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131 .IX Title "SSL_CTX_set_client_cert_cb 3"
132 .TH SSL_CTX_set_client_cert_cb 3 "2017-05-25" "1.0.2l" "OpenSSL"
133 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
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138 SSL_CTX_set_client_cert_cb, SSL_CTX_get_client_cert_cb \- handle client certificate callback function
140 .IX Header "SYNOPSIS"
142 \& #include <openssl/ssl.h>
144 \& void SSL_CTX_set_client_cert_cb(SSL_CTX *ctx, int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey));
145 \& int (*SSL_CTX_get_client_cert_cb(SSL_CTX *ctx))(SSL *ssl, X509 **x509, EVP_PKEY **pkey);
146 \& int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey);
149 .IX Header "DESCRIPTION"
150 \&\fISSL_CTX_set_client_cert_cb()\fR sets the \fB\f(BIclient_cert_cb()\fB\fR callback, that is
151 called when a client certificate is requested by a server and no certificate
152 was yet set for the \s-1SSL\s0 object.
154 When \fB\f(BIclient_cert_cb()\fB\fR is \s-1NULL,\s0 no callback function is used.
156 \&\fISSL_CTX_get_client_cert_cb()\fR returns a pointer to the currently set callback
159 \&\fIclient_cert_cb()\fR is the application defined callback. If it wants to
160 set a certificate, a certificate/private key combination must be set
161 using the \fBx509\fR and \fBpkey\fR arguments and \*(L"1\*(R" must be returned. The
162 certificate will be installed into \fBssl\fR, see the \s-1NOTES\s0 and \s-1BUGS\s0 sections.
163 If no certificate should be set, \*(L"0\*(R" has to be returned and no certificate
164 will be sent. A negative return value will suspend the handshake and the
165 handshake function will return immediately. \fISSL_get_error\fR\|(3)
166 will return \s-1SSL_ERROR_WANT_X509_LOOKUP\s0 to indicate, that the handshake was
167 suspended. The next call to the handshake function will again lead to the call
168 of \fIclient_cert_cb()\fR. It is the job of the \fIclient_cert_cb()\fR to store information
169 about the state of the last call, if required to continue.
172 During a handshake (or renegotiation) a server may request a certificate
173 from the client. A client certificate must only be sent, when the server
174 did send the request.
176 When a certificate was set using the
177 \&\fISSL_CTX_use_certificate\fR\|(3) family of functions,
178 it will be sent to the server. The \s-1TLS\s0 standard requires that only a
179 certificate is sent, if it matches the list of acceptable CAs sent by the
180 server. This constraint is violated by the default behavior of the OpenSSL
181 library. Using the callback function it is possible to implement a proper
182 selection routine or to allow a user interaction to choose the certificate to
185 If a callback function is defined and no certificate was yet defined for the
186 \&\s-1SSL\s0 object, the callback function will be called.
187 If the callback function returns a certificate, the OpenSSL library
188 will try to load the private key and certificate data into the \s-1SSL\s0
189 object using the \fISSL_use_certificate()\fR and \fISSL_use_private_key()\fR functions.
190 Thus it will permanently install the certificate and key for this \s-1SSL\s0
191 object. It will not be reset by calling \fISSL_clear\fR\|(3).
192 If the callback returns no certificate, the OpenSSL library will not send
196 The \fIclient_cert_cb()\fR cannot return a complete certificate chain, it can
197 only return one client certificate. If the chain only has a length of 2,
198 the root \s-1CA\s0 certificate may be omitted according to the \s-1TLS\s0 standard and
199 thus a standard conforming answer can be sent to the server. For a
200 longer chain, the client must send the complete chain (with the option
201 to leave out the root \s-1CA\s0 certificate). This can only be accomplished by
202 either adding the intermediate \s-1CA\s0 certificates into the trusted
203 certificate store for the \s-1SSL_CTX\s0 object (resulting in having to add
204 \&\s-1CA\s0 certificates that otherwise maybe would not be trusted), or by adding
205 the chain certificates using the
206 \&\fISSL_CTX_add_extra_chain_cert\fR\|(3)
207 function, which is only available for the \s-1SSL_CTX\s0 object as a whole and that
208 therefore probably can only apply for one client certificate, making
209 the concept of the callback function (to allow the choice from several
210 certificates) questionable.
212 Once the \s-1SSL\s0 object has been used in conjunction with the callback function,
213 the certificate will be set for the \s-1SSL\s0 object and will not be cleared
214 even when \fISSL_clear\fR\|(3) is being called. It is therefore
215 mandatory to destroy the \s-1SSL\s0 object using \fISSL_free\fR\|(3)
216 and create a new one to return to the previous state.
218 .IX Header "SEE ALSO"
219 \&\fIssl\fR\|(3), \fISSL_CTX_use_certificate\fR\|(3),
220 \&\fISSL_CTX_add_extra_chain_cert\fR\|(3),
221 \&\fISSL_get_client_CA_list\fR\|(3),
222 \&\fISSL_clear\fR\|(3), \fISSL_free\fR\|(3)