callback. If it wants to
30 set a certificate, a certificate/private key combination must be set
31 using the x509 and pkey arguments and "1" must be returned. The
32 certificate will be installed into ssl, see the NOTES and BUGS
33 sections. If no certificate should be set, "0" has to be returned and
34 no certificate will be sent. A negative return value will suspend the
35 handshake and the handshake function will return immediately.
36 SSL_get_error(3) will return SSL_ERROR_WANT_X509_LOOKUP to indicate,
37 that the handshake was suspended. The next call to the handshake
38 function will again lead to the call of client_cert_cb(). It is the job
39 of the client_cert_cb() to store information about the state of the
40 last call, if required to continue.
41
43 During a handshake (or renegotiation) a server may request a
44 certificate from the client. A client certificate must only be sent,
45 when the server did send the request.
46 When a certificate was set using the SSL_CTX_use_certificate(3) family
48 of functions, it will be sent to the server. The TLS standard requires
49 that only a certificate is sent, if it matches the list of acceptable
50 CAs sent by the server. This constraint is violated by the default
51 behavior of the OpenSSL library. Using the callback function it is
52 possible to implement a proper selection routine or to allow a user
53 interaction to choose the certificate to be sent.
54 If a callback function is defined and no certificate was yet defined
56 for the SSL object, the callback function will be called. If the
57 callback function returns a certificate, the OpenSSL library will try
58 to load the private key and certificate data into the SSL object using
59 the SSL_use_certificate() and SSL_use_private_key() functions. Thus it
60 will permanently install the certificate and key for this SSL object.
61 It will not be reset by calling SSL_clear(3). If the callback returns
62 no certificate, the OpenSSL library will not send a certificate.
63
65 SSL_CTX_get_client_cert_cb() returns function pointer of
66 client_cert_cb() or NULL if the callback is not set.
67
69 The client_cert_cb() cannot return a complete certificate chain, it can
70 only return one client certificate. If the chain only has a length of
71 2, the root CA certificate may be omitted according to the TLS standard
72 and thus a standard conforming answer can be sent to the server. For a
73 longer chain, the client must send the complete chain (with the option
74 to leave out the root CA certificate). This can only be accomplished by
75 either adding the intermediate CA certificates into the trusted
76 certificate store for the SSL_CTX object (resulting in having to add CA
77 certificates that otherwise maybe would not be trusted), or by adding
78 the chain certificates using the SSL_CTX_add_extra_chain_cert(3)
79 function, which is only available for the SSL_CTX object as a whole and
80 that therefore probably can only apply for one client certificate,
81 making the concept of the callback function (to allow the choice from
82 several certificates) questionable.
83 Once the SSL object has been used in conjunction with the callback
85 function, the certificate will be set for the SSL object and will not
86 be cleared even when SSL_clear(3) is being called. It is therefore
87 mandatory to destroy the SSL object using SSL_free(3) and create a new
88 one to return to the previous state.
89
91 ssl(7), SSL_CTX_use_certificate(3), SSL_CTX_add_extra_chain_cert(3),
92 SSL_get_client_CA_list(3), SSL_clear(3), SSL_free(3)
93
95 Copyright 2002-2018 The OpenSSL Project Authors. All Rights Reserved.
96 Licensed under the OpenSSL license (the "License"). You may not use
98 this file except in compliance with the License. You can obtain a copy
99 in the file LICENSE in the source distribution or at
100 <https://www.openssl.org/source/license.html>.
1011.1.1q 2022-07-21 SSL_CTX_SET_CLIENT_CERT_CB(3)